Residency · Residency · Nuclear Medicine
Meckel Diverticulum Scintigraphy
Overview
Meckel diverticulum is the most common congenital anomaly of the gastrointestinal tract, present in approximately 2% of the population. It results from incomplete obliteration of the omphalomesenteric (vitelline) duct. Approximately 50% of symptomatic Meckel diverticula contain ectopic gastric mucosa, which is the key to scintigraphic detection. Tc-99m pertechnetate is taken up by this ectopic gastric mucosa, allowing the diverticulum to be identified on imaging. The most common clinical presentation is painless lower GI bleeding in children.
Pathophysiology and Rule of 2s
The classic teaching mnemonic for Meckel diverticulum involves the number two. It is found in approximately 2% of the population, located within 2 feet (60 cm) of the ileocecal valve, approximately 2 inches (5 cm) in length, symptomatic in approximately 2% of affected individuals, and has a male-to-female symptomatic ratio of roughly 2:1. It most commonly presents before age 2, though it can present at any age. Bleeding occurs because the acid-secreting ectopic gastric mucosa causes ulceration of the adjacent normal ileal mucosa.
Radiopharmaceutical
Tc-99m pertechnetate is the radiopharmaceutical used for Meckel scanning. It is trapped by gastric mucosa, specifically by parietal and chief cells, via a chloride-perchlorate transport mechanism that is distinct from the sodium-iodide symporter. Both normal gastric mucosa and ectopic gastric mucosa within a Meckel diverticulum concentrate pertechnetate. The adult dose is 10 to 15 mCi, while the pediatric dose is 50 to 200 microCi/kg with a minimum of 1 mCi.
<image>Positive Meckel scan showing a focal area of Tc-99m pertechnetate uptake in the right lower quadrant appearing simultaneously with gastric activity, consistent with ectopic gastric mucosa in a Meckel diverticulum</image>
Technique
Patient Preparation
The patient should be NPO for 4 to 6 hours to minimize gastric secretion and bowel activity. The bladder should be emptied immediately before imaging, as bladder activity can obscure a Meckel diverticulum located in the pelvis. Medications that may affect gastric mucosa, such as loperamide and anticholinergics, should be discontinued.
Pharmacologic Enhancement
Several pharmacologic interventions can improve study sensitivity. H2 receptor blockers such as cimetidine or ranitidine are given orally for 1 to 2 days before the study. These agents block secretion of pertechnetate from gastric mucosa into the bowel lumen, thereby increasing the target-to-background ratio by retaining tracer within the ectopic mucosa. The typical dose of cimetidine is 20 mg/kg/day divided into two doses for 2 days. Pentagastrin, administered at 6 mcg/kg subcutaneously 15 minutes before tracer injection, stimulates pertechnetate uptake by gastric mucosa and increases sensitivity. It is less commonly used due to side effects including nausea and cramping and limited availability. Glucagon at 50 mcg/kg intravenously at the time of tracer injection reduces bowel peristalsis, preventing movement of secreted tracer and reducing false positives from bowel transit of gastric-secreted pertechnetate. It may be combined with cimetidine.
Imaging Protocol
Dynamic anterior abdominal images are acquired at 1-minute frames for 30 to 60 minutes. Anterior and lateral views are obtained as needed. Post-void images are essential to evaluate activity that may have been obscured by the bladder. Lateral views help distinguish anterior structures (such as a Meckel diverticulum) from posterior structures (such as kidney or ureter).
Interpretation
Positive Scan
A positive scan shows a focal area of ectopic uptake that appears at the same time as gastric activity. This is typically located in the right lower quadrant or periumbilical region. The intensity of the focus increases over time, paralleling the behavior of gastric activity, and it should remain fixed in position, unlike activity that is simply transiting through bowel.
Key Diagnostic Feature
Simultaneous appearance with gastric uptake is the critical diagnostic finding. Activity that appears after gastric activity may represent secreted pertechnetate that has moved into the bowel lumen, which is a false positive. Activity that appears before gastric activity is unusual and warrants careful evaluation.
Sensitivity and Specificity
The sensitivity of Meckel scintigraphy is approximately 85 to 90% in children and around 60% in adults. Specificity is approximately 95%. Accuracy is consistently higher in children than in adults. Pharmacologic enhancement with cimetidine improves sensitivity to greater than 95% in children. A negative scan does not completely exclude a Meckel diverticulum, especially one that lacks ectopic gastric mucosa.
| Enhancement Agent | Mechanism | Dose/Timing | Effect on Sensitivity |
|---|---|---|---|
| Cimetidine (H2 blocker) | Blocks pertechnetate secretion from gastric mucosa | 20 mg/kg/day × 2 days pre-study | Increases to >95% in children |
| Pentagastrin | Stimulates pertechnetate uptake by gastric mucosa | 6 μg/kg SC, 15 min before tracer | Increases uptake intensity |
| Glucagon | Reduces bowel peristalsis | 50 μg/kg IV at injection | Reduces false positives from transit |
False Positives
Several conditions can produce false positive results. Intestinal duplication cysts that contain ectopic gastric mucosa will also concentrate pertechnetate. Bowel inflammation from Crohn disease, intussusception, or appendicitis can cause hyperemic bowel to concentrate pertechnetate. Urinary tract activity from hydronephrosis, ectopic kidney, vesicoureteral reflux, or bladder diverticulum can be confused with a Meckel diverticulum. Uterine activity from menstrual bleeding or uterine pathology is another source of error. Barrett esophagus contains ectopic gastric mucosa in the esophagus, though it is usually above the field of view. Arteriovenous malformations or hemangiomas may produce blood pool activity that simulates uptake. Finally, bowel transit of secreted pertechnetate from normal gastric mucosa into the small bowel can create a misleading focus in the right lower quadrant.
False Negatives
False negative results occur when a Meckel diverticulum lacks ectopic gastric mucosa, which accounts for approximately 50% of all Meckel diverticula. Other causes include a very small amount of ectopic mucosa below the detection threshold, impaired blood supply to the diverticulum from torsion or ischemia, prior barium study (barium attenuates gamma rays), active bleeding that washes out pertechnetate from the diverticulum, and recent pertechnetate administration leaving residual isotope in the bowel from a prior study.
<image>False positive Meckel scan pitfalls: urinary tract activity from vesicoureteral reflux mimicking a Meckel diverticulum (left panel), and bowel transit of gastric-secreted pertechnetate creating a right lower quadrant focus (right panel)</image>
Pediatric Considerations
Most Meckel scans are performed in children between 2 and 10 years of age who present with painless rectal bleeding. Sensitivity is higher in children than adults. Weight-based dosing follows EANM and SNMMI pediatric guidelines. Cimetidine premedication is strongly recommended in children. Post-void images are essential because a small bladder can obscure a pelvic Meckel diverticulum. Sedation may be necessary for young children who cannot remain still during the acquisition.
Adult Meckel Diverticulum
Clinical presentation of Meckel diverticulum is less common in adults. When it does present, obstruction or inflammation is more likely than bleeding. Scintigraphy has lower sensitivity in adults, around 60%, due to larger body habitus and greater attenuation, a lower proportion of ectopic gastric mucosa in adult symptomatic cases, and a more complex differential diagnosis with many other causes of GI bleeding. CT enterography or video capsule endoscopy may serve as complementary studies in adults.
Surgical Correlation
A positive Meckel scan warrants surgical consultation for diverticulectomy, which is increasingly performed via a laparoscopic approach. Histologic confirmation of ectopic gastric mucosa in the resected specimen validates the scan result. It is important to recognize that incidental Meckel diverticula found at surgery that lack ectopic gastric mucosa would not have been detectable on scintigraphy.
<image>Correlation between a positive Meckel scan (right lower quadrant uptake) and surgical specimen showing a Meckel diverticulum with histologically confirmed ectopic gastric mucosa</image>
Clinical Pearls
The hallmark of a positive Meckel scan is focal uptake appearing simultaneously with gastric activity.
Always use cimetidine premedication to improve sensitivity, especially in children.
Post-void imaging is essential to prevent bladder activity from masking a pelvic Meckel diverticulum.
A negative Meckel scan does not exclude a Meckel diverticulum that lacks ectopic gastric mucosa.
In adults with suspected Meckel diverticulum, consider CT enterography as a complementary study.
Lateral views help distinguish an anterior Meckel diverticulum from posterior urinary tract activity.
If activity appears in the right lower quadrant after gastric activity has already appeared, consider secreted pertechnetate in the bowel rather than true ectopic gastric mucosa.
References
- SNMMI Procedure Standard for Meckel Diverticulum Scintigraphy.
- Defined Role of Meckel Scan in Pediatric GI Bleeding. Seminars in Nuclear Medicine, 2007.
- Defined Role of Cimetidine Enhancement in Meckel Scintigraphy. Pediatric Radiology, 1998.
- ACR Appropriateness Criteria: GI Tract Bleeding in Pediatric Patients.
