When you have a patient with a hepatic vascular abnormality, how do you confirm it? Even if you refer to a specialist, be sure to keep up on the latest in vascular imaging in order to enhance client communication and participate in ongoing treatment.
Figure 6. Dual-phase CT angiography. This axial CT image is of a normal dog. Note the contrast enhancement of the vasculature.
(Figures 6-11 courtesy of Michael Broome, DVM, MS, DABVP, and Rachel Moon, DVM, at Advanced Veterinary Medical Imaging.)
CT angiography is the gold standard for evaluating hepatic vascular anomalies in people and is being used to diagnose portosystemic
shunts in animals with increasing frequency. Methods of performing helical CT angiography to image portosystemic shunts in
dogs have been recently described.27,28 Single-phase CT angiography images only the portal venous phase of hepatic blood flow, while dual-phase images both the
arterial and portal phases.
Figure 7. Dual-phase CT angiography. This sagittal view of the same dog as in Figure 6 is a multiplanar reformat created
from the raw helical data file and built-in software in the helical CT scanner (HiSpeed NX/i dual-detector helical CT unit—GE
Healthcare) used for the study. The dogߣs head is to the left.
Procedure
Figure 8. Dual-phase CT angiography of an extrahepatic portosystemic shunt. The axial CT image reveals contrast enhancement
within a shunt vessel.
Patients must be anesthetized for CT. Before image acquisition, patients are hyperventilated to induce apnea, which reduces
respiratory motion artifact in the images. The start time for image acquisition is determined by an initial test intravenous
bolus of 0.55 ml/kg of iodinated contrast medium and scanning for the time of maximum opacification of either the hepatic
artery or portal vein. Images are then acquired after an injection of 2.2 ml/kg of iodinated contrast medium, while scanning
the entire abdomen from caudal to cranial for the arterial phase and cranial to caudal for the portal phase (Figures 6 & 7).
Usefulness
Figure 9. Dual-phase CT angiography of an extrahepatic portosystemic shunt. The image is a sagittal view of the dog in Figure
8 and a multiplanar reformat created from the raw helical data file. The shunt vessel (orange arrows) can be seen connecting
the portal vein (green arrows) to the caudal vena cava (purple arrow). Note the hypoplastic portal branches. The dog’s right
is to the left.
CT angiography has consistently diagnosed hepatic vascular anomalies, including intrahepatic and extrahepatic portosystemic
shunts and hepatic arteriovenous malformation, in recent reports.27-31 The origin, course, and insertion of extrahepatic and intrahepatic portosystemic shunts can be accurately detailed (Figures 8 & 9). This method is ideal for diagnosing dorsally located shunts that are difficult to image with ultrasonography, such as left
gastric vein shunts and portoazygous shunts.29,31
Figure 10. Dual-phase CT angiography of multiple acquired shunts. The orange circle is encompassing multiple acquired shunts
between the portal vein and caudal vena cava at the level of the left kidney.
CT can also aid in the diagnosis of multiple acquired portosystemic shunts, which are difficult to detect with ultrasonography
and cannot be distinguished from single macroscopic shunting through nuclear portal scintigraphy (Figures 10 & 11). CT is also especially useful in the diagnosis of hepatic arteriovenous malformation when a dual-phase scan is performed.
Opacification of the portal veins is seen concurrent with the arterial phase of the scan in cases of hepatic arteriovenous
malformation.30
Figure 11. Dual-phase CT angiography of multiple acquired shunts. The image is a sagittal view of the dog in Figure 10.
The primary disadvantages of CT compared with ultrasonography are the requirement of general anesthesia and reduced availability
of equipment. Cost may also be a limiting factor. Patient motion and poor resolution in small patients are additional limits
to CT. The information obtained from CT angiography is the same as provided by operative mesenteric portography.28 However, preoperative CT also allows surgical planning, which can reduce anesthetic time and tissue manipulation. At this
time, sensitivity and specificity values are not established for CT diagnosis of hepatic vascular anomalies.
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