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Cardiac MRI identifies nonobstructive disease, diagnoses microvascular angina
Cardiac MRI stress T1 mapping, which does not use contrast agents or radiation, adequately differentiated obstructive epicardial CAD from microvascular dysfunction, researchers reported.
In a separate study by the same team, cardiac MRI noninvasively correctly diagnosed microvascular angina in patients with nonobstructive CAD.
Distinguishing CAD from nonobstructive CAD
The researchers conducted a prospective validation of stress T1 mapping against invasive coronary measurements compared with first-pass perfusion imaging. Obstructive epicardial CAD was defined as fractional flow reserve less than 0.8, whereas coronary microvascular dysfunction was defined as FFR of at least 0.8 and index of microcirculatory resistance of at least 25 U.
Sixty patients with angina and 30 controls underwent cardiac MRI 1.5 T and 3 T to determine left ventricular function, ischemia and infarction. FFR and index of microcirculatory resistance were assessed within 7 days after cardiac MRI.
Normal myocardial T1 reactivity was 6.2% on 1.5 T and 3 T cardiac MRI, Alexander Liu, MBBS, from the Oxford Centre for Clinical Magnetic Resonance Research, division of cardiovascular medicine, Radcliffe Department of Medicine, University of Oxford, and colleagues wrote.
Ischemic viable myocardium downstream of obstructive CAD was associated with virtually no T1 reactivity (0.7%), which was not the case for myocardium downstream of nonobstructed coronary arteries with microvascular dysfunction (3%), the researchers wrote.
Stress T1 mapping outperformed gadolinium-based first-pass perfusion imaging for detecting obstructive CAD (area under the receiver-operating characteristic curve [AUROC], 0.97 vs. 0.91; P < .001), according to the researchers.
Myocardial T1 reactivity of 1.5% was an accurate detector of obstructive CAD (sensitivity = 93%; specificity = 95%; P < .001), whereas myocardial T1 reactivity of 4% was an accurate detector of microvascular dysfunction (AUROC = 0.95; sensitivity = 94%; specificity = 94%; P < .001), Liu and colleagues wrote.
Diagnosing microvascular angina
In the second study, Liu and colleagues validated cardiac MRI for diagnosing microvascular angina in patients with nonobstructive CAD, compared with patients with obstructive CAD, correlated to index of microcirculatory resistance during invasive coronary angiography.
Adenosine stress cardiac MRI 1.5 T and 3 T was administered to 50 patients with angina (mean age, 65 years) and 20 age-matched healthy controls to assess LV function, inducible ischemia, myocardial blood flow and infarction. Of the group with angina, 28 were found to have obstructive CAD and 22 were found to have nonobstructive CAD after angiography within 7 days of cardiac MRI.
Among those with nonobstructive CAD, myocardium with index of microcirculatory resistance less than 25 U was associated with normal myocardial perfusion reserve index (1.9 vs. 2 for controls; P = .49), according to the researchers.
However, those with nonobstructive CAD who had myocardium with index of microcirculatory resistance of at least 25 U had impaired myocardial perfusion reserve index (1.2 vs. 1.2 for obstructive CAD; P = .61).
A myocardial perfusion reserve index of 1.4 accurately detected impaired perfusion as a result of coronary microvascular dysfunction (AUC = 0.9; specificity = 95%; sensitivity = 89%; P < .001), according to the researchers.
The main driver behind impaired myocardial perfusion reserve index in patients with nonobstructive CAD was impaired augmentation of myocardial blood flow during stress, Liu and colleagues wrote.
Patients with FFR greater than 0.8 and index of microcirculatory resistance less than 25 U also had blunted stress myocardial blood flow, and this mild coronary microvascular dysfunction was distinguishable from controls by using a stress myocardial blood flow threshold of 2.3 mL/min/g (100% positive predictive value), the researchers wrote.
Diagnostic potential
In a related editorial, Theodoros D. Karamitsos, MD, PhD, from the first department of cardiology, Aristotle University of Thessaloniki, AHEPA Hospital, Thessaloniki, Greece, wrote that the studies were helped by the inclusion of invasive measurements and “the authors should be commended for performing two important studies that open new frontiers for myocardial ischemia testing.”
“Clinicians frequently face dilemmas as to how to diagnose and manage patients with coronary microvascular dysfunction,” he wrote. “There is no doubt that these novel, noncontrast [cardiac MRI] techniques offer important pathophysiological insights in myocardial ischemia and have a significant diagnostic potential that justifies the conduction of a large-scale study.” – by Erik Swain
Referen ces:
Karamitsos TD. J Am Coll Cardiol. 2018;doi:10.1016/j.jacc.2017.12.047.
Liu A, et al. J Am Coll Cardiol. 2018;doi:10.1016/j.jacc.2017.11.071.
Liu A, et al. J Am Coll Cardiol. 2018;doi:10.1016/j.jacc.2017.12.046.
Disclosures: Liu and Karamitsos report no relevant financial disclosures. Please see the studies for all other authors’ relevant financial disclosures.
Perspective
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Regarding the study of cardiac MRI stress T1 mapping differentiating obstructive epicardial CAD from microvascular dysfunction, this group is the first to use T1 reactivity with vasodilation to study various cardiac conditions. They had previously used it in patients with MI and aortic stenosis, but this is the first application in patients with suspected ischemia. It is quite impressive that one can make these precise quantitative myocardial measurements without using contrast. Should this hold up, this would be a way to avoid the use of gadolinium contrast to perform stress testing to identify ischemia.
It remains to be seen whether this use of CMR is viable in clinical practice. The differences in T1 reactivity between groups are quite tight and somewhat similar to other non-contrast techniques such as blood level dependent oxygenation, which is at the hairy edge in terms of signal-to-noise ratio. It remains to be seen if such excellent results can be reproduced in a multicenter study using different T1 mapping techniques. The Oxford center has been a pioneer in this arena, and it is unclear if other centers less well versed in these techniques will be able to perform as well.
Multicenter studies of patients with microvascular CAD and epicardial CAD using stress T1 imaging would need to be done to be confident that this approach is useful across multiple sites. In addition, it will need to be directly compared in accuracy to standard contrast-enhanced stress CMR.
The findings regarding using cardiac MRI to diagnose microvascular angina in patients with nonobstructive CAD suggest that one could perform a clinical CMR stress test and epicardial disease could be excluded by the absence of a visual perfusion defect. If microvascular CAD was a diagnosis being considered, semi-quantitative assessment of myocardial perfusion reserve could be performed. Thus, one could diagnose microvascular disease noninvasively, which would be a significant advance in noninvasive clinical diagnostics. If the myocardial perfusion reserve was < 1.4, then microvascular disease is likely. The authors suggest if the myocardial perfusion reserve is intermediate (1.4 to 1.6), then fully quantitative CMR would be required to further differentiate microvascular CAD from epicardial CAD.
The semi-quantitative approach for measuring myocardial perfusion reserve is straightforward enough to be done in most CMR labs. The fully quantitative approach is available in few labs at present, although there are ongoing efforts to make this more widely available. Our center published a paper at the same time as this paper (Zorach B, et al. J Cardiovasc Magn Reson. 2018;doi:10.1186/s12968-018-0435-1) showing that fully quantitative CMR does demonstrate microvascular disease, although without validation by measurement of microvascular resistance in the cath lab, as done in the study by Liu and colleagues.
Multicenter studies of patients with microvascular CAD and epicardial CAD would need to be done to be confident that this approach is practical and usable across many sites.