Meckel's Scan
The Meckel's scan is a technetium-99m pertechnetate study that finds ectopic gastric mucosa in a Meckel diverticulum.
Pertechnetate is taken up by the mucus secreting cells of normal and ectopic gastric mucosa, so a diverticulum that contains gastric mucosa appears as a focus of tracer in the abdomen at the same time as the stomach.
The adult dose is 185 to 370 MBq intravenously, and children receive about 1.85 MBq for each kg, with a minimum of 20 MBq.
The patient lies under the camera, and dynamic images are taken at 1 second a frame for the first minute, then every minute for 30 to 60 minutes. Lateral, anterior and post-void images are added. The effective dose is about 3 to 7 mSv.
Pre-treatment improves the study. Cimetidine, ranitidine or another H2 blocker (or a proton pump inhibitor) for 1 to 2 days raises uptake and lowers release of tracer. Pentagastrin raises uptake but is rarely available.
Glucagon (about 50 microgram for each kg) slows peristalsis and retention in the bowel. Fasting for 4 hours reduces stomach activity.
A Meckel diverticulum is found in about 2% of the population, and causes symptoms in 4 to 6% of those. Painless rectal bleeding in children under 5 years is the commonest presentation, and others are bowel obstruction, intussusception and diverticulitis.
The scan has a sensitivity of 85 to 95% in children, and about 60% in adults, with a specificity of 95%.
Indications.
- Painless rectal bleeding in a child or young adult, with suspected Meckel diverticulum.
- Unexplained gastrointestinal bleeding or iron deficiency anaemia with negative endoscopy.
- Recurrent abdominal pain, obstruction or intussusception with suspected diverticulum.
- Localisation of ectopic gastric mucosa in the oesophagus (Barrett oesophagus), duplication cysts and the small bowel.
- Before laparoscopy for obscure bleeding.
Not for diagnosis of a diverticulum without gastric mucosa, which is a common finding in asymptomatic people.
Contraindications and precautions.
- Pregnancy: avoid unless essential. Breast feeding: interrupt for about 12 to 24 hours.
- Barium studies or recent contrast in the bowel lower sensitivity. Perchlorate, potassium iodide and laxatives, and enemas, should be avoided before the scan.
- Pre-treatment with H2 blocker: usually 1 to 2 days at the usual dose, or a single intravenous dose before the study.
- Glucagon: caution in phaeochromocytoma and insulinoma, and in diabetes (glucose may rise).
- Pentagastrin: allergy and nausea.
- Active heavy bleeding makes the images hard to interpret, since blood can wash tracer away.
- Children need a calm environment, and sedation is rarely needed.
Preparation and technique.
The patient fasts for at least 4 hours, and the bladder is emptied before the injection, because bladder activity hides the pelvis. The H2 blocker is given beforehand, and glucagon is given after the first few images.
The camera covers the whole abdomen and pelvis in the anterior view. Images start at injection, with a flow phase, then frames of 1 minute for 30 to 60 minutes. The stomach, kidneys and bladder are always seen, as they take up the tracer.
A right lateral view helps to separate a diverticulum from the bladder.
Post-void images are taken at the end. Tracer in the diverticulum appears at the same time as the stomach (within 5 to 10 minutes) and changes in intensity in parallel with the gastric mucosa. SPECT-CT improves localisation.
