9.3 Gastrointestinal & Hepatobiliary Imaging

Nuclear medicine offers functional assessments of the gastrointestinal tract and hepatobiliary system that are critical for diagnosing motility disorders, acute inflammatory processes, and obscure bleeding. These studies demand strict adherence to protocols, precise timing, and an understanding of specific pharmacological interventions.

Solid Gastric Emptying Study

Gastroparesis is a debilitating condition characterized by delayed gastric emptying in the absence of mechanical obstruction, frequently seen in diabetic patients. The nuclear gastric emptying study is the gold standard for diagnosis.

The Standardized Meal

To ensure reproducibility across institutions, a standardized meal must be used. The Society of Nuclear Medicine and Molecular Imaging (SNMMI) mandates a specific formulation:

• Radiopharmaceutical: 0.5 to 1.0 mCi of Tc-99m Sulfur Colloid.
• The Meal: The tracer is mixed into liquid egg whites (or whole eggs) and cooked until firm. This physically binds the tracer to the solid protein, preventing it from leaching into the liquid phase. The egg is served with two slices of toasted white bread, 30 grams of jam, and 120 mL of water.
• Patient prep requires fasting for at least 8 hours and discontinuing prokinetic medications (like metoclopramide) and opiates for 2-3 days prior. Diabetic patients should have their glucose controlled (<275 mg/dL) as hyperglycemia independently delays emptying.

Imaging and Mathematics

Imaging consists of one-minute static images (anterior and posterior to calculate a geometric mean, correcting for tissue attenuation) acquired immediately after meal ingestion (Time 0), and then at 1, 2, and 4 hours.

• Emptying Rate Math: The time required for the stomach to empty half of its contents is the T1/2 (half-emptying time). A region of interest (ROI) is drawn around the stomach, and decay-corrected counts are plotted against time.
• Normal Values: A normal solid T1/2 is typically between 45 and 90 minutes. Normal SNMMI criteria state that at 4 hours, there should be less than 10% of the meal remaining in the stomach. Greater than 10% retention at 4 hours is definitive for delayed gastric emptying (gastroparesis).

Hepatobiliary Imaging (HIDA Scan)

Cholescintigraphy evaluates the function of the liver, gallbladder, and biliary tree. It is the most sensitive and specific test for diagnosing acute cholecystitis.

Radiopharmaceuticals

The agents used are iminodiacetic acid (IDA) derivatives: Tc-99m Mebrofenin (Choletec) and Tc-99m Disofenin (Hepatolite). These agents are extracted from the blood by the hepatocytes and excreted into the bile ducts, mirroring the pathway of bilirubin. Mebrofenin is favored in patients with elevated bilirubin levels (hepatic dysfunction) due to its higher extraction fraction.

Acute Cholecystitis Diagnosis

Acute cholecystitis is almost always caused by a gallstone obstructing the cystic duct.

• Normal Scan: The tracer is seen in the liver within 5 minutes, in the intrahepatic ducts and common bile duct by 15-20 minutes, and the gallbladder and small bowel are visualized by 60 minutes.
• Acute Cholecystitis Pattern: Because the cystic duct is blocked, the radiotracer cannot enter the gallbladder. The diagnostic hallmark is non-visualization of the gallbladder at 60 minutes, with prompt visualization of the small bowel.
• Morphine Augmentation: If the gallbladder is not seen at 60 minutes, the study must be extended. Intravenous morphine sulfate (0.04 mg/kg) is administered. Morphine causes contraction of the Sphincter of Oddi, increasing pressure in the common bile duct and forcing bile into the cystic duct. If the gallbladder still does not visualize 30 minutes post-morphine, the diagnosis of acute cholecystitis is confirmed. If it does visualize, it indicates a patent cystic duct and implies chronic cholecystitis.

Gallbladder Ejection Fraction (GBEF)

To evaluate for biliary dyskinesia (functional gallbladder disorder) or chronic cholecystitis, the gallbladder's ability to contract is measured.

• Pharmacological Intervention: Once the gallbladder is full of tracer, an artificial hormone, sincalide (Kinevac), a synthetic form of cholecystokinin (CCK), is administered via a slow sinusoidal IV infusion over 30-60 minutes to mimic a physiological meal. CCK stimulates gallbladder contraction and relaxes the Sphincter of Oddi.
• GBEF Calculation: Dynamic imaging captures the gallbladder emptying. Using ROIs, the calculation is: GBEF = [(Maximum GB counts - Minimum GB counts) / Maximum GB counts] × 100 (Background counts must be subtracted from both max and min prior to calculation).
• Normal Values: A normal GBEF is typically >35% or >38% (depending on institutional protocol). A value below this threshold indicates abnormal gallbladder motor function.

Gastrointestinal Bleed Scan

The GI bleed scan is used to localize the site of active lower gastrointestinal hemorrhage. It is exquisitely sensitive, capable of detecting bleeding rates as low as 0.1 mL/min (compared to angiography, which requires >1.0 mL/min).

Tc-99m Red Blood Cell (RBC) Labeling

The patient's own blood must be labeled with Tc-99m. The most critical step is ensuring the Tc-99m is bound tightly inside the RBCs so it doesn't detach and create false positives in the bowel or bladder. This involves "tinning" the blood with stannous ion (Sn+2), which acts as a reducing agent.

• In Vivo Method: Stannous pyrophosphate is injected into the patient, followed 20 minutes later by Tc-99m pertechnetate. Labeling efficiency is lowest (60-80%).
• Modified In Vivo Method: Stannous pyrophosphate is injected. Blood is withdrawn into a syringe containing Tc-99m pertechnetate, incubated for 10 minutes, and reinjected. Efficiency is ~90%.
• In Vitro Method (Ultrataag): Blood is withdrawn and the entire labeling process occurs outside the body in a specialized vial using stannous chloride and sodium hypochlorite, then reinjected. This provides the highest labeling efficiency (>95%) and is the preferred method, minimizing free pertechnetate artifacts in the gastric mucosa or urinary tract.

Image Interpretation

Dynamic images are acquired over 60-90 minutes, typically at 1 minute per frame. A positive scan shows a focal area of radiotracer accumulation outside of normal vascular structures that moves over time along the anatomical path of the bowel (e.g., retrograde in the colon or antegrade in the small bowel). This movement confirms active hemorrhage rather than a fixed vascular anomaly.

Meckel's Diverticulum Scan

A Meckel's diverticulum is a congenital remnant of the omphalomesenteric duct, often located in the distal ileum. Approximately 20-50% of these contain ectopic gastric mucosa, which secretes acid, leading to ulceration and painless rectal bleeding, most commonly in young children.

Procedure and Pharmacology

• Radiopharmaceutical: Tc-99m Pertechnetate. Because it is an analog of iodine and chloride, it is actively secreted by the mucin-producing cells of gastric mucosa, whether in the stomach or ectopic in a diverticulum.
• Pharmacological Interventions: To optimize the scan, certain drugs can be administered prior to the study:
  1. H2 Blockers (e.g., Cimetidine, Ranitidine): Prevent the release of pertechnetate from the gastric cells into the bowel lumen. This traps the tracer inside the ectopic tissue, increasing the target-to-background ratio.
  2. Pentagastrin: Stimulates gastric secretion and uptake of pertechnetate.
  3. Glucagon: Decreases bowel motility, preventing the washout of tracer secreted into the lumen.
• Interpretation: A positive scan shows a focal area of increased uptake in the lower abdomen (usually right lower quadrant) that appears simultaneously with the normal stomach uptake.