The Hepatobiliary Iminodiacetic Acid (HIDA) scan is a diagnostic imaging test used to visualize the flow of bile through the body’s biliary system, which includes the liver, bile ducts, and gallbladder. The primary purpose of the HIDA scan is to evaluate both the functional health and structural integrity of these organs to identify the cause of symptoms like abdominal pain. By tracking a specialized substance, the scan helps diagnose conditions such as inflammation or blockages.
Understanding the HIDA Scan Procedure
The HIDA scan, also known as cholescintigraphy, begins with the intravenous injection of a radioactive tracer, often a form of iminodiacetic acid (IDA). This chemical is designed to mimic the behavior of natural bile and is quickly taken up by the liver cells from the bloodstream. Once absorbed, the tracer is secreted into the bile ducts and begins its journey through the biliary tree.
A specialized device called a gamma camera then takes a series of images over a period of time, typically one to four hours. The camera tracks the tracer’s movement as it leaves the liver, travels through the ducts, and concentrates in the gallbladder for storage. Eventually, the radiotracer should pass from the gallbladder and the main ducts into the small intestine.
Indicators of a Normal Scan Result
A healthy HIDA scan establishes a clear timeline for the radiotracer’s journey through the biliary system. For the scan to be considered normal, the tracer must be taken up promptly by the liver and then successfully enter the gallbladder. This visualization of the gallbladder usually occurs within 30 to 60 minutes after the tracer injection.
In addition, the tracer must successfully pass from the main bile ducts into the small intestine, demonstrating an open pathway. When a functional test is performed using a drug like sincalide to stimulate gallbladder contraction, the organ’s emptying rate is measured as the Gallbladder Ejection Fraction (EF). A normal EF is generally defined as a percentage greater than 35% to 38%, which confirms that the gallbladder is contracting with adequate force to empty its contents.
Abnormal Result 1: Non-Visualization of the Gallbladder
One of the most definitive abnormal findings is the complete failure of the gallbladder to appear on the scan, even after several hours. This visual absence occurs because the radiotracer, despite passing through the liver and bile ducts, is physically blocked from entering the gallbladder. The most common cause of this non-visualization is an obstruction of the cystic duct, the small tube that connects the gallbladder to the main biliary system.
This pattern is a highly suggestive finding for acute cholecystitis, which is sudden inflammation of the gallbladder, often caused by a gallstone lodged in the cystic duct. In these instances, the tracer will stop at the site of the obstruction while the rest of the biliary system may appear normal.
Abnormal Result 2: Low Gallbladder Ejection Fraction
A different type of abnormality is a functional problem, which is identified by a calculated low Gallbladder Ejection Fraction (EF). This measurement is taken after a hormonal stimulant, such as sincalide, is administered to prompt the gallbladder to contract and empty its stored bile. The EF is the percentage of bile the gallbladder expels, and a result below the 35% to 38% threshold is considered abnormal.
A low EF indicates that the gallbladder muscle is not contracting with sufficient power to empty its contents effectively. This finding is often associated with chronic inflammatory conditions, such as chronic cholecystitis, or a functional disorder known as biliary dyskinesia.
Unlike acute blockage, a low EF suggests a long-standing issue with the gallbladder’s ability to perform its storage and release function, even in the absence of gallstones. The poor contractility leads to incomplete emptying, which can cause symptoms like pain and nausea after eating fatty foods.
Abnormal Result 3: Tracer Leakage or Delayed Flow
Other visual abnormalities relate to the movement of the radiotracer outside of the normal ducts or a significant delay in its transit time. Delayed flow refers to the tracer moving too slowly through the main bile ducts or taking an excessive amount of time to reach the small intestine. This slow transit can signal a partial obstruction in the common bile duct or it may suggest an issue with the liver’s ability to process the tracer effectively.
Tracer leakage is a finding where the radiotracer is seen collecting outside the defined borders of the biliary system, often appearing as a pool of activity in the abdominal cavity. This spillage indicates a bile leak, which is a serious complication often occurring after abdominal surgery, trauma, or a biopsy.