Novel cardiac MRI techniques pinpoint microvascular dysfunction

A team of researchers tested two novel forms of cardiac MRI, both of which showed strong potential for detecting microvascular coronary artery disease (CAD).

The group, led by Alexander Liu, MBBS, of University of Oxford, published its findings online Feb. 26 in two separate studies in the Journal of the American College of Cardiology.

Stress T1 mapping

Cardiac magnetic resonance (CMR) stress T1 mapping outperformed gadolinium-based first-pass perfusion imaging in a head-to-head comparison, detecting obstructive CAD with a 0.97 area under the curve (AUC) versus a 0.91 AUC. Obstructive CAD was defined by fractional flow reserve (FFR) less than 0.8 whereas coronary microvascular dysfunction was defined by FFR of 0.8 along with meeting a threshold for microcirculatory resistance (25 U).

In the study of 60 patients with angina and 30 healthy control subjects, a change in T1 of 1.5 percent detected obstructive CAD with 93 percent sensitivity and 95 specificity, while a threshold of 4 percent detected microvascular dysfunction with 94 percent sensitivity and 94 percent specificity. All of this was done noninvasively and without the need for contrast agents or radiation, the authors noted.

“Stress T1 mapping significantly outperformed the current CMR standard for detecting obstructive CAD with stress gadolinium-based first-pass perfusion imaging, whether using visual, semi-quantitative, or quantitative analysis,” Liu and colleagues wrote. “Furthermore, stress T1 mapping accurately detected coronary microvascular dysfunction defined invasively by a high IMR value (≥25 U) downstream of nonobstructive (FFR >0.8) coronary arteries.”

The second part of that conclusion, according to the researchers, could be key in increasing the utility of T1 mapping.

“Although these patients with ‘microvascular angina’ are often reassured as having no significant CAD or are treated empirically with antianginal medication, they experience reduced quality of life and adverse long-term prognosis. Therefore, a noninvasive test to accurately detect CMD can improve clinical risk stratification and guide targeted therapy in patients with microvascular angina,” they wrote.

In an accompanying editorial, Theodoros D. Karamitsos, MD, PhD, pointed out the superiority of T1 mapping is this study is based on per-vessel data. That doesn’t necessarily mean the T1 technique would allow for better diagnosis at the patient level for either single-vessel or multivessel CAD, he said.

In addition, the study analyzes only one T1 mapping technique, and results may be different using one of the many other methods—even for a similar patient population.

“Streamlining of the various T1 mapping protocols and developing a unified approach to T1 mapping is one of the most important challenges for the CMR community,” wrote Karamitsos, with Aristotle University of Thessaloniki, AHEPA Hospital in Greece.

Myocardial perfusion reserve index (MPRI)

Liu et al. performed another study in which a measure of myocardial perfusion accurately assessed the presence of CMD. Specifically, a MPRI of 1.4 detected impaired perfusion related to CMD with an AUC of 0.90, specificity of 95 percent and sensitivity of 89 percent.

The study’s goal was to validate the use of CMR for this purpose in 50 patients with nonobstructive CAD, who were compared to 20 control individuals. The average age of the study population was 65.

The MPRI was derived from CMR, while subsequent invasive angiography within seven days provided the measurement of microvascular resistance. Now validated, the authors believe MPRI has the potential to noninvasively assess microvascular angina.

“Integration of MPRI and (myocardial blood flow) assessment into the clinical CMR workflow can provide a noninvasive approach for evaluating both epicardial and microvascular CAD in patients with angina, which deserves further validation in an all-comers population,” they wrote.

Limitations of the study include that it was conducted in a single center and a small sample size.

Nevertheless, Karamitsos said “the authors should be commended for performing 2 important studies that open new frontiers for myocardial ischemia testing.”

“There is no doubt that these novel, noncontrast CMR techniques offer important pathophysiological insights in myocardial ischemia and have a significant diagnostic potential that justifies the conduction of a large-scale study,” Karamitsos wrote.