Friday, December 11, 2009

Meckel Scan

About 50% of Meckel diverticuli have gastric mucosa, which may cause bleeding by ileal mucosal ulceration from acid secretion. On the other hand, 95% of Meckel diverticula that bleed have ectopic gastric mucosa.

99mTc-pertechnetate accumulates in mucus-secreting cells of the gastric mucosa (not the acid-secreting cells) and can be used to identify ectopic gastric mucosa in a Meckel diverticulum in a patient who is not actively bleeding (sensitivity and specificity of about 90%). Patients with active bleeding are best evaluated with radiolabeled blood cells.

Although a Meckel diverticulum may appear anywhere within the abdomen, it is classically seen in the right lower quadrant. A positive scan will show activity in the ectopic gastric mucosa at the same time as the activity in the normal gastric mucosa. A small Meckel diverticulum may seem to appear at a later time than the stomach. This can lead to a potential pitfall: Activity in the kidneys, ureter or bladder usually first appears after activity is seen in the normal gastric mucosa.

Another pitfall is pertechnetate secreted by the gastric mucosa accumulating in the small bowel. This can be differentiated from a true positive scan by the delayed appearance of an area of mildly, ill-defined increased activity. A nasogastric tube or left lateral decubitus positioning can be used to decrease the likelihood of false positive results due to passage of radiotracer from the stomach into the small bowel.

Other causes of false positive results include hyperemia due to intussusception inflammatory bowel disease or other small bowel lesions; vascular lesions, intestinal duplication cysts containing gastric mucosa.

False negative scans can occur due to ileal malrotation, small amounts of gastric mucosa, and rapid clearance of 99mTc-pertechnetate due to localized bowel irritability.


While pre-treatment is not necessary, some pharmacologic interventions have been used to increase sensitivity. Pentagastrin, a gastrin analog, stimulates gastric secretions and increases gastric mucosa uptake of pertechnetate. Histamine H2 blockers (e.g., cimetidine and ranitidine) block secretion from the cells and increase gastric mucosa uptake.

Pentagastrin is administered subcutaneously 20 minutes prior to 99mTc-pertechnetate, while oral cimetidine is given 4 times a day 2 days prior to 99mTc-pertechnetate. Cimetidine can also be given intravenously one hour prior to imaging.

Giving an H2 blocker and pentagastrin in combination is not recommended, as H2 blockers antagonize pentagastrin.


No comments:

Post a Comment

Note: Only a member of this blog may post a comment.