Portosystemic shunt scintigraphy via transplenic injection of Tc99-Mebrofenin
Portosystemic shunt scintigraphy is the gold standard diagnostic test for a congenital portosystemic shunt. It is a quick, non-invasive, & accurate way to diagnose or exclude a portosystemic shunt. It is preferred over ultrasound in patients with a mild increase of bile acids (<100) since an ultrasound will not completely exclude the possibility of a shunt. In other words, a shunt cannot be excluded if not present on ultrasound given the possibility of a false-negative result. Ultrasound evaluation is very operator and machine-dependent and its sensitivity varies but has been previously estimated to be 70%. Therefore, ultrasound is capable of identifying 7 of 10 dogs that have portosystemic shunts.
Nuclear scintigraphy via transplenic injection is ideal because it leads to a better radionuclide bolus, which makes it easier to interpret. Additionally, the use of Tc99-Mebrofenin is also very important since the Tc99-Mebrofenin is uptaken by the hepatocytes in the liver. Therefore, a normal study shows that the vast majority of the radionuclide will go to the liver and remain in the liver. By contrast, a patient with a shunt will have a significant amount of radionuclide bypassing the liver and spreading through the body.
Nuclear scintigraphy is cheaper, quicker, & safer than an abdominal CTA as it is performed in 5 minutes and does not require general anesthesia. Oftentimes, patients will receive a very mild and short sedative so the patient remains still during the image capture (5 minutes). However, many patients are not even sedated for the test. A CTA should be considered in patients with a higher likelihood of having a shunt (post-prandial bile acids >100) if a surgical correction is being considered to treat the shunt.
General recommendations of when to use scintigraphy:
1) If post-prandial bile acids are less than 100 mg/dl and a portosystemic shunt need to be ruled out.
2) If a shunt is suspected, surgical correction of the shunt is not an option (owners will not consider surgical correction).
Consider CTA if post-prandial bile acids at greater than 100 mg/dl and surgery is a possible treatment option if a shunt is found.
This nuclear scintigraphy test is performed by injecting a very small amount of a radionuclide (radioactive dye) into the spleen (Tc-99 Mebrofenin). This dye has a high uptake into the liver. Then, you monitor the movement of the dye through a gamma camera.