Gastric emptying scintigraphy of a radiolabeled solid meal is the gold standard for the diagnosis of gastroparesis.   Measurement of emptying of solids is more sensitive by scintigraphy. This is due to the fact that liquid emptying may remain normal despite advanced disease.

Consensus recommendations for a standardized gastric emptying procedure have recommended a universally acceptable 99-m technetium sulphur-colloid labelled low fat, egg-white meal. Retention of over 10% of the solid meal after 4 h is abnormal. A grading of severity based on 4 h values might be used: grade 1 (mild), 11%-20% retention at 4 h; grade 2 (moderate), 21%-35% retention at 4 h; grade 3 (severe), 36%-50% retention at 4 h.

What is gastroparesis and how is it diagnosed?

Gastroparesis is diagnosed when symptoms such as nausea, vomiting, early satiety, postprandial fullness, abdominal discomfort, and pain are associated with objective evidence of delayed GE in the absence of obstruction, and typically with impairment in maintenance of normal nutrition using standard food (see reference).

Rapid gastric transit (dumping syndrome) can also be diagnosed.

What are the potential problems with interpretation of gastric emptying scintigraphy?

As Gastric emptying scintigraphy is the gold standard, and there is difficulty in comparing it with any other investigation, the difficulty is establishing normal values in normal patients. There is obviously an ethical concern in doing the study in normal patients to establish normal values, owing to the radiation dose.

The recent consensus statement recommends imaging at 0, 1, 2 and 4 hours, although an older measure of gastric emptying was the T50, or time for the gastric activity to reach half of its original value. By standardizing the protocol (for timings of imaging and the nature of the meal) wherever it is used, it is hoped that normal values can be established and interpreted. Variations in the imaging protocol result in variations in the values obtained, caused by test rather than patient factors and decreases the reliability of the test.

Reference

Consensus Recommendations for Gastric Emptying Scintigraphy: A Joint Report of the American Neurogastroenterology and Motility Society and the Society of Nuclear Medicine

Edited by Dr Iain Au-Yong

• A blood sample is obtained from the patient. The leucocytes are separted from erythrocytes and platelets. Leucocytes  labelling is achieved by incubation with the radioisotope  in plasma or saline. The radioisotopes used are either Indium-111 tropolonate (111 In) or Technetium-99m hexamethyl propylene  amine oxime (99m Tc HMPAO). This process takes a couple of hours (thus the patient is asked to return after a couple of hours). Results from 99m Tc HMPAO  in small bowel disease are better than those obtained from 111 In
• The tagged white cells are injected intravenously in the patient.  After injection, the labelled white cells migrate into the inflammed lesion before being shed into the bowel lumen.
• Images are obtained with a gamma camera at 2-6 hrs later. A late image (24 hrs) may be required if  the technique is used to detect infection.

What are the disadvantages of the white cell scan?

• False negative scans. However, this is rarely seen in symptomatic patients with active disease. A negative scan in a symptomatic patient virtually excludes Crohn’s disease.
• Useful only for active disease
• Difficulty in identifying the various bowel segments

What is the indication for SeHCAT test?

Bile Acid Malabsorption

This can cause symptoms of chronic diarrhoea. Three types of bile acid malabsorption are recognised:
Type 1: following ileal disease or resection or bypass surgery.
Type 2: primary idiopathic malabsorption.
Type 3: associated with cholecystectomy, peptic ulcer surgery, chronic pancreatitis, coeliac disease and diabetes mellitus.
Bile acids are cathartic to colonic mucosa, and impair sodium and water absorption.

• Upper GI bleed: limited role due to the widespread use of endoscopy as a first-line modality for such bleeding.
• Lower GI bleed: Tc-99m RBC scintigraphy plays a larger role in the evaluation of lower GI bleeding due to the limited sensitivity of endoscopy within the lower GIT and has typically been used as a screening examination to identify patients who require angiography or surgery.
• Obscure GI bleed

How is red cell scan performed?

• Blood is taken from the patient. The red blood cells are separated from the rest of the blood sample and then mixed with the radioactive material. The red cells are tagged with 99m Technetium and injected back in the patient
• A second method involves injection of the radioactive material directly in the vein. This attaches to the red cells in the circulation.
• Images are subsequently obtained using the gamma camera.

What are the advantages and disadvantages of red cell scan?
Advantages

• Non-invasive
• More sensitive than angiography (detects bleeding at a rate more than 0.5ml/min). Tc-99m RBC scintigraphy is 93% sensitive and 95% specific for detecting a bleeding site with active arterial or venous bleeding rates as low as 0.04 mL/min (Zuckier LS. Semin Nucl Med 2003) anywhere within the gastrointestinal tract.
• Patients can be scanned up to 24 hrs after injection to detect intermittent bleeding.

Disadvantages

• Anatomical localisation is insufficiently accurate. Peristalsis can cause antegrade or retrograde movement of the radioisotope within the bowel, further increasing the difficulties in localisation.

Discuss the indication for Meckel’s scan?

It is done when GI bleeding is suspected to be due to a Meckel’s diverticulum. Sometimes the lining of the Meckel’s diverticulum contains ‘gastric mucosa’ instead of the normal intestinal lining and this can lead to bleeding by acid induced damage of the adjacent mucosa. When bleeding occurs within a Meckel’s diverticulum, gastric mucosa is nearly always present. A Meckel’s scan is used to look for the presence of gastric mucosa in the intestine.

What is a Meckel’s scan?

• Meckel’s scan is a technetium-99m pertechnetate scintiscan. The technetium-99m pertechnetate is injected in a vein and is taken up by gastric mucosa and hence concentrate in areas where gastric mucosa is present.
• After intravenous injection of the isotope, the gamma camera is used to scan the abdomen. This procedure usually lasts approximately 30 minutes. Gastric mucosa secretes the radioactive isotope; thus, if the diverticulum contains this ectopic tissue, it is recognized as a hot spot.
• Administration of ranitidine enhances the accuracy of the scan. Ranitidine enhances the uptake and blocks the secretion of technetium-99m pertechnetate from ectopic gastric mucosa.

What are the disadvantages of Meckel’s scan?

• Meckel’s scan is specific for gastric mucosa (i.e. in the stomach or ectopic) and not specifically diagnostic of Meckel’s diverticulum, false positive results occur whenever ectopic gastric mucosa is present.
• False negative results can occur when gastric mucosa is very slight or absent in the diverticulum
• Low sensitivity and specificity in adults