Gastrointestinal CT: Enterography, Colonography & Acute Abdomen

Acquisition settings and contrast timings below are illustrative adult protocol examples. Select the authorized protocol for the indication, scanner, body size, access device, and clinical condition. Treatment decisions belong to the responsible clinical team.

CT Enterography (CTE) Protocols & Small Bowel Imaging

Evaluating the small bowel has historically challenged diagnostic imaging due to its extensive length (approx. $5\text{ to }7\text{ meters}$), overlapping loops, active peristalsis, and physiological collapse. Conventional positive oral contrast agents opacify the lumen with high radiodensity ($+200\text{ to }+400\text{ HU}$), which completely obscures mucosal enhancement, subtle mucosal ulcerations, low-attenuation wall edema, and intraluminal bleeding. CT Enterography (CTE) is a specialized technique optimized for visualizing small bowel mucosal and mural pathology, serving as the frontline cross-sectional modality for inflammatory bowel disease (Crohn's disease), obscure gastrointestinal bleeding (OGIB), and small bowel neoplasms (carcinoids, adenocarcinoma, lymphoma, and gastrointestinal stromal tumors [GISTs]).

Neutral Enteral Contrast Selection & Administration

• Mechanism of Neutral Contrast: Neutral enteral contrast agents exhibit an attenuation profile equivalent to water, ranging between $-20\text{ and }+20\text{ HU}$. While plain water rapidly diffuses across the jejunal mucosa—resulting in collapsed, non-diagnostic distal loops—commercial CTE agents (such as VoLumen, a $0.1\%\text{ w/v}$ dilute barium sulfate suspension) incorporate non-absorbable osmotic additives, specifically sorbitol and locust bean gum. Sorbitol exerts an intraluminal osmotic gradient that prevents mucosal water absorption, ensuring robust, uniform luminal distension from the duodenum to the terminal ileum.
• Ingestion Schedule: Total volume of $1500\text{ mL}$ in this illustrative schedule, split into timed aliquots split into timed aliquots:
  • $45\text{ minutes}$ prior to scan: Ingest first bottle ($450\text{ mL}$).
  • $30\text{ minutes}$ prior to scan: Ingest second bottle ($450\text{ mL}$).
  • $15\text{ minutes}$ prior to scan: Ingest third bottle ($450\text{ mL}$).
  • Immediately before scanning: Ingest a final glass of water ($150\text{ mL}$) on the scanner table to distend the stomach and proximal duodenum.

Intravenous Contrast & Enteric Phase Delay

• Injection Parameters: High injection rate of $4.0\text{ to }5.0\text{ mL/s}$ of non-ionic iodinated contrast ($100\text{ to }125\text{ mL}$ at $350\text{ to }370\text{ mg I/mL}$) to produce sharp arterial-capillary contrast bolus delivery.
• Enteric Phase Timing ($45\text{--}50\text{ s}$ Delay): Scanned at $45\text{ to }50\text{ seconds}$ post-injection (an intermediate phase between late arterial and portal venous phases). This timing precisely captures the peak mucosal enhancement of the small bowel wall while the neutral intraluminal fluid remains dark (water attenuation), producing maximum diagnostic contrast between the bright, hyperenhancing bowel mucosa and the dark lumen.

Imaging Biomarkers of Active Crohn's Disease on CTE

1. Mucosal Hyperenhancement: Intense iodine uptake in the inflamed small bowel mucosa compared to adjacent normal loops, reflecting acute capillary engorgement and neovascularity.
2. Mural Thickening ($>3\text{ mm}$): Normal distended small bowel wall measures $\le 2\text{ to }3\text{ mm}$. In active Crohn's, mural thickness typically reaches $5\text{ to }10\text{ mm}$.
3. Mural Stratification ("Target" or "Double-Halo" Sign): Visualized on axial and coronal reformations as alternating concentric layers: an inner hyperenhancing mucosal ring, a middle hypoattenuating submucosal layer (representing inflammatory submucosal edema or chronic fat deposition), and an outer hyperenhancing muscularis propria/serosa.
4. The "Comb Sign": Marked engorgement, dilation, and tortuosity of the mesenteric vasa recta supplying the inflamed bowel loop. The parallel alignment of these hyperemic vessels mimics the teeth of a comb, serving as a reliable sign of active transmural inflammation.
5. Fibrofatty Proliferation ("Creeping Fat"): Hypertrophy of mesenteric adipose tissue along the mesenteric border of the diseased segment, causing wide separation of adjacent bowel loops.
6. Complicated Penetrating & Stenosing Disease: Identifies transmural sinus tracts, fistulas (enteroenteric, enterovesical, enterocutaneous), mesenteric inflammatory phlegmons, abscesses, and assesses inflammatory and fibrotic components of strictures, which can coexist. Imaging does not always separate them completely.

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The Acute Abdomen on CT

Computed tomography has become the definitive diagnostic modality for acute, non-traumatic abdominal pain, rapidly diagnosing surgical emergencies and preventing unnecessary exploratory laparotomies.

Acute appendicitis, diverticulitis and obstruction

For appendicitis, identify the appendix from its cecal origin and assess caliber together with wall enhancement, periappendiceal fat changes, fluid and complications. Diameter above 6 mm alone is not diagnostic, since a normal appendix can exceed that value. An appendicolith may be present without acute inflammation. Coverage and reconstruction must allow the appendix to be traced, including a pelvic or retrocecal course.

Diverticulitis can produce localized bowel wall thickening and adjacent fat inflammation near diverticula. Look for abscess, extraluminal gas, obstruction or fistula. An abscess size does not automatically determine drainage in every patient; clinical stability, location and accessibility influence treatment. If a leak question is added, select enteral or rectal contrast through the responsible clinician's plan.

For obstruction, identify dilated proximal bowel, a transition and decompressed distal bowel, then assess causes and signs of compromised perfusion. Closed-loop configuration, mesenteric distortion, reduced wall enhancement or concerning fluid warrant prompt attention. Pneumatosis or portal venous gas can occur in different settings and is not universal proof of irreversible necrosis. Communicate potentially urgent findings instead of assigning viability from one sign.

A high-grade obstruction examination should not be unnecessarily delayed to force large oral volumes. IV enhancement can help assess bowel-wall perfusion when appropriate. The protocol must address the actual question and the patient's condition.

CT Colonography (CTC / Virtual Colonoscopy)

CT Colonography (virtual colonoscopy) provides a minimally invasive, high-resolution endoluminal and cross-sectional evaluation of the entire large intestine, serving as a primary screening tool for colorectal cancer and polyps, and as the gold standard alternative when optical colonoscopy is incomplete due to severe tortuosity, stricture, or dolichocolon.

Patient Preparation & Tagging Protocols

1. Cathartic Bowel Cleansing: Low-residue diet followed by osmotic laxatives (polyethylene glycol, magnesium citrate, or sodium picosulfate) to clear solid feces from the mucosal surface.
2. Fecal and Fluid Tagging:
   • Stool and fluid tagging: follow the approved agent concentrations, volumes and drinking schedule. Tagged material becomes conspicuous against untagged soft tissue. Distinguish tagged stool from a wall-attached polyp by multiple views and positions. Electronic cleansing can help but can also introduce artifacts; inspect source images.

Carbon Dioxide ($CO_2$) Insufflation & Dual-Position Scanning

• Automated Mechanical Insufflation: A thin, flexible balloon catheter is placed in the rectum, and carbon dioxide ($CO_2$) is insufflated into the colon under automated pressure control (typically $15\text{ to }25\text{ mmHg}$) until complete distension is achieved ($3\text{ to }4\text{ liters}$ total volume).
• Why Carbon Dioxide Over Room Air? Carbon dioxide is absorbed across the colonic mucosa more rapidly than room air and is eliminated through pulmonary respiration. More rapid absorption can improve postprocedure comfort, but does not eliminate pain or other complications associated with manual room-air pumping.
• Dual Positioning (Supine & Prone): Imaging must be acquired in both the supine and prone positions (or supine and lateral decubitus if the patient cannot lie prone). Positional shifts cause mobile residual fluid and tagged fecal pellets to tumble to the dependent wall, allowing the nondependent mucosal surfaces to be fully evaluated. A true polyp is fixed to the bowel wall and remains in the same anatomic position on both scans, whereas residual stool shifts to the dependent wall.

Current C-RADS categories

CT colonography evaluates the distended lumen in complementary views and positions. Distinguish a true polyp from tagged stool, fluid or a fold by attenuation, attachment and behavior across positions. The 2023 C-RADS update distinguishes C2a, including one or two 6–9 mm polyps, from C2b, a likely benign mass-like finding such as a diverticular stricture. C3 includes a polyp of at least 10 mm or at least three 6–9 mm polyps. Follow the complete reporting system and clinical pathway; these category descriptions do not authorize the technologist to prescribe follow-up independently.

Reference: ACR C-RADS 2023 changes.