Contrast the differences between the sulfur colloid scan and the tagged RBC scan for detecting GI bleeding
A 99m Tc sulfur colloid scan can be started immediately when the patient arrives in the nuclear medicine department because the agent can be prepared ahead of time. The sulfur colloid remains in the intravascular space for approximately 25 minutes. An active bleed can be detected quickly, whereas an intermittent bleed might be missed if the patient bleeds after more than 25 minutes following injection of the radiotracer. Because sulfur colloid does not remain in the intravascular space, if there is a negative scan the first time, a repeat scan can be performed any time when the patient begins to bleed again.
A tagged RBC scan requires drawing some of the patient’s blood, which must be radiolabeled, a process that takes 15 to 20 minutes. After injection of 99m Tc-labeled RBCs, the radiotracer activity persists in the intravascular space, and imaging can be performed continuously for several hours after injection; this can help to detect intermittent bleeds. If the initial scan results are negative, however, a repeat scan cannot be performed for approximately 24 hours until the 99m Tc decays sufficiently. Otherwise, the bowel would have too much background radiotracer activity.