Applied Radiology

Dennis Foley, MD
Director, Section of Digital Imaging, Medical College of Wisconsin, Milwaukee, WI

CT angiography (CTA) of the extremities has proven to be extremely useful for evaluating vascular anatomy and patency, demonstrating the source of limb ischemia, and aiding in endovascular and surgical planning. One reason for the success of CT angiography is that, unlike other imaging techniques, it is capable of demonstrating arteries, including lumen, thrombus, and calcification; skeletal tissue, and soft tissue in detail, and in relation to one another.

Figure 1 provides an example of the remarkable detail that can be achieved with CTA, in this case, in an upper-extremity study. The 3-dimensional volume rendering in Figure 1A demonstrates the thoracic outlet in a young person who has had a cervical rib resection, with bone detail provided as background. Figures 1B and 1C show the right subclavian artery in a curved planar reformation of the data, with the patient in the neutral position and the Adson maneuver, respectively. Not only do these CT images together demonstrate the vascular, skeletal and soft tissues in detail; they do so relatively dynamically.

Like other CT applications, extremity arteriography has benefited from impressive advances in multidetector helical scanner technology. The introduction of first 4-, and now 8- and 16-detector channels, along with improvements in scan rotation speeds, have made it possible to image the vasculature more quickly and with thinner slices, thus improving both temporal and spatial resolution. ¹

Taking advantage of the advanced capabilities of new, faster scanners requires that adjustments be made in scanning parameters and contrast administration. This article will provide a brief overview of some of the ways that contrast injection protocols and scanning methods can be modified to achieve high-quality extremity CTA.

The protocols described here represent projections only, rather than protocols that we use in everyday practice. They are designed to conform to a basic principle that holds true of angiographic imaging in general, that of maintaining high intra-arterial iodine concentrations, so that attenuation is maintained at about 300 HU from the distal aorta all the way through to the feet.

Lower-Extremity CTA

The lower extremities can be evaluated well with a detector collimation of 2.5 mm. Figure 2 is an example of lower-extremity CTA covering the length of the femoral arteries to the popliteal arteries at the knee. Figure 3 shows a subsequent lower-extremity CTA in another patient, who was scheduled to undergo surgery to donate a fibula as part of mandibular reconstruction for head and neck cancer. This 3-dimensional volume rendering demonstrates the circulation below the knee, with bone detail present. It is possible to identify the tibioperoneal vessels and the pedal arches to the level of the intermetatarsal vessels.

While it is easy to perform CTA below the knee and retain the bone in the image, lower-extremity CTA presents other, more sophisticated technical challenges. General acceptance of lower-extremity CTA may hinge on the efficiency of accomplishing 3 tasks: bone segmentation below the knee, vessel tracking, and stenosis sizing throughout the lower-extremity circulation.

Certain automated techniques may help in accomplishing that goal. Automated vessel analysis, for example, uses center-line tracking and edge detection, and can do a superb job of stenosis sizing. It excels not only in determining the diameter of the vessel but also area reduction. This technology can be effectively applied throughout the vascular system, particularly in the lower extremities.

Contrast Administration

In extremity CTA, the issues of injection rate, injection duration, and acquisition interval are the same as for CTA in other parts of the body. It is important that image acquisition be timed to correspond with the first circulation of contrast; that the duration of the acquisition interval match that of the injection interval; and that the intra-arterial iodine concentration produce an arterial attenuation of approximately 300 HU throughout the full sequence of the acquisition.

Vascular coverage depends on several factors, including the number of detectors, detector collimation, beam width, pitch, and scan rotation speed. In general, CT angiography is optimized by coupling the thinnest possible image slices with the table speed needed to cover the targeted cephalocaudad dimension during the first circulation of a bolus of contrast media.

Table 1 demonstrates how the details of contrast injection and scanning may vary with scanner configuration. Starting with the assumption of a cephalocaudad coverage of 120 cm, the schematic provides examples of how that coverage could be accomplished using a 4-channel scanner with a 0.8-second scan rotation speed; a 4-channel scanner with a 0.5-second scan rotation speed; and an 8-channel scanner with a 0.5-second scan rotation speed.

Consider a modern 4-channel scanner. At a scan rotation speed of 0.5 sec, a table speed of 15 mm per rotation produces a coverage speed of 3 cm/sec. Combined with a coverage requirement of 120 cm, this results in an acquisition interval of 40 seconds. As a result, the injection rate at which 160 mL of contrast is delivered must drop from the standard 5 mL/sec to 4 mL/sec in order to lengthen the injection interval in accordance with the relatively long acquisition interval.

When scanning on an 8-channel system, detector collimation can be reduced by half, from 2.5 mm to 1.25 mm, which will increase z-axis resolution. Because the beam width remains the same as with a 4-detector system, the acquisition interval remains the same, as does the injection interval. An alternative in using an 8-channel system that is not shown in Table 1 involves maintaining detector collimation at 2.5 mm. By doubling imaging speed in this way, it may be possible to perform lower-extremity CTA with less than 100 mL of contrast material, while at the same time covering all the way from the renal vascular pedicle to the feet.

Higher-concentration contrast media with an iodine concentration of 370 mg/mL is useful in achieving higher intra-arterial iodine concentrations and thereby improving the definition of stenoses. I anticipate that the use of higher-concentration contrast will prove superior for lower-extremity arteriography because it should maintain a high intra-arterial iodine concentration throughout the study. *