Portal scintigraphy is a noninvasive method of diagnosing portosystemic shunts by using radioisotopes to image portal blood
flow. Patients with primary hypoplasia of the portal vein can have normal or abnormal scans, but calculated shunt fractions
are usually much lower in these patients than in patients with macroscopic portosystemic shunts. Nuclear portal scintigraphy
is not a described imaging modality to diagnose hepatic arteriovenous malformation.
Per-rectum portal scintigraphy
Per-rectum portal scintigraphy (PRPS) is performed by administering 10 to 20 millicuries (mCi) of 99mTc-pertechnetate through
the distal colon. The radioisotope is absorbed by the colonic veins, followed by the caudal mesenteric vein, portal vein,
liver, and, finally, heart (Figure 4). In the presence of a portosystemic shunt, the radioisotope bypasses the liver to reach the heart first (Figure 5).
Figure 4. A normal per-rectum portal scintigram using technetium 99mTc-pertechnetate in a dog. The flow of the radionucleotide
is depicted as it is absorbed into the portal vein, perfuses the liver, and then reaches the heart. (Figures 4 & 5 courtesy
of Michael Broome, DVM, MS, DABVP, and Rachel Moon, DVM, at Advanced Veterinary Medical Imaging.)
Patients are sedated and placed in right lateral recumbency for PRPS. The locations of the heart and liver are marked as regions
of interest, and a gamma camera collects images over two or three minutes. Specialized software is used to measure shunt fraction.
Shunt fractions > 15% are considered positive for portosystemic shunts, and a mean shunt fraction of 84% was found in patients
with portosystemic shunts in one study using PRPS.23
Figure 5. A portal scintigram from a dog with an extrahepatic portosystemic shunt. Note how the radionucleotide bypasses
the liver and is seen highlighting the heart and lungs.
PRPS is simple to perform, noninvasive, and yields rapid quantitative results. Distinct disadvantages include the need for
special certification and radiation safety measures. PRPS does not provide detailed images of the shunting vessels and cannot
distinguish between single congenital portosystemic shunts and multiple acquired portosystemic shunts. A false positive study
may occur if the radioisotope is absorbed into the systemic circulation through the caudal rectal vein. Portal streamlining,
which is the nonuniform distribution of blood (or radioisotope) to the liver due to preferential delivery by discrete portal
channels, is a recognized cause of falsely interpreted studies.24 The nonuniform appearance of radioactivity within the liver is a normal physiologic phenomenon.
Transsplenic portal scintigraphy
Transsplenic portal scintigraphy was developed to overcome the poor absorption rate and lack of anatomic detail of PRPS. Ultrasound-guided
injection of 1 or 2 mCi of 99mTc-pertechnetate into the splenic parenchyma results in absorption by the splenic vein, followed
by the left gastric vein and main portal vein.25
Scans are evaluated similar to PRPS.
A nuclear portovenogram is imaged in most transsplenic portal scintigraphy studies because of the greater absorption and density
of radioisotope, which allows for identification of shunt number and termination.26 Another advantage over PRPS is the decreased dose of radioisotope required. Disadvantages of transsplenic portal scintigraphy
include risk of splenic hemorrhage, intraperitoneal injection resulting in a nondiagnostic scan, and the possibility of missing
a portosystemic shunt caudal to the splenic vein.25
Comparison of the two methods
A recent study comparing PRPS and transsplenic portal scintigraphy concluded that both methods are 100% sensitive for detection
of macroscopic shunts in dogs with portosystemic shunts. The reported specificity from the study was 100% for transsplenic
portal scintigraphy and 95% for PRPS. It also concluded that transsplenic portal scintigraphy provides significantly better
scan quality than PRPS does and has significantly improved ability to delineate shunt anatomy.26