A Hepatobiliary Iminodiacetic Acid scan, commonly called a HIDA scan, is a nuclear medicine imaging test that evaluates the function of the body’s biliary system, which includes the liver, bile ducts, and the gallbladder. This procedure uses a radioactive tracer injected into a vein, which is then absorbed by the liver and excreted with the bile. A specialized camera tracks the tracer’s movement, creating images that show how well the bile flows through the system and if there are any blockages.
The Purpose of the HIDA Scan
A HIDA scan is typically ordered when a person experiences severe abdominal pain, often concentrated in the upper right quadrant. This pain can suggest inflammation or blockage in the gallbladder or bile ducts. It is frequently used when gallbladder issues are suspected but standard ultrasound imaging does not show gallstones.
The scan is designed to help rule in or rule out conditions like acute cholecystitis (sudden gallbladder inflammation) or chronic cholecystitis (repeated episodes of inflammation). It also helps determine if the cystic duct, the small tube connecting the gallbladder to the main bile duct, is blocked. The HIDA scan also measures the gallbladder’s ability to contract and empty, which gauges its muscular function.
Interpreting the Gallbladder Ejection Fraction
One of the primary quantitative results provided by a HIDA scan is the Gallbladder Ejection Fraction (EF). The EF measures the percentage of bile the gallbladder releases after chemical stimulation. This stimulation is usually achieved by administering a synthetic hormone, such as cholecystokinin (CCK) or sincalide, which mimics the body’s natural response to a meal.
The EF value directly reflects the muscular function of the gallbladder wall. A normal EF is generally considered to be 38% or greater, though this threshold can vary slightly between facilities. This range indicates that the gallbladder is contracting with sufficient strength to empty bile into the small intestine.
A low EF, typically defined as less than 35% or 38%, suggests a functional problem with the gallbladder’s muscle. This finding is often associated with diagnoses like chronic cholecystitis or biliary dyskinesia, where the gallbladder is not contracting adequately. A very low EF in a patient experiencing typical pain may prompt a recommendation for gallbladder removal, even without gallstones.
An EF that is unusually high, sometimes greater than 80%, may indicate a condition called biliary hyperkinesia. This condition can also cause pain due to excessive contraction.
Understanding Tracer Flow and Visualization
In addition to the quantitative EF, the HIDA scan provides qualitative data by tracking the radioactive tracer’s path and timing through the biliary system. The liver should take up the tracer almost immediately after injection, and it should rapidly move into the main bile ducts. In a typical result, the gallbladder should visualize within about one hour, and the tracer should also be seen moving into the small intestine within this timeframe.
Delayed visualization of the gallbladder (appearing more than an hour after injection) can suggest a partial or mild obstruction in the cystic duct. This slow movement may also point to chronic inflammation or a problem with the liver’s ability to process the tracer. The most significant finding is non-visualization, where the gallbladder never fills with the tracer.
Non-visualization, especially when the tracer successfully enters the small intestine, is a strong indicator of acute cholecystitis. This result suggests the cystic duct is completely blocked, usually by a gallstone, which physically prevents the bile from entering the gallbladder. Conversely, if the tracer enters the gallbladder normally but does not move into the small intestine, it may suggest a blockage in the common bile duct.
Connecting Results to Diagnosis
The final HIDA scan report synthesizes the quantitative ejection fraction and the qualitative tracer flow findings to arrive at a clinical summary. A normal result is confirmed by prompt visualization of the gallbladder and bile flow into the small intestine, paired with an ejection fraction above the normal threshold. This indicates a fully functioning system, suggesting the patient’s symptoms may stem from another cause.
When the tracer flow is normal but the Ejection Fraction is low (under 35-38%), the diagnosis is typically functional gallbladder disorder, such as biliary dyskinesia or chronic cholecystitis. This means the plumbing is open, but the pump muscle is weak. A finding of non-visualization of the gallbladder, where the tracer cannot enter, points directly to acute cholecystitis caused by a complete obstruction of the cystic duct.
The report may also note other findings, such as the tracer appearing outside the biliary system, which suggests a bile leak. Clinicians use these precise details to determine the appropriate next steps, which may range from further testing to a surgical consultation for gallbladder removal.