A Hepatobiliary Iminodiacetic Acid (HIDA) scan is a common nuclear medicine test. It evaluates the health and function of the hepatobiliary system, assessing the liver, gallbladder, and bile ducts. The scan identifies issues related to bile production and flow through these organs.
Understanding the HIDA Scan: Purpose and Principle
The HIDA scan assesses the functional aspects of the hepatobiliary system. It tracks how bile is produced by the liver, transported through the bile ducts, stored in the gallbladder, and released into the small intestine. This process reveals problems with bile flow and organ function.
The scan involves injecting a radioactive tracer, typically Technetium-99m iminodiacetic acid (Tc-99m IDA), into a patient’s vein. This tracer mimics bile behavior. As the liver processes and excretes the tracer, its path is tracked by a gamma camera. The camera detects emitted gamma rays, creating images that identify blockages, leaks, or impaired gallbladder activity.
Conditions Diagnosed by HIDA Scan
The HIDA scan diagnoses several conditions affecting the hepatobiliary system. One of its primary uses is identifying acute cholecystitis, which is the sudden inflammation of the gallbladder. If the gallbladder does not visualize on the scan after a certain period, it strongly suggests an obstruction of the cystic duct, a hallmark of acute cholecystitis.
The scan also helps in diagnosing chronic cholecystitis, a long-term inflammation of the gallbladder. In this scenario, cholecystokinin (CCK) may be administered to stimulate gallbladder contraction. A low gallbladder ejection fraction (GBEF) measured after CCK stimulation can indicate chronic inflammation or functional impairment of the gallbladder.
Biliary obstruction, a blockage within the bile ducts, is another condition a HIDA scan can detect. If the tracer fails to reach the small intestine within a few hours, it suggests a significant blockage in the bile ducts preventing normal bile flow. This blockage can be caused by gallstones, tumors, or strictures.
Bile leaks, often occurring after surgery or trauma to the abdomen, can also be identified. The scan shows extravasation, or leakage, of the tracer outside the normal biliary pathways. This indicates where bile is escaping from the system, pinpointing the location of the leak.
For infants, the HIDA scan assists in diagnosing biliary atresia, a rare congenital condition where the bile ducts are underdeveloped or blocked. Non-excretion of the tracer into the bowel in an infant strongly suggests this condition. The scan can also help in cases of Sphincter of Oddi dysfunction, where delayed drainage of the tracer into the small intestine can suggest spasms or narrowing of the Sphincter of Oddi, a muscle that controls bile and pancreatic juice flow.
Interpreting HIDA Scan Findings
Interpreting HIDA scan findings involves analyzing the flow and distribution of the radioactive tracer through the hepatobiliary system. A normal scan shows the tracer quickly appearing in the liver, followed by its visualization in the bile ducts, gallbladder, and eventually the small intestine, typically within an hour. This rapid and complete progression indicates healthy organ function and unobstructed bile flow.
Abnormal findings provide specific clues about underlying conditions. Non-visualization of the gallbladder, even after several hours, is a key indicator of acute cholecystitis, suggesting the cystic duct is blocked and preventing tracer entry. Delayed filling of the gallbladder or delayed appearance of the tracer in the small intestine can point towards chronic gallbladder issues or partial bile duct obstructions.
A low gallbladder ejection fraction, after administering CCK, indicates that the gallbladder is not contracting effectively. This finding suggests chronic inflammation or functional problems with the gallbladder’s ability to empty. If the tracer is seen outside the normal biliary system, it signifies a bile leak, often appearing as a localized collection or diffuse spread of radioactivity in the abdominal cavity. If no tracer reaches the small intestine, it suggests a complete obstruction of the common bile duct. Delayed clearance of the tracer from the bile ducts, without a clear obstruction, might suggest Sphincter of Oddi dysfunction.
Complementary Diagnostic Approaches
While the HIDA scan offers functional information about the hepatobiliary system, it is often used alongside other diagnostic tools for a comprehensive assessment.
Ultrasound
Ultrasound, for example, is frequently employed to visualize gallstones within the gallbladder or to detect dilation of bile ducts, providing anatomical details that complement the HIDA scan’s functional insights. It is a readily available and non-invasive imaging method.
CT and MRI Scans
Computed Tomography (CT) scans and Magnetic Resonance Imaging (MRI) offer detailed anatomical views of the liver, pancreas, and bile ducts. These techniques can identify tumors, cysts, or other structural abnormalities that might be causing biliary issues. They provide cross-sectional images, allowing for a thorough evaluation of surrounding organs and tissues.
ERCP and MRCP
More specialized procedures like Endoscopic Retrograde Cholangiopancreatography (ERCP) or Magnetic Resonance Cholangiopancreatography (MRCP) are used for detailed imaging of the bile ducts. ERCP is an invasive procedure that can also allow for therapeutic interventions, such as stone removal or stent placement, while MRCP is a non-invasive imaging technique that provides detailed images of the bile and pancreatic ducts without radiation.
Blood Tests
Blood tests, which assess liver function enzymes and inflammatory markers, also contribute to the overall diagnostic picture by indicating the presence of inflammation or liver damage.