13.3 GI Tract CT & Spleen Imaging
Key Takeaways
- The normal spleen enhances heterogeneously ("zebra"/arciform pattern) in the early arterial phase; trauma injury assessment should be read on the homogeneous portal venous phase to avoid a false-positive laceration call.
- AAST grades I-V stage true splenic injury; a vascular "blush" that increases in attenuation over time signals active hemorrhage, often warranting angioembolization.
- CT enterography for Crohn's disease uses large-volume neutral oral contrast (1,350-2,000 mL) and an enteric phase (~45-50 seconds) so mural hyperenhancement is not masked by positive contrast.
- Active Crohn's findings include mural hyperenhancement, wall thickening over 3 mm, the "comb sign" (engorged vasa recta), and "creeping fat."
- Acute appendicitis on CT is diagnosed by an appendix diameter over 6 mm plus periappendiceal fat stranding, with an appendicolith or abscess signaling more advanced disease.
Why This Topic Matters
Two of the highest-yield emergency-department presentations — right lower quadrant pain (appendicitis) and chronic abdominal pain with diarrhea (Crohn's disease) — are tested through dedicated GI-tract CT technique choices, and the spleen carries one of the classic "don't get fooled" traps in trauma imaging: a completely normal enhancement pattern that can be mistaken for injury if scanned at the wrong phase or read without context. This section covers the spleen and GI tract leaf items under Procedures → Abdomen and Pelvis → Abdomen, and it also connects to the cross-cutting "Additional Procedures" focus theme (trauma) that ARRT explicitly calls out as spanning every organ system in the Procedures domain.
Core Terms
Spleen — the arciform/"zebra" enhancement trap. In the early arterial phase, the normal spleen enhances heterogeneously — a mottled, striped, "zebra" or arciform pattern — because blood flows through the red pulp's venous sinuses at markedly different rates than through the white pulp and trabeculae. If a trauma CT is scanned in a pure early arterial phase, or interpreted without knowing this is a normal variant, that heterogeneity can be mistaken for a laceration, contusion, or infarct. The spleen becomes homogeneous by the portal venous phase (roughly 70 seconds), which is exactly why abdominal trauma protocols are read primarily on the portal venous phase for parenchymal injury assessment rather than the arterial phase. True splenic injury is graded using the American Association for the Surgery of Trauma (AAST) organ injury scale (grades I through V, based on laceration depth, hematoma size, and vascular involvement). A focus of contrast that pools and increases in attenuation between the arterial/portal venous phase and a delayed phase — a vascular "blush" — indicates active hemorrhage and often triggers angioembolization rather than observation alone. A splenic infarct, by contrast, appears as a peripheral, wedge-shaped area of hypoattenuation with its base at the capsule — a distinct, focal finding rather than the diffuse heterogeneity of normal early-phase enhancement.
CT enterography — dedicated small-bowel technique. Ordered primarily for suspected or known Crohn's disease. The single most important technical choice is oral contrast strategy: rather than positive (high-density, barium- or iodine-based) oral contrast, enterography uses a large volume of neutral, low-density oral contrast (a polyethylene glycol/mannitol-based solution, commonly 1,350-2,000 mL given over 45-60 minutes before scanning) to distend the small bowel lumen without obscuring enhancement of the bowel wall itself. Positive contrast would mask the very finding — mural hyperenhancement — that the study exists to detect. Imaging is performed in an enteric phase, roughly 45-50 seconds post-injection, timed to peak small-bowel mural enhancement. Findings of active inflammation include mural hyperenhancement (sometimes a layered "target" or "stratified" appearance), bowel wall thickening greater than 3 mm, engorged vasa recta feeding the inflamed segment (the "comb sign"), mesenteric fibrofatty proliferation ("creeping fat"), and reactive mesenteric lymphadenopathy.
Appendicitis on CT. A CT of the abdomen and pelvis for suspected acute appendicitis (right lower quadrant pain, low-grade fever, elevated white blood cell count) is read for: an appendiceal diameter greater than 6 mm, appendiceal wall thickening and enhancement, periappendiceal fat stranding, and sometimes an appendicolith (a calcified fecalith within the appendiceal lumen) or a periappendiceal abscess if perforation has already occurred. Positive oral contrast (or rectal contrast via enema) can help by opacifying adjacent bowel loops, making the non-filling, blind-ending appendix easier to distinguish from normal bowel, though many institutions now perform contrast-optional (IV-contrast-only) studies in adults with adequate mesenteric fat, reserving oral contrast for thin patients or when obstruction/leak is also suspected.
| Finding / Sign | Organ or Study | Clinical Meaning |
|---|---|---|
| "Zebra"/arciform heterogeneous enhancement | Spleen, early arterial phase | Normal enhancement variant — not injury |
| Vascular "blush" (increasing attenuation over time) | Spleen, trauma CT | Active hemorrhage — often needs angioembolization |
| Wedge-shaped peripheral hypodensity | Spleen, portal venous phase | Splenic infarct |
| Comb sign (engorged vasa recta) | CT enterography | Mesenteric hyperemia — active Crohn's inflammation |
| Creeping fat | CT enterography | Chronic fibrofatty mesenteric change from long-standing Crohn's |
| Appendix diameter greater than 6 mm plus fat stranding | Abdomen/pelvis CT | Acute appendicitis |
Realistic Exam Scenario
A trauma patient undergoes a single early arterial-phase acquisition. The spleen appears mottled and heterogeneous, with no discrete hypodense laceration or perisplenic fluid. The exam expects you to recognize this as the normal arciform ("zebra") enhancement pattern rather than injury, and to know that confirming a true splenic injury requires assessment on the portal venous phase, where the parenchyma should appear homogeneous. A second scenario: a 27-year-old with a known history of Crohn's disease presents with a symptom flare; the correct study is a CT enterography with large-volume neutral oral contrast timed to the enteric phase, specifically so any mural hyperenhancement and comb sign are not masked by dense positive oral contrast. A third scenario: a 22-year-old woman presents with right lower quadrant pain, low-grade fever, and leukocytosis; on CT, the appendix measures 9 mm with surrounding fat stranding and a visible appendicolith — findings that together are most specific for acute appendicitis, distinguishing it from nonspecific findings like free pelvic fluid alone.
A trauma CT of the spleen, acquired in the early arterial phase, shows a mottled, striped enhancement pattern with no discrete hypodense laceration. What is the most likely explanation?
Why does CT enterography use a large volume of neutral, low-density oral contrast instead of positive (high-density) oral contrast?
Which combination of CT findings is most specific for acute appendicitis?