Basic Abdomen and Pelvis CT Interpretation

Nate Beech

Structured search pattern and frequent windowing are important for evaluation of various structures and organs in the abdomen and pelvis. In the abdomen, a technique called drilling, or focusing on one area while scrolling through the image set, is helpful for focusing on areas of interest throughout the scan while windowing to assess for different pathologies.

Soft tissue window: for evaluation of solid organ and soft tissue structures
Bone windows: to identify fractures, osseous lesions, calcifications, kidney stones
Lung windows: to evaluate lung bases. Also helpful for identifying intraperitoneal free air in the abdomen and pelvis or pathology such as pneumatosis intestinalis.

Example of routine CT abdomen pelvis search pattern and dictation template:

Lower chest

Lung bases: consolidation, aspiration, atelectasis, interstitial thickening, emphysema.
Pleural effusion, pneumothorax, pulmonary emboli, lung nodules, granulomas.
Pericardial effusion, cardiomegaly, coronary calcification.

Liver

Assess for attenuation, morphology, enhancement, masses, abscesses, vascular/intrahepatic biliary abnormalities, trauma (laceration, hematoma, hemorrhage).

Attenuation: normal, hyperattenuating (hemochromatosis, Wilson’s), hypoattenuating (steatosis, amyloidosis).
Morphology: size, shape, contour (smooth or nodular).
Abscess: variable appearance, generally peripheral enhancement with central hypoattenuation.
Laceration: irregular linear/branching areas of hypoattenuation (top image).
Hematoma: can be chronic (hypodense) or acute (hyperdense) when compared to normal liver parenchyma.
Vascular injury.
Active hemorrhage: typically hyperdense compared to normal parenchyma.

Gallbladder and Biliary Tract

Evaluate for dense gallstones, surrounding fat stranding or fluid, dilated biliary ducts, gallbladder wall thickening, pneumobilia.

Pancreas

Assess for morphology, enhancement, lesions, ductal dilation, peripancreatic fluid or fat stranding, calcification, masses.

Pancreatitis: two subtypes; interstitial edematous (majority) [seen below] and necrotizing.
Parenchymal enlargement, alterations in attenuation, indistinct margins, surrounding fat stranding, lack of parenchymal enhancement suggests necrosis.
Gas or a circumscribed fluid collection could indicate infection or abscess.

Spleen

Size, enhancement, trauma, masses, infarct.

Infarcts are characterized by hypoenhancement

Adrenal glands

Morphology, masses

Similar density to liver and spleen; enhance with contrast.
Hyperenhancement is concerning for shock.

Kidneys and Ureters

Renal size, morphology, enhancement, symmetry masses/cysts, hydronephrosis, hydroureter, renal or ureteral stones

Urolithiasis: obstructing ureteral calculus (arrows) with upstream hydronephrosis.
Pyelonephritis: often appear normal in non-contrast CT.
Can see perinepheric fat stranding, edema in non-con CT.
Wedge-like regions of parenchymal swelling and reduced enhancement relative to normal parenchyma, cortex affected (less likely in infarct) in post-contrast CT.
Complications: renal or perinephric abscess, renal papillary necrosis, emphysematous pyelonephritis (noted by bubbly or linear streaks of gas, fluid collections with air-fluid levels), renal infarct, renal vein thrombosis.

Urinary bladder

Not well assessed when decompressed. Assess for wall thickening, masses, intraluminal debris, periserosal fat stranding; can assess area around bladder for free fluid (Pouch of Douglas in women).

Gastrointestinal Tract

assess for morphology of distal esophagus and stomach, bowel caliber, wall thickness, masses, mural enhancement, stool burden, appendix, free air, fat stranding

Bowel obstruction: Dilated gas or fluid-filled loops of bowel, air-fluid levels, fecal matter in small bowel loops, transition point between dilated and collapsed loops of bowel, bowel wall thickening, fat stranding (right image).
3-6-9 cm rule for upper limit of normal size for small bowel, colon, and cecum diameter.
Bowel perforation: assess for free air, pneumatosis, portal venous gas, localized fat stranding, bowel wall. thickening/enhancement, bowel wall discontinuity, contrast leak.
Pneumatosis intestinalis: Intramural bowel gas; linear lucencies or rounded cystic/bubbly collections that, when joined together, create outline of the margin of the bowel (left image).
Can be life-threatening secondary to ischemia, obstruction, enteritis/colitis, organ transplantation, caustic ingestion.
Can also be incidental and secondary to benign etiology, systemic disease (Lupus, AIDS), procedures, medications (steroids, chemo), making clinical context crucial.

Diverticulitis: Pericolonic fat stranding, segmental bowel wall thickening, mural hyperenhancement, engorged mesenteric vessels (right image).
- Complicated diverticulitis: perforation (free air and fluid), abscess formation, fistula formation (usually chronic complication).
Appendicitis
- Appendix arises from cecum and is inferior to ileocecal valve.
- Location of the tip of the appendix and length of the appendix are variable.
- Defined by appendiceal dilatation (classically >6mm outer-to-outer diameter but some studies reporting up to ≥8-9mm), wall thickening (classically >2mm but some studies up to >3mm) and enhancement, periappendiceal fat stranding/inflammation, adjacent cecal thickening.

Peritoneum

Assess for free air, free fluid, fluid collections, peritoneal or omental nodularity/implants

Small volume of peritoneal fluid may be physiologic in female pts, particularly around menses.
Fluid is generally hypodense. Hyperdense fluid may suggest hemoperitoneum, especially if history of trauma.
Should assess dependent areas if suspecting free fluid, such as Pouch of Douglas or paracolic gutters.

Vasculature

Suboptimally assessed without intravenous contrast, best assessed with CT angiography.

Portal, splenic, superior mesenteric veins: evaluate patency.
IVC: contrast reflux in IVC can appear similar to a hypodense filling defect and is specific but insensitive for right heart dysfunction.

Abdominal aorta
- Abdominal aortic aneurysm: focal dilatation >3 cm in maximum transverse diameter.
- Ruptured AAA: retroperitoneal hemorrhage adjacent to aneurysm, blood extending into perirenal or pararenal spaces or psoas muscles (image above).
- Signs of impending rupture include high attenuation crescent sign (crescent of increased attenuation due to fresh blood within thrombus of aneurysm), large size (>7cm), increasing size, reduced thrombus size.
Visceral arteries (celiac, splenic, common hepatic, renal, SMA, IMA).
Iliac, pelvic, and femoral arteries and veins.

Lymph Nodes

Throughout the abdomen and pelvis. Evaluate size, morphology, enhancement.

5mm in short axis is considered normal, with upper limit being around 9-11mm, which could indicate abnormality/pathology.
Lower chest, upper abdomen (gastrohepatic ligament, celiac, portocaval, porta hepatis), retroperitoneum, mesentery, pelvis (inguinal, mesorectal, sidewall).

Reproductive Organs

Generally not well evaluated with CT, can be used as an adjunct to imaging with ultrasound or MRI

Ovarian/Adnexal torsion: enlarged ovary shifted medially, distended fallopian tube, twisted ovarian pedicle in the adnexa, adnexal fat stranding, underlying edematous/hemorrhagic ovarian mass.
Pelvic inflammatory disease: tubular adnexal “mass”, fallopian tube thickening (>5mm with enhancing wall), uterosacral ligament thickening, complex pelvic free fluid (within Pouch of Douglas), pelvic fat stranding or haziness.

Abdominal Wall

Hernias, hematomas, solid or cystic masses, skin/soft tissue infection, anasarca

Assess hernias for signs of incarceration, strangulation, obstruction.

Musculoskeletal

Important to assess using bone windows

Assess for fractures, destructive osseous lesions, degenerative changes

Additional Resources for Abdomen and Pelvis CT Interpretation:

For additional info regarding organ systems, please see radiopaedia.org reference articles.