A HIDA scan, formally known as a Hepatobiliary Iminodiacetic Acid scan, is a specialized nuclear medicine test that provides a dynamic view of the biliary system. This system includes the liver, bile ducts, and gallbladder, which work together to produce, transport, and store bile. The HIDA scan uses a small radioactive tracer to track bile flow, allowing providers to observe how these organs are performing their functions in real-time. Unlike imaging methods like ultrasound, which primarily show the structure and anatomy of the organs, the HIDA scan focuses on the function and movement of bile.
Why Healthcare Providers Order a HIDA Scan
The HIDA scan is a valuable diagnostic tool, often reserved for situations where symptoms strongly suggest a problem with the gallbladder or bile flow that other tests cannot confirm. A primary reason for ordering the scan is to diagnose acute cholecystitis, the sudden inflammation of the gallbladder often caused by a blockage in the cystic duct. If the tracer cannot enter the gallbladder due to a blockage, it indicates an acute problem. The test also evaluates chronic gallbladder issues, such as recurring, severe abdominal pain without gallstones. This can signal a functional disorder where the gallbladder is not contracting efficiently. Providers also use the HIDA scan to identify bile duct blockages or obstructions and to detect bile leaks after surgery or trauma.
The HIDA Scan Procedure: Step-by-Step
The HIDA scan begins with the intravenous injection of a small amount of a radioactive tracer, typically technetium-99m labeled iminodiacetic acid. This tracer mimics bile components, allowing it to be taken up by the liver cells and excreted into the biliary system. Following the injection, the patient lies still on an examination table while a gamma camera is positioned over the abdomen to capture images. The camera tracks the tracer’s journey from the liver, through the bile ducts, and into the gallbladder and small intestine. Imaging usually starts immediately and continues for about an hour, allowing the medical team to see if the tracer successfully enters the gallbladder and moves into the small bowel. If the gallbladder does not visualize within 60 minutes, it is a strong indicator of acute inflammation or cystic duct obstruction.
Assessing Gallbladder Function
To assess the gallbladder’s pumping ability, a drug like cholecystokinin (CCK) or sincalide is often administered intravenously after the initial images. This medication is a synthetic version of a hormone naturally released in the body after eating, which stimulates the gallbladder to contract and empty its contents. The gamma camera then takes a second set of images to measure how much of the tracer is expelled from the gallbladder. Patients may sometimes experience temporary, mild abdominal cramping or nausea when the CCK is given, which mimics the body’s natural response to a fatty meal.
Understanding the Results and Gallbladder Function
The interpretation of a HIDA scan focuses on two primary observations: the visualization pattern of the tracer and the calculation of the Gallbladder Ejection Fraction (EF). If the tracer does not appear in the gallbladder at all, even after several hours, it strongly suggests a complete blockage of the cystic duct, often associated with acute cholecystitis. Delayed visualization, where the tracer takes a long time to reach the small intestine, can indicate a partial obstruction or a problem with liver function.
Gallbladder Ejection Fraction (EF)
The Gallbladder Ejection Fraction is a critical measurement that quantifies the percentage of bile the gallbladder expels after stimulation by CCK. It is calculated by comparing the amount of tracer in the gallbladder before and after the CCK injection. The normal range for the EF is typically considered to be 35% to 38% or greater. A low ejection fraction, generally below 35%, suggests that the gallbladder is not contracting with sufficient force to empty properly. This finding is often associated with functional gallbladder disease, also known as chronic cholecystitis or biliary dyskinesia, which can be the cause of a patient’s recurring abdominal pain even without gallstones. This functional assessment is the primary advantage of the HIDA scan over structural imaging tests like ultrasound.
Preparing for the Scan and Post-Procedure Care
Proper preparation is necessary to ensure the accuracy of the HIDA scan results, with the main requirement being a period of fasting. Patients are typically asked to fast from food and drink, other than plain water, for a minimum of four hours and often up to 12 hours before the procedure. Fasting ensures the gallbladder is full of bile, allowing the tracer to fill it completely for accurate imaging.
In addition to fasting, patients may be instructed to temporarily stop certain medications, such as opioid pain relievers, for up to 24 hours before the scan, as these can interfere with normal gallbladder function and skew the results. The procedure itself typically takes between one and two hours, but the technologist will advise if delayed images are necessary, which could extend the time.
After the HIDA scan is complete, patients are generally allowed to resume their normal activities immediately. The radioactive tracer administered is temporary and leaves the body naturally through urine and stool over the next few days. To help flush the tracer out more quickly, drinking extra water after the scan is often recommended.