Coronary Vessel Wall Imaging using MRI

Imaging of the coronary artery vessel wall is probably the most challenging task in cardiac MRI because of the small dimension and constant motion of the coronary arteries. Simultaneously, the need for high contrast between the coronary lumen blood pool and the surrounding coronary vessel wall is mandatory. However, this is very similar to the challenges faced by coronary MRA in general.

The first successful implementation of coronary vessel wall imaging in humans involved the use of a dual-inversion fast spin echo sequence. Using this method, single slices of the coronary artery wall could be acquired during a prolonged breath-hold period, permitting the demonstration of relative wall thickening in selected cases [59]. Subsequently, and to overcome the limitations associated with breath-holding, this technique was adapted for use with navigators for free-breathing data acquisition [76]. More recently, the free-breathing navigator approach has been combined with 3D spiral imaging in conjunction with a 'local inversion' technique [81]. Using this method, excellent image quality can be obtained because of the high SNR associated with 3D imaging on the one hand and the signal-efficient spiral read-out on the other. This enables larger anatomical coverage with much thinner reconstructed slices than those of the earlier 2D approaches. Therefore, it is now possible to visualize long, contiguous sections of the coronary artery vessel wall as shown in Fig. 6. Additionally, the spiral approach permits data

Coronary Vessel Wall Imaging

acquisition within a short acquisition window of only 50 ms, permitting the effects of intrinsic my-ocardial motion to be suppressed more effectively while at the same time rendering the technique less susceptible to R-R variability. A disadvantage of the technique is a prolonged scanning time of ~ 12 min during free breathing with image acquisition during alternate R-R intervals. Preliminary evaluation of this local-inversion 3D spiral technique has been performed in 12 adult subjects comprising 6 clinically healthy subjects and 6 patients with non-significant coronary artery disease (10% to 50% diameter reduction on x-ray angiography). Examinations were performed on a commercial 1.5 Tesla scanner with free-breathing 3D coronary vessel wall imaging performed along the major axis of the right coronary artery with isotropic spatial resolution (1.0x1.0x1.0 mm3). The proximal vessel wall thickness and luminal diameter were objectively determined with an automated algorithm [32]. The 3D vessel wall scans allowed for visualization of the contiguous proximal right coronary artery in all subjects. The mean vessel wall thickness (1.7 ± 0.3 versus 1.0 ± 0.2 mm) was significantly increased in the patients compared with the healthy subjects (p<0.01). However, the lumen diameter measurement (3.6 ± 0.7 versus 3.4 ± 0.5 mm, p=0.47) was similar in both groups. The findings suggest that free-breathing 3D black-blood coronary MRI may serve as an appropriate non-invasive technique for the identification of increased coronary vessel wall thickness with preservation of lumen size in patients with non-significant coronary artery disease, consistent with "Glagov-type" outward arterial remodeling [3]. This novel approach may have the potential to quantify sub-clinical disease. Future developments will include the use of higher magnetic field strengths, contrast agents for plaque characterization [71, 72], and studies of vessel wall thickness following intervention [82].

Fig. 6a, b. Coronary vessel wall imaging in a healthy adult subject. The SSFP image (a) of a right coronary artery (arrow) was used as a scout scan for (b) the local-inversion 3D spiral navigator-gated and corrected coronary vessel wall (arrowsin b). A 5-6 cm segment of the coronary vessel wall is clearly seen and high visual contrast between the coronary lumen and the surrounding vessel wall is apparent. The spatial resolution is 0.8x0.8x1 mm and the scanning time during free-breathing is ~12min

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