Acute lower gastrointestinal bleeding: NUCLEAR SCINTIGRAPHY Part 1
Radioisotope scanning can detect gastrointestinal bleeding rates as low as 0.1 mL/min. The general principle is to inject a radioactive substance that will extravasate into the bowel at the bleeding point, thereby localizing the site of bleeding. By viewing the initial site of extravasation, and allowing the contrast to accumulate and travel distally through peristalsis, the exact site of bleeding (small bowel versus colon) can theoretically be determined. Two types of scintigrams are available: those using sulphur colloid and those using autologous red blood cells, both of which are ‘tagged’ with technetium99m (Tc99m). The advantage of Tc99m sulphur colloid is that it requires no preparation and can, therefore, be injected into the patient immediately. Unfortunately, it is rapidly cleared by the reticuloendothelial system, having a half-life of only 2 to 3 min. This is its primary disadvantage; if the scan is not immediately positive, the radioactive substance cannot be detected on delayed images. Furthermore, it tends to accumulate in the spleen and liver (areas of uptake by the reticuloendothelial system), occasionally obscuring the site of bleeding. Your drugs could cost you less – buy Mircette birth control to start the treatment soon.
Generally, Tc99m-labelled red blood cell scanning is preferred over sulphur colloid scanning because of its long halflife (bleeding can be detected on images taken up to 24 h later). If bleeding is rapid, a positive result can be seen within as little as 5 min. Patients whose initial scans are negative may show bleeding on subsequent scans over the following 12 to 24 h as labelled red blood cells accumulate in the lumen near the site of hemorrhage.