CT Abdomen and Pelvis

Computed tomography of the abdomen and pelvis acquires a volume from the diaphragm to the pubic symphysis in 5 to 15 seconds, with slices of 1.0 to 5 mm reconstructed in axial, coronal and sagittal planes. It is the main cross-sectional test for acute abdominal pain, trauma, cancer staging and follow up, inflammatory disease, infection and complications.

Oral contrast (water or dilute positive medium, 1 to 1.5 litres over 60 to 90 minutes) and intravenous contrast (100 to 120 mL non-ionic at 2 to 3 mL each second) are used depending on the question: portal venous phase at 60 to 80 seconds is routine.

An arterial phase (25 to 35 seconds) is added for hypervascular tumours, bleeding and vascular disease. A delayed phase (3 to 5 minutes or longer) for the urinary tract and washout.

Effective dose is about 8 to 15 mSv; low dose techniques give 2 to 4 mSv for stones.

Acute abdominal pain accounts for about 5 to 10% of emergency visits, and CT changes the working diagnosis in about 50% and management in about 25 to 30%. CT has a sensitivity of 95 to 98% for appendicitis, over 95% for urinary stones, 90 to 95% for diverticulitis and bowel obstruction, and about 95% for solid organ injury in trauma.

Indications. Acute abdominal pain with peritonism, suspected appendicitis, diverticulitis, bowel obstruction, perforation, ischaemia, pancreatitis, cholecystitis (when ultrasound is inconclusive), abscess, and renal colic (non-contrast). Abdominal trauma. Suspected or known malignancy: staging, response and recurrence.

Palpable mass, unexplained weight loss, jaundice, and fever of unknown origin. Inflammatory bowel disease complications. Gastrointestinal bleeding with unstable state (with arterial phase).

Aortic aneurysm and mesenteric vascular disease. Pre-operative planning and post-operative complications (leak, collection). Planning of image directed biopsy and drainage.

Retroperitoneal disease, lymphadenopathy and hernias. Not for uncomplicated dyspepsia or irritable bowel symptoms without alarm features, and not as a first test for biliary pain, gynaecological pain in young women or suspected appendicitis in children and pregnant women, where ultrasound or MR is used first.

Contraindications and precautions. Pregnancy: avoid unless essential and ultrasound or MR cannot answer. Children: use ultrasound first; low dose settings if CT is needed.

Intravenous contrast: previous severe reaction (premedication or non-contrast study), renal impairment (estimated glomerular filtration rate under 30 mL each minute: weigh risk, hydrate, avoid nephrotoxic drugs), metformin (follow local advice depending on renal function), thyroid disease, and myeloma.

Oral contrast is omitted when perforation, leak or complete obstruction is suspected needing urgent surgery, or aspiration risk is high. Cannula 18 to 20 G, and watch for extravasation.

Preparation and technique. Fasting for 4 hours for elective contrast scans. Oral contrast is taken 60 to 90 minutes before the scan in elective cases; positive contrast is avoided before CT urography or angiography. The patient lies supine, with arms raised over the head.

Scan from the dome of the diaphragm to the lesser trochanters (or to the symphysis). Intravenous contrast is given by pump with bolus tracking for arterial studies. Water as a neutral oral agent helps in assessing the stomach and small bowel wall.

In acute stone colic, a low dose non-contrast study of the kidneys, ureters and bladder is done. Rectal contrast is added for suspected leak, fistula or low colonic lesions. Coronal and sagittal reformats and thin slab images are reviewed in every study.

Normal appearances. The liver has uniform attenuation (50 to 65 HU non-contrast, higher than the spleen is not normal in fatty liver), with hepatic veins and portal vein enhancing in the portal phase. The gallbladder has thin walls under 3 mm with fluid at 0 to 20 HU.

The spleen is homogeneous and under 12 cm. The pancreas enhances uniformly with a thin main duct under 3 mm, the adrenals are thin limbs under 10 mm, and the kidneys enhance in the cortex then medulla, with thin ureters. The bowel has walls under 3 mm when distended, with a collapsed colon thicker.

The appendix is under 6 mm with no wall thickening or stranding. The aorta is under 3 cm, retroperitoneal nodes are under 10 mm in short axis, and there is no free fluid beyond a small amount in the pelvis in women.

The uterus, ovaries, prostate and bladder have regular outlines, and the bladder wall is under 5 mm when distended.

Interpretation.

• Appendicitis: dilated appendix over 6 to 7 mm, wall thickening and enhancement, periappendiceal fat stranding, appendicolith, and complications such as perforation (gas, abscess, phlegmon), with free fluid and caecal wall thickening.
• Diverticulitis: colonic wall thickening with pericolic stranding adjacent to diverticula, abscess (rim enhancing collection), perforation (extraluminal gas), fistula, and stricture; mimics include colon cancer if the wall thickening is long, with shouldering and lymph nodes.
• Bowel obstruction: dilated small bowel over 3 cm proximal to a transition point with collapsed distal bowel; causes include adhesions, hernia, tumour, volvulus (whirl sign), and intussusception; signs of strangulation are wall thickening, reduced enhancement, mesenteric oedema, free fluid and pneumatosis.
• Mesenteric ischaemia: absent or reduced bowel wall enhancement, wall thickening, pneumatosis, portal venous gas, and arterial or venous thrombosis on contrast.
• Perforation: free intraperitoneal gas, focal wall defect, extraluminal fluid and fat stranding at the site (duodenal ulcer, diverticulitis, malignancy).
• Pancreatitis: pancreatic enlargement with peripancreatic stranding and fluid, necrosis (non-enhancing areas after 72 hours), collections (acute fluid collection, pseudocyst, walled off necrosis), splenic vein thrombosis and pseudoaneurysm; pancreatic cancer shows a hypoenhancing mass with upstream duct dilatation (double duct sign), vascular encasement, and liver metastases.
• Biliary disease: gallstones (some are isodense), gallbladder wall thickening and pericholecystic fluid, dilated bile ducts over 6 to 7 mm with a stone or mass at the obstruction, and cholangitis with liver perfusion changes.
• Liver: hepatic steatosis (liver darker than spleen), cirrhosis (nodular contour, caudate enlargement, splenomegaly, varices, ascites), focal lesions (cyst, haemangioma with peripheral nodular enhancement and fill in, hepatocellular carcinoma with arterial enhancement and portal washout, metastases hypoattenuating or ring enhancing), abscess (rim enhancement, gas), and vascular disease (portal vein thrombosis, Budd-Chiari syndrome).
• Kidney and urinary tract: stones (hyperattenuating, with hydronephrosis and perinephric stranding), pyelonephritis (wedge shaped reduced enhancement, abscess), renal mass (enhancing, with fat in angiomyolipoma, heterogeneous in renal cell carcinoma), cysts, obstructive uropathy, bladder wall thickening or mass, and trauma (laceration, perinephric haematoma, urine leak on delayed images).
• Adrenal: adenoma (under 10 HU on non-contrast, or rapid washout), and indeterminate masses needing further tests; phaeochromocytoma, metastases and myelolipoma (fat).
• Spleen: infarct (wedge), laceration, abscess, lymphoma (masses), and splenomegaly.
• Gastrointestinal tumours: gastric wall thickening, small bowel tumours, colorectal cancer (focal circumferential mass, nodes, liver metastases), lymphoma (bulky nodes, bowel wall thickening), and peritoneal disease (nodules, omental caking, ascites).
• Inflammatory bowel disease: wall thickening with enhancement, comb sign, fibrofatty proliferation, strictures, fistulae and abscess (Crohn disease), and continuous colitis (ulcerative colitis).
• Vascular: abdominal aortic aneurysm (over 3 cm; rupture with retroperitoneal haematoma and contrast extravasation), dissection, mesenteric and renal artery disease, and venous thrombosis.
• Pelvis: ovarian cysts and masses (adnexal torsion with an enlarged ovary and twisted pedicle), pelvic inflammatory disease (tubo ovarian abscess), fibroids, endometrial thickening, prostate enlargement, and pelvic fluid.
• Trauma: liver, spleen and kidney lacerations (graded by depth), haemoperitoneum, active contrast extravasation (arterial phase blush), mesenteric and bowel injury (free fluid without solid organ injury, wall thickening), pancreatic injury, and pelvic fractures.
• Hernias and abdominal wall: inguinal, femoral, incisional hernia with obstruction or strangulation, haematoma, and abscess.
• Retroperitoneum: lymphadenopathy, fibrosis, haematoma, and tumours.
• Post-operative: anastomotic leak (extraluminal contrast or gas), abscess, retained surgical material, and ileus.

Critical values and findings needing immediate action.

• Free intraperitoneal gas or extraluminal contrast: surgical review for perforation or leak.
• Ruptured aortic aneurysm or active arterial bleeding: emergency vascular surgery or embolisation.
• Mesenteric ischaemia or strangulated bowel: emergency laparotomy and vascular intervention.
• Closed loop obstruction or volvulus: urgent surgical review.
• Complicated appendicitis or diverticulitis with abscess: drainage and antibiotics.
• Solid organ injury with active bleeding: angioembolisation or surgery.
• Obstructed infected urinary tract: urgent decompression (nephrostomy or stent).
• Ovarian torsion: urgent surgery.
• Necrotising pancreatitis with infected collections: specialist care.
• Ischaemic or perforated gallbladder, cholangitis with obstruction: antibiotics and urgent drainage.
• Cancer with obstruction or perforation: urgent surgical and oncological review.

Limitations and artefacts.

• Radiation dose is substantial, especially with multiphase studies; young patients and repeated imaging for chronic conditions should be restricted, and ultrasound or MR used where they answer the question.
• Contrast timing errors miss arterial bleeding, hypervascular tumours and venous thrombosis; the phase must suit the question.
• Poor fat planes in thin patients, no oral contrast and collapsed bowel limit bowel assessment; water or positive oral contrast improves it.
• Motion, bowel gas, metal (hip prostheses) and beam hardening in obese patients degrade the pelvis.
• Early appendicitis, early pancreatitis (first 24 to 72 hours), small bowel tumours, mild inflammatory bowel disease, small peritoneal deposits, and early ischaemia may be missed; a normal CT does not exclude disease if symptoms persist.
• Small liver lesions and pancreatic tumours may be isoattenuating; MR or endoscopic ultrasound is more sensitive.
• Incidental findings (renal and liver cysts, adrenal nodules, pancreatic cysts, gallstones, aneurysms) are common and need defined follow up.
• Contrast reactions and acute kidney injury risk should be weighed, particularly in patients with renal impairment, diabetes and dehydration.
• Normal fat stranding after surgery or trauma can be mistaken for inflammation.

Special groups.

• Children and young adults: ultrasound first for appendicitis and intussusception; low dose CT only when needed.
• Pregnancy: ultrasound and MR first; CT only for life threatening conditions where other tests fail.
• Older patients: atypical presentations of serious disease; lower threshold for CT, with attention to renal function.
• Patients with cancer: staging and follow up with consistent technique so that comparison is valid.
• Patients with renal impairment: non-contrast or alternative modalities when the question allows.
• Obese patients: image quality is lower, so adjust technique; ultrasound is limited, so CT is used more.
• Regions: typhoid perforation, tuberculous abdomen (ascites, omental thickening, necrotic nodes), amoebic and pyogenic liver abscess, hydatid cysts, schistosomal periportal fibrosis, obstructed labour complications, abdominal trauma, and late presenting cancer are common; CT is limited, so ultrasound and the clinical picture decide most management.

Follow up tests. Ultrasound for the gallbladder, kidneys, pelvis and liver. MR (including MR cholangiopancreatography and enterography) for the liver, pancreas, bile ducts, small bowel and pelvis. Endoscopic ultrasound for pancreatic and biliary lesions. Colonoscopy for colon lesions. Endoscopy. PET-CT for staging and recurrence. Image directed biopsy or drainage.

Tumour markers (carcinoembryonic antigen, CA 19-9, alpha fetoprotein, CA-125). Blood tests (full blood count, inflammatory markers, lipase, liver function, creatinine). Urinalysis and culture. Stool tests. Laparoscopy. And referral to general surgery, gastroenterology, urology, gynaecology or oncology.