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Commentary |
1 Department of Radiology, MetroHealth Medical Center, 2500 MetroHealth Dr., Cleveland, OH 44109-1998.
Received September 25, 2007; accepted after revision September 27, 2007.
A. J. Duerinckx is on the CT advisory board of Philips Medical Systems.
Keywords: coronary angiography • coronary arteries • coronary artery disease • coronary artery stenosis • MDCT angiography • MR angiography
Targeted coronary MR angiography (MRA) offers hope when encountering the very common clinical problem of a "blind spot" on coronary CT angiography (CTA) due to a focal calcified lesion with a calcium score of > 100 based on a preliminary study of 33 lesions in 18 patients [1]. In this issue of the American Journal of Roentgenology, an interdisciplinary group of investigators, Liu et al. [1] from Northwestern University in Chicago, IL, report their findings that coronary MRA had a better diagnostic performance than coronary CTA for the detection of significant stenosis in patients with high calcium scores. They base this conclusion on a comparison of 33 calcified plaques with a calcium score of > 100 detected in 18 patients using CTA and MRA and compared with conventional coronary angiography (CAG).
Both the MRA and the CTA examinations performed for the study [1] were state-of-the art (i.e., volume-targeted and whole-heart coronary MRA with 3D slabs and 64-MDCTA), and both are widely available. Nitroglycerine was used for both studies to achieve maximal coronary vasodilation. Liu et al. [1] report the highest coronary MRA image quality in coronary segments with a nodal pattern of calcification, defined as a single arterial segment with three or fewer calcified plaques resulting in a combined calcium score of > 100. Most significant was the improved specificity—fewer false-positives—of MRA over CTA for such lesions.
Why are these findings worth noting today? Compared with CTA, coronary MRA is the older noninvasive coronary imaging technique that has been around for quite some time [2]. It is, however, well accepted that the image quality of coronary MRA is inferior to that of newer techniques, such as 64-MDCTA, because of its limited spatial resolution and increased motion artifact from longer imaging times. Although 64-MDCTA has almost become a routine clinical test to identify significant coronary stenosis, it is limited in its assessment of coronary segments with moderate and severe calcifications. This shortcoming of CTA creates a vexing problem that we encounter often in emergency departments: A patient with a low to moderate pre-test probability for coronary artery disease (CAD) admitted to the emergency department for chest pain undergoes 64-MDCTA and minimal disease is detected except for a single calcified focal lesion (described as a nodal pattern in [1]) with a calcium score of > 100. What is the emergency department's doctor or consulting cardiologist then supposed to do—especially if the calcium score due to this single focal lesion is in the 100–300 range? Trying to evaluate lesion severity in such segments is a guessing game [3], and such segments are often referred to as "blind spots" for coronary CTA. If the calcium score in the focal lesion is very high (i.e., > 300), one should become more suspicious for CAD and probably order another imaging test. However, for focal lesions with a calcium score in the 100–300 range, one is left with a dilemma and the need to make a purely clinical decision about future cardiac risk using all of the available data—medical history, physical examination findings, laboratory results, and ECG data. Confidence in this clinical decision could be increased by performing another potentially expensive and high-risk test, such as a stress test or conventional CAG. The study by Liu et al. [1] offers a third alternative: targeted coronary MRA focused on the one problem lesion to perform a site-to-site comparison with a specific calcific lesion previously detected on CTA.
Targeted coronary MRA is less challenging to perform than whole-heart coronary MRA to detect just any lesion. A targeted MRA examination also may help avoid additional radiation from other tests if MRA results can provide a definite answer. Moreover, chances are very good that targeted coronary MRA findings will help: Liu et al. [1] report that the sensitivity of MRA was similar to that of CTA (0.81 vs 0.75, respectively; p > 0.5), but the specificity was much higher (0.75 vs 0.48; p = 0.002) if using a site-to-site comparison of lesions. The performance of coronary MRA versus CTA in general, obviously does not show the same results as one would expect [4]. Liu et al. [1] do not recommend coronary MRA as an alternative test for detecting significant stenosis in patients with high calcium scores, but their results open the door for future studies about the use of MRA for targeted site-to-site comparison studies after coronary CTA stumbles onto a blind spot caused by a high focal concentration of calcium.
Obviously, additional multicenter studies need to confirm these preliminary findings. Also, in this preliminary study, the authors did not mention other important study factors, such as the average heart rate and heart rate variability, total time duration of the scanning procedure, and associated costs. The results might someday also be validated for diffuse patterns of calcification, although that may be more challenging because of the lesser image quality of coronary MRA in segments with such lesions.
Liu et al. [1] should be commended for presenting such daring and provocative results. Their study forces us to constantly rethink our beliefs in the power of one technique over another and offers hope for a new approach when encountering this common clinical problem of a blind spot on coronary CTA due to a focal calcified lesion with a calcium score of > 100. Coronary CTA is a powerful and promising technique. It offers tremendous information and can help us with many patient management issues, but it is not perfect. The appropriate targeted use of the older technique—that is, coronary MRA—may, if confirmed in future studies, offer an answer for specific types of calcific coronary lesions.
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