During the past decades, a considerable amount of effort has been invested to achieve noninvasive visualization of the coronary arteries and bypass grafts with CMR. The larger size, straight course, and immobility during the cardiac cycle allowed the evaluation of graft patency even in the earliest studies, while assessment of the coronary arteries could not be achieved. In these initial investigations, two-dimensional (2D) spin-echo and gradient-echo techniques were applied to acquire successive axial slices during repetitive breath-holds (1,2,3,4,5,6,7 and 8). With the spin-echo technique the presence of blood flow is depicted as an absence of signal. If absence of signal at different levels of the bypass graft occurred, the graft was considered to be patent. In contrast, flowing blood is depicted as a bright signal during imaging with gradient MR techniques. As shown in Table 23.1, both acquisition techniques have been evaluated in several studies with conventional angiography as the standard of reference, demonstrating sensitivities and specificities varying from 71% to 100% and 88% to 100%, respectively. Pooled analysis of these eight studies (with 180 patients and 381 grafts) revealed a weighted mean sensitivity and specificity of 81% and 94%, with inclusion of 95% of grafts. Despite these promising results in the distinction between patent and occluded grafts, these 2D techniques were still limited by their low signal-to-noise ratio and low spatial resolution.

Substantial progress in image quality was achieved by the development of three-dimensional (3D) imaging techniques, allowing the acquisition of volume slabs with several thin slices, which resulted in a higher spatial resolution. To improve patient comfort, navigator techniques have been developed that permit real-time monitoring of diaphragm motion and free-breathing during data acquisition. In addition, improved enhancement of blood/muscle contrast can be expected by the administration of intravascular contrast agents. An example of a typical MR angiography (MRA) acquisition protocol with navigator respiratory gating is depicted in Figure 23.1. In Figure 23.2 an example of a patent, nonstenosed vein graft as confirmed by conventional angiography is provided. Pooled analysis of nine studies using 3D techniques, with over 200 patients included, revealed a slight increase in weighted sensitivity from 81% to 85%, with no loss in specificity (9,10,11,12,13,14,15,16 and 17) (Table 23.1). No difference in diagnostic accuracy for the evaluation of graft patency was noted between arterial and vein grafts. The sensitivity and specificity of arterial grafts were 85% and 95% as compared to 86% and 93% in vein grafts, respectively (10,11,13,14,15,16 and 17). An initial study was performed to assess graft patency postoperatively with MRA at 3 Tesla (T) (18). MRA confirmed all grafts to be patent; however, no comparison with invasive angiography was made. An example of MR angiogram at 3 T from this study is shown in Figure 23.3. These studies illustrate the potential of MRA to evaluate graft patency in clinical routine. Its safe and noninvasive nature in combination with the high specificity (˜94%) and negative predictive value (˜96%) suggests that in patients presenting with recurrent complaints after bypass grafting, MRA may function as a first-line investigation tool to rule out graft occlusion prior to more invasive diagnostic procedures.

In addition to assessment of patency, few attempts have been made to evaluate graft stenosis. A sensitivity and specificity of 82% and 88% was observed for the detection of >50% luminal narrowing in vein grafts (15). The number of evaluable grafts was 50 of 56. In contrast, a discouraging sensitivity of 38% was reported (13). Evaluation of graft stenosis remains difficult due to the presence of metallic clips, stents, or calcifications in the course of the graft, which may cause signal voids. Since the presence of graft stenosis rather than occlusion may be the cause of recurrent complaints and the remaining nongrafted coronary vasculature cannot be evaluated in sufficient detail in a single MR session at this stage, the value of MRA to evaluate graft stenosis needs to be further explored before this technique can be recommended for routine use.

Flow through a blood vessel may or may not be impaired by a stenosis, visualized by angiography (19,20). The assessment of blood flow through coronary arteries and bypass grafts has gained wide attention. To evaluate the hemodynamic impairment of a lesion, flow is measured at rest and during pharmacologically induced stress with adenosine or dipyridamole (21). By dividing the flow value during stress by the flow value at rest, the coronary flow reserve (CFR) is calculated (22,23). Flow-limiting stenoses cause a compensatory vasodilation at rest to maintain sufficient blood flow to the myocardium. As a consequence, the blood vessel cannot respond adequately to an increase in absolute flow by vasodilation during stress and CFR will be reduced. At first, blood flow was determined by Doppler flow transducers at open-chest procedures (19,22,23,24,25 and 26), limiting extensive use of CFR in clinical practice. When the diameter of intravascular catheter-based Doppler ultrasonographic devices could be reduced to 0.018 in., it became feasible to measure the velocity of the blood flow and calculate the coronary flow velocity reserve (CFVR) for coronary arteries in patients during catheterization. Blood flow correlated well with Doppler-derived velocity of blood flow both in vitro and in vivo (27,28 and 29). Invasive Doppler-derived CFVR has proven its potential in numerous clinical applications, such as in identifying hemodynamic significant stenoses in native coronary arteries and vein grafts (30,31), in the functional assessment of stenoses of intermediate severity (32), in the determination of the need for and the outcome after coronary intervention (33,34 and 35), and in the prediction of restenosis (36).

With the use of CMR, blood flow velocity can be measured using phase-contrast, velocity-encoded sequences (37). To use such a sequence accurately, an imaging plane is planned perpendicular to the target vessel. Throughout the cardiac cycle, the acquisition yields anatomic modulus images paired with phase images, in which every pixel contains a different velocity value. Volume flow (mL/min) can be obtained by calculating the integrated volumetric flow rate of all pixels in the vessel lumen per heartbeat and multiply it with the heart rate. To display the flow pattern, a flow rate-versus-time graph is drawn. Alternatively, the central peak velocity (cm/sec) can be obtained by selecting several pixels in the vessel center. Figure 23.4 depicts a typical example of a CMR examination with velocity mapping in a vein graft.