Clinical History:
75 year old male presents to emergency room with gross blood in stool.

Findings:

Initial flow images are normal. Sequential planar images taken at 1 minute interval show increase radiotracer uptake at the ascending colon near the hepatic flexure. The increase radiotracer uptake progresses in an ante grade flow from the hepatic flexure to the sigmoid colon.

Discussion:
Findings consistent with GI bleed of the ascending colon. Tc99m RBC is shown to be affective detecting bleeds with a bleeding rate as low as 0.05 to 0.1 ml/min. Angiography require a bleeding rate of 1ml/ min to detect bleeding.

Certain criteria are used to diagnose gastrointestinal bleeds on Tc99m RBC studies. The extra vascular activity must be intraluminal and increase over time. The increase in radiotracer activity must move through the GI tract. It can move ante grade and retrograde.

Interpretation errors can occur if Tc99m is not labeled correctly. Free Tc99m pertechnetate is taken up by the gastric mucosa and released, simulating a GI bleed. Genitourinary activity can cause false positives.

Final Diagnosis: Gastrointestinal bleed from the ascending colon in the Superior mesenteric destribution.

Ref: Thrall, James, Nuclear Medicine:The Requisites, 1995, pp 241-248.

EDITOR'S NOTE: In addition to what the author noted as causes of false positive bleeding scans, penile blood pool activity can be quite confounding. Lateral scintiphotos, and imaging following movement of the penis are usually adequate to distinguish penile blood pool activity from a GI bleed.