A Hepatobiliary Iminodiacetic Acid scan (HIDA scan) is a specialized nuclear medicine imaging test that provides a dynamic view of the biliary system. This procedure uses a small amount of radioactive material, called a radiotracer, to track the flow of bile from the liver through the bile ducts, into the gallbladder, and finally into the small intestine. By tracking this movement, the HIDA scan reveals how well these organs are functioning, offering insights that traditional static imaging methods often cannot provide.
The Purpose of a HIDA Scan
The HIDA scan is performed specifically to evaluate the function and patency of the biliary tract, which is the system responsible for producing, storing, and releasing bile. Unlike imaging techniques such as ultrasound, which primarily show the structure and anatomy of the organs, the HIDA scan assesses the physiological process of bile transport. This functional assessment is useful when a patient experiences acute upper abdominal pain that may point to gallbladder issues.
The primary reason to perform this scan is to diagnose acute cholecystitis, which is sudden inflammation of the gallbladder, often caused by a gallstone blocking the cystic duct. It can also help identify bile duct obstructions, which impede the flow of bile from the liver, or locate bile leaks that can occur following abdominal surgery. Furthermore, the scan can be used to measure the gallbladder’s ability to contract and empty, a metric known as the gallbladder ejection fraction. This measurement is used to diagnose chronic gallbladder dysfunction, sometimes referred to as biliary dyskinesia.
Essential Preparation Steps
Accurate results from a HIDA scan depend heavily on the patient’s preparation, especially regarding fasting and medication use. Patients are typically required to fast for about four hours before the scan. This fasting ensures the gallbladder is adequately distended and ready to accept the radiotracer, as recent food intake would cause the gallbladder to contract and empty. However, fasting for too long (more than 12 to 24 hours) can cause the gallbladder to become overly full and sluggish, potentially leading to a false positive result where it appears blocked. To counteract this, patients who have fasted excessively may receive an intravenous dose of a medication like sincalide (CCK) to prepare the gallbladder.
Another important preparation step involves temporarily stopping certain medications, particularly opioid or morphine-based pain relievers. These drugs cause the sphincter of Oddi, which controls bile flow into the small intestine, to constrict severely. This constriction can prevent the radiotracer from entering the small intestine and gallbladder, mimicking an obstruction and leading to a false positive diagnosis. Patients are usually instructed to abstain from these specific medications for at least four to six hours before the scheduled procedure.
The Imaging Procedure Explained
The HIDA scan procedure begins with the patient lying still on a padded table underneath a gamma camera. A nuclear medicine technologist establishes an intravenous (IV) line, typically in the arm, through which the technetium-labeled radiotracer is injected directly into the bloodstream. This tracer is chemically designed to mimic the body’s natural bile components, allowing the liver cells to quickly absorb and excrete it into the biliary system.
Immediately following the injection, the gamma camera begins taking a series of images over 60 to 90 minutes, tracking the chronological movement of the tracer. In a healthy system, the radiotracer should be visible in the liver within minutes, in the bile ducts shortly after, and then in the gallbladder and small intestine within the first hour. This initial imaging sequence confirms that the liver is functioning correctly and that the major bile pathways are open.
If the gallbladder does not visualize within the expected timeframe, the technologist may administer a dose of morphine through the IV line. Morphine causes the sphincter of Oddi to temporarily constrict, increasing pressure within the bile ducts and encouraging the radiotracer to enter the gallbladder. If the gallbladder is still not visualized 30 minutes after the morphine injection, it strongly suggests a complete blockage of the cystic duct, a classic sign of acute cholecystitis.
For cases where chronic dysfunction is suspected, the radiologist will proceed with a gallbladder ejection fraction study once the organ is visualized. This involves slowly infusing a medication, often sincalide or CCK, which stimulates the gallbladder to contract. The gamma camera then continues to take images as the gallbladder empties, allowing a calculation of the percentage of bile ejected (the ejection fraction). The entire procedure usually takes between one and four hours, depending on how quickly the tracer moves and whether additional medications or delayed imaging are required.
Interpreting the Scan Results
The interpretation of a HIDA scan focuses on the timing of the radiotracer’s movement and the calculated gallbladder ejection fraction. A normal result is demonstrated when the tracer moves freely and quickly, appearing in the liver, bile ducts, gallbladder, and finally the small intestine within 60 minutes. If an ejection fraction study is performed, a normal result is typically considered to be greater than 35 percent.
Abnormal findings are categorized based on which part of the biliary system fails to visualize or function correctly. Non-visualization of the gallbladder, even after morphine administration, strongly indicates a complete obstruction of the cystic duct and suggests acute cholecystitis. If the gallbladder is visualized but the calculated ejection fraction is abnormally low (less than 30 to 35 percent), it suggests chronic gallbladder disease or biliary dyskinesia, meaning the gallbladder walls are not contracting forcefully enough. The presence of the radiotracer outside the normal pathways signals a bile leak, a complication sometimes seen after surgery or trauma.