Inflammatory processes

Inflammatory processes

Appendicitis Historically , a straightforward clinical diagnosis of appendicitis obviated any need for imaging, but with the proven accuracy of available modalities imaging has become increasingly popular to reduce negative appendicectomy rates and to George Kenneth Mallory , 1900–1986, Professor of Pathology , Boston University , Boston, MA, USA. Soma Weiss , 1898–1942, Professor of Medicine, Harvard University Medical School, Boston, MA, USA. demonstrate a calcified appendicolith in the right iliac fossa, it is insu ffi ciently sensitive or specific to be reliable. In children, who typically have a fav ourable body habitus, ultrasound is the best test as it reduces radiation exposure. This also applies to females of childbearing age, again to reduce radiation exposure, but also because the symptoms may be mimicked by gynaecological pathology , such as ectopic pregnancy , haem - orrhagic ovarian cyst and tubo-ovarian abscess, all diagnoses that are best made with ultrasound. The definitive exclusion of appendicitis, however, hinges on the identification of a normal appendix, measuring less than 6 /uni00A0 mm in diameter . Retrocaecal appendicitis can readily escape detection with ultrasound, and thus CT is the next modality of choice; indeed, frequently it is the first requested in most adults ( Figure 8.39 ). The diagnosis of appendicitis on CT requires the identification of a thick - ened appendix (>7 /uni00A0 mm), with periappendiceal inflammatory change as evidenced by stranding in the surrounding fat. Other signs that may be sought include free fluid, thickening of the caecal pole, possible localised small bowel ileus and right iliac - fossa lymphadenopath y . Both CT and ultrasound can also - identify collections if an inflamed appendix ruptures, and can be used to guide percutaneous drainage as a bridge to definitive surgery . -

Figure 8.39 Acute appendicitis. Contrast-enhanced computed tomography scan reconstructed in the coronal plane demonstrates a thickened appendix in the right iliac fossa (arrow) with in /f_l ammatory changes in the surrounding fat and reactive thickening of the caecal pole.

Inflammation of an obstructed diverticulum typically presents with left iliac fossa pain and pyrexia ( Figure 8.40 ). While some authors have promoted the use of focused ultrasound for this indication, in general it is best diagnosed with a CT scan. The typical CT appearance is of pericolic inflammatory change around a diverticulum, most commonly in the sigmoid colon. Complications of diverticulitis include perforation, abscess formation, fistulation to adjacent structures and strictures in the bowel. CT is also the modality of choice to identify these; as with appendicitis, it can be used to guide percutaneous abscess drainage as a bridge to definitive surgery . Inflammatory bowel disease The diagnosis of inflammatory bowel disease is made histo logically . Radiologically , the diagnosis and monitoring of inflammatory bowel disease has changed significantly in recent years. Previously a barium study of the small bowel, either a follo w-through (where barium is ingested orally) or enteroclysis (where dilute barium is infused via a nasojejunal tube) was used as a screening tool if symptoms are vague. If the diagnosis of Crohn’s disease is established, barium studies can still be useful to demonstrate the extent of disease, particularly to demonstrate the length and number of strictures if surgery is planned. Increasingly , however, the role of barium studies has been superseded by cross-sectional imaging, particularly MRI enterography , which entails an abdominopelvic MRI scan Burrill Bernard Crohn , 1884–1983, gastroenterologist, Mount Sinai Hospital, New Y ork, NY , USA, described regional ileitis in 1932 along with Leon Ginzburg and Gordon Oppenheimer. John Benjamin Murphy , 1857–1916, Professor of Surgery , Northwestern University , Chicago, IL, USA, described his sign in 1903. He was the son of Irish immigrants fleeing the potato famine in Ireland, and was known as the ‘Stormy Petrel’ of American surgery , demonstrating the benefit of appendicectomy over conservative treatment among many things. distend the small bowel. The other obvious advantage of MRI is the lack of radiation, which is particularly relevant in young patients with Cr ohn’s disease, who often undergo multiple imaging studies over their lifetime; for this reason it is gaining in popularity for inflammatory bowel disease follow-up. An acute flare-up may also require imaging, and an ultra - sound is usually a good first test to look for dilated bowel loops and any abscess, though CT may ultimately be required as gas-filled bowel loops can obscure visualisation of an abscess on ultrasound. MRI is the imaging modality of choice to assess perianal fistulae and abscesses. Acute pancreatitis As with acute appendicitis, when the diagnosis is straight - - forward clinically there may be no need for imaging, though increasingly it is used to confirm the diagnosis, to assess the severity of the process and to look for complications. While ultrasound may show gallstones and can demonstrate an enlarged pancr eas with peripancreatic fluid and inflammatory changes, the optimal modality is CT . CT performed too early in the course of the attack, e.g. in the first 12 hours, can be equivocal and the optimal timing of imaging is 48–72 hours. In mild acute pancreatitis, CT may be normal or may show an enlarged oedematous gland, but in more severe attacks other findings which should be sought include peripancreatic fluid collections, v ascular complications such as arterial pseudoaneurysm formation or venous thrombosis and necrosis, either of the gland itself or of the surrounding fat. Necrosis typically develops 48–72 hours after the onset of symptoms and is manifest on CT as lack of enhancement of the necrotic areas. CT with intravenous contrast is therefore essential to look for necrosis, which is potentially catastrophic, particularly if it becomes infected. While CT is not always reliable to diagnose infected necrosis, it is suggested by bubbles of air in the necrotic segment. As with other intra-abdominal inflammatory processes, either ultrasound or more usually CT can be used to guide percutaneous drainage of inflammatory fluid collections. Acute cholecystitis/biliary colic/jaundice While acute cholecystitis is usually due to mechanical obstruc - tion of the cystic duct or gallbladder neck by a gallstone, acute acalculous cholecystitis can occur in critically ill patients from a number of causes. In any case ultrasound is the modality of c hoice should this diagnosis be suspected, and the classic diagnostic features are of gallbladder distension with wall thickening (>3 /uni00A0 mm). A gallstone obstructing the gallbladder neck or cystic duct may be visualised; alternatively , in acal - culous cholecystitis sludge may be seen layering in the gall - bladder lumen. Associated signs include pericholecystic fluid and hyperaemia on Doppler examination. Ultrasonographic Murphy’s sign refers to tenderness over the gallbladder when

Figure 8.40 Coronal computed tomography reformatted images showing a diverticular perforation. There is stranding around the sigmoid colon with an extraluminal track of gas (arrow). Because of surrounding in /f_l ammatory changes diverticular perforation usually leads to pericolic localised gas collections rather than generalised pneumoperitoneum.

in making the diagnosis. As a second-line investigation CT is also accurate for this condition, demonstrating similar signs of gallbladder distension and wall thickening with surrounding inflammatory changes. CT is also useful to diagnose complica tions such as gangrenous cholecystitis, gallbladder perforation and emphysematous cholecystitis, which may necessitate emergency cholecystectomy . If cross-sectional studies are equivocal, he patobiliary scintigraphy can be useful, with the diagnosis of acute cholecystitis suggested by non-visualisation of the gallbladder 3 hours after radioisotope administration. A frequent limitation of ultrasound is failure to visualise the common bile duct throughout its length owing to overlying bowel gas, and elective cholecystectomy was typically accom panied by bile duct imaging or exploration to look for duct calculi. Increasingly , however, MRCP has been shown to be highly accurate in excluding bile duct calculi before surger Ultrasound is also a useful first-line investigation for jaun dice of unknown cause as it can demonstrate duct dilatation and gallstones. If a definitive cause is not shown with ultra sound, or a mass is identified but its precise nature and extent is uncertain, CT is indicated to look for common causes, includ ing stones, c holangiocarcinoma and pancreatic carcinoma. CT can not only identify malignant lesions but also demonstrate the extent of local infiltration, including the ver y important assess ment of vascular involvement if surgery is consider ed, and the presence of metastases to determine potential resectability . If the ducts are of normal calibre in a jaundiced patient, liver biopsy should be consider ed. Renal colic The historical methods of imaging for renal colic all have their limitations. Plain film radiography may not demonstrate all calculi, will not show renal tract obstruction and is unreliable for alternative diagnoses. IVU necessitates the administration of intravenous contrast and, if a level of obstruction is sought, delayed films up to 8 hours after injection may be required; it also will not provide alternative diagnoses. Ultrasound will demonstrate hydronephrosis and hydroureter, and calculi in the kidneys and either the proximal or distal ureters can usually be identified as echogenic foci with posterior acoustic shadowing; however, the ureter from just below the kidneys to the pelvis is usually obscured by bowel gas, which significantly impairs stone detection. For these reasons the optimal investigation is now CT of the kidneys, ureters and bladder, a non-contrast, low-dose (2–3 /uni00A0 MSv if a low mA scan is perf ormed, equivalent to the dose from a limited IVU series) scan from the upper poles of the kidneys to the pubic symphysis. Contrast administration, either orally or intravenously , is not employed as it does not aid stone detection and may even impair it. Stones are readily identified as high-attenuation (typically calcific) foci, and the secondary signs of acute ureteric obstruction may also be seen, includ ing hydronephrosis and hydroureter, renal enlargement and perinephric fat stranding. The most common sites for stones to be seen are at the areas of ureteric narrowing, namely the pelviureteric junction, the pelvic brim and vesicour eteric junc tion. CT also o ff ers unrivalled capability for making alternative diagnoses when compared with other modalities. If a pulsatile mass is felt in the abdomen and the diagnosis of a possible abdominal aortic aneurysm (AAA) is suspected, ultrasound is a useful modality; provided the aorta is not - obscured by bowel gas, an aneurysm can usually reliably be excluded. If, however, ultrasound visualisation is suboptimal and the diagnosis is as a result equivocal, or if an aneurysm is identified and information regarding the extent and exact size is required, for example for surgical or endovascular repair planning, CT angiography is indicated, with the aorta typically scanned from the arch to the pubic symphysis in the arterial phase after intravenous contrast. MR angiography is a useful alternative if iodinated contrast is contraindicated. - In the case of suspected aneurysm rupture, provided the patient is su ffi ciently haemodynamically stable to undergo CT , CT angiography should be urgently perf ormed; a sup - y . plementary non-enhanced initial scan is useful to look for - retroperitoneal haematoma, which is typically of relatively high attenuation compared with the blood in the lumen on a - non-contrast scan. - Inflammatory processes

Appendicitis Historically , a straightforward clinical diagnosis of appendicitis obviated any need for imaging, but with the proven accuracy of available modalities imaging has become increasingly popular to reduce negative appendicectomy rates and to George Kenneth Mallory , 1900–1986, Professor of Pathology , Boston University , Boston, MA, USA. Soma Weiss , 1898–1942, Professor of Medicine, Harvard University Medical School, Boston, MA, USA. demonstrate a calcified appendicolith in the right iliac fossa, it is insu ffi ciently sensitive or specific to be reliable. In children, who typically have a fav ourable body habitus, ultrasound is the best test as it reduces radiation exposure. This also applies to females of childbearing age, again to reduce radiation exposure, but also because the symptoms may be mimicked by gynaecological pathology , such as ectopic pregnancy , haem - orrhagic ovarian cyst and tubo-ovarian abscess, all diagnoses that are best made with ultrasound. The definitive exclusion of appendicitis, however, hinges on the identification of a normal appendix, measuring less than 6 /uni00A0 mm in diameter . Retrocaecal appendicitis can readily escape detection with ultrasound, and thus CT is the next modality of choice; indeed, frequently it is the first requested in most adults ( Figure 8.39 ). The diagnosis of appendicitis on CT requires the identification of a thick - ened appendix (>7 /uni00A0 mm), with periappendiceal inflammatory change as evidenced by stranding in the surrounding fat. Other signs that may be sought include free fluid, thickening of the caecal pole, possible localised small bowel ileus and right iliac - fossa lymphadenopath y . Both CT and ultrasound can also - identify collections if an inflamed appendix ruptures, and can be used to guide percutaneous drainage as a bridge to definitive surgery . -

Figure 8.39 Acute appendicitis. Contrast-enhanced computed tomography scan reconstructed in the coronal plane demonstrates a thickened appendix in the right iliac fossa (arrow) with in /f_l ammatory changes in the surrounding fat and reactive thickening of the caecal pole.

Inflammation of an obstructed diverticulum typically presents with left iliac fossa pain and pyrexia ( Figure 8.40 ). While some authors have promoted the use of focused ultrasound for this indication, in general it is best diagnosed with a CT scan. The typical CT appearance is of pericolic inflammatory change around a diverticulum, most commonly in the sigmoid colon. Complications of diverticulitis include perforation, abscess formation, fistulation to adjacent structures and strictures in the bowel. CT is also the modality of choice to identify these; as with appendicitis, it can be used to guide percutaneous abscess drainage as a bridge to definitive surgery . Inflammatory bowel disease The diagnosis of inflammatory bowel disease is made histo logically . Radiologically , the diagnosis and monitoring of inflammatory bowel disease has changed significantly in recent years. Previously a barium study of the small bowel, either a follo w-through (where barium is ingested orally) or enteroclysis (where dilute barium is infused via a nasojejunal tube) was used as a screening tool if symptoms are vague. If the diagnosis of Crohn’s disease is established, barium studies can still be useful to demonstrate the extent of disease, particularly to demonstrate the length and number of strictures if surgery is planned. Increasingly , however, the role of barium studies has been superseded by cross-sectional imaging, particularly MRI enterography , which entails an abdominopelvic MRI scan Burrill Bernard Crohn , 1884–1983, gastroenterologist, Mount Sinai Hospital, New Y ork, NY , USA, described regional ileitis in 1932 along with Leon Ginzburg and Gordon Oppenheimer. John Benjamin Murphy , 1857–1916, Professor of Surgery , Northwestern University , Chicago, IL, USA, described his sign in 1903. He was the son of Irish immigrants fleeing the potato famine in Ireland, and was known as the ‘Stormy Petrel’ of American surgery , demonstrating the benefit of appendicectomy over conservative treatment among many things. distend the small bowel. The other obvious advantage of MRI is the lack of radiation, which is particularly relevant in young patients with Cr ohn’s disease, who often undergo multiple imaging studies over their lifetime; for this reason it is gaining in popularity for inflammatory bowel disease follow-up. An acute flare-up may also require imaging, and an ultra - sound is usually a good first test to look for dilated bowel loops and any abscess, though CT may ultimately be required as gas-filled bowel loops can obscure visualisation of an abscess on ultrasound. MRI is the imaging modality of choice to assess perianal fistulae and abscesses. Acute pancreatitis As with acute appendicitis, when the diagnosis is straight - - forward clinically there may be no need for imaging, though increasingly it is used to confirm the diagnosis, to assess the severity of the process and to look for complications. While ultrasound may show gallstones and can demonstrate an enlarged pancr eas with peripancreatic fluid and inflammatory changes, the optimal modality is CT . CT performed too early in the course of the attack, e.g. in the first 12 hours, can be equivocal and the optimal timing of imaging is 48–72 hours. In mild acute pancreatitis, CT may be normal or may show an enlarged oedematous gland, but in more severe attacks other findings which should be sought include peripancreatic fluid collections, v ascular complications such as arterial pseudoaneurysm formation or venous thrombosis and necrosis, either of the gland itself or of the surrounding fat. Necrosis typically develops 48–72 hours after the onset of symptoms and is manifest on CT as lack of enhancement of the necrotic areas. CT with intravenous contrast is therefore essential to look for necrosis, which is potentially catastrophic, particularly if it becomes infected. While CT is not always reliable to diagnose infected necrosis, it is suggested by bubbles of air in the necrotic segment. As with other intra-abdominal inflammatory processes, either ultrasound or more usually CT can be used to guide percutaneous drainage of inflammatory fluid collections. Acute cholecystitis/biliary colic/jaundice While acute cholecystitis is usually due to mechanical obstruc - tion of the cystic duct or gallbladder neck by a gallstone, acute acalculous cholecystitis can occur in critically ill patients from a number of causes. In any case ultrasound is the modality of c hoice should this diagnosis be suspected, and the classic diagnostic features are of gallbladder distension with wall thickening (>3 /uni00A0 mm). A gallstone obstructing the gallbladder neck or cystic duct may be visualised; alternatively , in acal - culous cholecystitis sludge may be seen layering in the gall - bladder lumen. Associated signs include pericholecystic fluid and hyperaemia on Doppler examination. Ultrasonographic Murphy’s sign refers to tenderness over the gallbladder when

Figure 8.40 Coronal computed tomography reformatted images showing a diverticular perforation. There is stranding around the sigmoid colon with an extraluminal track of gas (arrow). Because of surrounding in /f_l ammatory changes diverticular perforation usually leads to pericolic localised gas collections rather than generalised pneumoperitoneum.

in making the diagnosis. As a second-line investigation CT is also accurate for this condition, demonstrating similar signs of gallbladder distension and wall thickening with surrounding inflammatory changes. CT is also useful to diagnose complica tions such as gangrenous cholecystitis, gallbladder perforation and emphysematous cholecystitis, which may necessitate emergency cholecystectomy . If cross-sectional studies are equivocal, he patobiliary scintigraphy can be useful, with the diagnosis of acute cholecystitis suggested by non-visualisation of the gallbladder 3 hours after radioisotope administration. A frequent limitation of ultrasound is failure to visualise the common bile duct throughout its length owing to overlying bowel gas, and elective cholecystectomy was typically accom panied by bile duct imaging or exploration to look for duct calculi. Increasingly , however, MRCP has been shown to be highly accurate in excluding bile duct calculi before surger Ultrasound is also a useful first-line investigation for jaun dice of unknown cause as it can demonstrate duct dilatation and gallstones. If a definitive cause is not shown with ultra sound, or a mass is identified but its precise nature and extent is uncertain, CT is indicated to look for common causes, includ ing stones, c holangiocarcinoma and pancreatic carcinoma. CT can not only identify malignant lesions but also demonstrate the extent of local infiltration, including the ver y important assess ment of vascular involvement if surgery is consider ed, and the presence of metastases to determine potential resectability . If the ducts are of normal calibre in a jaundiced patient, liver biopsy should be consider ed. Renal colic The historical methods of imaging for renal colic all have their limitations. Plain film radiography may not demonstrate all calculi, will not show renal tract obstruction and is unreliable for alternative diagnoses. IVU necessitates the administration of intravenous contrast and, if a level of obstruction is sought, delayed films up to 8 hours after injection may be required; it also will not provide alternative diagnoses. Ultrasound will demonstrate hydronephrosis and hydroureter, and calculi in the kidneys and either the proximal or distal ureters can usually be identified as echogenic foci with posterior acoustic shadowing; however, the ureter from just below the kidneys to the pelvis is usually obscured by bowel gas, which significantly impairs stone detection. For these reasons the optimal investigation is now CT of the kidneys, ureters and bladder, a non-contrast, low-dose (2–3 /uni00A0 MSv if a low mA scan is perf ormed, equivalent to the dose from a limited IVU series) scan from the upper poles of the kidneys to the pubic symphysis. Contrast administration, either orally or intravenously , is not employed as it does not aid stone detection and may even impair it. Stones are readily identified as high-attenuation (typically calcific) foci, and the secondary signs of acute ureteric obstruction may also be seen, includ ing hydronephrosis and hydroureter, renal enlargement and perinephric fat stranding. The most common sites for stones to be seen are at the areas of ureteric narrowing, namely the pelviureteric junction, the pelvic brim and vesicour eteric junc tion. CT also o ff ers unrivalled capability for making alternative diagnoses when compared with other modalities. If a pulsatile mass is felt in the abdomen and the diagnosis of a possible abdominal aortic aneurysm (AAA) is suspected, ultrasound is a useful modality; provided the aorta is not - obscured by bowel gas, an aneurysm can usually reliably be excluded. If, however, ultrasound visualisation is suboptimal and the diagnosis is as a result equivocal, or if an aneurysm is identified and information regarding the extent and exact size is required, for example for surgical or endovascular repair planning, CT angiography is indicated, with the aorta typically scanned from the arch to the pubic symphysis in the arterial phase after intravenous contrast. MR angiography is a useful alternative if iodinated contrast is contraindicated. - In the case of suspected aneurysm rupture, provided the patient is su ffi ciently haemodynamically stable to undergo CT , CT angiography should be urgently perf ormed; a sup - y . plementary non-enhanced initial scan is useful to look for - retroperitoneal haematoma, which is typically of relatively high attenuation compared with the blood in the lumen on a - non-contrast scan. - Inflammatory processes

Appendicitis Historically , a straightforward clinical diagnosis of appendicitis obviated any need for imaging, but with the proven accuracy of available modalities imaging has become increasingly popular to reduce negative appendicectomy rates and to George Kenneth Mallory , 1900–1986, Professor of Pathology , Boston University , Boston, MA, USA. Soma Weiss , 1898–1942, Professor of Medicine, Harvard University Medical School, Boston, MA, USA. demonstrate a calcified appendicolith in the right iliac fossa, it is insu ffi ciently sensitive or specific to be reliable. In children, who typically have a fav ourable body habitus, ultrasound is the best test as it reduces radiation exposure. This also applies to females of childbearing age, again to reduce radiation exposure, but also because the symptoms may be mimicked by gynaecological pathology , such as ectopic pregnancy , haem - orrhagic ovarian cyst and tubo-ovarian abscess, all diagnoses that are best made with ultrasound. The definitive exclusion of appendicitis, however, hinges on the identification of a normal appendix, measuring less than 6 /uni00A0 mm in diameter . Retrocaecal appendicitis can readily escape detection with ultrasound, and thus CT is the next modality of choice; indeed, frequently it is the first requested in most adults ( Figure 8.39 ). The diagnosis of appendicitis on CT requires the identification of a thick - ened appendix (>7 /uni00A0 mm), with periappendiceal inflammatory change as evidenced by stranding in the surrounding fat. Other signs that may be sought include free fluid, thickening of the caecal pole, possible localised small bowel ileus and right iliac - fossa lymphadenopath y . Both CT and ultrasound can also - identify collections if an inflamed appendix ruptures, and can be used to guide percutaneous drainage as a bridge to definitive surgery . -

Figure 8.39 Acute appendicitis. Contrast-enhanced computed tomography scan reconstructed in the coronal plane demonstrates a thickened appendix in the right iliac fossa (arrow) with in /f_l ammatory changes in the surrounding fat and reactive thickening of the caecal pole.

Inflammation of an obstructed diverticulum typically presents with left iliac fossa pain and pyrexia ( Figure 8.40 ). While some authors have promoted the use of focused ultrasound for this indication, in general it is best diagnosed with a CT scan. The typical CT appearance is of pericolic inflammatory change around a diverticulum, most commonly in the sigmoid colon. Complications of diverticulitis include perforation, abscess formation, fistulation to adjacent structures and strictures in the bowel. CT is also the modality of choice to identify these; as with appendicitis, it can be used to guide percutaneous abscess drainage as a bridge to definitive surgery . Inflammatory bowel disease The diagnosis of inflammatory bowel disease is made histo logically . Radiologically , the diagnosis and monitoring of inflammatory bowel disease has changed significantly in recent years. Previously a barium study of the small bowel, either a follo w-through (where barium is ingested orally) or enteroclysis (where dilute barium is infused via a nasojejunal tube) was used as a screening tool if symptoms are vague. If the diagnosis of Crohn’s disease is established, barium studies can still be useful to demonstrate the extent of disease, particularly to demonstrate the length and number of strictures if surgery is planned. Increasingly , however, the role of barium studies has been superseded by cross-sectional imaging, particularly MRI enterography , which entails an abdominopelvic MRI scan Burrill Bernard Crohn , 1884–1983, gastroenterologist, Mount Sinai Hospital, New Y ork, NY , USA, described regional ileitis in 1932 along with Leon Ginzburg and Gordon Oppenheimer. John Benjamin Murphy , 1857–1916, Professor of Surgery , Northwestern University , Chicago, IL, USA, described his sign in 1903. He was the son of Irish immigrants fleeing the potato famine in Ireland, and was known as the ‘Stormy Petrel’ of American surgery , demonstrating the benefit of appendicectomy over conservative treatment among many things. distend the small bowel. The other obvious advantage of MRI is the lack of radiation, which is particularly relevant in young patients with Cr ohn’s disease, who often undergo multiple imaging studies over their lifetime; for this reason it is gaining in popularity for inflammatory bowel disease follow-up. An acute flare-up may also require imaging, and an ultra - sound is usually a good first test to look for dilated bowel loops and any abscess, though CT may ultimately be required as gas-filled bowel loops can obscure visualisation of an abscess on ultrasound. MRI is the imaging modality of choice to assess perianal fistulae and abscesses. Acute pancreatitis As with acute appendicitis, when the diagnosis is straight - - forward clinically there may be no need for imaging, though increasingly it is used to confirm the diagnosis, to assess the severity of the process and to look for complications. While ultrasound may show gallstones and can demonstrate an enlarged pancr eas with peripancreatic fluid and inflammatory changes, the optimal modality is CT . CT performed too early in the course of the attack, e.g. in the first 12 hours, can be equivocal and the optimal timing of imaging is 48–72 hours. In mild acute pancreatitis, CT may be normal or may show an enlarged oedematous gland, but in more severe attacks other findings which should be sought include peripancreatic fluid collections, v ascular complications such as arterial pseudoaneurysm formation or venous thrombosis and necrosis, either of the gland itself or of the surrounding fat. Necrosis typically develops 48–72 hours after the onset of symptoms and is manifest on CT as lack of enhancement of the necrotic areas. CT with intravenous contrast is therefore essential to look for necrosis, which is potentially catastrophic, particularly if it becomes infected. While CT is not always reliable to diagnose infected necrosis, it is suggested by bubbles of air in the necrotic segment. As with other intra-abdominal inflammatory processes, either ultrasound or more usually CT can be used to guide percutaneous drainage of inflammatory fluid collections. Acute cholecystitis/biliary colic/jaundice While acute cholecystitis is usually due to mechanical obstruc - tion of the cystic duct or gallbladder neck by a gallstone, acute acalculous cholecystitis can occur in critically ill patients from a number of causes. In any case ultrasound is the modality of c hoice should this diagnosis be suspected, and the classic diagnostic features are of gallbladder distension with wall thickening (>3 /uni00A0 mm). A gallstone obstructing the gallbladder neck or cystic duct may be visualised; alternatively , in acal - culous cholecystitis sludge may be seen layering in the gall - bladder lumen. Associated signs include pericholecystic fluid and hyperaemia on Doppler examination. Ultrasonographic Murphy’s sign refers to tenderness over the gallbladder when

Figure 8.40 Coronal computed tomography reformatted images showing a diverticular perforation. There is stranding around the sigmoid colon with an extraluminal track of gas (arrow). Because of surrounding in /f_l ammatory changes diverticular perforation usually leads to pericolic localised gas collections rather than generalised pneumoperitoneum.

in making the diagnosis. As a second-line investigation CT is also accurate for this condition, demonstrating similar signs of gallbladder distension and wall thickening with surrounding inflammatory changes. CT is also useful to diagnose complica tions such as gangrenous cholecystitis, gallbladder perforation and emphysematous cholecystitis, which may necessitate emergency cholecystectomy . If cross-sectional studies are equivocal, he patobiliary scintigraphy can be useful, with the diagnosis of acute cholecystitis suggested by non-visualisation of the gallbladder 3 hours after radioisotope administration. A frequent limitation of ultrasound is failure to visualise the common bile duct throughout its length owing to overlying bowel gas, and elective cholecystectomy was typically accom panied by bile duct imaging or exploration to look for duct calculi. Increasingly , however, MRCP has been shown to be highly accurate in excluding bile duct calculi before surger Ultrasound is also a useful first-line investigation for jaun dice of unknown cause as it can demonstrate duct dilatation and gallstones. If a definitive cause is not shown with ultra sound, or a mass is identified but its precise nature and extent is uncertain, CT is indicated to look for common causes, includ ing stones, c holangiocarcinoma and pancreatic carcinoma. CT can not only identify malignant lesions but also demonstrate the extent of local infiltration, including the ver y important assess ment of vascular involvement if surgery is consider ed, and the presence of metastases to determine potential resectability . If the ducts are of normal calibre in a jaundiced patient, liver biopsy should be consider ed. Renal colic The historical methods of imaging for renal colic all have their limitations. Plain film radiography may not demonstrate all calculi, will not show renal tract obstruction and is unreliable for alternative diagnoses. IVU necessitates the administration of intravenous contrast and, if a level of obstruction is sought, delayed films up to 8 hours after injection may be required; it also will not provide alternative diagnoses. Ultrasound will demonstrate hydronephrosis and hydroureter, and calculi in the kidneys and either the proximal or distal ureters can usually be identified as echogenic foci with posterior acoustic shadowing; however, the ureter from just below the kidneys to the pelvis is usually obscured by bowel gas, which significantly impairs stone detection. For these reasons the optimal investigation is now CT of the kidneys, ureters and bladder, a non-contrast, low-dose (2–3 /uni00A0 MSv if a low mA scan is perf ormed, equivalent to the dose from a limited IVU series) scan from the upper poles of the kidneys to the pubic symphysis. Contrast administration, either orally or intravenously , is not employed as it does not aid stone detection and may even impair it. Stones are readily identified as high-attenuation (typically calcific) foci, and the secondary signs of acute ureteric obstruction may also be seen, includ ing hydronephrosis and hydroureter, renal enlargement and perinephric fat stranding. The most common sites for stones to be seen are at the areas of ureteric narrowing, namely the pelviureteric junction, the pelvic brim and vesicour eteric junc tion. CT also o ff ers unrivalled capability for making alternative diagnoses when compared with other modalities. If a pulsatile mass is felt in the abdomen and the diagnosis of a possible abdominal aortic aneurysm (AAA) is suspected, ultrasound is a useful modality; provided the aorta is not - obscured by bowel gas, an aneurysm can usually reliably be excluded. If, however, ultrasound visualisation is suboptimal and the diagnosis is as a result equivocal, or if an aneurysm is identified and information regarding the extent and exact size is required, for example for surgical or endovascular repair planning, CT angiography is indicated, with the aorta typically scanned from the arch to the pubic symphysis in the arterial phase after intravenous contrast. MR angiography is a useful alternative if iodinated contrast is contraindicated. - In the case of suspected aneurysm rupture, provided the patient is su ffi ciently haemodynamically stable to undergo CT , CT angiography should be urgently perf ormed; a sup - y . plementary non-enhanced initial scan is useful to look for - retroperitoneal haematoma, which is typically of relatively high attenuation compared with the blood in the lumen on a - non-contrast scan. -