Imaging modalities for peripheral arterial disease (PAD) cover the gamut from to noninvasive approaches to invasive digital subtraction angiography (DSA), which historically has been the reference standard. The advent of percutaneous revascularization has seen shift in imaging strategies to predominantly noninvasive approaches that can provide adequate visualization of arterial stenoses to allow accurate treatment planning. Because of the arterial puncture, DSA requires postprocedural monitoring and has a 2% to 3% major complication rate.1,2 The advent of computed tomography (CT) and magnetic resonance angiography (MRA) has relegated DSA to an adjunctive role that may occasionally be required in some patients at the time of percutaneous revascularization. In this chapter, we will discuss the application of CT technology for the diagnosis of vascular disease in the specific vascular beds (extremities, extracranial vasculature, and thoracic and abdominal aorta).
Current generation CT scanners employ the so-called "rotate/rotate" geometry, in which both x-ray source and detector are mounted onto a rotating gantry and rotate around the patient (Figure 19-1). Key requirement for the mechanical design of the gantry is the stability of both the x-ray source and detector position during rotation, in particular with regard to the rapidly increasing rotational speeds of modern CT systems (0.33 s for 64 slice systems). In a multidetector CT system (MDCT), the detector comprises several rows of 700 and more detector elements, which cover a scan field of view of usually 50 cm. The x-ray attenuation of the object is measured by the individual detector elements. Each detector element consists of a radiation-sensitive solid-state material (such as cadmium tungstate, gadolinium oxide, or gadolinium oxisulfide with suitable dopings), which converts the absorbed x-rays into visible light. The light is then detected by a silicone photodiode. The resulting electrical current is amplified and converted into a digital signal. The detectors laid out in the z-axis are an important determinant of volume coverage (along with other factors, see factors below). The "volume concept" pursued by GE, Philips, and Toshiba aims at a further increase in volume coverage by increasing the number of detectors (256 and beyond) without changing the physical parameters of the scanner compared to the 16-slice version. The "resolution concept" pursued by Siemens uses a set number of physical detector rows in combination with a oscillating x-ray source, enabled by a periodic motion of the focal spot in the z-direction, to simultaneously acquire twice the number of overlapping slices as the number of detectors using an overlapping approach (Figure 19-2).
Crossectional view of the inside of a modern CT gantry.
Reproduced with permission from Debabrata M, Rajagopalan S, eds. CT and MR Angiogrpahy of the Peripheral Circulation: Practical Approach With Clinical Protocols. 1st ed. Boca Raton, FL: Informa Healthcare, Taylor and Francis; 2007.