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via Medscape Radiology heartwire Whole-Heart Coronary Magnetic Resonance Angiography Advance Shows Promise [Free registration required. Emphases mine].

A study comparing the diagnostic value of contrast-enhanced whole-heart coronary magnetic resonance angiography (MRA) with conventional X-ray angiography has shown that MRA can accurately detect coronary artery stenosis with high sensitivity and moderate specificity. The findings, published online April 15, 2009 in the Journal of the American College of Cardiology by Dr Qi Yang (Capital Medical University, Beijing, China) and colleagues, are from a single center using the 3.0-T MRA, which has a higher signal-to-noise ratio (SNR) than existing 1.5-T MRA machines. As the researchers point out, whole-heart coronary MRA using 1.5-T has demonstrated promising results, particularly for high negative predictive value. “However, long scan time and relatively low spatial resolution have prevented its wide acceptance as a routine test for coronary artery stenosis detection,” write Yang and colleagues. “The 3.0-T systems have the potential to improve SNR by a factor of two as compared with 1.5-T with the same imaging sequence.”

In this study of coronary MRA at 3.0 T, investigators included 96 patients with suspected coronary artery disease scheduled for conventional angiography, and of these, 62 patients underwent successful imaging with MRA. Among the excluded patients, nine had previous stents or bypass surgery, while others were excluded because of claustrophobia, unstable angina, atrial fibrillation, or impaired renal function. Overall, on a per-patient basis, the sensitivity of coronary MRA was 94.1%, with significant coronary artery disease, defined as stenosis >50%, detected in 32 of 34 patients. Coronary disease was correctly ruled out in 23 of 28 patients, for a specificity of 82.1%. The imaging test also correctly identified the presence or absence of stenosis in 55 of 62 patients, which corresponded to 88.7% accuracy.

The acquisition time for the images was nine minutes. Reduced imaging time is important for whole-heart coronary MRA, “as long scan times tend to cause lower image quality from increased motion artifacts and reduced coronary SNR,” note the researchers. The 3.0-T MRA also allowed researchers to improve depictions of distal coronary artery segments and to assess segments with diameters >1.5 mm, compared with 2.0 mm in previous 1.5-T studies. “A major challenge for coronary MRA remains respiration-induced motion artifacts,” the researchers note. “Patient training and practice before data acquisition for maintaining regular breathing should be useful to improve the gating efficiency and image quality of coronary MRA.”

Commenting on the results of the study for heartwire , Dr Raymond Kim (Duke University, Durham, NC) said 3.0-T imaging with MR allows clinicians better image clarity, but “what this papers actually tells us is that we’re not quite ready for prime time with 1.5 T, because at 3.0 T this is becoming more and more usable. The vast majority of MR sites don’t have 3.0 T available, at least for cardiac imaging.” Like the researchers, Kim noted that 3.0 T does have some downsides regarding motion artifacts, although the promise is that these issues can be solved in the future. “As of right now, the study is intriguing, and it suggests that the way to go to improve coronary imaging is to go to higher fields and then to work on some of the limitations related to imaging quality,” he told heartwire. Kim noted that the sensitivity, specificity, and accuracy reported by Yang and colleagues is in line with existing coronary MRA data, but caution should used when interpreting the findings, because 27 patients did not meet the criteria for MRA. Also, seven patients were not scanned successfully. “Like all imaging studies, you have to be very careful where people drop off on the wayside before you get these final numbers,” said Kim. “That’s not just an issue with this study, but with all imaging studies.”

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My personal opinion: (Full disclosure: I perform & interpret Coronary CT Angiograms in a privately owned community hospital. I or my hospital do not have any commercial ties with CT scan manufacturers. We do not have an MRI scanner.)

First things first: Nine minutes acquisition time is insane!! My team can get a patient into the CT gantry room, insert 18G IV cannula, attach ECG leads, prepare and attach the pressure injector, position the patient, do the entire CCTA including Calcium Scoring and get the patient out in about 12 minutes. We could tweak it to 9 minutes if we tried and rushed. I have done 60-odd CCTAs since February 2009, and I’ve never had a scan time of more than 7 seconds.

Next the Exclusion Criteria: Patients who are post-CABG and post-coronary-stenting form a large proportion of referrals to CCTA. These are patients in whom cardiologists prefer CCTA over invasive angiography despite the turf warfare. They are mandatorily excluded from Coronary MRA. There is usually no claustrophobia in CT scans. Even if a patient is apprehensive, their fear(s) can be allayed by saying the entire procedure will be over in a few seconds. Unstable angina is not a contraindication for CCTA. Nor is atrial fibrillation (at least with the newer faster CT scanners). I had one patient with AF whose irregularly irregular pulse rate varied from 66 to 90 beats per minute. I could get good quality images with no motion artifacts with a standard retrospective ECG-gated CT acquisition protocol. Impaired renal function is a total no-no for CCTA. But then so is the case for MRA and catheter angiography. At best we could offer to do Coronary artery Calcium Scoring for those patients.

“Patient training and practice before data acquisition for maintaining regular breathing should be useful to improve the gating efficiency and image quality of coronary MRA.” << Good luck with that one. I know how difficult it is to train, beg, even threaten patients to remain immobile inside the MRI tunnel, leave alone instructing them to maintain a steady regular respiratory pattern.

In this part of the world the truth is: The vast majority of sites do not have MRI. If available, they are usually 0.2 T or 0.3 T permanent magnets.

Speed is one of the two most important criteria when imaging a fast moving organ like the heart, the other being image resolution. CCTA with the newer multislice CT scanners from all major manufacturers meet both criteria. It’s going to take a long time before MRA even comes into the same league.

3.0 T MRI probably has hundreds of other clinically useful applications. It would serve everyone’s purpose if clinical research was done to improve upon those applications instead of trying to reinvent the Coronary Angiography wheel.