A Hepatobiliary Iminodiacetic Acid (HIDA) scan is a specialized imaging test used to evaluate the function of the body’s biliary system, which includes the liver, gallbladder, and bile ducts. Unlike other imaging methods that focus on organ structure, the HIDA scan is a functional study that tracks how well bile is produced and transported. This technique uses a radioactive tracer that mimics bile, allowing physicians to observe its flow from the liver, through the ducts, into the gallbladder for storage, and finally into the small intestine. By following this path, the scan provides a dynamic assessment of the system’s ability to create, store, and release bile, helping to pinpoint problems.
Specific Conditions Evaluated by the Scan
The HIDA scan is often performed to diagnose acute cholecystitis, which is sudden inflammation of the gallbladder. This condition is typically caused by a blockage in the cystic duct, often by a gallstone, preventing bile from entering or leaving the gallbladder. A positive finding for acute inflammation occurs when the radiotracer travels through the bile ducts but fails to enter the gallbladder itself within a specific timeframe.
The scan also evaluates chronic gallbladder dysfunction, often referred to as biliary dyskinesia. This condition involves the gallbladder’s inability to contract and empty properly, even without a physical blockage. Assessing this function requires measuring the gallbladder’s ejection fraction (GEF), which provides a quantitative value for its performance.
Beyond the gallbladder, the HIDA scan detects obstructions within the main bile ducts that transport bile to the small intestine. If the tracer enters the liver and bile ducts but does not proceed into the intestine, it suggests a blockage exists further down the biliary tree. The rate of tracer movement can also indicate a partial obstruction or an issue with the liver’s ability to excrete bile.
The test is utilized to identify bile leaks, which are a common concern following abdominal surgery or trauma. A bile leak is indicated when the radioactive tracer is detected outside the normal confines of the biliary system, showing up in unexpected areas of the abdomen. This ability to track the path of bile makes the HIDA scan useful for evaluating post-operative complications.
The Step-by-Step Procedure
The HIDA scan is a nuclear medicine procedure involving a small, safe dose of a radiotracer. The process begins with the patient lying down underneath a specialized imaging device called a gamma camera. A healthcare professional then injects the radiotracer, typically a Technetium-99m labeled iminodiacetic acid compound, into a vein in the patient’s arm.
The radiotracer is absorbed by the liver cells, mimicking the natural metabolic pathway of bile. The gamma camera immediately captures a series of images, tracking the tracer as it travels from the liver, through the bile ducts, and into the gallbladder and small intestine. The patient must remain still during this imaging process to ensure the captured pictures are clear and accurate.
The initial phase of the scan typically lasts about an hour, but the total duration varies depending on the findings. If the gallbladder does not fill with the tracer in the expected timeframe, the technologist may administer a synthetic hormone, such as cholecystokinin (CCK). This drug causes the gallbladder to contract and empty, simulating the effect of a meal, which is necessary for assessing gallbladder function.
Interpreting the Scan Findings
Physicians analyze the HIDA scan results by observing the timing and distribution of the radiotracer throughout the biliary system. A normal result shows the tracer moving freely into the liver, filling the gallbladder, and passing into the small intestine within 60 minutes of the injection. Deviations from this pattern allow for a diagnosis.
Acute cholecystitis is indicated when the gallbladder fails to visualize, meaning the tracer does not enter it, even after several hours. This non-visualization is interpreted as a complete obstruction of the cystic duct, the hallmark of acute inflammation. Conversely, if the tracer is seen in the liver and gallbladder but not in the small intestine, it suggests a blockage in the common bile duct.
When chronic gallbladder dysfunction is suspected, the focus shifts to calculating the Gallbladder Ejection Fraction (GEF). The GEF is the percentage of bile the gallbladder expels after being stimulated by the synthetic hormone (CCK). A normal GEF is generally above 35% to 40%. A GEF significantly lower than this threshold suggests biliary dyskinesia, indicating that the gallbladder muscle is not contracting effectively.
Patient Preparation and Safety
Proper patient preparation is necessary for accurate HIDA scan results, particularly regarding food intake. Patients are required to fast for at least four hours before the test, but they should not fast for longer than 24 hours. Fasting is necessary because recent eating can cause the gallbladder to contract and empty, potentially leading to inaccurate results.
Patients must inform their healthcare provider about all current medications, especially opioid pain drugs. Opioid medications interfere with the test by causing a spasm of the sphincter of Oddi, which can mimic a blockage and produce a false-positive result. Physicians generally instruct patients to stop taking these specific medications for a period before the scan.
The HIDA scan is considered a safe procedure with minimal risk. The procedure involves a small exposure to ionizing radiation from the radiotracer, but the dosage is low and comparable to a standard X-ray. Minor side effects are usually limited to slight bruising at the injection site or temporary nausea and cramping if the gallbladder-stimulating drug is administered.