What are the key imaging findings to look for on a contrast-enhanced computed tomography (CECT) abdomen and pelvis in a patient with lower gastrointestinal (GI) bleed to identify the source and extent of bleeding?

How to Identify Bleeding on CECT Abdomen and Pelvis in Lower GI Bleed

For lower GI bleeding, perform CTA (CT angiography) of the abdomen and pelvis WITHOUT and WITH IV contrast in a multiphasic protocol (noncontrast + arterial + portal venous phases), and look for active contrast extravasation into the bowel lumen as the primary sign of bleeding. 1, 2

Critical Technical Requirements

Imaging Protocol

• Use CTA technique specifically, not standard contrast-enhanced CT—these are distinct procedures with different protocols 1
• Obtain multiphasic imaging: noncontrast phase + arterial phase (0.5 minutes post-contrast) + portal venous phase (5 minutes post-contrast) 1, 3
• Do NOT give oral contrast—positive oral contrast will mask intraluminal extravasation and render the study nondiagnostic 1, 2
• The noncontrast phase is essential for identifying a "sentinel clot" (high-density blood collection that marks the bleeding site before contrast administration) 1

Detection Threshold

• CTA can detect bleeding rates as low as 0.3 mL/min, which is more sensitive than catheter angiography (requires 0.5-1.0 mL/min) 1, 2

Primary Imaging Finding: Active Extravasation

What to Look For

• Active contrast extravasation: Pooling of IV contrast material within the bowel lumen that appears on arterial phase imaging 1, 3
• This appears as high-attenuation (bright) material within the bowel that was not present on noncontrast images 3
• The extravasation may increase in size or change configuration on delayed portal venous phase images 1, 3

Arterial Phase Findings

• The arterial phase identifies the beginning of intraluminal extravasation and can locate the specific culprit arterial branch feeding the bleeding site 1
• This information allows for more focused transcatheter arteriography and embolization if needed 1

Portal Venous Phase Findings

• The portal venous phase helps evaluate the nature of the underlying pathology causing the bleeding (tumor, vascular malformation, diverticular disease) 1
• CTA identifies both the bleeding site AND the underlying culprit lesion in 92% of cases 2

Secondary Imaging Findings

Sentinel Clot Sign

• High-density blood collection visible on noncontrast images that marks the bleeding location before contrast administration 1
• This finding helps confirm the bleeding site even if active extravasation is not captured during the scan 1

Underlying Pathology

• Diverticular disease: Look for colonic diverticula near the extravasation site 1, 3
• Vascular malformations: Abnormal vascular structures or angiodysplasia 1
• Tumors: Mass lesions with abnormal enhancement 1
• Ulcers: Mucosal defects in small bowel or colon 3

Diagnostic Performance

Accuracy Metrics

• Sensitivity: 79-100% across individual studies, with meta-analysis showing 90% sensitivity 1
• Specificity: 50-100%, with meta-analysis showing 92% specificity 1
• Localization accuracy: 90.9% for identifying the specific bleeding site 2
• CTA identifies active bleeding or potential bleeding lesion in 92% of cases versus only 29% with catheter angiography 1

Clinical Context

• The volume of contrast extravasation correlates with need for hemostatic therapy, intraprocedural active bleeding, and massive transfusion requirements (though not with mortality) 1
• Negative CTA is highly predictive: approximately 75% of patients with negative CTA settle spontaneously with conservative management 1

Critical Timing Considerations

Speed of Diagnosis

• CTA can be completed within 15 minutes from start to finish 3
• This is significantly faster than colonoscopy (requires bowel preparation), nuclear medicine scans, or catheter angiography 1, 2, 3

Intermittent Bleeding Challenge

• GI bleeding is extremely intermittent—in one study, 5 of 19 patients with active bleeding on CTA had negative catheter angiography despite a median interval of only 33 minutes between studies 1
• This highlights why capturing the bleeding during the brief arterial phase is critical 1, 2

Common Pitfalls to Avoid

Technical Errors

• Using standard CT with IV contrast instead of CTA protocol—these are different procedures and standard CT lacks the arterial phase timing needed to detect extravasation 1
• Administering oral contrast—this will obscure intraluminal blood and extravasation 1, 2
• Omitting the noncontrast phase—this prevents identification of sentinel clot and makes it harder to distinguish extravasation from bowel content 1

Clinical Errors

• Assuming all hematochezia is lower GI bleeding—up to 11% may be from upper GI sources, particularly in hemodynamically unstable patients 4
• Delaying CTA in unstable patients—for patients with shock index >1 or requiring >5 units of blood, CTA should be performed immediately as the first diagnostic test, NOT colonoscopy 1, 4
• Performing CTA when extensive positive oral contrast is already present—if pre-existing positive oral contrast is seen extensively in the bowel, CTA should not be performed 1

Interpretation Errors

• Confusing virtual noncontrast images with true noncontrast images—dual-energy CT can generate virtual noncontrast images, but these are not yet standardized and should not replace true unenhanced images 1
• Missing the arterial phase extravasation—the key finding may only be visible on arterial phase, so all phases must be carefully reviewed 1, 3

Clinical Decision Algorithm

For Hemodynamically Stable Patients

• CTA is usually appropriate as first-line imaging alongside colonoscopy or RBC scan (these are equivalent alternatives) 1
• CTA can guide subsequent colonoscopy, increasing detection of culprit lesions from 31% to 60% in diverticular bleeding 4, 2

For Hemodynamically Unstable Patients (Shock Index >1)

• CTA is the mandatory first diagnostic test—it provides the fastest and least invasive means to localize bleeding 1, 4, 2
• Following positive CTA, proceed to catheter angiography with embolization within 60 minutes to maximize success 1, 4, 2
• Colonoscopy is explicitly contraindicated in unstable patients until after successful localization and stabilization 4, 5

Special Consideration: Diffuse Abdominal Tenderness

• If the patient has peritoneal signs with diffuse abdominal tenderness, this suggests bowel ischemia, perforation, or intra-abdominal catastrophe requiring immediate surgical exploration rather than imaging 5