CT when performed correctly has a high accuracy for detecting the presence and cause of a lower GI bleed. We will give the patients 750-1000 cc of water to distend the stomach and proximal small bowel. We will do a dual phase acquisition from the diaphragm to the symphysis with both arterial (30-35 sec delay) and venous phase imaging (70 second delay). We inject 100-120 cc of Iohexol-350 at 4-5 cc/second. Fast injection rates are critical. Images are reconstructed with thin (1mm or less) sections and thick sections (3mm) and reconstructed at .5mm and 3 mm intervals. At a minimum coronal reconstructions and MIP coronal images (10 mm slab) are needed. There is no need for non contrast studies. Bleeds can be subtle so narrow windows will need to be reviewed.

There are several “pearls” to share about detecting a lower GI bleed. These include;

1. Make sure the patient is given water as an oral contrast agent and not positive contrast.
2. We do not do non contrast or delayed scans for this protocol. In the past people would do 2 phases at non contrast and arterial and assume anything that appeared was a bleed. This is true but many patients show the bleed better on the venous phase which Is why we do arterial and venous. In our experience the bleed always changes between arterial and venous phase
3. If a bleed increases significantly between arterial and venous phase, then a subsequent angiogram is likely to be positive as well (not always)
4. In the past nuclear medicine tagged red blood cell studies were done but they are not as sensitive as CT and could not localize the site of bleed or define its cause.
5. Depending on the site of bleed we can often determine its cause of the CTA.
6. To increase sensitivity, make certain you review images with multiplanar reconstruction (especially coronal views) and thin slab MIPS. Volume rendering and Cinematic rendering may also prove helpful.
7. In the face of a negative CTA study classic angiography will not be performed. Colonoscopy may also be postponed for similar reasons.
8. Although the clinicians are usually correct when selecting upper GI bleed from lower GI bleed, they are not always correct. That is why we give 750-1000cc of water to distend the stomach and proximal small bowel. Take a careful look at the stomach and small bowel. CT is very good at picking up lower esophageal or gastric bleeds.
9. Bleeds can be subtle, so it is helpful to review prior positive cases if you do not have significant experience. We have many cases in our teaching file and several lectures as well.
10. In addition to defining the presence of a bleed we can often define its cause. This may include diverticulitis of the colon, colon cancer, rectal varices due to cirrhosis, small bowel tumors like GIST or angiodysplasia.