A Hepatobiliary Iminodiacetic Acid scan, commonly called a HIDA scan, is a specialized nuclear medicine imaging procedure used to evaluate the function of the liver, gallbladder, and bile ducts. This test tracks the flow of bile, a digestive fluid produced by the liver, as it moves through the biliary system and into the small intestine. By using a harmless radioactive tracer, the HIDA scan provides a dynamic view of how these organs process and transport bile, helping doctors diagnose various digestive conditions.
Diagnostic Applications and Required Preparation
The primary purpose of ordering a HIDA scan is to investigate potential issues within the biliary tract, which often present as right upper quadrant abdominal pain. The test is highly effective for diagnosing acute cholecystitis, a sudden inflammation of the gallbladder, by determining if the cystic duct is blocked, often by a gallstone. It also helps in identifying chronic inflammation of the gallbladder, known as chronic cholecystitis, which may not involve a complete blockage but impairs function.
Physicians also use this scan to check for bile duct obstructions, which can be caused by stones or tumors, or to detect bile leaks that may occur following surgery or trauma. In some cases, the scan is requested to assess the function of a liver transplant or to evaluate a rare congenital condition in infants called biliary atresia.
To ensure the gallbladder is in a receptive state for testing, the patient must follow specific preparation guidelines. Most patients are required to fast for a minimum of four hours before the test, though fasting for longer than 12 to 24 hours might negatively affect the results. Fasting ensures the gallbladder is full of bile and ready to contract upon stimulation during the procedure.
It is also necessary to temporarily discontinue certain medications, particularly opioid pain relievers, for several hours before the exam. Opioids can cause the sphincter of Oddi, the muscle that controls bile flow into the small intestine, to constrict, which could lead to inaccurate or misleading results. Patients are instructed to discuss all current medications with their healthcare provider beforehand.
Step-by-Step Description of the HIDA Scan Procedure
The HIDA scan is performed in a nuclear medicine department and begins with the patient lying down on an examination table beneath a gamma camera. A technologist first inserts an intravenous line into a vein, typically in the arm, to administer the radioactive tracer. The tracer, an Iminodiacetic Acid compound linked to a small amount of a radioactive isotope, mimics the body’s natural bile components.
Once injected, the radiotracer travels through the bloodstream to the liver, where it is absorbed by liver cells and then excreted into the bile ducts. The gamma camera, positioned over the abdomen, immediately begins taking a series of images to track the tracer’s movement through the liver, the bile ducts, and the common bile duct. Within the first hour, the tracer should normally move into and fill the gallbladder.
If the tracer fills the gallbladder within 60 to 90 minutes and no obstruction is suspected, the next step involves assessing the gallbladder’s ability to contract. The technologist administers a synthetic version of the hormone cholecystokinin (CCK) through the IV line. This hormone naturally stimulates the gallbladder to contract and release bile into the small intestine after a meal.
The CCK is infused slowly over a period of 30 to 60 minutes to mimic a physiological response. As the gallbladder contracts, the gamma camera continues to capture images to measure how much of the tracer is expelled from the organ. This allows for the calculation of the gallbladder’s functional capacity.
Understanding the Scan Results and Ejection Fraction
The interpretation of the HIDA scan results depends on two main factors: the visualization of the gallbladder and the calculated Gallbladder Ejection Fraction (GB-EF). If the gallbladder fails to visualize—meaning the tracer does not enter it—within four hours of injection, it strongly suggests a complete blockage of the cystic duct, characteristic of acute cholecystitis. In this scenario, the bile-like tracer bypasses the obstructed gallbladder and proceeds directly into the small intestine.
Conversely, if the tracer successfully fills the gallbladder, the results focus on the GB-EF, which quantifies the percentage of bile volume expelled after the CCK stimulation. This fraction is a direct measure of the gallbladder’s muscular function and its ability to contract. The ejection fraction is calculated by comparing the amount of radiotracer remaining after stimulation to the amount present before stimulation.
A normal Gallbladder Ejection Fraction is greater than 35% to 40%. A GB-EF value below this accepted range indicates gallbladder dysfunction, medically referred to as biliary dyskinesia. This low ejection fraction suggests that the organ is not squeezing bile out effectively, which can cause chronic pain even without gallstones present.
The results may also show slow movement of the tracer through the liver or bile ducts, which can suggest a partial obstruction or reduced liver function. If the tracer appears outside of the normal biliary path, it can indicate a bile leak, a serious condition requiring immediate attention.