Blood in the Stool (Rectal Bleeding) (cont.)
Bhupinder Anand, MD
In this Article
There are two types of radionuclide scans that are used for determining the site of gastrointestinal bleeding; a Meckel's scan, and a tagged red blood cell (RBC) scan.
The Meckel's scan is a scan for detecting a Meckel's diverticulum. A radioactive chemical is injected into the patient's vein, and a nuclear camera (like a Geiger counter) is used to scan the patient's abdomen. The radioactive chemical will be picked up and concentrated by the acid-secreting tissue in the Meckel's diverticulum and will appear as a "hot" area in the right lower abdomen on the scan.
Tagged RBC scans are used to determine the location of the gastrointestinal bleeding. After drawing blood from the bleeding patient, a radioactive chemical is attached to the patient's red blood cells and the "tagged" red blood cells are injected back into the patient's vein. If there is active gastrointestinal bleeding, the radioactive red blood cells leak into the intestine where the bleeding is occurring and will appear as a hot area on the scan. One drawback of the tagged RBC scan is that it will not show as a hot area if there is no active bleeding at the time of the scan. Thus, it can fail to diagnose the site of bleeding if bleeding is intermittent and the scan is done between bleeding episodes. Another drawback of the scan is that it requires a minimum amount of bleeding to form a hot area. Thus, it can fail to diagnose the site of the bleeding if bleeding is too slow. The tagged RBC scan is safe, and can be done quickly and without discomfort to the patient.
Unfortunately, the tagged RBC scans are not very accurate in defining the exact location of the bleeding; there is often a poor correlation between where the tagged RBC scan shows the bleeding to be and the actual site of bleeding found at the time of surgery. Therefore, tagged RBC scans cannot be relied upon to help surgeons decide what area of the gastrointestinal tract to remove in the event the bleeding is severe or persistent and requires surgery. However, if the scan shows a hot area, it usually means there is active bleeding, and the patient may be a candidate for a visceral angiogram to more accurately locate the site of bleeding.
A visceral angiogram is an X-ray study of the blood vessels of the gastrointestinal tract. The doctor (usually a specially trained radiologist) will insert a thin, long catheter into a blood vessel in the groin and, under X-ray guidance, will advance the tip of the catheter into one of the mesenteric arteries (arteries that supply blood to the gastrointestinal tract). A radio-opaque dye is injected through the catheter and into the mesenteric artery. If there is active bleeding, the dye can be seen leaking into the gastrointestinal tract on the X-ray film. Visceral angiograms are accurate in locating rapid bleeding in the gastrointestinal tract, but it is not useful if the bleeding is slow or has stopped at the time of the angiogram.
The visceral angiogram is not widely used because of its potential complications such as kidney damage from the dye, allergic reactions to the dye, and the formation of blood clots in the mesenteric arteries. It is reserved for patients who have severe and continuous bleeding and in whom colonoscopy cannot locate the site of the bleeding.
Medically Reviewed by a Doctor on 12/4/2014
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