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Basic Abdomen and Pelvis CT Interpretation

  • Structured search pattern and frequent windowing are important for evaluation of various structures and organs in the abdomen and pelvis.

  • Soft tissue window: for evaluation of solid organ and soft tissue structures

  • Bone windows: to identify fractures or osseous lesions

  • Lung windows: to evaluate lung bases. Also helpful for identifying intraperitoneal free air in the abdomen and pelvis.

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

  • Lower chest

  • Lung bases: consolidation, aspiration, atelectasis
  • Pleural effusion, pneumothorax

  • Pericardial effusion

  • Liver: assess morphology, enhancement, masses, abscesses, vascular abnormalities, trauma

  • Liver trauma: laceration, hematoma, hemorrhage

    • Laceration: irregular linear/branching areas of hypoattenuation
    • Hematoma: intermediate to hyperdense collection between liver parenchyma and capsule
    • 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

  • Pancreas: assess morphology, enhancement, lesions, ductal dilation, peripancreatic fluid or stranding.

  • Pancreatitis: two subtypes (interstitial edematous [seen in image] and necrotizing)

  • Parenchymal enlargement, alterations in attenuation, indistinct margins, surrounding fat stranding, lack of parenchymal enhancement suggests necrosis

  • Spleen: size, enhancement, trauma

  • Adrenal Glands: morphology, masses

  • Kidneys and Ureters: renal size, morphology, enhancement, masses, hydronephrosis, hydroureter, renal or ureteral stones.

  • Urolithiasis: distal obstructing ureteral calculus with upstream hydronephrosis

  • Pyelonephritis: wedge-like regions of parenchymal swelling and reduced enhancement relative to normal parenchyma

    • Complications: renal or perinephric abscess, renal papillary necrosis, emphysematous pyelonephritis (look for bubbly or linear streaks of gas, fluid collections with air-fluid levels)

  • Urinary Bladder: not well assessed when decompressed.

  • Assess for wall thickening, masses, intraluminal debris, periserosal fat stranding.

  • Gastrointestinal Tract: morphology of distal esophagus and stomach, bowel caliber, wall thickness, masses, mural enhancement, stool burden, appendix

  • Bowel obstruction: Dilated gas or fluid-filled loops of bowel (3-6-9 cm rule for upper limit of normal size for small bowel, colon, and cecum diameter), air-fluid levels, fecal matter in small bowel loops, transition point between dilated and collapsed loops of bowel
  • Bowel perforation: assess for free air, pneumatosis, portal venous gas
  • Pneumatosis intestinalis: Intramural bowel gas. Linear lucencies or rounded bubbly collections.
    • Can be life-threatening secondary to ischemia, obstruction, enteritis/colitis, organ transplantation. Can also be incidental and secondary to benign etiology, making clinical context crucial.
  • Diverticulitis: Pericolonic fat stranding, segmental bowel wall thickening, mural hyperenhancement.
    • Complicated diverticulitis: perforation (free air and fluid), abscess formation, fistula formation (usually a chronic complication).

  • Appendicitis

    • Base of appendix located between the ileocecal valve and apex of the cecum.
    • Location of the tip of the appendix and length of the appendix are variable.
    • Appendiceal dilatation (classically >6mm outer diameter), wall thickening and enhancement, intraluminal fluid, periappendiceal fat stranding, adjacent cecal thickening.

  • Peritoneum: 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 in context of trauma.

  • Vasculature: suboptimally assessed without intravenous contrast, best assessed with CT angiography

  • Portal, splenic, superior mesenteric veins: evaluate patency
  • IVC: contrast mixing in IVC can appear similar to a hypodense filling defect. -
  • 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. High attenuation crescent representing acute hematoma within mural thrombus or aneurysm wall suggests impending rupture.
  • 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 (general guideline is <10mm short axis), morphology, enhancement.

  • 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 torsion: enlarged ovary, ovary shifted medially, twisted ovarian pedicle in the adnexa, adnexal fat stranding, underlying ovarian mass.

  • Pelvic inflammatory disease: tubular adnexal “mass”, fallopian tube thickening, uterosacral ligament thickening, complex pelvic free fluid, pelvic fat stranding or haziness.

  • Abdominal Wall: hernias, hematomas, solid or cystic masses, skin/soft tissue infection

-Musculoskeletal: important to assess using bone windows - Fractures - Destructive osseous lesions - Degenerative changes