Technetium-99m mebrofenin (Choletec) is a synthetic radiotracer that is crucial for non-invasive evaluation of hepatobiliary function. Technetium-99m, a metastable nuclear isomer of technetium-99, serves as the radioactive component, while mebrofenin is a derivative of the iminodiacetic acid (IDA) family, serving as the chelating agent. The complex formed, Tc-99m mebrofenin, has become a pivotal tool in nuclear medicine for the evaluation of liver function, biliary tree patency, and gallbladder diseases.
Properties and Mechanism of Action
Technetium-99m emits gamma rays with an energy of 140 keV, ideal for gamma camera detection. It has a half-life of about 6 hours, providing sufficient time for image acquisition while minimizing radiation exposure to the patient. Mebrofenin, on the other hand, has high hepatic extraction efficiency, ensuring rapid uptake by the hepatocytes.
Upon intravenous administration, Tc-99m mebrofenin is quickly taken up by the hepatocytes and excreted into the biliary system. Its uptake is dependent on proper liver cell function and blood flow. Once in the biliary system, the agent travels through the bile ducts, into the gallbladder, and eventually into the small intestine, assuming there is no obstruction or dysfunction.
Clinical Applications
The primary use of Tc-99m mebrofenin is in hepatobiliary scintigraphy, also known as a hepatobiliary iminodiacetic acid (HIDA) scan. This scan assesses liver function, evaluates the patency of the bile ducts, and investigates gallbladder diseases such as cholecystitis and bile leaks. Additionally, it can be employed to calculate gallbladder ejection fraction, aiding in the diagnosis of biliary dyskinesia.
Acute Cholecystitis
In cases of suspected acute cholecystitis, a HIDA scan with Tc-99m mebrofenin is highly sensitive and specific. If the gallbladder is not visualized within 4 hours of radiotracer administration, and the bile ducts and small intestine are seen, it suggests cystic duct obstruction, consistent with acute cholecystitis.
Biliary Obstruction and Leaks
The agent can also identify biliary obstructions and leaks. Obstructions lead to delayed transit or non-visualization of specific biliary structures, while leaks result in abnormal radiotracer accumulation outside the biliary tree.
Liver Function
The rapid hepatic uptake of Tc-99m mebrofenin makes it useful for assessing liver function and blood flow. It provides valuable information on various hepatic conditions, including cirrhosis and hepatic tumours.
Preparation and Administration
The preparation of Tc-99m mebrofenin involves reconstituting the mebrofenin kit with Tc-99m pertechnetate, obtained from a technetium generator. Quality control procedures are necessary to ensure the purity and stability of the radiotracer.
The standard adult dose ranges from 74 to 370 MBq (2-10 mCi), administered intravenously. The dose can be adjusted based on patient size, age, and clinical indication. For accurate gallbladder ejection fraction calculation, sincalide, a cholecystokinin analogue, may be administered to stimulate gallbladder contraction.
Imaging Protocol
The typical imaging protocol involves acquiring dynamic images immediately post-injection, followed by static images at specific time intervals. Delayed images may be necessary for cases of slow transit or non-visualization of certain structures.
Benefits
Tc-99m mebrofenin scintigraphy offers several advantages. Its high sensitivity and specificity make it a valuable tool for diagnosing and evaluating various hepatobiliary conditions. It provides functional information, complementing anatomical imaging modalities like ultrasound, CT, and MRI. Being non-invasive, it is well-tolerated by patients and carries minimal risk.
Drawbacks and Limitations
Despite its benefits, there are limitations. False positives can occur in acute cholecystitis diagnosis, especially in cases of prolonged fasting or severe hepatic dysfunction. The radiation exposure, although relatively low, is a consideration, particularly in pregnant or breastfeeding women and young children.
Conclusion
Technetium-99m mebrofenin (Choletec) has established itself as an invaluable radiotracer in nuclear medicine for hepatobiliary imaging and HIDA scans. Its ability to provide functional insights into liver and biliary system function, alongside its high diagnostic accuracy, makes it a preferred choice in many clinical scenarios. With proper preparation, administration, and interpretation, it significantly contributes to patient care in hepatobiliary medicine.
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