Computed Tomographic Angiography of the Lower Extremities

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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.

TECHNIQUES

Indications

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).

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

Technique Description

Technique involves image acquisition and contrast agent administration.

Image Acquisition

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.

Background Principles

Further discussion of this area involves recapitulation of some concepts.

Increasing the pitch results in the following:

A pitch greater than 1 broadens the slice sensitivity profile and in essence increases the effective section thickness.12

The z-axis is along the long axis of the body (i.e., in the direction the images are obtained). Clearly, the greater the number of detectors in the CT scanner, the greater the z-axis coverage in one gantry rotation. For example, if there are 64 detectors, each of width 0.625 mm, the area coverage is 4 cm in one rotation. If the pitch is 1.5 and the gantry rotation time is 500 ms, then 120 cm will be covered in 10 seconds. Assume the same parameters with 256 detectors; the scan time will be 2.5 seconds, which means that the acquisition will almost certainly outpace the contrast agent delivery to the lower extremity arteries. Thus, more detectors are not necessarily more beneficial in CTA runoff and after a certain number can actually be detrimental.

Consider this situation with a 4-detector scanner and a 1-mm individual detector width. It will take 100 seconds to cover 120 cm. This is too long and will result in one or more of the following: increased radiation dose, tube heating effects, increased contrast agent requirement, lower contrast agent administration rate, reduced arterial enhancement, and greater chance of venous enhancement. A choice must be made between the section thickness and the field of view, that is, one might have the entire field of view scanned for the thicker slices (e.g., 2.5-mm slices, which would mean a scan time of 40 seconds) or a smaller field of view scanned for thinner slices.

The optimal number of detectors that can provide sub-millimeter isotropic spatial resolution and coverage in a reasonable time without outrunning the bolus or incurring venous enhancement is 16 detectors. For proper visualization of the suprageniculate arteries in the absence of heavy calcification, a 4-detector scanner with 2.5-mm detector width can suffice. As a general rule, the larger the detector configuration, the lower the scanning pitch and the slower the gantry rotation speed.1

Scanning Parameters
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