Background Information and Definitions
Gastrointestinal bleeding scintigraphy is performed
in patients suspected of active gastrointestinal bleeding using Tc-99m labeled
red blood cells (RBCs). Sites of active bleeding are identified by the
accumulation and movement of labeled Red Blood Cells within the bowel lumen.
Since activity within the lumen of the bowel can move antegrade and retrograde,
frequent images (1 image every 10 to 60 sec) will increase the accuracy of
localizing the bleeding site. Tc-99m sulfur colloid (SC) is rarely used today
because of the short residence time within the blood. Tc-99m SC is cleared from
the blood by the reticuloendothelial system with a half-time as short as 2 to 3
min while radiolabeled RBCs last for hours.
Gastrointestinal bleeding (GI) is either upper, originating above the ligament of Treitz, or lower,
distal to the ligament of Treitz. Frequent causes of upper GI bleeding include
esophageal varices, gastric and duodenal ulcers, gastritis, esophagitis,
Mallory-Weiss tear or neoplasm. Causes of lower GI hemorrhage include
angiodysplasia, diverticula, neoplasms and inflammation, and, in children,
Meckel's diverticulum. Endoscopy and angiography provide accurate localization
of bleeding sites and potentially therapeutic control. Scintigraphy with labeled
RBCs is complementary to endoscopy and angiography because it permits continuous
monitoring over hours. This is a major advantage over intermittent sampling
since most GI bleeds are intermittent and therefore frequently missed.
The clinical findings for active gastrointestinal hemorrhage are often
unreliable and misleading. There is frequently a marked temporal lag between the
onset of bleeding and the clinical findings. While it may be clinically apparent
that the patient has bled from the presence of melena or hematochezia, the blood
may pool in the colon for hours before being evacuated. A drop in the hematocrit
and elevated serum blood urea nitrogen (BUN) also lack the temporal resolution
needed to indicate active bleeding. Orthostatic hypotension and tachycardia
occur more acutely but are insensitive and non-specific.
In cases where there is only occult bleeding detected by guaiac positive
stools, gastrointestinal bleeding scintigraphy is unlikely to be useful.
Gastrointestinal bleeding scintigraphy can detect bleeding rates as low as 0.1
to 0.35 ml per min. The guaiac test detects bleeding at rates well below the
level necessary to be seen on gastrointestinal bleeding scintigraphy.
Meckel's Diverticulum Scintigraphy
A Meckel's diverticulum is a vestigial remnant of the omphalomesenteric
duct located on the ileum about 50 to 80 cm from the ileocecal valve. About half
of Meckel's diverticuli have gastric mucosa. Bleeding may result from
ileal mucosal ulceration from acid secretion. Tc-99m pertechnetate avidly
accumulates in gastric mucosa and is the study of choice for identifying ectopic
gastric mucosa in a Meckel's diverticulum.
Common Indications
Gastrointestinal Bleeding Scintigraphy
The goals of gastrointestinal bleeding scintigraphy are to locate the
bleeding site and to determine who requires aggressive treatment versus those
who can be medically managed. It is usually in those patients that require
urgent care that the bleeding site is identified. In some patients, the bleeding
site is identified with sufficient confidence for specific surgical intervention
(e.g. right hemicolectomy in the case of a bleeding site in the ascending
colon). If bleeding is detected, the site is usually localized well enough to
direct the next diagnostic test (e.g. endoscopy or arteriography).
Gastrointestinal scintigraphy should be done as soon as possible after the
patient presents for medical care, since active bleeding is more likely at early
times and is needed for correct localization.
Meckel's Diverticulum Scintigraphy
The indication for a Meckel's scintiscan is to localize ectopic gastric
mucosa in a Meckel's diverticulum as the source of unexplained
gastrointestinal bleeding. Bleeding Meckel's diverticula usually occur in
young children. The Meckel's scintiscan should be used when the patient is
not actively bleeding. Even in young children, active bleeding is best studied
by radiolabeled RBC scintigraphy.
Procedure
The detailed procedure recommendations in the original guideline address the following
areas: facility/personnel, patient preparation; information pertinent to
performing the procedure (i.e., important data that the physician should have
about the patient at the time the exam is performed and interpreted);
precautions; information regarding the radiopharmaceutical (i.e., ranges of
administered activity, organ receiving the largest radiation dose, effective
dose), image acquisition; interventions; processing; interpretation/reporting;
quality control, and sources of error.
