Computed tomography findings of abnormal gas in the abdomen and pelvis

Pneumoperitoneum and pneumoretroperitoneum

The presence of gas within the peritoneal cavity (pneumoperitoneum) and the retroperitoneal space (pneumoretroperitoneum) is a crucial imaging finding for gastrointestinal perforation. The direct imaging finding of gastrointestinal perforation is bowel wall discontinuity (Fig. 6b). There are several findings supporting the correct diagnosis of gastrointestinal perforation, including pneumoperitoneum/pneumoretroperitoneum, surrounding fat stranding, focal bowel wall thickening, extraluminal fluid and extraluminal abscess formation.(4) The most common causes of gastric and duodenal perforation are peptic ulcer (Fig. 6) and malignancy.

Fig. 6

Pneumoperitoneum in a 71-year-old woman caused by duodenal ulcer at the first portion, presenting as epigastric pain and vomiting. (a) Contrast-enhanced axial CT image shows an area of air attenuation around the liver, suggesting pneumoperitoneum (arrows). (b) Coronal CT image shows duodenal wall discontinuity due to an ulcer at the first portion (dotted arrow), with surrounding fat stranding (arrowhead).

Pneumoperitoneum may be detected owing to reasons other than gastrointestinal perforation, including postoperative state or peritoneal dialysis. However, these typically have no symptoms. Pneumoperitoneum due to open laparotomy decreases over time. However, it can remain for up to two weeks.(4) In patients with peritoneal dialysis, pneumoperitoneum can occur owing to catheter insertion or inappropriate exchange technique for a bag or extension tube, and its incidence rate has been reported to range between 4% and 34%.(7) Therefore, it is important to review the patient’s treatment history to avoid misinterpreting pneumoperitoneum due to postoperative state and peritoneal dialysis as life-threatening pneumoperitoneum. The common causes of pneumoretroperitoneum include duodenal or rectal perforation and extensions from pneumomediastinum. The common causes of pneumoperitoneum and pneumoretroperitoneum are different, but both are potentially serious signs.

Abscess

An abscess may form because of inflammation in the peritoneal or retroperitoneal cavity. Common causes of an intra-abdominal abscess include surgical trauma, anastomotic leaks, perforation of a peptic ulcer, perforated appendicitis or diverticulitis, and pelvic inflammatory disease.(8) An abscess is demonstrated as a low-attenuation central necrotic component with an enhancing wall that can be irregular and thick on contrast-enhanced CT. Abnormal gas is often contained in an abscess when gas-producing bacteria are the cause of the abscess (Fig. 7).

Fig. 7

Abscess formation in a 22-year-old woman caused by perforated acute appendicitis, presenting as right lower abdominal pain. Contrast-enhanced CT image shows a low-attenuation central necrotic component and gas with enhancing wall in the lower abdomen, suggesting abscess (arrow). An enlarged appendix (dotted arrows) that has an appendicolith (arrowhead) at the tip is seen adjacent to the abscess.

Walled-off pancreatic necrosis

Walled-off pancreatic necrosis (WOPN) is a late complication of necrotising pancreatitis, as defined in the revised Atlanta classification.(9) Peripancreatic fluid collections developing after necrotising pancreatitis have been classified into two categories: (a) acute necrotic collections, defined as non-encapsulated heterogeneous solid and liquid necrotic material within four weeks from necrotising pancreatitis attack; and (b) WOPN, defined as encapsulated heterogeneous solid and liquid necrotic material after four weeks from necrotising pancreatitis attack.(9) Life-threatening complications such as infected WOPN, bleeding or fistulisation into the gastrointestinal tract can occur.(10) On CT, WOPN is demonstrated as a heterogeneous lesion around the pancreas that comprises a non-enhancing area surrounded by a wall (Fig. 8). However, it may be difficult to distinguish necrotic tissue in WOPN from serous fluid on CT, and MR imaging has been shown to have better diagnostic accuracy to differentiate between the two.(10) The presence of gas within WOPN on CT is an important finding of infected WOPN and fistulisation into the gastrointestinal tract (Fig. 8).(10) Fistulas between WOPN and the gastrointestinal tract can be visualised on contrast-enhanced CT and thin-slice CT, and multiplanar reconstructions are helpful (Fig. 8c).

Fig. 8

Fistulisation between walled-off pancreatic necrosis (WOPN) and the descending colon in a 74-year-old man 12 weeks after a necrotising pancreatitis attack, presenting as progression of anaemia.(a) Unenhanced; (b) contrast-enhanced; and (c) thin-slice CT images show heterogeneous lesions around the pancreas comprising non-enhancing areas surrounded by a wall, suggesting WOPN. Abnormal gas is observed in the WOPN (arrows), and fistulas between WOPN and the descending colon are clearly visualised on thin-slice image (arrowhead). A biliary stent (dotted arrows) is seen.

Postoperative pancreatic fistula

Postoperative pancreatic fistula (POPF) is one of the most serious complications after pancreatic resection. It is defined as an abnormal communication between the pancreatic ductal system and another epithelial surface containing pancreas-derived, enzyme-rich fluid.(11) The incidence of POPF ranges between 3% and 45% of pancreatic operations at high-volume centres.(11) For diagnosis, any measurable volume of drain fluid on or after postoperative day 3 with amylase levels > 3 times the upper limit of normal amylase for each specific institution is the necessary threshold, and this condition is defined as biochemical leak (BL) in the latest guidelines.(11) BL is not considered a true pancreatic fistula or an actual complication and implies no deviation in the normal postoperative pathway. Clinically significant POPF is defined as a condition that requires persisting peripancreatic drainage or other treatment in addition to BL.(11) On CT, fluid collection around the pancreatic stump or anastomosis is an indicator of an underlying POPF (Fig. 9). Gas can be observed within the fluid collection on CT owing to anastomosis with the jejunum or infection (Fig. 9). Demonstrating the communication between the fluid collection and the pancreatic duct is especially useful for imaging diagnosis of POPF (Fig. 9b).

Fig. 9

(a) Biochemical leak (BL) in a 76-year-old woman after subtotal stomach-preserving pancreaticoduodenectomy 13 days prior to presentation for pancreatic ductal adenocarcinoma (PDAC) with complaints of fever and vomiting. Contrast-enhanced CT image demonstrates a fluid collection containing abnormal gas (arrow) around the anastomosis site, indicating leak. It was diagnosed as BL because of the high amylase level in the drainage. (b) Grade B postoperative pancreatic fistula (POPF) in a 67-year-old man after distal pancreatectomy and distal gastrectomy 17 days prior to presentation for PDAC and gastric cancer with a complaint of fever. Contrast-enhanced CT image shows a fluid collection with abnormal gas (dotted arrows) adjacent to the pancreatic stump. Communication between the fluid collection and the pancreatic duct (arrowheads) is clearly demonstrated. Percutaneous drainage was performed and a diagnosis of grade B POPF was established.

Pneumatosis intestinalis

Pneumatosis intestinalis is defined as the presence of gas in the bowel wall owing to various causes. It occurs as a primary (idiopathic) form or secondary form in 15% and 85% of cases, respectively.(3) It can be found with portal venous gas and mesenteric venous gas, rendering them related phenomena. Pneumatosis intestinalis is commonly caused by fatal diseases including bowel necrosis, bowel obstruction and bowel perforation. Nevertheless, it has been reported in various pathological conditions including inflammatory bowel disease and chronic pulmonary disease, with corticosteroids or molecular targeted drugs, and trauma that can show better clinical outcomes (so-called benign pneumatosis intestinalis).(3) Three patterns of pneumatosis on CT have been described: a circumferential form in the bowel wall (Fig. 10), a linear pattern in which the gas has a curvilinear or a crescent shape (Fig. 10), and a bubble-like or cystic pattern characterised by separate bubbles of gas with a cystic appearance (Figs. 10 & 11).(3) These can all be observed at the same time (Fig. 10). It is not possible to judge the presence or absence of necrosis only by the shape of the gas. It should be evaluated for bowel necrosis along with the patient’s condition and blood test results. Among benign pneumatosis intestinalis, a disease in which multiple gas cysts form in the bowel walls is classified as an independent disease called pneumatosis cystoides intestinalis (Fig. 11). Although the prognosis is good, it may be associated with pneumoperitoneum. Thus, it is important to differentiate it from bowel perforation.(3)

Fig. 10

Pneumatosis intestinalis due to bowel necrosis in a 76-year-old man with a complaint of abdominal pain. Contrast-enhanced CT image shows abnormal gas in the small intestinal wall with various patterns, including circumferential (arrows), linear (dotted arrows) and bubble-like (arrowheads) patterns.

Fig. 11

Pneumatosis cystoides intestinalis in a 79-year-old man who underwent corticosteroid therapy for lymphoproliferative disease, presenting with abdominal distension and nausea. Contrast-enhanced CT image (lung window) shows multiple areas of circumferential/curvilinear (arrow) and cystic (arrowhead) gas in the small intestinal wall. Abnormal gas in the mesentery is also observed (dotted arrow).

Emphysematous cholecystitis

Emphysematous cholecystitis is a rare form of acute cholecystitis. Vascular compromise of the cystic artery is thought to play a significant role in the evolution of the emphysematous form.(2) The overall mortality rate of patients with emphysematous cholecystitis has been reported as 15%.(2) Emphysematous cholecystitis is characterised by the presence of air within the gallbladder lumen or wall in the absence of abnormal communication between the biliary system and gastrointestinal tract, and CT is the most sensitive and specific imaging modality (Fig. 12).(2)

Fig. 12

Emphysematous cholecystitis in an 80-year-old man with complaints of abdominal pain and fever, and a history of diabetes mellitus. Contrast-enhanced (a) axial and (b) sagittal CT images show bubble-like gas in the gallbladder wall (arrows). Intraluminal gas showing air-fluid level (dotted arrows) is also observed. These findings suggest emphysematous cholecystitis.

Emphysematous pyelonephritis

Emphysematous pyelonephritis has been defined as a necrotising infection of the renal parenchyma and perirenal tissue, with abnormal gas in these areas (Fig. 13).(2,12,13) Obstruction of the urinary collecting system due to urinary stones or urothelial neoplasm is commonly observed.(2) It is important to assess the imaging features, including parenchymal destruction with gas, fluid content, and the extent of gas or abscess on CT.(12,13)

Fig. 13

Emphysematous pyelonephritis in a 56-year-old woman with complaints of abdominal pain and fever, and a history of diabetes mellitus. (a) Unenhanced; (b) contrast-enhanced; and (c) coronal CT images show an enlarged, destroyed renal parenchyma with gas and fluid collection in the right kidney (arrows). The gas and fluid collection extends to the pararenal space (dotted arrows). Fat stranding and thickened renal fascia are also observed. Acute focal bacterial nephritis is observed in the left kidney (arrowhead).

Emphysematous cystitis

Emphysematous cystitis is a rare form of complicated infection, with abnormal gas within the bladder wall and lumen. Chronic urinary tract infections, bladder outlet obstruction and a neurogenic bladder are often present.(2) Abdominal radiographs can demonstrate curvilinear or mottled gas in the region of the urinary bladder. However, CT is more useful for detailed evaluation of the distribution and amount of the gas (Fig. 1). Intraluminal gas forms an air-fluid level, and extraluminal gas or gas in the bladder wall is also observed (Fig. 1).(2)