A Computed Tomography (CT) scan is a sophisticated medical imaging technology that uses specialized X-rays to create detailed cross-sectional pictures of the body’s internal structures. When dealing with bleeding from the lower gastrointestinal tract, commonly referred to as colonic bleeding, physicians need a rapid and accurate way to locate the source. Colonic bleeding, or hematochezia, occurs when blood originates in the colon or rectum and can range in severity from mild to life-threatening. While a standard CT scan is generally used to look at organ structure, a specialized version of this technology can be used to detect active bleeding in the colon under specific conditions. This imaging tool has become a valuable part of the diagnostic process, particularly in urgent or emergency situations.
CT Angiography: The Technique for Identifying Active Bleeding
The ability of a CT scan to detect active bleeding relies entirely on a specialized protocol called CT Angiography (CTA). A standard CT scan is not sensitive enough to reliably visualize blood actively leaking from a vessel. CTA requires the injection of an iodine-based contrast material directly into a vein, which rapidly travels through the arterial system.
This intravenous contrast material acts as a temporary marker, making the blood vessels appear bright white on the CT images. If a blood vessel in the colon is actively bleeding, the contrast material will leak out of the damaged vessel and into the surrounding tissue or the lumen of the colon itself. This visible leakage, known as contrast extravasation, shows up as a distinct, bright spot against the darker background of the bowel contents.
The timing of the scan is exceptionally important for this technique to work effectively. Images are rapidly acquired in multiple phases, including an arterial phase when the contrast is primarily in the arteries, and a venous phase shortly after. The presence of contrast extravasation that changes in shape or density between these phases confirms active bleeding.
This technique is highly effective for acute, rapid blood loss, with the best detection rates for bleeding that is occurring at a rate of 0.3 to 0.5 milliliters per minute or faster. If the bleeding is intermittent or has temporarily stopped at the moment of the scan, the CTA may not capture the active extravasation, potentially resulting in a false-negative result. In such cases, the radiologist must look for secondary signs, such as clotted blood or a pooling of high-density material within the colon lumen, which suggests a recent bleed.
Diagnosing Underlying Causes of Colonic Bleeding
Beyond pinpointing the exact site of active leakage, the CT scan provides comprehensive anatomical information for determining the underlying cause of the bleeding. Colonic bleeding is a symptom, not a diagnosis, and the CT scan helps physicians understand the pathology driving the hemorrhage. This information is often visible even if active extravasation is not captured during the scan.
The high-resolution images allow for excellent visualization of the colon wall and surrounding structures. The CT scan is highly effective at identifying several common causes of lower GI bleeding:
- Diverticulosis, which are small pouches that form in the wall of the colon. The scan can show the presence of these pouches or signs of inflammation (diverticulitis).
- Structural abnormalities such as masses or tumors, which may indicate colorectal cancer. These lesions often show abnormal enhancement patterns with the contrast material.
- Colitis, which is inflammation of the colon, showing characteristic thickening of the bowel wall.
- Vascular malformations, such as angiodysplasias, which are abnormal clusters of small, fragile blood vessels.
By providing this detailed map of the underlying pathology, the CT scan helps guide the next steps for a definitive diagnosis and treatment plan.
Comparing CT Scans to Other Diagnostic Procedures
While CT angiography is a powerful tool for locating active colonic bleeding, its role is contextualized by other available diagnostic procedures. The traditional gold standard for diagnosing and treating lower GI bleeding is colonoscopy, which involves inserting a flexible tube with a camera into the rectum to directly visualize the colon. Colonoscopy has the advantage of being both diagnostic and therapeutic, allowing the physician to stop the bleed immediately with techniques like cauterization or clipping.
However, colonoscopy often requires several hours of bowel preparation, which may be impossible for a patient with rapid, severe bleeding or one who is too unstable for a lengthy procedure. In these time-sensitive situations, the CT scan is often preferred because it is fast, readily available in most hospital emergency settings, and requires no bowel preparation. This speed allows for rapid localization of the bleeding site, which can then guide an urgent intervention like catheter angiography or surgery.
CT angiography is less invasive than traditional angiography, which involves threading a catheter directly into the artery supplying the colon. Traditional angiography is typically reserved for therapeutic purposes, such as performing embolization to plug the bleeding vessel once the CT scan has localized the source. For very slow or intermittent bleeds that the CTA might miss, nuclear medicine scans using tagged red blood cells can detect bleeding rates as low as 0.1 milliliters per minute, but these scans only localize the general region, not the precise spot. The CT scan fills a specific gap in the diagnostic pathway, providing a quick, non-invasive method for localizing brisk, active bleeding to facilitate rapid, targeted treatment.