CHAPTER 115 Computed Tomographic Angiography of the Lower Extremities

Computed tomographic angiography (CTA) is at the forefront of noninvasive assessment of the peripheral arteries (CTA runoff).¹ The advantage of CTA over catheter angiography is the absence of complications attributed to its more invasive predecessor, such as pseudoaneurysm and arteriovenous fistula.² In addition, CTA requires a shorter stay for the patient, is less costly than catheter angiography, and has the potential to be more cost-effective than magnetic resonance angiography (MRA).³

CTA can assess the entire arterial tree from the aortic arch to the toes, if required, potentially making it a “one-stop” examination for extended field of view arterial illustration. With multidetector CT, large volume of coverage and high spatial resolution can be dually achieved, and the speed of acquisition allows reduction in the volume of iodinated contrast agent. CT can assess the arterial wall and the arterial lumen, making it an invaluable evaluator of atherosclerotic plaque burden and aneurysm morphology; this yields information beyond the lumen of the artery that can assist preoperative determination of suitability of vessels for grafting.

With the aid of postprocessing tools now available, CTA is able to provide the interpreting physician and vascular surgeon with information concerning the site of obstruction, extent of vascular disease, and strategy for the appropriate type of vascular intervention.⁴

The availability of CTA and the relative simplicity of its operation in comparison to MRI have led to the rapid adoption of CTA for noninvasive imaging for peripheral vascular disease. However, CTA has limitations and hazards. It involves the use of ionizing radiation and iodinated contrast agent, which are not appropriate in certain populations. In addition, there are inherent limitations to the examination itself; in such situations, MRA or catheter angiography is required to provide the clinical information.

TECHNICAL REQUIREMENTS

An appropriate CTA protocol must provide both large field of view coverage and spatial resolution optimized for maximal contrast agent bolus enhancement of the arterial tree while minimizing ionizing radiation exposure and contrast agent dose.¹

The individual requirements are as follows:

• Multidetector CT (4 channels or more)

• Contrast agent with appropriate concentration of iodine and a dual injector. Typically, the concentrations used are 320 to 370 mg of iodine per milliliter.

• Software that allows optimal timing of the acquisition with respect to the contrast agent injection

• Postprocessing workstation

• Storage such as PACS (picture archiving and communication system)

Preparation of the patient involves appropriate hydration, and local policies with respect to prevention of contrast-induced nephropathy should be followed. Intravenous access must be of sufficient diameter (usually 20-gauge intravenous catheter) to allow rapid injection of iodinated contrast agent, typically around 4 mL/sec. Peripheral intravenous access is encouraged; the use of central catheters should strictly adhere to the manufacturer’s guidelines with respect to the pressure limitations and maximum injection rates.

TECHNIQUES

Indications

• Systemic arterial disease due to atherosclerosis (Figs. 115-1 and 115-2)

• Nonatherosclerotic arterial disease (Buerger disease, vasospastic disease)

• Fibular graft vascular assessment ⁵

• Trauma (Fig. 115-3)⁶

• Vascular masses and malformations ⁷

• Congenital

• Acquired: aneurysm and pseudoaneurysm, arteriovenous fistula

• Compression of artery

• Vascular invasion by musculoskeletal tumors (Fig. 115-4)⁸

• Cystic adventitial disease ⁹

• Popliteal entrapment syndrome ¹⁰

• Venous disease (Fig. 115-5)

FIGURE 115-1 Sagittal (A) and axial (B) images show occlusion of the left superficial femoral artery (A and B, arrows), the superior aspect of which has a meniscal appearance (A, arrowhead), suggesting an acute or an acute on chronic event. CTA runoff is useful in the evaluation of patients with acute and chronic ischemic symptoms.

FIGURE 115-2 CTA runoff in patient with atherosclerotic peripheral vascular occlusive disease. Reformatted coronal maximum intensity projection (MIP) image of the calves shows segmental high-grade narrowing or occlusion in the mid and distal anterior tibial arteries (white arrows) and peroneal arteries (arrowheads) with relatively disease free posterior tibial arteries (yellow arrow). In this patient, the posterior tibial arteries were patent in their entirety and thus would be the preferred choice for distal attachment of a bypass procedure for a mid superficial femoral artery occlusion.

FIGURE 115-3 A, Pelvic CTA. Note the bright contrast in the left common iliac vein. B, Volume rendering of the pelvis demonstrates a large arteriovenous fistula system (arrow) between the femoral artery and femoral vein.

(From Fleiter TR, Mervis S. The role of 3D-CTA in the assessment of peripheral vascular lesions in trauma patients. Eur J Radiol 2007; 64:92-102.)

FIGURE 115-4 Extremity CTA of a 30-year-old man with a left fibular giant cell tumor. A, Three-dimensional volume rendered image demonstrates vascular supply of the proximal fibular mass (asterisk). Note early venous filling as a result of hypervascularity of the malignant giant cell tumor (arrow). Bones are removed on the side of the giant cell tumor. B, Coronal MIP image demonstrates vascular supply of the mass from the popliteal and anterior tibial arteries (arrows). C, Axial MIP image shows arterial supply of the mass from the popliteal (long arrow) and anterior tibial (short arrow) arteries.

(From Karcaaltincaba M, Aydingoz U, Akata D, et al. Combination of extremity computed tomography angiography and abdominal imaging in patients with musculoskeletal tumors. J Comput Assist Tomogr 2004; 28:273-277.)

FIGURE 115-5 Axial (A) and coronal (B) images of the calf show dilated subcutaneous veins (arrowheads) in this patient with varicose veins. Venous disease is frequently discovered in patients imaged for peripheral ischemia.

By far, the most common indication is peripheral arterial disease (also known as peripheral arterial occlusive disease) in both the acute and chronic setting and in patients for follow-up and surveillance after surgical or percutaneous revascularization (Figs. 115-6 and 115-7).

FIGURE 115-6 Axial (A) and reformatted sagittal (B) images show an occluded left limb of the aortobifemoral graft (arrows). CTA is a useful modality for postoperative surveillance.

FIGURE 115-7 Multiplanar reformatted images in this patient with a right popliteal to distal posterior tibial graft (arrow; A, sagittal; B, proximal coronal; C, distal coronal; D, axial). The graft is patent, other than narrowing in its distal aspect (C, arrowhead). The right calf is edematous. The native posterior tibial artery is occluded. A rim-enhancing collection (E, arrowhead, axial view) posterior to the knee joint in the vicinity of the proximal anastomosis is suggestive of a graft infection.

Trauma is an increasing indication for imaging, and CTA is readily accessible. CTA is able to assess coexisting injuries of neighboring and distant organs, making it preferable to conventional angiography in this setting. Notably, CTA is able to detect a host of vascular complications, such as hematoma, pseudoaneurysm, vascular compression, intimal tear, and vasospasm.

Assessment of the crural vessels for vascular variants is of importance in patients who may require fibular transfer, such as in complex craniofacial surgery, and thus recruitment of the peroneal artery.

Because of its limited ability to provide dynamic vascular information and soft tissue contrast, it may not be the best modality for the assessment of a vascular mass or popliteal entrapment syndrome. MRA and ultrasonography are more appropriate in these circumstances. Ultrasonography is more appropriate for the assessment of venous disease, which nonetheless may be encountered in surprisingly high frequency among those referred because of peripheral ischemia.

Contraindications

Renal Impairment

The ultimate decision to administer an intravenous contrast agent is a shared one between the interpreting physician and the referring physician after informed consent of the patient in the event of renal impairment. The timing of dialysis in relation to the time of the scan is of importance. MRA is an option and may have to be performed without intravenous gadolinium-chelate contrast agent if the patient is at risk for development of nephrogenic systemic fibrosis (e.g., glomerular filtration rate less than 30 mL/min/1.73 m²).¹¹

Contrast Allergy

It is important to determine if there is a true allergy to nonionic iodinated contrast agent and the nature of the allergic reaction. Steroid preparation can be given for those with documented history of mild to moderate contrast agent reactions.

Orthopedic Hardware or High-Attenuation Object

Although it is not technically a contraindication, the artifact resulting from a high-attenuation object ¹² may obscure the artery of interest and limit the utility of the examination. Catheter angiography may be required because MRI may also be inflicted with significant artifact in such situations (Fig. 115-8).

FIGURE 115-8 Reformatted coronal (A) and axial (B) images of the right calf in this patient with a gunshot wound, comminuted tibial-fibular fracture, and suspicion of a vascular injury. The CT images are degraded by high-attenuation artifact from the ballistic fragments, which is causing considerable streak artifacts. Under these circumstances, catheter angiography may be more appropriate.

Radiation-Sensitive Population

The risk of radiation-induced carcinogenesis must not be forgotten, and the principles of ALARA (as low as is reasonably achievable) should be respected in terms of radiation dose. The risk/benefit of CTA in the younger patient should be evaluated carefully.

Technique Description

Technique involves image acquisition and contrast agent administration.

Image Acquisition

Field of View

The field of view must answer the clinical question and provide information about associated vascular pathologic changes that would affect management. In the case of peripheral arterial occlusive disease, such as in patients with claudication, this involves the assessment of the abdominal aorta to assess the inflow and to exclude coexisting abdominal aortic aneurysm (Fig. 115-9).

FIGURE 115-9 Frontal and lateral scout topograms show the extended field of view in a typical CTA runoff study that is able to visualize the vasculature from the celiac artery to the toes. The lateral topogram ensures that the feet are included.

An initial image through the abdomen and pelvis before the administration of the contrast agent is followed by arterial phase images from the celiac artery origin to the toes in a single acquisition. An immediate second acquisition is prudent through the calves, particularly with the newer scanners, in which outrunning of the contrast bolus can be a problem. Venous phase images are not routine and are obtained as clinically necessary. Breath-hold is required for the acquisition through the abdomen and pelvis.¹^,¹³

It is important to keep the patient’s knees and legs together, close to the isocenter. Excessive plantar flexion can erroneously depict occlusion of the dorsalis pedis and should be avoided.¹⁴

Scanning Protocols

The acquisition parameters depend on the number of detectors and are vendor specific (Tables 115-1 and 115-2). Regardless, the scan time should be such that the entire arterial tree of interest is covered in a reasonable time during which the arteries remain maximally opacified (i.e., in the first and single pass of contrast bolus). With the latest multidetector CT scanners, this does not involve a tradeoff between spatial resolution and z-axis coverage. With the 4-detector CT, a choice usually needs to be made, as will be explained.

TABLE 115-1 CT Runoff: Typical Protocol for 16-Detector Scanner