This prospective, single-center study demonstrated high diagnostic performance of 3.0T contrast enhanced whole-heart coronary MRA using 32-channal cardiac coils in intermediate symptomatic patients for the detection of obstructive CAD. The patient-based sensitivity and specificity of whole-heart coronary MRA with 32-channel coils in the detection of significant stenoses were 95.9% and 86.5%, respectively. Thus, our data establish coronary MRA at 3.0T using a 32-channal cardiac coil as a robust technique to perform noninvasive and radiation-free coronary angiography.
To the best of our knowledge, this is the first study in which 32-channel cardiac coils, 3.0T MRI system, and highly accelerated parallel imaging were combined to perform whole-heart coronary MRA. A very recent study has investigated the use of 32-channel coils to perform whole-heart coronary MRA at 1.5T with high parallel imaging factors12
. In their study, substantially shortened imaging time and high study success rate were observed. Nevertheless, it is known that the straightforward application of parallel imaging at field strengths of 1.5T dramatically reduces acquisition time, but at the cost of SNR. The baseline SNR gain at 3.0T can be used to tradeoff spatial and temporal resolution of coronary MRA with parallel imaging13, 14
In our study, whole-heart coronary MRA was obtained with a substantially reduced acquisition time (7.0 ±1.8 min) compared to previous studies without using 32-channel coils15
. Reduced acquisition time can be translated into an improvement of in-plane and through-plane spatial resolution, which resulted in an improved delineation of distal segments of the coronary arteries. In most of the previous coronary MRA studies, evaluation was limited to branches having a diameter >2 mm15, 16
. We evaluated all segments being >1.5 mm in diameter, while only 6.5% of the coronary segments were non-diagnostic, significantly lower than findings of our previous study (12%)7
. In addition, the time savings improves the clinical throughput of coronary MRA and potentially decrease patient discomfort resulting from long measurements. The resulting measurable improvements of coronary MRA using 32-channal cardiac coils in image quality are likely to translate into more stable, more diverse, and more widely accepted clinical applications.
MSCT has emerged as a rapid and noninvasive tool for the detection of CAD and numerous methods for dose reduction have been developed recently17, 18
. However, the penetration of these techniques into widespread clinical practice has not yet established, and current guidelines recommend a heart rate of than 60 beats per minute both for optimal image quality and for reduction of radiation exposure19
. MRI is the most promising cardiac imaging test due to its unique advantage of not requiring radiation exposure, allowing concurrent assessment of myocardial structure, function, myocardial edema, fibrosis, and coronary arteries in a single setting. In our study only patients with heart rate higher than 75 beats per minute received an oral beta-blocker before the scan. Slow heart rates relatively prolong cardiac phases with little cardiac motion, so a data acquisition window of patients with high heart rates can be safely set to allow artifact-free imaging. The high sensitivity (95.9%) of coronary MRA for CAD shown in our study is comparable to the sensitivity of 64-slice CT studies performed in multicenter trials1, 20
. The specificity (86.5%) is on par with magnetic resonance myocardial perfusion imaging, whereas the diagnostic sensitivity and NPV are higher21
. The NPV was 98.4%, 97.4%, and 95.7% on per-segment, per-vessel and per-patient basis, respectively, indicating that this technique can reliably rule out significant stenoses, consistent with findings from previous studies7, 15, 16
The low PPV (50.8%) on segment basis is explained for the most part by the non-assessable segments on coronary MRA. We did not exclude these segments from the analysis but tended to grade these lesions as having a significant obstruction. Our coronary stenosis grading policy is based on the premise that patients with either positive coronary MRA results or non-assessable segments will undergo QCA in an intention-to-read approach. Because of this, coronary MRA is not ready to challenge invasive coronary angiography as a true alternative.
Nevertheless, developments in parallel imaging22, 23
and multi-channel phased-array coils may further reduce the imaging time24, 25
, this will have a considerable impact on improving spatial resolution and image quality due to inconsistent cardiac and respiratory motion. Thus, if the diagnostic performance of coronary MRA can be further improved, this test may become the most important imaging tool for noninvasively and comprehensively assessing patients with suspected CAD. Whole-heart coronary MRA at 3.0T has great potential to become a valuable complement to other non-invasive imaging modalities if current limitations, such as navigator failure rate and low spatial resolution, can be overcome.
There are several limitations that need to be acknowledged in this study. The capability to perform cardiac function, perfusion, and viability, as well as coronary imaging in the same setting for a comprehensive exam of CAD is a major strength of cardiac MR imaging. The need for contrast agent in both coronary MRA and cardiac perfusion scans will either lead to increased total contrast dose, or decreased dose from optimal value for coronary MRA and/or perfusion scans. The performance of coronary MRA with further reduced contrast dose is not yet established. Second, coronary MRA could not be acquired from about 8% of the patients due to unstable breathing patterns or poor ECG signal. Not including all subjects may result in overestimation of the diagnostic accuracy of coronary MRA. Future studies are needed to define the method’s precise role in the diagnostic algorithm for the evaluation of patients with suspected CAD in multi-center trials. Third, despite the use of partial Fourier acquisition and parallel imaging, it is desirable to further improve the imaging speed with advanced acceleration and reconstruction techniques without compromising image quality. Finally, to acquire consistent coronary MRA images requires highly attentive and experienced operators on the setting of timing and imaging parameters. Broader utilization and acceptance of coronary MRA could be improved by simplifying or automating the protocol settings.
In conclusion, among patients who were scheduled to obtain conventional X-ray coronary angiography, we found that coronary MRA at 3.0 T using 32-channal cardiac coils demonstrates high accuracy for detection of significant coronary artery stenosis. The high NPV (95.8%) establishes coronary MRA as an effective noninvasive method to rule out significant coronary artery stenosis without exposure to ionizing radiation. The speed advantage and extra diagnostic value afforded by 32-channel cardiac coils at 3.0 T may be expected to drive future technological developments of more robust and reliable coronary MRA.