Gastrointestinal Bleeding Scintigraphy

Michael A. McDonald, MD, PhD; Harvey A. Ziessman, MD

Disclosures

Appl Radiol. 2016;45(5):19-22. 

In This Article

Radiopharmaceuticals

GIB scintigraphy can play an important and unique role in characterizing and risk stratifying patients presenting with lower GIB. The superior diagnostic efficacy of esophagogastroduodenoscopy makes the use of red blood cell (RBC) scintigraphy for the detection of upper GIB less advantageous, although proximal small bowel, duodenal and even distal gastric bleeds are often detected. Patient's presenting with acute GIB usually undergo an initial clinical assessment and resuscitation, nasogastric lavage and esophagogastroduodenoscopy, which serve to stratify the bleeding into either suspected upper or lower GI sources.[6] If a lower GIB is suspected or an upper GI source has been excluded GIB scintigraphy can be performed as the next step in the diagnostic evaluation. Both bleed location (or suspected location) and rate of bleeding help determine the therapeutic strategy utilized, with the three main interventional approaches being colonoscopy, angiography and surgery.[4]

Tc99m sulfur colloid (SC) was the first radiopharmaceutical used for evaluation of lower GIBs. It is cleared from the blood pool by 10–15 minutes after administration, with a half-life of 2 to 3 minutes.[2] The ability of Tc99m SC to detect GIB detection is due to the high target to background ratio between the bleeding site and the surrounding soft tissues, caused by rapid clearance of the background by the RES cells in the liver, spleen and bone marrow. However, the rapid clearance of radiotracer from the blood pool means that the patient must be bleeding at the time of injection. Delayed imaging is not possible. Tc99m SC is now mostly used when there are time constraints in preparing Tc99m RBCs or a lack of availability. In a prospective study comparing Tc99m RBC to Tc99m SC scintigraphy in 100 patients imaged with both agents under identical clinical conditions, Tc99m RBCs was diagnostically superior in all cases with sensitivity of 95%, specificity of 93% and an overall accuracy of 94%. This is in contrast to a sensitivity of 12%, 100% specificity and 62% overall accuracy for sulfur colloid.[6]

There are various in vivo and in vitro methods available for labeling RBCs with Tc99m. An often used in vitro method involves utilization of a simple commercial kit and permits >97% labeling efficiency in less than 30 minutes preparation time. A minimum detectable bleeding rate of 0.04 mL/minute was reported for Tc99m RBCs. Detection of lower GIBs at low flow rate is influenced by the volume of extravasated RBCs at the bleeding site. A focal volume of 3 mL can be readily detected however the sensitivity of the procedure can be reduced by hyperactive peristalsis, which can cause the volume to be distributed over a significant length of bowel.[3]

The relatively stable persistence of Tc99m RBCs in the blood pool enables intermittent (and venous) bleeding to be detected, permitting repeat imaging without additional radiation exposure. RBC scintigraphy can be acquired for up to 24h, making it unique among diagnostic imaging methods in being able to provide monitoring of patients with intermittent bleeding.[7]

RBC scintigraphy bleeding scan will be negative unless the patient is actively bleeding. In patients with active, intermittent bleeding who are hemodynamically stable between episodes, RBC scintigraphy compliments clinical findings by possible localization of ongoing bleeding and risk assessment for future interventions, reducing the rate of negative angiograms from 22 to 53%.[1]

Tc99m RBCs accurately localize the site of bleeding in 88% to 97% of patients, with positive findings resulting in a 5-fold greater likelihood that the patient will require surgery.[4] False positive scans, which can be avoided with proper methodology and interpretation, are generally due to incorrectly identified vascular structures or free 99m Tc pertechnetate.

RBC scintigraphic has been proven to be a useful early diagnostic tool in Lower GIB risk-stratification and in guiding decisions regarding surgical or angiographic intervention. This is critical in a disease process characterized by spontaneous cessation in 75–89% of cases and potential high mortality in those who continue to bleed. Equally important is that there is a significant risk associated with potential interventions such as angiography and surgery.[8]

A positive scan is predictive of increased hospital morbidity and mortality, helping to identify a relatively high risk population requiring more aggressive intervention such as transfusion or surgery.[9] Conversely, a negative study has been shown to correlate with a good clinical outcome, stratifying those patients who can be managed conservatively. A negative study may play a role in identifying those patients who are not actively bleeding and, due to very low volume of blood loss or slow rate, are more likely to stop bleeding spontaneously. This prevents overaggressive surgical management and may lead to a reduction both in morbidity and mortality.[10]

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