Meckel Scan

Specimen Collection

Traditionally, specimen collection was through open diverticulectomy; however, recent advancements in laparoscopic techniques have made it possible to manage laparoscopically.[6]

Procedures

99mTc-Pertechnetate scintigraphy, also known as Meckel's scan, is considered as the modality of choice to evaluate patients with suspected Meckel diverticulum, when used correctly, is an effective method for the detection of Meckel diverticulum with an approximately 100% sensitivity and specificity.[5] Therefore, various modalities are under consideration for the diagnosis of Meckel's diverticulum, especially in adults, where the diagnostic accuracy often is deemed to be unsatisfactory for clinical practice.[7] 

Mucoid cells of the gastric mucosa in Meckel's diverticulum secrete chloride into the intestinal lumen. This excretion is independent of the presence of the parietal cells. 99mTc-pertechnetate works like a halide anion (e.g., chloride, iodide). Normal or ectopic mucoid cells of gastric mucosa will actively accumulate the pertechnetate from the bloodstream and secrete it into the intestine.

Based on The Society of Nuclear Medicine and Molecular Imaging (SNMMI) guidelines, fasting is not necessary for the examination. Although, preexamination fasting of 3 to 4 hours may improve sensitivity for the detection of ectopic gastric mucosa. If possible, all drugs or procedures that may irritate the gastrointestinal tract should be stopped for 2 to 3 days before the study.  Patient premedicated with Histamine-2 (H2)  blocker or proton pump inhibitors and glucagon can improve the sensitivity of the study. The patient is premedicated with an H2 blocker or proton pump inhibitor before injecting the radiopharmaceutical agent.[8] Then, a dose between 296 to 444 MBq (8 to 12 mCi) is given intravenously in adults. The recommended dose for children is 1.85 MBq/kg (0.05 mCi/kg).[9]

The images are taken with the patient positioned supine with the abdomen and pelvis in the field of view. In the case of infants and small children (up to 2 years of age), the thorax should be included in the imaging field to assess for bronchopulmonary foregut malformation with ectopic mucosa. Dynamic flow images up to 1 min are taken to identify any vascular blood pool that may be confused with the ectopic gastric mucosa.  

Anterior abdominal images are obtained for at least 30 min. Imaging beyond 60 min may compromise sensitivity and study interpretation due to the passage of activity from the stomach to the small intestine. Additional static images are recommended to improve study interpretation, especially, lateral view to localize renal pelvic activity. Postvoiding images may be useful to detect the activity of a Meckel diverticulum obscured by the bladder.  

Indications

Meckel's scan is indicated for the identification of a Meckel's diverticulum or Barrett's esophagus with dysplasia. Barrett's esophagus without dysplasia can't be detected as there are no mucoid cells that will secret the 99mTc-pertechnetate.[10] 

This study is contraindicated in patients that have undergone a recent in vivo RBC labeling scan with a pyrophosphate kit, where the circulating RBCs are treated with stannous ion via intravenous administration. Such labeling could compromise the Meckel's scan since intravenous 99mTc-pertechnetate will label RBCs rather than be secreted by the ectopic gastric mucosa; this might not be the case if the RBC labeling for the RBC scan is performed by the in vitro labeling method.[11]

Potential Diagnosis

• Meckel's diverticulum 
• Barretts' esophagus with dysplasia

Normal and Critical Findings

Usually patient with Meckel's diverticulum and Barrett's esophagus shows increase secretion of pertechnetate.[12][13] While administering the tracer, the flow phase is visible. Typical structures that can appear in this phase include the heart, lungs, major arteries and veins, and vascular organs like the spleen, liver, and kidneys. The most prominent accumulation of radioactivity occurs in the stomach in the dynamic phase after 10-15 min. Activity in kidneys, ureters, and bladder is normal. Duodenal, jejunal, and occasionally ileal activity can be seen and should not be confused with ectopic mucosa.[14][15] 

Meckel diverticula are usually located in the right lower quadrant, but they can occur elsewhere. Lateral views at the end of the study are essential to differentiate anterior structures like Meckel diverticula from posterior structures like kidney, ureters, and bladder. Usually, the hyperactivity of the Meckel's diverticula is prominent, rounded, of small size, and appears simultaneously with stomach activity between 10 to 20 min after the injection of the tracer. Radioactivity in the Meckel's diverticula increases along with stomach activity.[14][15]

For Barrett's esophagus, continuous suction of the radioactive saliva is necessary. Cephalad activity of the stomach indicates the existence of gastric mucosa in the esophagus, but first, the clinician must rule out hiatal hernia and gastroesophageal reflux.

Interfering Factors

A patient with a negative Meckel's scan but with a high clinical suspicion of Meckel's diverticulum can have a repeat of Meckel's scan to confirm the diagnosis.[16]

• False Positives results are caused by ectopic gastric mucosa on an enteric duplication cyst, or other bowel diseases like intussusception, and small bowel obstruction. Bowel cleansing with enema or laxative can cause bowel inflammation, which can accumulate the tracer. Peptic ulcer and vascular lesions with increased blood pool like arteriovenous malformations and hemangiomas can also collect the tracer longer than a healthy tissue giving a false-positive impression.

• Focal pooling of the tracer in the genitourinary tract like uterine blush, hydronephrosis, ectopic kidney, extrarenal pelvis, hydroureter, vesicourethral reflux, and bladder diverticulum can result in a false positive study.[14][15][17]

• False-negatives are most likely caused by gastrointestinal bleeding during the circulation of the tracer, tracer activity obscured by the urinary bladder, or dilated ureter activity that normally appears in studies. Post-voiding images are essential for the distinction of a Meckel's diverticula activity from a normal urinary system excretion. Also, small foci of ectopic mucosa less than 1.8 cm^2 with motion artifact in the study can result in a false-negative. Prior barium fluoroscopy examination or perchlorate administration and presence of other ectopic tissue instead of ectopic gastric tissue can result in a false-negative result.[14][15][17]

Complications

• Radiopharmaceutical side effects (most common *) [18]
  • Chills*
  • Nausea
  • Vomiting
  • Diffuse rash
  • Pruritus
  • Hives/urticaria*
  • Chest pain or discomfort
  • Hypertension
  • Dizziness, vertigo
  • Headache
  • Diaphoresis
  • Anaphylaxis*
• Misadministration
• Vasovagal responses
• Injury from poor injection technique

The liposomal form of the drug reportedly has lower side effects.[19]

Patient Safety and Education

It is essential to educate the patient about the possible side effect of using pertechnetate and the need for a retest in case of a negative result, especially in situations with high suspicion of Meckel's diverticulum. Epinephrine administration can be via subcutaneous, intramuscular, or in severe conditions, intravenous, in case of anaphylaxis. In the case of inadequate response to antihistamine, aminophylline is an option. The International Commission on Radiological Protection (ICRP) recommends interruption of breastfeeding for 12 h after administration of 99mTc-pertechnetate.[20] A pregnancy test is recommended before any radiopharmaceuticals administration that will expose the fetus to >50 mGy. 

Clinical Significance

Meckel's Diverticula can present with signs and symptoms of small bowel obstruction. But can also present as painless rectal bleeding, or rectal bleeding with peritonitis. Commonly presents with abdominal pain, vomiting, fever, and bloody stools.

Multiple imaging modalities used for the diagnosis of Meckel's diverticula like an abdominal ultrasound, abdominal X-ray, abdominal angiography, abdominopelvic CT, and magnetic resonance imaging, have lower sensitivity and specificity when compared with the Meckel's Scan.[3]

Meckel's Scan with 99mTc-pertechnetate can detect the Meckel's Diverticula, by the physiological advantages of the secretion of the radiopharmaceutical by the gastric tissues like the ectopic gastric tissues found in Meckel's diverticula, when used correctly, is an effective method for the detection of Meckel diverticulum with an approximately 100% sensitivity and specificity.[5]

Symptomatic Meckel's treatment is resection, either laparoscopically or by open surgery.

Complications following resection can occur, with wound infections being the most common complication, followed by postoperative ileus.