Computed Tomography of Vascular Disease
ACVIM 2008
Gabriela Seiler, Dr.Med.Vet., DACVR, DECVDI
Philadelphia, PA, USA

Introduction

Computed Tomography (CT) is now widely available in the Veterinary Specialist Practice and is an excellent tool to evaluate patients with suspected vascular disease. Compared with ultrasound which is in many places the method of choice for evaluation of abdominal vascular disease, CT has the advantage of superior anatomical overview which is operator independent and available for repeated review, an advantage especially for surgical planning.

Basic Principles of CT-Angiography

Basic requirements for most CT angiographic studies are a helical CT unit and a power injector. Helical CT technique allows rapid image acquisition as the table is moving while the x-ray tube and detectors are rotating around the patient in a spiral pathway. A power injector is needed for most angiographic studies to provide a bolus injection of contrast. This is especially important if arterial and venous phases have to be investigated separately (dual-phase imaging), if injection is too slow overlap of the two phases will result. Hand injections can be used in some smaller patients, especially when only a venous phase of longer duration is evaluated.

CT angiography is performed in several steps. Good planning of the study is essential. Before starting the CT, the investigator should decide which vascular system is going to be analyzed (arterial, portal, venous) and which the main vessels of interest are. Based on that information the study can be planned. The first step is acquisition of a helical image series of the area of interest. On these images, the vessels of interest are identified and a slice is selected where the vessels can be seen, for example the portal vein and aorta in a patient with suspected portosystemic shunt. The second step is then to find out when and for how long after contrast injection the vessels of interest are maximally filled with contrast. For this, a dynamic scan of the previously identified slice is then performed after a test injection with a small dose of contrast. The same slice is scanned repeatedly over a time period of about 60 seconds. Using CT software, contrast enhancement within the vessels of interest over time can be analyzed and a time vs. attenuation graphs is plotted to determine the beginning, end and duration of the enhancement of the different vessels of interest (Figure 1).

Click on the image to see a larger view.

Figure 1.
Figure 1.

Time vs. attenuation curve of the aorta (A) and portal vein (P).
 

For example, from the curve shown in figure 1 it can be determined that the arterial phase (A) starts at 5 seconds after contrast injection and last until second 12 post injection. The portal phase reaches its peak at 18 seconds post contrast injection. Delay between injection and duration of the different vascular phases (arterial vs. portal etc) is determined from the graph and the actual angiogram can thus be planned. The angiogram is the third and final step, and it can be performed as a single scan if only one vascular system is of interest (for example a venogram of the caudal vena cava) or as a dual-phase scan if arterial and venous phases should be evaluated. For dual-phase scan the region of interest is scanned in caudo-cranial direction for example during the arterial phase, after a short delay, the patient is then scanned in cranio-caudal direction during the venous phase. The patient has to be hyperventilated to induce expiratory breath-hold for all image series to avoid motion artifact.1

Hepatic Vascular Diseases

For a long time the gold standard for diagnosing hepatic vascular disease in veterinary medicine has been angiography. However the method is invasive, and unable to fill the whole portal system simultaneously with contrast medium. In human medicine it has been long replaced by CT angiography. Ultrasound and Doppler ultrasound is often the method of choice for evaluation of the portal vasculature in small animals and in the hands of an experienced ultrasonographer is a sensitive and specific method.2 Atypical shunt vessels, less than ideal scanning conditions or lack of experience of the examiner reduces the accuracy of the method considerably however. CT angiography has now also been established in veterinary medicine and proven to be a minimally invasive procedure that allows reliable imaging of all portal tributaries and braches during a single peripheral venous injection.3

Portosystemic Shunts

Typical CT-angiographic findings in dogs with portosystemic shunts include a small liver, increased arterial blood supply with tortuous arteries, and small or absent portal vasculature. The shunt vessel can directly be identified by following the portal vein tributaries and looking for vessels joining the systemic circulation. An abrupt decrease in the diameter of the portal vein at the origin of the shunt vessel is a helpful marker to determine presence and origin of a portosystemic shunt.

Extrahepatic shunts may originate from the portal vein or any tributary; however the most common extrahepatic shunts originate from the splenic or left gastric vein. Occasionally the shunt connects the gastroduodenal vein from the right with the systemic circulation. Portocaval shunts most commonly enter the caudal vena cava cranial to the right renal vein. Porto-azygos shunts are easily identified by a vessel coursing dorsally and joining the azygos vein which then becomes abnormally large.

Intrahepatic shunts are divided into left divisional (ductus venosus), central divisional and right divisional. In left-divisional or patent ductus venosus, the shunt vessel curves to the left side and forms a focal widening at the entrance of the left hepatic vein into the caudal vena cava. In right divisional shunts the anomalous vessel curves to the right before entering the caudal vena cava. In central-divisional shunts, the portal vein in the central liver becomes enlarged and enters the caudal vena cava through a short shunt or a foramen.4 The prenatal development of the caudal vena cava is complex, and the final vessel is composed of multiple segments originating from the cardinal venous system in the embryo.5 Malformations of the caudal vena cava are seen in a small percentage of dogs with portosystemic shunts, such as interrupted caudal vena cava with continuation as azygos vein.6

Multiple extrahepatic shunts can be difficult to identify ultrasonographically but are readily identified as clusters of small tortuous vessels using CT angiography. Extrahepatic shunts usually terminate in the renal veins or directly in the caudal vena cava. Small vessels may also terminate in the phrenicoabdominal veins.3

Arteriovenous and Arterioportal Fistulas

Rare vascular anomalies in dogs and cats are arteriovenous or arterioportal fistulas. These animals usually present with a large amount of ascites caused by the portal hypertension. In the arterial phase of the CT angiography simultaneous strong contrast enhancement of the portal vein and aorta can be observed. Multiple extrahepatic shunts are often present and an interesting finding is the small size of the caudal vena cava and aorta in the caudal abdomen, caused by "steal of blood" by the fistula. Hypoperfusion of the gastrointestinal tract can contribute to ascites and malabsorption commonly seen in those animals.7

Arteriovenous fistulas can be congenital or acquired. Acquired fistulae are commonly post-traumatic, resulting from rupture of an artery into an adjacent vein with blunt trauma or from penetrating injury producing direct arteriovenous communication. They can occur at any point in the vascular system; and vary in size, length, location, and number. CT angiographic findings include an aneurysmal dilation of the artery proximal to the fistula, and early and strong enhancement of the dilated vein during the arterial phase.8

Tumor Characterization

Tumor growth is directly dependent on vascular supply. Imaging the tumor vessels can give information about tissue of origin and intravascular tumor extension. The vascular supply to a tumor is also a potential target for treatment such as tumor embolization. Description of the main vascular supply and follow-up after treatment can be performed using CT angiography.

Determination of Tissue of Origin

Large abdominal masses tend to compress all adjacent organs and it can be difficult to determine the tissue of origin, which may be important for surgical planning and prognosis. CT angiography can be used to delineate the main blood supply to the mass. A dual-phase CT angiogram is the method of choice, the arterial phase centered over the suspected arterial supply (celiac artery if the question is pancreatic vs. liver mass for example), and the venous phase then covers the mass and surrounding organs to look for venous drainage of the mass. Certain types of neoplasia can be differentiated by their vascular characteristics such as insulinomas. Insulinomas have a strong arterial blood supply and can be detected as very hyperattenuating nodules compared with the surrounding pancreas during the arterial phase of the CT scan.9

Tumor invasion of Major Vessels

Surgical respectability depends often on presence or absence of vessel invasion of a tumor. Especially adrenal tumors are frequently growing into the caudal vena cava, renal veins or even the aorta, forming a tumor thrombus. Tumor thrombus is a term used for a mass formed by the extension of a nonvascular tumor into a blood vessel. Adrenal tumors, especially pheochromocytomas tend to invade the caudal vena cava intraluminally through the phrenicoabdominal vein rather than by direct erosion of the wall of the caudal vena cava. Both left and right sided adrenal tumors can invade the caudal vena cava, pre-operative assessment of the cava is therefore indicated.10 For detection of intravascular tumor extension into the caudal vena cava a venogram is performed. A bolus injection is very important as incomplete contrast filling of the caudal vena cava can mimic filling defects caused by tumor invasion. Vascular invasion leads to incomplete contrast column with persistent filling defects adjacent to the mass, in some instances the vessel can be widened or completely obstructed with tumor tissue leading to an interruption of the contrast column.

Blood Supply Before and After Tumor Embolization

Tumor embolization is a treatment choice for non-resectable abdominal masses.11 Visualization of the main arterial supply of a mass is important for planning of the procedure. Follow up studies are performed to determine successful embolization of the main tumor vasculature.

Thrombosis

Major Abdominal Vessel Thrombosis

The imaging procedure for caudal vena cava thrombosis has already been addressed above. Portal vein thrombosis can be diagnosed using a portal phase CT angiogram. Caudal vena cava obstruction can be caused by an intravascular neoplasia or thrombosis. Surgical treatment depends on reliable assessment of tumor extent, especially extension into the right atrium and invasion of the vessel wall and identification of a primary tumor. Typical CT angiographic findings include a filling defect causing complete or partial obstruction of the lumen seen as attenuation of the contrast column.12 Older thrombi sometimes have hyperattenuating foci in the center consistent with mineralizations. Differentiation between a caudal vena cava mass and a thrombus is difficult. An attempt can be made to evaluate the thrombus during the arterial phase and to look for contrast enhancement.

Pulmonary Thromboembolism

Pulmonary thromboembolism (PTE) is a concern in many critically ill patients, but it is challenging to diagnose. Radiographic findings are very unspecific and more advanced imaging methods such as CT angiography and/or scintigraphy have to be used. Multidetector CT angiography is the state of the art procedure for diagnosis of pulmonary thromboembolism in human medicine.13 In veterinary medicine, most institutions work with single slice helical CT units. Compared with multislice units, single slice units have a decreased scan speed and resolution. Visualization of segmental and subsegmental pulmonary arteries is difficult to obtain, and we are limited to diagnosis of thrombi and emboli in major lung vessels.14 Respiratory pause is mandatory for PTE-CT angiography, as motion artifact can lead to inconsistencies in contrast flow and the appearance of small filling defects which may be misinterpreted as small thrombi. All pulmonary vessels have to be carefully scrutinized for enlargement, blunt ending and filling defects indicative of pulmonary thrombi or emboli. Changes in the pulmonary parenchyma range from hypoattenuation (hyperlucency) due to lack of blood supply to areas of hyperattenuation, ground glass opacity or consolidation.15

References

1.  Zwingenberger AL, et al. Vet Radiol Ultrasound 2004;45(2): 117.

2.  Szatmari V, et al. J Am Vet Med Assoc 2004; 224:717.

3.  Zwingenberger AL, et al. Vet Radiol Ultrasound 2005;46(1): 27.

4.  D'Ánjou MA. Clin Tech Sm Anim Pract 2007; 22: 104.

5.  Cornillie P, et al. Anatomia, Histologia, Embryologia: Journal of Veterinary Medicine 2005; Series C 34: 364.

6.  Hunt GB, et al. Vet Surg 1998; 27(3): 203.

7.  Zwingenberger AL, et al. Vet Radiol Ultrasound 2005;46(6): 472.

8.  Crusco F, et al. Br J Radiol 2007; 80: 216.

9.  King AD, et al. Br J Radiol 1998; 71(841):20.

10. Kyles AE, et al. J Am Vet Med Assoc 2003;223:654.

11. Sun F, et al. J Am Anim Hosp Assoc 2002; 38(5):542.

12. Cuevas C, et al. Curr Probl Diagn Radiol 2006; 35: 90.

13. Stein PD, et al. N Engl J Med 2006; 354:2317.

14. Raptopoulos V, et al. Radiology 2001; 221: 606.

15. Bhalla S, et al. Eur J Radiol 2007;64(1):54.

Speaker Information
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Gabriela Seiler, DMV, DACVR, DECVDI
University of Pennsylvania
Philadelphia, PA


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