18.3.2 Thoracic imaging 3970 Susan J. Copley and D

18.3.2 Thoracic imaging 3970 Susan J. Copley and David M. Hansell

section 18  Respiratory disorders 3970 Miscellaneous tests Analysis of expired air has traditionally been limited to oxygen and carbon dioxide, but recently attention has turned to other gases which are present in very low concentrations. The concentration of exhaled carbon monoxide has been used for some years as a guide to its inhal- ation and as a valuable method for confirming nonsmoking claims. The measurement can now be made very simply with a portable analyser. Breath carbon monoxide is also increased in nonsmoking subjects with asthma, where it appears to be released as a result of airway inflamma- tion. In similar fashion, expired nitric oxide concentration is increased as a consequence of airway inflammation and it has been proposed as a noninvasive way of assessing airway inflammation and monitoring its treatment, particularly in asthma. Care needs to be taken to avoid contamination of expired air from the bronchial tree with that from the nose and nasal sinuses, which contain higher concentrations of NO. FURTHER READING General Gibson GJ (2009). Clinical tests of respiratory function, 3rd edition. Hodder Arnold, London. Hughes JMB (2009). Physiology and practice of pulmonary func- tion. association for respiratory technology and physiology. Boldmere, UK. West JB (2012). Respiratory physiology:  the essentials, 9th edition. Lippincott, London. Performance and interpretation of respiratory function tests American Thoracic Society/​European Respiratory Society (2002). ATS/​ERS statement on respiratory muscle testing. Am J Respir Crit Care Med, 166, 518–​624. Culver BH, et al. (2017). Recommendations for a standardized pul- monary function report: an official American Thoracic Society statement. Am J Respir Crit Care Med, 196, 1463–72. MacIntyre N, et al. (2005). Standardisation of the single-​breath deter- mination of carbon monoxide uptake in the lung. Eur Respir J, 26, 720–​35. Miller MR, et  al. (2005). General considerations for lung function testing. Eur Respir J, 26, 153–​61. Miller MR, et al. (2005). Standardisation of spirometry. Eur Respir J, 26, 319–​38. Oostveen E, et al. (2003). The forced oscillation technique in clinical practice: methodology, recommendations and future developments. Eur Respir J, 22, 1026–​41. Pellegrino R, et al. (2005). Interpretative strategies for lung function tests. Eur Respir J, 26, 948–​68. Wanger J, et al. (2005). Standardisation of the measurement of lung volumes. Eur Respir J, 26, 511–​22. Sources of normal reference values Cerveri I, et al. (1995). Reference values of arterial oxygen tension
in middle-​aged and elderly. Am J Respir Crit Care Med, 152, 934–​41. Hankinson JL, Odenkrantz JR, Fedan KB (1999). Spirometric ref- erence values from a sample of the general U.S. population. Am J Respir Crit Care Med, 159, 179–​87. Quanjer PH, et al. (2012). Multi-​ethnic reference values for spirometry for the 3–​95-​yr age range: the global lung function 2012 equations. Eur Respir J, 40, 1324–​43. Stanojevic S, et al. (2017). Official ERS technical standards: Global lung function initiative reference values for the carbon monoxide transfer factor for Caucasians. Eur Respir J, 50, 1700010. doi.org/10.1183/13 993003.00010.2017. Interpretation of blood gases Berend K, de Vries AP, Gans RO (2014). Physiological approach to as- sessment of acid-​base disturbances. N Engl J Med, 371, 1434–​45. Seifter JL (2014). Integration of acid-​base and electrolyte disorders.
N Engl J Med, 371, 1821–​31. Exercise tests Ward SE, Palange P (eds) (2007). Clinical exercise testing. Eur Respir Monogr, 40, 1–​35. Wasserman K, et al. (2011). Principles of exercise testing and interpreta- tion, 5th edition. Lippincott, London. Miscellaneous Dweil RA, et al. (2011). An official ATS clinical practice guideline:
interpretation of exhaled nitric oxide levels (FeNO) for clinical ap- plications. Am J Respir Crit Care Med, 184, 602–15. 18.3.2  Thoracic imaging Susan J. Copley and David M. Hansell ESSENTIALS Radiographic findings should always be interpreted in conjunction with the clinical picture. Chest radiography—​this remains the commonest technique in the investigation of suspected thoracic disease. Advantages are cost, availability, and a lower radiation dose than CT, but even with an op- timal technique nearly one-​third of the lungs are partially obscured by the overlying mediastinum, diaphragm, and ribs. CT—​is more sensitive and specific than chest radiography in a range of pulmonary disorders, including airways disease and diffuse interstitial lung disease. In the latter condition, high-​resolution CT images of the lung correlate closely with the microscopic appear- ances of pathological specimens and are a substantial improvement over chest radiography in terms of sensitivity, specificity, and diag- nostic accuracy. In many centres CT has supplanted ventilation–​ perfusion radionuclide imaging in the investigation of patients with suspected pulmonary embolism. Radiation dose is always a con- sideration in CT, particularly in children and young adults, however recent advances make this less of an issue. Ventilation/​perfusion radionuclide scanning—​is the commonest radionuclide study of the lungs and is most frequently used to con- firm or exclude the diagnosis of suspected pulmonary embolism, but is increasingly being supplanted by CT. Due to the reduced radiation

18.3.2  Thoracic imaging 3971 dose (particularly to breast tissue) of a perfusion only study, the tech- nique may be useful in specific circumstances (e.g. in young females with a normal chest radiograph and suspected pulmonary embolism). Positron emission tomography (PET) and CT/​PET—​usually employed with the isotope 18F-​fluorodeoxyglucose for investigation and staging of lung cancer. Transthoracic ultrasonography—​the use of this technique for the imaging of lung parenchyma is limited because high-​frequency sound waves do not traverse normally aerated lung, but fluid can be readily detected and the main use of ultrasound is for the localization of small or loculated pleural effusions and guiding biopsy of periph- eral lung lesions, anterior mediastinal masses, and intercostal chest drain insertion. Ultrasound is also used for evaluating diaphragmatic movement in suspected diaphragmatic paralysis or dysfunction. MRI—​imaging of the mediastinum by CT scanning and MRI are comparable, but MR images of the lungs are generally inferior to those obtained by CT because of their very low water (and therefore proton) content. Other disadvantages are respiratory and cardiac motion arte- fact (unless respiratory and cardiac gating are used), relatively long scan- ning time, and difficulties with the monitoring of critically ill patients. However, there have been recent advances in techniques evaluating lung ventilation and perfusion. MRI may also have a role in patients who are allergic to iodinated intravenous contrast for the evaluation of suspected pulmonary embolus or malignancies such as malignant mesothelioma. Introduction Despite recent technological advances, chest radiography remains the cornerstone of thoracic imaging. The chest radiograph is justifiably re- garded as an integral part of the examination of the patient in respira- tory medicine. Because of the wealth of information available from chest radiography, careful interpretation of the chest radiograph remains a necessary clinical skill. Advances in cross-​sectional imaging have had a great impact in improving the diagnosis of thoracic pathology, not only for the assessment of mediastinal disease but also in the evaluation of patients with suspected diffuse lung disease. Nevertheless, a chest radio- graph should be obtained and looked at carefully before submitting a patient to more sophisticated imaging techniques. In the case of CT, the expense and radiation burden are important considerations. Techniques in thoracic imaging Chest radiography The first chest radiograph was taken over 100 years ago and chest radiography is now the most frequently requested radiological in- vestigation worldwide. The technique has changed surprisingly little over the years, although digital technology has recently been used to overcome some of the shortcomings of film-​based radiography. Technical considerations An ideal chest radiograph is taken with the patient standing erect, sus- pending respiration at total lung capacity and with the X-​ray beam traversing the thorax from back to front (the posteroanterior (PA) or frontal view). Because of the wide range of densities within the chest (soft tissues of the mediastinum through to aerated lung), perfect exposure of every part of the chest radiograph is impossible. The ­resulting ­suboptimal exposure of the denser part of the chest can be ­partially overcome with a high-​kilovoltage technique (120–​150  kVp). With this technique there is greater penetration of the mediastinum, which improves visualization of the trachea and main bronchi. However, a disadvantage of high-​kilovoltage radiography is the relatively poor demonstration of calcified structures so that rib fractures and calcified pulmonary nodules or pleural plaques are less conspicuous. Even with optimal technique, nearly one-​third of the lungs are partially obscured by the overlying mediastinum, diaphragm, and ribs. Automatic exposure devices have been developed to optimally ­expose the various parts of the chest. Digital image capture devices (including flat panel detectors and high-​density line-​scan solid state detectors) have largely replaced conventional film radiography. High-​density line-​ scan solid state detectors consist of a photostimulable phosphor plate in a conventional cassette (which does not contain film) and which is exposed in the normal way. The energy of the incident X-​ray beam is stored as a latent image. The phosphor plate is then scanned with a laser beam and the light emitted from the excited latent image is de- tected by a photomultiplier. Thereafter, this signal is processed in digital form. The digital image is then viewed on a monitor. The advantage of phosphor plate computed radiography is that it can retrieve an image of diagnostic quality from an imperfect exposure which would result in a non-​diagnostic conventional film radiograph. Manipulation or post-​ processing of the digital image (e.g. ‘edge enhancement’), aids the detec- tion of linear structures, such as the edge of a pneumothorax or central venous catheters (see Fig. 18.3.2.1). Some centres may have access to techniques such as digital tomosynthesis, when X-​ray tube rotation and flat panel detectors produce multiple ‘slices’ through the thorax in a single exposure, which may be of value in pulmonary nodule ­detection and characterization without the distraction of overlying bony and soft tissue structures. Digital subtraction radiography utilizes the different absorption of high and low kV photons by varying anatomical struc- tures (e.g. bone and soft tissue), allowing bony structures to be ‘sub- tracted’, which may be of value in pulmonary nodule detection. With the advent of picture archiving and communication systems that ­enable storage and transfer of digital images, most radiology departments in the developed world are now ‘filmless’, with images available to view simultaneously on both local and distant workstations. Standard radiographic views of the chest The PA projection is the standard view (see Fig. 18.3.2.20a). The pa- tient is positioned with the anterior chest wall against the detector panel and the arms are abducted to rotate the scapulae away from the pos- terior chest. Chest radiographs in the anteroposterior (AP) projection are usually taken when the patient is too ill to stand for a formal PA radiograph. A consequence of this view is that the heart is magnified because it lies further from the detector. Moreover, the shorter distance from X-​ray tube to detector, which is inevitable when a portable AP radiograph is taken, causes further magnification that must be taken into account when assessing the heart size on an AP chest radiograph. The lateral radiograph is obtained by placing the patient at right ­angles to the detector. The lateral projection provides the third dimen- sion and helps to determine the site of a lesion identified on the PA projection, although it is surprising how often an opacity clearly seen on the PA radiograph is invisible on the lateral radiograph. As well as allowing accurate localization of lesions and devices, the lateral radio- graph may reveal abnormalities that lie behind the heart or diaphragm.

section 18  Respiratory disorders 3972 Over the years a number of supplementary projections have been developed to provide information about areas that are not easily seen on the standard PA and lateral radiograph. With the advent of cross-​ sectional imaging, notably CT, many of these extra views have be- come obsolete. However, even with access to CT, some of these views supply extra anatomical detail readily and inexpensively. The lateral decubitus projection is sometimes useful for the demonstration of small pleural effusions, for which view the patient lies on their side (suspected effusion downwards). However, ultrasonography is a reli- able technique for demonstrating small pleural effusions and can be performed at the patient’s bedside. Other supplementary projections (e.g. apical and lordotic views), used to improve visualization of the lung at the extreme apices, are now less commonly performed: CT is much more effective at showing pathology in these difficult areas. Transthoracic ultrasonography High-​frequency sound waves do not traverse air and are completely reflected at interfaces between soft tissue and air. The use of this technique for the routine evaluation of lung parenchyma is there- fore limited because of normally aerated lung. However, fluid can be readily detected, and the main use of ultrasound examination is for the localization of pleural effusions (Fig. 18.3.2.2). Furthermore, ultrasonography can differentiate between pleural fluid and pleural thickening in cases where radiography cannot make this distinction. Detection of a pneumothorax is also possible with transthoracic ultrasonography, and may be of particular use in the context of major trauma (Fig. 18.3.2.3). Ultrasonography is also an extremely useful technique for guid­ ing percutaneous needle biopsy of masses arising from the chest wall or pleura (see Video 18.3.2.1), or peripheral pulmonary masses or consolidation, and for aiding the accurate placement of a chest drain within a pleural collection. Ultrasound-​guided bi- opsy of supraclavicular lymph nodes is of use as an alternative to Fig. 18.3.2.1  Chest radiographs of an individual post right subclavian central venous catheter insertion. The image (b) has been inverted post-​ processing to aid detection of the tip of the catheter, which is incorrectly placed within the right internal jugular vein (arrow). Fig. 18.3.2.2  Ultrasonography showing an empyema. Thick fibrinous septations traverse the pleural space. The diaphragm and liver are seen on the right of the image. Fig. 18.3.2.3  Ultrasound of the pleura demonstrating normal ‘comet’ tails (arrows) in (a) and a lack of comet tails at the chest wall/​pleural interface (b) in a patient with a pneumothorax.

18.3.2  Thoracic imaging 3973 bronchoscopic or image-​guided lung biopsy in the diagnosis and staging of lung cancer, especially in patients with poor respiratory reserve. CT CT depends on the same basic principle as conventional radiog- raphy, namely the differential absorption of X-​rays by tissues of disparate densities, although CT has much greater sensitivity to dif- ferences in attenuation of X-​rays by various tissues. A CT machine consists of an X-​ray source and an array of detectors that surround the patient. The X-​ray source rotates around the patient and the resulting attenuated beam is measured by the detectors. The sig- nals from the detectors are used to construct an image by a math- ematical technique. The reconstructed images are transverse (axial) cross-​sections of the patient and are viewed as if from the feet end of the patient (i.e. on the image, the patient’s right side is to the viewer’s left). Each CT section is a matrix of three-​dimensional elements (voxels) containing a measurement of X-​ray attenuation, arbitrarily expressed as Hounsfield units (HU): water measures 0 HU and air –​ 1000 HU (so that lung parenchyma are approximately –​600 HU), fat is –80 HU, soft tissue 40–​80 HU, and bone 800 HU. If a voxel is completely occupied by a tissue of uniform density (most fre- quently the case with narrow sections, e.g. 1 mm), then the HU will be truly representative of that tissue. If the section contains tis- sues of two different densities (more likely to occur with thicker sections, e.g. 5 mm), for example, half a lung and a half dome of diaphragm, then the attenuation value will be a weighted average of the two components—​the so-​called ‘partial volume’ effect. The cross-​sectional nature of CT means that it can accurately lo- calize lesions seen on only one view on chest radiography. The su- perior contrast resolution of CT gives exquisite detail of the various components of mediastinal anatomy (e.g. lymph nodes and ves- sels) and density differences (e.g. calcifications within a pulmonary nodule). Different image settings are needed to view the soft tissue structures of the mediastinum and the aerated lung parenchyma, re- spectively (Fig. 18.3.2.4). The principle of continuous volume acquisition (formerly known as spiral or helical CT) involves continuous rotation of the X-​ray beam and detectors around the patient while the table moves into the gantry. Markedly reduced scan times are possible with this tech- nique, allowing the entire thorax to be imaged in a single breath-​ hold. An examination of sufficient diagnostic quality can be obtained in breathless patients and young children during quiet respiration. Another advance has been the development of multidetector CT (MDCT), where multiple rows of detectors rotate around the pa- tient acquiring volumetric data, allowing even further reduced scan times—​hence most modern CT scanners are termed ‘volumetric’. The technique allows for accurate timing of an intravenous in- jection of contrast medium for optimum opacification (e.g. of the pulmonary arteries), enabling pulmonary emboli to be detected (Fig. 18.3.2.5). In many centres, contrast-​enhanced CT has sup- planted ventilation/​perfusion (V/​Q) radionuclide imaging in the investigation of patients with suspected pulmonary embolism and often demonstrates an alternative cause for the patient’s symptoms in ‘negative’ cases. Computer software can perform multiplanar two-​ and three-​ dimensional image reconstructions of volumetric data sets, including views of the bronchial tree which can elegantly demonstrate normal variants (Fig. 18.3.2.6) and aid interventional techniques such as bronchial stent placement (Fig. 18.3.2.7). In addition MIP (max- imum intensity projection) and MinIP (minimum intensity projec- tion) images project the voxels with either high or low density from a ‘slab’ of thin sections. MIPS are used to increase the conspicuity of spherical pulmonary nodules by comparison with branching pulmonary vessels (Video 18.3.2.2). MinIPs increase the conspi- cuity of low attenuation areas within the lung such as emphysema (Fig. 18.3.2.8). With the advent of MDCT and the use of ECG gating, evaluation of structures such as coronary arteries is routine due to reduction in motion artefact and increased spatial resolution. High-​resolution CT utilizes thin sections (1–​2 mm) and a high spatial frequency reconstruction algorithm to produce highly de- tailed sections of the lung parenchyma (Fig. 18.3.2.9). Most centres with MDCT routinely obtain high-​resolution reconstructions from the volumetric data, by contrast to the dedicated ‘interspaced’ 1 mm sections every 10 mm obtained with the first-​generation single slice CT scanners. Reformats in the coronal and sagittal planes can be helpful for demonstrating the distribution of interstitial lung dis- eases (Fig. 18.3.2.10) and may allow more accurate and confident Fig. 18.3.2.4  CT section through the mid-​thorax. The window settings have been adjusted to show details of (a) the lungs and (b) the soft tissues of the mediastinum.

section 18  Respiratory disorders 3974 identification of individual features (particularly the distinction be- tween honeycomb cysts and traction bronchiectasis). Submillimetre structures can be resolved with this technique, allowing the subtle and sometimes complex morphology of interstitial lung diseases to be shown with great clarity. Since the mid-​1980s, the development of high-​resolution CT has changed the radiological approach to the diagnosis of diffuse lung disease. High-​resolution CT images of the lung correlate closely with the macroscopic appearances of patho- logical specimens, and high-​resolution CT represents a substantial improvement over chest radiography in terms of sensitivity, specifi- city, and diagnostic accuracy. Furthermore, CT samples a far greater volume of lung than even the most generous lung biopsy, making it less prone to sampling errors. High-​resolution CT has also been shown to provide useful in- formation regarding prognosis and response to treatment in some diffuse lung diseases. Nevertheless, despite the increased confi- dence with which a specific diagnosis of diffuse lung disease can often be made with high-​resolution CT, a multidisciplinary ap- proach (including histopathological examination of a lung biopsy in some cases) is still required. The extent of diffuse lung disease (a) (b) Fig. 18.3.2.5  CTPA of a patient with acute pulmonary emboli. Coronal (a) and axial (b) images show filling defects in contrast media opacification within the pulmonary arteries bilaterally (arrows). Fig. 18.3.2.6  Volume-​rendered 3D reconstruction of the trachea and main bronchi. Note the anatomical variant (tracheal or ‘pig bronchus’ where a bronchus supplying the right upper lobe arises from the trachea; see arrow). Fig. 18.3.2.7  Reconstructed three-​dimensional image from spiral CT showing the carina and the right and left main bronchi viewed from above.

18.3.2  Thoracic imaging 3975 can be precisely estimated on high-​resolution CT and, when a bi- opsy is indicated, the distribution of disease will indicate whether a transbronchial biopsy or an open lung biopsy is more likely to obtain a representative specimen. Dual source CT was initially designed to decrease gantry rotation time by utilizing two orthogonal X-​ray beams and detectors sim- ultaneously rotating around the patient. A spin-​off from this tech- nology was the use of X-​ray beams of different energy, so-​called dual-​energy. This utilizes the different absorption characteristics of photons of different energy (keV) to improve differentiation of various tissues, particularly post-​iodinated intravenous contrast en- hancement. Either a single ‘source’ (with fast kilovoltage switching to produce photons of different energy levels) or a dual source CT scanner can be used. With both types of scanner, the patient is sim- ultaneously exposed to X-​ray photons of both high (140 kVp) and low (80 kVp) energies. Clinical applications include the assessment of lung perfusion, particularly in the context of pulmonary embolic disease. The disadvantages of all CT techniques are relatively high cost and high radiation exposure to the patient, particularly by comparison with chest radiography. Manufacturers have responded to the chal- lenge of dose reduction, but CT should still not be regarded as a routine investigation (especially in children and young adults) and examinations should always be tailored to solve questions not an- swered by less sophisticated investigations. The commonest indica- tions for thoracic CT are summarized in Box 18.3.2.1. MRI The physical principles of MRI are very different from those governing CT. An MR image is obtained by placing the subject in a strong magnetic field that polarizes some of the ubiquitous hydrogen protons (which can be thought of as behaving like randomly orien- tated bar magnets) in the body so that they have the same alignment. The application of radiofrequency wave pulses of specified lengths and repetition (pulse sequences) displaces the protons and some of this transmitted energy is absorbed by them. With the cessation of the radiofrequency pulse, the protons return to their initial alignment and in so doing they emit, as a weak signal, some of the energy they have absorbed; this signal is received and then amplified and handled in digital form and is subsequently reconstructed into an image. (a) (b) Fig. 18.3.2.8  Standard coronal reconstructions (a) compared with coronal MinIP reconstructions (b) in a patient with emphysema. Note the areas of emphysema are more conspicuous on the MinIPs (b). Fig. 18.3.2.9  High-​resolution CT of a patient with lymphangioleiomyomatosis showing thin-​walled cysts throughout the lungs. These cysts were not apparent on chest radiography. Fig. 18.3.2.10  MDCT with coronal reconstructions of a patient with advanced pulmonary Langerhan’s cell histiocytosis. Note the upper and mid-​zone distribution of cystic lung destruction with relative sparing of the lung bases. Box 18.3.2.1  Indications for CT of the thorax • Elucidation of abnormal mediastinal or hilar contour on chest radiography • As part of the staging procedure in the evaluation of a patient with known lung cancer (the findings on CT must be interpreted in con- junction with other investigations) • Detection of pulmonary disease in the face of a questionably ab- normal or apparently normal chest radiograph (notably diffuse inter- stitial disease and bronchiectasis) • Investigation of a patient with haemoptysis in whom chest radio­ graphy and bronchoscopy are normal • Detection of pulmonary embolism • Assessment of complex pleural or chest wall pathology when chest radiography does not adequately show the extent of disease • As a means of guiding the percutaneous needle biopsy of pulmonary lesions or mediastinal masses

section 18  Respiratory disorders 3976 The advantages of MRI include the improved contrast resolution between different soft tissues compared with CT (Fig. 18.3.2.11), and the use of special sequences which give functional information (e.g. the velocity of blood flow). An important advantage of MRI is the lack of any known hazard to the patient, in contrast to CT with its small attendant risk from ionizing radiation. The technique may also be of value in patients who are allergic to iodinated contrast for the detection of pulmonary emboli, or in the evaluation of chronic thomboembolic disease (Fig. 18.3.2.12). Disadvantages of MRI in- clude the long scan time (although this is continually being short- ened), reduced spatial resolution compared with CT, the inability to image calcium, reduced acceptability to patients because of the claustrophobic bore of the magnet, and important contraindications such as permanent cardiac pacemaker devices and ferromagnetic intraocular foreign bodies. In many respects, the imaging of the mediastinum by CT and MRI are comparable. However, MR images of the lungs are currently in- ferior to CT because of their very low water (and therefore proton) content, meaning that the signal produced by normal lung is small and not visualized by conventional sequences. However, the rela- tively recent introduction of hyperpolarized gases, including helium-​ 3, has enabled evaluation of pulmonary function. The use of such agents by inhalation is largely still a research tool, but may provide valuable insights into pulmonary ventilation and small airways func- tion in the future. The use of oxygen ventilation MRI may be a more practical technique and avoid the inherent difficulties in the pro- duction and storage of hyperpolarized inert gases (Fig. 18.3.2.13). Diffusion-​weighted MRI allows the mapping of diffusion processes of molecules (particularly water) in vivo. The technique is particu- larly valuable in neuroimaging, but may be useful in the future for differentiating benign from malignant pulmonary masses and pleural disease, and for assessing tumour response to therapy. Radionuclide imaging V/​Q radionuclide scanning is an effective noninvasive method of providing both anatomical and physiological information about the lung. It is the commonest radionuclide study of the lungs and most frequently used to confirm or exclude the diagnosis of suspected pulmonary embolism. Regional pulmonary capillary perfusion can be assessed following the intravenous injection of a bolus of particles that have been labelled with technetium-​99m. The minute particles are microspheres or macroaggregates of human albumin (15–​70 µm in diameter). These are evenly dispersed by the time they reach the pulmonary circula- tion and they can become temporarily lodged in a very small fraction (<0.5%) of the precapillary arterioles and capillaries of the lungs. The distribution of γ-​ray emission from the technetium-​labelled particles is directly proportional to the regional pulmonary flow and a sig- nificant defect in perfusion is usually readily detected. However, it is important to appreciate that such defects may be due to a variety of conditions other than pulmonary embolism, including any cause of hypoxic vasoconstriction such as an area of subsegmental collapse or a space-​occupying lesion not supplied by the pulmonary circulation. However, in these cases the affected area of lung will be neither venti- lated nor perfused, whereas in acute pulmonary embolism there is no corresponding defect of ventilation. Thus, to improve the specificity of the diagnosis of pulmonary embolism, ventilation scintigraphy is usually performed at the same time as perfusion scanning. Evaluation of ventilation of the lungs depends on filling the distal air spaces with a γ-​ray-​emitting radionuclide. The radionuclides suit- able for inhalation are the inert gases xenon-​133 and krypton-​81m, Fig. 18.3.2.12  Reformatted image from an MRI of the thorax
(viewed posteriorly) in a patient with chronic thomboembolic pulmonary hypertension showing tortuous pulmonary arteries and intravascular webs (arrows). Fig. 18.3.2.11  MRI (T2-​weighted coronal section) showing the relationship of an apical bronchial carcinoma (Pancoast tumour) to the chest wall and adjacent mediastinum. Note the multiple high signal (bright) areas within the vertebral bodies consistent with bony metastases.

18.3.2  Thoracic imaging 3977 or a technetium-​99m aerosol (Technegas). The characteristic abnor- mality of pulmonary embolism is the so-​called ‘mismatched defect’ in which a regional defect in perfusion is not matched by a defect in ventilation (Fig. 18.3.2.14). Because of the importance of establishing a correct diagnosis of pulmonary embolism, V/​Q scans should always be interpreted in the light of current chest radiographs and clinical information. Even then a substantial proportion of V/​Q scans remain indeterminate, (B) 0.0 0 94 188 ∆pO2 (mm Hg) 282 370 470 564 60.0 120.0 Time (sec) Change in partial pressure of oxygen (∆pO2) in healthy lungs 180.0 240.0 (A) (C) (D) Fig. 18.3.2.13  Example of changes in partial pressure of oxygen as measured with dynamic oxygen-​enhanced MRI in a healthy individual. The colour map overlaid on the dynamic images
(A to D, left to right) represents the changes in partial pressure of oxygen (oxygen-​enhanced MRI) in a healthy individual. Images provided courtesy of Jose Ulloa and Geoff Parker, Bioxydyn Limited. Fig. 18.3.2.14  Images from ventilation/​perfusion lung scintigraphy showing multiple perfusion defects (top two of images), which are not matched by ventilation defects (bottom two images) consistent with a high probability of pulmonary emboli.

section 18  Respiratory disorders 3978 hence the increasing use of CT angiography. However, due to the decreased radiation burden, low-​dose perfusion scanning remains a satisfactory first-​line investigation in young patients with no pre-​ existing lung disease and a low pretest probability for pulmonary embolism, but availability ‘out of hours’ remains a problem. Positron emission tomography Positron emission tomography (PET) relies on tissue uptake of radio-​ isotopes that decay by positron emission. Detectors located around the patient map the site of origin of the two resultant photons emitted at 180° from each other. The most widely used isotope for the detec- tion of pulmonary malignancy is 18F-​fluorodeoxyglucose (FDG), a d-​glucose analogue. The increased uptake and retention of glucose by malignant cells allows differentiation of benign from malignant pulmonary masses, detection of lymph node involvement by tumour, and identification of distant metastases. Limitations of the technique include false positive results caused by granulomatous infection and acute inflammation, and false negative results with certain tumours (e.g. indolent adenocarcinomas and carcinoid tumours). Small (<1 cm diameter) malignant lesions may also give false negative results. CT/​PET is where a helical CT is performed simultaneously with PET and the images then coregistered (Fig. 18.3.2.15). The fusion of CT images (which give good anatomical resolution) with PET im- ages (which provide functional data) is an advantage in the staging of thoracic malignancies including lung carcinoma and mesothelioma, particularly for the detection of unsuspected distant metastases. The avidity of tumour uptake may provide prognostic information and assessment of response to treatment. Pulmonary and bronchial arteriography:
Superior vena cavography The ‘gold standard’ for identifying emboli within the pulmonary arteries has traditionally been pulmonary arteriography, which re- quires the catheterization of an antecubital, jugular, or femoral vein, and guidance of the catheter through the right heart under fluoro- scopic control. Although the complication rate is low, it is a time-​ consuming procedure that requires an experienced angiographer (now a rare species). The technique allows embolization of pul- monary arteriovenous malformations—​a specialized technique available in only a few centres (Fig. 18.3.2.16). The bronchial arteries that supply the airways become hyper- trophied in chronic inflammatory pulmonary disease, notably bronchiectasis. Rupture of these vessels can cause severe and life-​ threatening haemoptysis. The bronchial arteries are selectively catheterized by the passage of a catheter via the femoral artery and aorta. Having identified the abnormally hypertrophied bronchial arteries (Fig. 18.3.2.17), they can be therapeutically embolized. This technique is usually successful in treating massive haemoptysis. Superior vena cavography was previously used to evaluate the exact site of narrowing in patients with symptoms of obstruction of the su- perior vena cava, but has largely been supplanted by CT. However, patients with symptoms of superior vena cava obstruction—​most frequently due to neoplastic nodal involvement—​may be success- fully palliated by radiotherapy or the insertion of an expandable metallic wire stent at the site of the narrowing (Fig. 18.3.2.18). A transvenous biopsy at the time of stent insertion may also provide a histopathological diagnosis. Percutaneous lung biopsy Percutaneous needle biopsy of a pulmonary lesion or mediastinal mass is a useful method of obtaining tissue for diagnostic purposes (including for genomic studies) in patients with suspected lung cancer. The procedure is often performed as a day case. Complications include pneumothorax, haemoptysis, and extremely rarely, air em- bolism. Percutaneous biopsy is performed under local anaesthesia with either CT or ultrasound guidance (if the mass abuts the pleura). Contraindications to the procedure include any patient with poor Fig. 18.3.2.15  CT/​PET image showing increased uptake of 18F-​fluorodeoxyglucose (FDG) in the left lung corresponding to a primary bronchial carcinoma. The image on the left is the coronal MDCT image, the central image is the coronal PET, and the image on the right is the coregistered CT/​PET. Note the central necrosis resulting in central photopenia, also the physiological uptake of tracer in the liver, spleen, kidneys, and bladder.

18.3.2  Thoracic imaging 3979 respiratory reserve who is unable to withstand a pneumothorax, clotting disorders, and pulmonary arterial hypertension. Very small lesions (less than a centimetre in diameter), central lesions and those difficult to access because of overlying bony structures may not be possible sample using this technique however. There has been increased interest in the use of CT-​guided ‘labelling’ of small subcentimetre pulmonary nodules using methylene blue dye and fi- ducial markers to aid subsequent surgical identification and resection. Image-​guided ablation Radiofrequency ablation (which employs a small electrode that produces radiofrequency waves), and more recently microwave ab- lation, are usually performed under CT guidance and general an- aesthesia to treat either primary or secondary lung tumours using thermal energy (Fig. 18.3.2.19). The common indications are small primary lung tumours in a patient too unwell to undergo thora- cotomy, debulking of large primary tumours, and treatment of small numbers of pulmonary metastases. Complications are similar to those of percutaneous lung biopsy. Normal radiographic anatomy The mediastinum On a PA chest radiograph (see Fig. 18.3.2.20a) the mediastinal struc- tures are superimposed on one another and thus cannot be distin- guished individually. The mediastinum is conventionally divided into superior, anterior, middle, and posterior compartments: the practical use of these arbitrary divisions is that specific mediastinal patholo- gies show a definite predilection for individual compartments (e.g. a superior mediastinal mass is most frequently due to intrathoracic extension of the thyroid gland, a middle mediastinal mass is usually due to enlarged lymph nodes). However, it should be borne in mind that the position of a mass within one of these compartments is no guarantee of a specific diagnosis, nor do these boundaries preclude disease from spreading from one compartment to the next. Because only the outline of the mediastinum and the air-​ containing trachea and bronchi are clearly seen on a PA chest radio- graph, the mediastinal anatomy will be considered in more detail in the description of CT anatomy. On a chest radiograph, the right superior mediastinal border is formed by the right brachiocephalic vein and superior vena cava. The mediastinal border to the left of the trachea above the aortic arch represents the sum of the left carotid and left subclavian arteries together with the left brachiocephalic and jugular veins. The left cardiac border comprises the left atrial appendage which merges inferiorly with the left ventricle. The car- diac silhouette is always sharply outlined: any blurring of the border denotes replacement of the aerated lung, usually by collapse or con- solidation, in the immediately adjacent lung (see ‘Silhouette sign’ in ‘Common radiological signs of disease’). The density of the cardiac shadow to the left and right of the ver- tebral column should be identical and any difference signals pul- monary pathology (e.g. consolidation in a lower lobe). A density with a convex lateral border is often seen through the right heart border on a well-​penetrated film: this apparent mass is due to the confluence of the pulmonary veins as it enters the left atrium and is of no pathological significance. The trachea and main bronchi are visible through the upper and middle mediastinum. The trachea is rarely straight and is often to the right of the midline at its midpoint. In elderly patients, the tra- chea may appear dramatically displaced by a dilated aortic arch. The angle of the carina is usually somewhat less than 80°. Splaying of the carina is a sign of gross disease, either in the form of massive subcarinal lymphadenopathy, or a markedly enlarged left atrium. A  more sensitive sign of a subcarinal mass is obliteration of the azygo-​oesophageal line which is usually visible on a well-​penetrated chest radiograph. The origins of the lobar bronchi, where they are projected over the mediastinal shadow, can usually be made out but the segmental bronchi within the lungs are not generally seen on plain radiography. The hilar structures The hilar shadows on a chest radiograph are a complex summa- tion of the pulmonary arteries and veins with virtually no contri- bution from the overlying bronchial walls or normal-​sized lymph nodes. The hila are approximately the same size and the left hilum always lies between 0.5 cm and 1.5 cm above the level of Fig. 18.3.2.16  Images from a digital subtraction angiogram of a patient with multiple pulmonary arteriovenous malformations (note the multiple metallic embolization coils) demonstrating a pre-​ (a) and post-​ (b) embolization pulmonary arteriovenous malformation at the right base.

section 18  Respiratory disorders 3980 the right hilum. The size and shape of the hila in normal individ- uals show remarkable variation so that subtle abnormalities are difficult to detect. At least as important as an abnormal contour in detecting a mass at the hilum is a discrepancy in density be- tween the two hila: both hilar shadows, at equivalent points, will be of equal density and a mass at the hilum (or an intrapulmonary mass in line with the hilum) will be evident as increased density of that hilum. The pulmonary fissures, vessels, and bronchi The lobes of each lung are surrounded by visceral pleura: the upper and lower lobes of the left lung are separated by the major (or ob- lique) fissure. The upper, middle, and lower lobes of the right lung are separated by the major (or oblique) and minor (horizontal or transverse) fissures. The minor fissure is visible on about 60% of normal PA chest radiographs. In normal individuals, this fissure runs horizontally and any deviation from this course represents loss of volume of a lobe. The major fissures are inconstantly identifiable on lateral radiographs. Other fissures are occasionally seen (e.g. in Fig. 18.3.2.17  A patient with massive haemoptysis from an aspergilloma at the right apex (arrows) (a). Digital subtraction images demonstrating hypertrophied bronchial arteries which have subsequently been embolized (c). Fig. 18.3.2.18  Images from a superior vena cava stent procedure. Image (a) shows narrowing of the superior vena cava (SVC) from extrinsic compression due to a small cell lung cancer. Post-​stent image shows stent placement across the previously narrowed segment.

18.3.2  Thoracic imaging 3981 the left lung a minor fissure can occur which separates the lingula from the remainder of the upper lobe). All of the branching structures seen within the lungs on a chest radiograph represent either pulmonary arteries or veins. The larger pulmonary vessels can be traced back to the hila and mediastinum. The pulmonary veins can sometimes be differentiated from the pul- monary arteries:  the superior pulmonary veins have a distinctly vertical course, but in practice it is often impossible to distinguish arteries from veins in the outer two-​thirds of the lung on chest radi- ography. In the erect position, there is a gradual increase in the diameter of the vessels, at equidistant points from the hilum, travel- ling from lung apex to base; this is a gravity-​dependent effect and is abolished if the patient is supine or in cardiac failure. The lobes of the lung are divided into segments, each of which is supplied by its own segmental bronchi. The walls of the segmental bronchi are rarely seen on the chest radiograph, except when lying parallel with the X-​ray beam, when they are seen end-​on as ring shadows measuring up to 8 mm in diameter. The diaphragm and thoracic cage The interface between aerated lung and the domes of the diaphragm is sharp and in general the highest point of each dome is medial to the midclavicular line. The right dome of the diaphragm is higher than the left by up to 2 cm in the erect position unless the left dome is temporarily elevated by air in the stomach. Laterally, the diaphragm dips steeply downwards to form an acute angle with the chest wall. Filling in or blunting of these costophrenic angles usually represents pleural disease, either pleural thickening or an effusion. Localized humps on the dome of the diaphragm are common and represent minor weaknesses or defects of the diaphragm. Similarly, interposition of the colon in front of the right lobe of the liver is a frequently seen normal variant. Deformities of the thoracic cage may cause distortion of the normal mediastinum and so simulate disease. One of the com- monest deformities is pectus excavatum which, by compressing the heart between the depressed sternum and vertebral column, causes displacement of the apparently enlarged heart to the left and causes blurring of the right heart border. High-​kilovoltage chest radiographs often allow the vertebral bodies to be seen through the cardiac shadow. However, with this technique the ribs, and particularly their posterior parts, are often rendered invisible. Anatomy on the lateral chest radiograph It is useful to become accustomed to viewing a lateral film (Fig. 18.3.2.20b) in the same orientation, whether it is a right or left lateral projection. Familiarity with the same orientation improves the viewer’s ability to detect deviations from normal. The trachea is angled slightly posteriorly as it runs towards the carina, and the posterior wall of the trachea is always visible as a fine stripe. Furthermore, the posterior walls of the right main bron- chus and the right intermediate bronchus are outlined by air and are also seen as a continuous stripe on the lateral radiograph. The spines of the scapulae are invariably seen running almost vertically in the upper part of the lateral radiograph and they should not be confused with intrathoracic structures. Further spurious shadows are formed by the soft tissues of the outstretched arms which are projected over the anterior and superior mediastinum. Although the carina is not visible on the lateral radiograph, the two transradiancies projected over the lower trachea represent the right main bronchus (super- iorly) and the left main bronchus (inferiorly). More lung is obscured by overlying structures on a lateral radio- graph than on the PA view. The unobscured lung in the retrosternal and retrocardiac regions should be of the same transradiancy. Furthermore, as the eye travels down the dorsal spine, the viewer should be aware of a gradual increase in transradiancy. The loss of this phenomenon suggests the presence of disease in the posterobasal segments of the lower lobes (sometimes not visible on the frontal radiograph). The two major fissures are seen as diagonal lines, often incom- plete and of a hair’s breadth, running from the upper dorsal spine to the anterior surface of the diaphragm. Care must be taken not to Fig. 18.3.2.19  Image from a CT-​guided radiofrequency ablation procedure for colorectal metastasis demonstrating the radiofrequency probe in situ (a) and subsequent follow-​up CT (b) demonstrating typical post-​procedural changes within the lung.

section 18  Respiratory disorders 3982 confuse the obliquely running edges of ribs with fissures. The minor fissure extends horizontally from the mid-​right major fissure. It is often not possible to distinguish the right from the left major fissures with confidence. Similarly, although the two hemidiaphragms may be identified individually (especially if the gastric bubble is visible under the left dome of the diaphragm), the distinction between the right and the left is often not possible. A helpful sign is the relative heights of the two domes: the dome furthest from the film is usually higher because of magnification. The summation of both hila on the lateral radiograph generates a complex shadow. However, there are some generalizations which aid the interpretation of this difficult area. The right pulmonary artery lies anterior to the trachea and right main bronchus, whereas the left pulmonary artery hooks over the left main bronchus so that a large part of it lies posterior to the major bronchi. As a result, any mass identified on a PA and lateral radiograph that lies anterior to the left hilum or posterior to the right hilum is not vascular in origin and is most likely to represent enlarged hilar lymph nodes. A band-​like opacity is often seen along the lower third of the anterior chest wall behind the sternum. This represents a normal density and occurs because there is less aerated lung in contact with the chest wall because the space is occupied by the heart; it should not be confused with pleural disease. Points in the interpretation of a chest radiograph Even when there is an obvious radiographic abnormality, there is much to recommend a careful and systematic method in reviewing a chest radiograph. Such an approach will allow an appreciation of normal variations of anatomy to be built up with time. With increasing experience, an appreciation of deviation from normal ap- pearances becomes more rapid and this leads quickly to a directed search for related abnormalities. Before interpreting a chest radiograph, it is vital to establish whether there are any previous radiographs for comparison: the sequence and pattern of change is often as important as the iden- tification of a radiographic abnormality. Information gained from preceding radiographs, particularly the lack of serial change, will often prevent needless further investigation. Demographic details, particularly the age and racial origin of the patient, should be noted since this information may increase the probability of a differential diagnosis which is based on the radiographic findings alone. A quick check that the radiograph is of satisfactory quality in- cludes an estimation of the radiographic exposure, depth of inspir- ation, and position of the patient. As a general rule, the intervertebral disc spaces of the entire dorsal spine should be visible on a correctly exposed radiograph; the midpoint of the right hemidiaphragm lies at the level of the anterior end of the sixth rib if the patient has taken a satisfactory breath in. The patient is axially rotated if the medial ends of the clavicles are not equidistant from the spinous process of the thoracic vertebral body at that level. The order in which the structures on a chest radiograph are analysed is unimportant. A  suggested sequence is to start with a scrutiny of the position of the trachea, of the mediastinal con- tour (which should be sharply outlined in its entirety), and then the position, outline, and density of the hilar shadows. Only then are the lungs examined, taking into account their size, the relative transradiancy of each zone, and the position of the horizontal fis- sure (and any other indirect signs of volume loss—​see later section on lobar collapse). Pulmonary vessels are seen as far as the outer third of the lung and the number of vessels should be roughly symmetrical on the two sides. Next, the position and clarity of the hemidiaphragms should be noted, followed by an assessment of the ribs and soft tissues of the chest wall. Special care should be taken to look for pleural thickening along the lateral chest walls, which may be easily overlooked. Before deciding that a chest radiograph is normal, it is worth re- viewing areas that are either poorly demonstrated on chest radi- ography or often misinterpreted. These include:  (1) the central mediastinum, where even a large mass may be barely noticeable on Fig. 18.3.2.20  Normal radiographic anatomy on (a) pa and (b) lateral chest radiographs. (a) 1, trachea; 2, aortic arch; 3, left main pulmonary artery; 4, right main pulmonary artery; 5, right atrial border; 6, left atrial appendage; 7, left ventricular border; 8, right ventricle; 9, right dome of diaphragm; 10, costophrenic angle; 11, breast shadow. (b) 1, trachea; 2, scapulae; 3, anterior aortic arch; 4, right pulmonary artery; 5, left pulmonary artery; 6, right ventricle; 7, breast shadows; 8, gastric bubble under the left hemidiaphragm; 9, left main bronchus.

18.3.2  Thoracic imaging 3983 the PA view; (2) the areas behind the heart and hemidiaphragms; (3) the lung apices, often obscured by overlying clavicle and ribs; and (4) the lung and pleura just inside the chest wall. Once a radiographic abnormality has been detected it should be considered in terms of gross pathology. Both the site and the radio- graphic characteristics of the lesion will allow the observer to pro- duce, at the very least, a generic diagnosis. A precise diagnosis can only occasionally be achieved from the radiographic appearances alone without knowledge of the clinical context. Normal CT anatomy of the mediastinum CT provides unique information about the anatomy of the medi- astinum and is often used to provide further information about abnormalities which are seen merely as a deformity of the medias- tinal contour on chest radiography. The normal structures that are always identified on a CT of the mediastinum are the blood vessels (which make up the bulk of the superior mediastinum), the major airways, the oesophagus, and mediastinal fat. An appreciation of the relationship of these structures to each other is crucial for the correct interpretation of CT scans; four important levels are shown in Fig. 18.3.2.21. Normal lymph nodes surrounded by fat may be identified throughout the mediastinum. Many schemes have been de- vised to map their precise locations but they can be broadly div- ided into (1) anterior mediastinal, (2) posterior mediastinal, and (3)  tracheobronchial. The tracheobronchial can be further sub- divided into the following regions: (1) right and left paratracheal, (2) subaortic, (3) pretracheal, and (4) subcarinal. It is important to appreciate that the absolute size of lymph nodes identified on CT (or by direct inspection at mediastinoscopy) should not be regarded as a foolproof criterion for significant disease, particularly in the context of lung cancer. Although markedly enlarged lymph nodes (>2 cm diameter) almost invariably signify important pathology, moderate enlargement of mediastinal lymph nodes may represent reactive hyperplasia of little clinical significance. Conversely, small volume lymph nodes or lymph nodes not identified by CT may sometimes contain micrometastases, particularly if the primary neoplasm is an adenocarcinoma. Fig. 18.3.2.21  CT with contrast enhancement to show the normal anatomy at four levels through the mediastinum. 1, trachea; 2, superior vena cava; 3, brachiocephalic artery; 4, left common carotid artery; 5, left subclavian artery; 6, oesophagus; 7, aortic arch; 8, azygos vein; 9, ascending aorta; 10, descending aorta; 11, main pulmonary artery; 12, right pulmonary artery; 13, left pulmonary artery; 14, right main bronchus; 15, left main bronchus; 16, left atrium; 17, left inferior pulmonary vein; 18, segmental bronchi of the left lower lobe; 19, right atrium; 20, right ventricular outflow; 21, left ventricle.

section 18  Respiratory disorders 3984 The thymus gland occupies a large part of the anterior medias- tinum in children. In adult life remnants of the normal thymus are usually small or inconspicuous on CT. Common radiological signs of disease Pulmonary consolidation Consolidation is a pathological description of the state of the lungs when the normal air-​filled spaces, distal to the bronchi, are occu- pied by the products of disease (e.g. water, pus, or blood). The most important radiographic signs of pulmonary consolidation are: (1) an area of increased opacification in the lungs which obscures the underlying blood vessels and has a poorly defined margin—​unless it is surrounded by a fissure; (2) an ‘air bronchogram’; and (3) the ‘silhouette sign’ (Fig. 18.3.2.22). The air bronchogram is a dis- tinctive and certain sign of intrapulmonary pathology and is seen as a radiolucent (grey) branching structure of the bronchi against a more opaque (white) background of airless lung. Although an air bronchogram is seen almost invariably in consolidation, a lung which has become collapsed and airless—​for example, due to a large surrounding pleural effusion—​ may also show an air bronchogram. The silhouette sign is seen when the normally clear border of a struc- ture is lost because the air-​filled lung outlining the border is replaced by fluid or a mass. Recognition of this sign can help to localize the area of abnormality within the lungs; for example, consolidation in the lingula will make the left heart border indistinct. As with the air bronchogram sign, the silhouette sign may be seen in either pul- monary consolidation or collapse; for example, loss of a clear right heart border may be due to right middle lobe consolidation with or without lobar collapse; the common feature is loss of normal aeration of the affected lung. The causes of widespread pulmonary consolidation are numerous but may be broadly divided into five categories, as shown in Table 18.3.2.1. Pulmonary collapse This is the term used to describe loss of aeration and therefore infla- tion in part or all of a lung. Depending on the cause, collapse may occur at any level from small, subsegmental areas of lung through to an entire lung. Small areas of subsegmental collapse occur very com- monly in debilitated and postoperative patients, where they are seen as linear, usually horizontal, opacities. At the other end of the spec- trum, collapse of an entire lung, usually due to an endobronchial lesion or inhaled foreign body, has a dramatic radiographic appear- ance with complete opacification of the affected lung and loss of volume of that hemithorax. At the lobar level, the signs of collapse of an individual lobe are characteristic, but depending on the lobe, may be very subtle. Recognition of the collapse of individual lobes is important and these are described in detail. Collapse of individual lobes Right upper lobe On the frontal radiograph there is elevation of the minor fissure and of the right hilum. If the collapse is complete the nonaerated lobe is seen as a density alongside the superior mediastinum (Fig. 18.3.2.23). On the lateral view, the minor fissure moves up- wards and the major fissure moves forwards. The retrosternal area becomes progressively more opaque and the anterior margin of the ascending aorta becomes obscured. Right middle lobe On the frontal radiograph the lateral part of the minor fissure moves down. There is blurring of the normally sharp right heart border and this may be a subtle abnormality which is easily overlooked Table 18.3.2.1  Causes of widespread pulmonary consolidation Pulmonary oedema Cardiogenic/​fluid overload Diffuse alveolar damage (acute respiratory distress syndrome) Inhalational injury (noxious gases) Sepsis Aspiration Traumatic (fat embolism) Exudate Infective consolidation Eosinophilic lung disease Cryptogenic organizing pneumonia Radiation pneumonitis Neoplasm Mucinous adenocarcinoma Lymphoproliferative disorders Blood Contusion Infarction Idiopathic pulmonary haemorrhage Other Alveolar proteinosis Sarcoidosis Fig. 18.3.2.22  Widespread pulmonary consolidation in a patient with alveolar proteinosis. The right heart border is obscured confirming that a large part of the consolidation is in the right middle lobe (the silhouette sign).

18.3.2  Thoracic imaging 3985 (Fig. 18.3.2.24). On the lateral view, the minor fissure moves down- wards and lower half of the major fissure moves forwards giving rise to a triangular shadow with its apex at the hilum and the base behind the lower sternum. Right lower lobe There is an increase in density overlying and obscuring the medial portion of the right hemidiaphragm, and the right hilum is displaced inferiorly on the frontal radiograph (Fig. 18.3.2.25). In contrast to right middle lobe collapse, the right heart border usu- ally remains sharply defined since this is in contact with the aer- ated right middle lobe. On the lateral view the major fissure moves backwards and downwards; with increasing collapse there is a loss of definition of the posterior part of the right hemidiaphragm as well as increased density overlying the lower dorsal vertebral column. Left upper lobe The main finding on the frontal radiograph is a veil-​like increase in density, without a sharp margin (quite unlike right upper lobe col- lapse), spreading outwards and upwards from the elevated left hilum (Fig. 18.3.2.26). The outlines of the aortic knuckle, left hilum, and left heart border become ill defined. As the collapse increases, the lobe moves centrally and the apical segment of the left lower lobe expands to fill the space left by the collapsed upper lobe—​this is the cause of the relatively transradiant lung apex. With complete left upper lobe collapse, a sharp border may return to the aortic arch because it is surrounded by the hyperinflated apical segment of the lower lobe. On the lateral view the major fissure moves superiorly and anteriorly while remaining relatively vertical and roughly par- allel to the anterior chest wall (Fig. 18.3.2.26b). Left lower lobe On the frontal radiograph there is a triangular density behind the heart with loss of the medial part of the left hemidiaphragm (Fig. 18.3.2.27), but even on a properly exposed radiograph it may be difficult to appreciate the collapsed lobe. Supplementary signs in- clude inferior displacement of the left hilum, loss of volume, and in- creased transradiancy of the left hemithorax. On the lateral view there is posterior displacement of the major fissure. As with right lower lobe collapse, there is increased density over the lower dorsal vertebral column and the posterior part of the left hemidiaphragm is effaced. Fig. 18.3.2.24  Right middle lobe collapse. Fig. 18.3.2.23  Posterior–​anterior chest radiograph showing a right upper lobe collapse secondary to a right hilar mass. Note the superior displacement of the horizontal fissure (arrow). Fig. 18.3.2.25  A chest radiograph showing right lower lobe collapse. Note the incorrect position of the nasogastric tube tip within a left lower lobe segmental bronchus (and patchy adjacent consolidation).

section 18  Respiratory disorders 3986 Complete opacification (a ‘white-​out’) of a hemithorax is generally due to either collapse of a lung or a large pleural effusion or tumour. Shift of the mediastinum to the affected side implies that volume loss (i.e. collapse of the lung) has occurred. In contrast, a pleural effusion or soft tissue mass which is large enough to cause com- plete opacification of a hemithorax will almost invariably displace the mediastinum away from the side of the opacified hemithorax. An important exception is an advanced mesothelioma which may encase one lung and ‘freeze’ the mediastinum and prevent contralat- eral mediastinal shift. Occasionally, when there is no obvious shift of the mediastinum, it is surprisingly difficult to differentiate between these two completely different causes of an opacified hemithorax. In these instances, ultrasonography and CT allow the distinction to be made with confidence and may give further information about the underlying disease. Increased transradiancy of a hemithorax There are many causes of increased transradiancy (darkening) of one lung. These may range from a loss of soft tissues of the chest wall (e.g. a mastectomy) through to reduced perfusion of one lung due to hypoxic vasoconstriction resulting from underventilation of the lung because of an inhaled foreign body or a tumour in a main bronchus. It is surprisingly easy to overlook this important radio- graphic abnormality, especially when the density difference between the two lungs is slight: a subtle discrepancy in density between the two hemithoraces is more readily appreciated by viewing the radio- graph from a distance of at least 1.5 m. The commonest causes of a relatively transradiant hemithorax are shown in Table 18.3.2.2. Close scrutiny of the chest radiograph will usually indicate which one of the categories of causes is responsible for this radiographic sign. If there is any clinical suggestion that the cause of the increased transradiancy is due to an obstructing lesion in a central airway, a chest radiograph taken in full expiration will accentuate the in- creased transradiancy and will show that the lung fails to empty. Once it has been established that the difference in density of the lungs is not due to a technical problem (e.g. rotation of the patient), points to look for are (1) loss of symmetry of the soft tissues of the chest wall, (2) discrepancy in the volumes and vas- cular pattern between the two lungs, and (3) a visceral pleural Fig. 18.3.2.26  Posteroanterior (a) and left lateral (b) radiographs demonstrating left upper lobe collapse due to a central obstruction squamous cell carcinoma. Note the anterior displacement of the oblique fissure (arrow) (b). Fig. 18.3.2.27  Left lower lobe collapse. Table 18.3.2.2  Causes of increased transradiancy of one hemithorax Technical Rotation of the patient Chest wall Loss of soft tissues, most commonly due to a mastectomy Pneumothorax Particularly in supine patients Compensatory overinflation Postlobectomy Overlooked lobar collapse (e.g. left lower lobe) Reduced pulmonary perfusion Hypoxic vasoconstriction due to underventilation caused by an inhaled foreign body or endobronchial tumour Obliterative bronchiolitis following childhood viral infection (MacLeod’s/​Swyer–​James syndrome) Recurrent pulmonary emboli (rarely unilateral)

18.3.2  Thoracic imaging 3987 edge (denoting a pneumothorax). The identification of a pneumo- thorax on an erect chest radiograph is usually straightforward because of the appearance of the collapsed lung which is clearly demarcated by the fine edge of the visceral pleura. However, in the supine patient, such an edge is often not seen because air in the pleural space drifts anteriorly to the least dependent part of the chest. In this situation, a pneumothorax is only seen as a vague area of increased transradiancy over the lower zone of the chest. It is vital to recognize when the pressure of the air trapped in the pleural space exceeds alveolar pressure—​a so-​called tension pneumothorax. The typical signs are of contralateral mediastinal shift with straightening and flattening of the ipsilateral dome of the diaphragm (Fig. 18.3.2.28). The pulmonary nodule/​mass Many pulmonary nodules or masses are discovered incidentally on a chest radiograph. Whenever possible, previous films should be obtained so that the growth rate of the lesion can be estimated. The growth rate is a more reliable indicator of the likely nature of a pulmonary mass than any one of its radiographic features: if a lesion doubles in volume (increases in diameter by approximately 25% on serial chest radiographs) in less than 1 week, it is very unlikely to be malignant. The doubling time of most malignant lesions is between 1 and 6 months, but some neoplasms in the adenocarcinoma spectrum may be more indolent. Over the years much importance has been attached to the radio- logical characteristics of a solitary pulmonary mass in an attempt to make the crucial distinction between benign and malignant le- sions. With the possible exception of heavy calcification within the lesion (most commonly seen in ancient granulomas), no radio- logical appearance will reliably differentiate a benign from a ma- lignant mass. Although generalizations can be made, for example, that bronchial carcinomas have irregular and spiculated margins whereas benign lesions are more likely to have smooth outlines, in the individual patient it is not safe to rely on these radiographic features alone to make the distinction between a benign and ma- lignant lesion. After the discovery of a pulmonary nodule or mass on chest radi- ography, further imaging with CT is usually required. CT is valu- able in evaluating extension of a central mass into the mediastinum (Fig. 18.3.2.29), for demonstrating the presence or absence of en- larged mediastinal lymph nodes which may, but do not invariably, indicate local tumour spread, and for the detection of distant me- tastases, such as to the contralateral lung, adrenal glands, and liver. When surgical resection is a potential option, additional imaging of the brain (CT and/​or MRI) and CT/​PET is usually obtained. If required, mediastinal node sampling may be performed with mediastinoscopy, endobronchial or endoscopic ultrasound (EBUS/​ EUS). Ultrasound-​guided sampling of supraclavicular lymph nodes may be an important tool to both confirm nodal involvement and obtain a diagnosis, particularly in patients who are unable to tolerate percutaneous lung biopsy due to poor pulmonary reserve. Local invasion of mediastinal structures, chest wall, and brachial plexus by an adjacent lung cancer may not always be demonstrated by CT, and MRI may be useful in specific circumstances. When surgery is not indicated and a histological diagnosis is needed, percutaneous needle biopsy of central lesions can be safely performed under CT guidance. Similarly, smaller peripheral lesions that are not access- ible by bronchoscopy may be biopsied, under CT or, if abutting the pleura, ultrasound guidance. Lung cancer screening with CT has been evaluated in several large multicentre studies, and there have been promising results from one large trial in the United States. However, the applicability to other populations (including the developing world and areas with a high incidence of tuberculosis) and comparison of mortality reduction with smoking cessation still need to be addressed to determine if CT screening is an optimal use of resources in any given country. However, with the increased use of multidetector CT scanning in a range of conditions, many small (<1 cm) nodules are now detected ‘incidentally’. Practical guidelines have been produced regarding the follow-​up and management of such findings in both low-​risk and high-​risk individuals and continue to evolve. Assessment of nodule Fig. 18.3.2.28  Chest radiograph demonstrating a left tension pneumothorax with mediastinal shift to the right and depression of the left hemidiaphragm, which is a medical emergency. Fig. 18.3.2.29  An intravenous contrast-​enhanced image of a patient with non-​small cell lung cancer demonstrating involvement of the left main pulmonary artery which is significantly narrowed (arrow).

section 18  Respiratory disorders 3988 volume with CT has been a useful adjunct to assessment of growth (Fig. 18.3.2.30) and allows calculation of volume doubling time. Some nodules (e.g. juxtapleural nodules) cannot be assessed using this method as the computer cannot segment the abnormal soft tissue from normal adjacent pleura. Automatic pulmonary nodule detection using specially designed software packages may aid as- sessment of increasingly larger image data sets in the future, but at present these techniques are time-​consuming and throw up large numbers of false positives. Cavitating pulmonary lesions The radiological definition of cavitation is a lucency, representing air, within a mass or area of consolidation. The cavity may or may not contain a fluid level or an intracavitary body, and is surrounded by a wall of variable thickness. The two most likely diagnoses in an adult presenting with a cavitating pulmonary mass on chest radiog- raphy are lung cancer (central, large, and often squamous in type) (Fig. 18.3.2.31) or a lung abscess (usually peripheral and sometimes multiple). Cavitation is recognized in a variety of bacterial pneumo- nias, particularly those due to tuberculosis, staphylococcus, anaer- obes, klebsiella, and chronic aspergillosis. Less commonly, cavitation is seen within pulmonary infarcts and in areas of pulmonary contu- sion due to trauma. Long-​standing cavities in lungs scarred by pre- vious tuberculosis predispose to the formation of mycetomas; once these fungus balls occupy most of the cavity, a characteristic trans- lucent ‘air-​crescent sign’ may be seen between the upper surface of Fig. 18.3.2.30  Examples of pulmonary nodule volumetry software. Initial volume measurements (a) and (b) and subsequent increase in nodule size six months later (c) and (d). The nodule was resected and adenocarcinoma confirmed on histopathology.

18.3.2  Thoracic imaging 3989 the fungus ball and the margin of the cavity on chest radiography (Fig. 18.3.2.32). Multiple pulmonary nodules Many conditions are characterized by multiple small pulmonary nodules. Only by combining the relevant clinical information with a precise description of the size and distribution of the nodules can the differential diagnosis be narrowed. In the United Kingdom, metastatic deposits are by far the commonest cause of multiple pul- monary nodules of varying sizes in an adult. In some parts of the southern United States of America, histoplasmosis is endemic and multiple granulomatous nodules are commoner than those due to disseminated malignancy. In the absence of a known malignancy and when clinical findings and laboratory investigations are in- conclusive, biopsy of one of the nodules may be the only means of establishing a diagnosis. A myriad of small nodules, less than 5 mm in diameter, produces a pattern that is often described as miliary (Fig. 18.3.2.33). A list of causes of miliary shadowing is given in Box 18.3.2.2. An important diagnosis to consider in any patient with this radiographic pattern is miliary tuberculosis. Other differential diagnoses in an asymptom- atic patient with numerous pulmonary nodules include sarcoidosis, metastatic disease or, if there is a relevant occupational history, a pneumoconiosis. As always, comparison with previous radiographs will give invaluable information about the rate of progression and thus the likely nature of the pulmonary nodules. To a lesser extent the distribution of nodules is a consideration in refining the dif- ferential diagnosis of multiple pulmonary nodules; for example, the small nodules of pulmonary sarcoidosis tend to be mid-​zone and perihilar, whereas haematogenous metastases are generally of varying sizes and have a predilection for the lower lobes (probably because of increased blood flow to these regions). The density of nodules sometimes provides conclusive evidence that the nodules are of benign aetiology—​such as the heavily calci- fied nodules which are seen following histoplasmosis or chickenpox (varicella) pneumonia. Most multiple pulmonary nodules are of soft tissue density, and it may be extremely difficult to judge whether small Fig. 18.3.2.31  Chest radiograph of a large cavitating squamous cell bronchial carcinoma adjacent to the right hilum. The right hemidiaphragm is raised because of phrenic nerve invasion by the tumour. Fig. 18.3.2.32  An air crescent (arrow) around a fungus ball at the left apex. This had developed in a tuberculous fibrotic cavity. Fig. 18.3.2.33  Multiple small pulmonary nodules in a patient with miliary tuberculosis.

section 18  Respiratory disorders 3990 nodules are of calcific or soft tissue density because their apparent density depends so critically on the radiographic technique used. Numerous poorly defined, low-​density nodules approximately 8 mm in diameter may be seen around areas of pulmonary con- solidation. In other areas they may be confluent and so make up a larger poorly defined opacity; occasionally these nodules will be uniformly distributed throughout the lungs. At a pathological level these nodules correspond to individual acini which are full of the products of disease, such as pulmonary oedema, inflammatory ex- udates, or haemorrhage. Radiological features of specific diseases Pleural and chest wall disease Because of the two-​dimensional nature of a PA chest radiograph, abnormalities arising from the pleura or chest wall are often difficult to assess. The appearance of a pleural mass on chest radiography depends on whether it is face on or tangential to the X-​ray beam. Generally, a pleural mass will produce a rounded opacity with a sharp medial border and a less well-​defined lateral margin, and an obtuse angle with the pleura. Although abnormality of an adjacent rib will usually indicate that an apparent ‘pleural’ mass is of chest wall origin (Fig. 18.3.2.34), the distinction between a pleural and chest wall mass often cannot be made from a chest radiograph alone. With extensive pleural pathology it may be difficult to distinguish between a pleural effusion, chronic pleural thickening, or even a neoplasm of the pleura such as a mesothelioma. In such cases, a lat- eral decubitus film will distinguish between pleural fluid or thick- ening by demonstrating redistribution of the shadowing if it is due to an effusion. Ultrasonography is also useful in identifying pleural fluid. CT will show even more precisely the site and extent of an abnormality which is apparently ‘pleural’ on a chest radiograph. Furthermore, CT will reveal subtle abnormalities not shown on a plain chest radiograph, such as flecks of calcification within the wall of a chronic empyema, or underlying rib abnormalities in the case of a neoplastic tumour. Similarly, masses arising from the chest wall that give the appearance of a ‘pleural’ mass, such as an intercostal neurofibroma or lipoma, are most accurately assessed by CT. Chronic obstructive pulmonary disease Most patients with chronic obstructive pulmonary disease show remarkably little radiographic abnormality despite often consider- able symptoms. Of the two principal components falling under this disease, emphysema can be detected on chest radiography when it is severe, whereas chronic bronchitis is a clinical diagnosis with no specific radiographic features. Although emphysema is correctly regarded as a pathological diag- nosis, the destruction of alveolar walls distal to the terminal bron- chiole results in certain radiographic features in more advanced cases: overinflation of the lungs causes flattening of the domes of the diaphragm, which may have a scalloped appearance; a lateral chest radiograph may show striking translucency of the enlarged retrosternal and retrocardiac regions. The pattern of the pulmonary vasculature is deranged, with the smooth tapering of the vessels re- placed by an abrupt change in calibre from the larger proximal pul- monary arteries to spindly and attenuated peripheral vessels, giving a so-​called pruned appearance. Depending on the aetiology of the emphysema, there may be an upper zone (e.g. smokers) or lower zone (e.g. α1-​antitrypsin deficiency) predominance; the relatively spared lung often shows a prominent vascular pattern due to blood diversion to these areas. Bullous emphysema is characterized by cystic air spaces bound by extremely thin walls. They may become extremely large and occupy a large part of the lung (Figs. 18.3.2.35a, b). A fluid level within a bulla represents either infection or haemorrhage. Another compli- cation is a pneumothorax, which may be chronic and is sometimes difficult to distinguish from a large bulla. CT is far more sensitive than chest radiography in the detection of emphysema and in some early cases will show evidence of emphy- sema before lung function tests become abnormal. Bronchiectasis Bronchiectasis, whatever the aetiology, is defined as damage to the bronchial wall causing irreversible dilatation of the bronchi. The diagnosis of bronchiectasis is rarely made with certainty from the chest radiograph alone, unless the disease is extensive and severe. Box 18.3.2.2  Differential diagnosis of widespread nodular (0.5–​ 3 mm diameter) shadowing • Miliary tuberculosis • Fungal diseases • Metastatic disease • Pneumoconiosis • Sarcoidosis • Subacute hypersensitivity pneumonitis • Idiopathic pulmonary haemosiderosis Fig. 18.3.2.34  A malignant chest wall lesion resulting in rib destruction. Note the obtuse angle between the lesion and the pleural surface superiorly and inferiorly.

18.3.2  Thoracic imaging 3991 On a chest radiograph the abnormal bronchi may be visible as ei- ther ring shadows and curvilinear shadows that represent thickened bronchial wall seen end-​on, or as parallel thin lines or ‘tramlines’, particularly in the lower lobes; this latter sign can be very subtle and may be more obvious on the lateral chest radiograph. Other radiographic signs of bronchiectasis include round or oval nodular opacities, and sometimes band shadows representing grossly dilated fluid-​filled bronchi. High-​resolution CT is the imaging technique of choice in investi- gation of patients with suspected bronchiectasis. Abnormally dilated and thickened bronchi are readily identified on high-​resolution CT (Fig. 18.3.2.36); a normal bronchus is of approximately the same diameter as its accompanying pulmonary artery. In addition to al- lowing a confident diagnosis of bronchiectasis to be made, often in the face of a normal chest radiograph, high-​resolution CT will show how many lobes are involved—​a consideration in deciding on op- timal medical or surgical management. Chronic diffuse lung disease Many conditions are characterized by diffuse shadowing of the lungs on a chest radiograph. The lung has few ways of responding to in- jury (capillary leak, cellular infiltration, or interstitial fibrosis) and the resulting spectrum of radiographic patterns is correspondingly limited. It is important that reproducible terms are used in the de- scription of widespread pulmonary shadowing. Vague terms that may convey a pathological meaning (which in fact cannot be in- ferred from the gross signs of disease on a chest radiograph, e.g. ‘in- flammatory shadowing’) should not be used. Instead, descriptions of the radiographic pattern should be limited to strictly morpho- logical terms such as ‘reticular’—​a fine network, ‘nodular’—​small dots of a specified size, ‘linear’—​fine lines which are not vessels, ‘ground glass’—​a greying-​out of the lungs that makes the vascular markings indistinct, and finally ‘air-​space shadowing’ or ‘consolida- tion’—​opacification of the lungs in which an air bronchogram may be visible. These descriptors are more reproducible and are prefer- able to the wide range of imprecise and whimsical terms that have been coined in the past. An analysis of the distribution of the disease is often at least as important as defining the radiographic pattern in reaching a dif- ferential diagnosis. This involves an assessment of whether the disease involves all parts of the lung uniformly, or whether there is a zonal predominance (upper, mid, or lower; central or per- ipheral). The perihilar, mid-​ and upper-​zone distribution of the reticulonodular pattern in sarcoidosis is quite different from the lower-​zone peripheral distribution of idiopathic pulmonary fi- brosis; these differences in distribution are even more obvious on the cross-​sectional images of CT. The differential diagnosis can be Fig. 18.3.2.35  The chest radiograph (a) and an axial CT section (b) in a patient with severe bullous emphysema. The absence of lung markings within the right upper zone makes it difficult to exclude a pneumothorax, however the CT demonstrates the multiple thin-​walled bullae. Fig. 18.3.2.36  Axial high-​resolution CT section of a patient with allergic bronchopulmonary aspergillosis showing bilateral varicose and cystic bronchiectasis.