Optimal Utilization of Magnetic Resonance Imaging Javier Sanz, Zahi A Fayad and Valentin Fuster
Commonly available modalities for carotid disease imaging include duplex ultrasonography (DUS), MRI, computed tomography angiography (CTA), and digital subtraction angiography (DSA). The use of positron emission tomography for the depiction of plaque inflammatory activity 5 offers great potential, but to date remains an investigational application. Although each of these modalities is reviewed in detail in other chapters of this book, the discussion of strengths and weaknesses of MRI is only meaningful in comparison with other techniques. DUS is by far the most widely employed imaging modality, and the screening method of choice. DUS is relatively inexpensive, portable, and completely non-invasive, it can be repeated as often as necessary, and it has good diagnostic accuracy for the detection of significant stenoses. However, DUS evaluation is restricted to a limited portion of the carotid system, image quality is not always optimal, and reproducibility is limited because of patient-, equipment-, and operatordependent factors. Thus, there is substantial variability in the performance of the test amongst different centers. 6 For many years DSA has been the confirmatory imaging modality before surgical intervention, and still today it is considered the reference standard for the detection and quantification of carotid stenoses. The invasive nature of DSA constitutes its most obvious limitation: neurological complications (including a small risk of mortality) occur in 1.3-1.6% of cases, plus additional vascular complications resulting from arterial cannulation. 7 Moreover, DSA involves ionizing radiation and nephrotoxic contrast agents, arterial access may be limited in patients with extensive atherosclerotic disease, and it is expensive and relatively lengthy. Not surprisingly, patients experience DSA as more stressful and painful than MRA, and are more willing to undergo repeated
testing with the latter. 8 Interestingly, the validity of conventional DSA as a reference standard has also been questioned, as discussed below. CTA represents a robust alternative to DSA, particularly with newer generations of multidetector spiral scanners. CTA can provide complete threedimensional (3D) coverage of the carotid tree and is highly accurate in detecting luminal stenoses, 9 and preliminary data suggest a potential role in the characterization of non-calcified plaque composition. Although almost completely noninvasive, CTA shares some of the limitations of DSA, namely radiation exposure and risk of contrast-induced nephrotoxicity. CTA is excellent for the depiction of calcified lesions, but extensive calcification may limit the ability to detect and quantify luminal stenoses.