MRA-identified impaired coronary vasodilation indicates advanced CAD
Using MR angiography, researchers have found that impaired nitroglycerin-induced coronary vasodilation was significantly and independently associated with advanced coronary artery disease (CAD) in asymptomatic older patients, according to a study in the March issue of JACC Cardiovascular Imaging.

The researchers, from Stanford University School of Medicine and the University of California, San Francisco, suggest that “nitroglycerin coronary vasodilation by MRA may be a functional measure of subclinical coronary atherosclerosis.”

  Comparison of Nitroglycerin-Induced Coronary Vasodilation and CAC
The degree of coronary vasodilation is shown for patients with and without advanced coronary artery calcium. Coronary MRA images were analyzed in 212 patients and coronary vasodilation to nitroglycerin was quantified. Coronary artery calcium scoring was performed by CT. Box and whisker plots show the median, interquartile range, and full range of percent coronary vasodilation between groups with and without advanced CAC (>400). Patients with CAC >400 had significantly impaired nitroglycerin-induced coronary vasodilation compared to patients with CAC <400.
(Source: JACC: Cardiovascular Imaging)
While CT imaging of coronary artery calcium has been widely studied for structural assessment, the standard for functional assessment is the measurement of epicardial coronary artery vasodilation by catheter angiography.

Although invasive studies have suggested that impaired nitroglycerin-induced vasodilation has adverse prognostic significance, the association with plaque burden, an important “missing” link, has not previously been recognized, because invasive angiography does not directly assess plaque burden, according to an accompanying editorial comment by Nathaniel Reichek, MD, and Deepu Alexander, MD, from St. Francis Hospital-The Heart Center in Roslyn, New York.

Led by Masahiro Terashima, MD, from the division of cardiovascular medicine at Stanford, the researchers developed a noninvasive method to measure nitroglycerin-induced coronary vasodilation with MRA. They studied 236 older patients (33 percent female) without a history of cardiovascular disease.

Imaging was performed on a 1.5-T Signa MRI scanner (GE Healthcare) equipped with high-performance gradients (40mT/m, 150 mT/m/ms). A commercial four-channel cardiac phased-array surface coil provided signal reception (GE Healthcare). A real-time interactive MRI system (iDrive, GE Healthcare) was used for coronary localization.

High-resolution coronary MRA was performed using a multislice spiral sequence, with cardiac gating, breath-holding, and acquisition during diastole (field of view = 22 cm, in-plane spatial resolution = 0.7 mm, slice thickness = 5 mm, three slices, repetition time = one heart beat, echo time = 2.5 ms, 18 interleaves, flip angle = 60°, acquisition window = 35 ms).

After real-time interactive MRI was used to prescribe in-plane views of the right coronary artery (RCA), a cross-sectional view of a linear portion of the proximal- to mid-RCA was then prescribed. Cross-sectional high-resolution coronary MRA images were then acquired both before and five minutes after the administration of nitroglycerin while the patient was in the magnet.

Researchers assessed the presence and severity of coronary artery calcium (CAC) by four- or 16-slice CT (GE Healthcare and Siemens Healthcare, respectively). An experienced technologist analyzed the CT sections using an AccuImage coronary calcium scoring workstation (Merge/eFilm) to determine the Agatston score.

Researchers excluded 10% of subjects because of poor image quality on either the pre- or post-nitroglycerin MRA, leaving 212 subjects for quantitative analysis.

Forty patients (19%) had advanced CAC, defined as a score greater than 400. These patients had significantly impaired coronary vasodilation (15.9 percent) compared to patients with CAC less than 400 (21.5 percent)

The degree of coronary vasodilation was independently and inversely associated with CAC greater than 400 after adjustment for age, gender, body mass index, diabetes, systolic blood pressure, total cholesterol/HDL cholesterol ratio, and current cigarette smoking.

Reichek and Alexander comment that these findings “provide a potential explanation for earlier invasive studies demonstrating an association between impaired coronary vasodilator response to nitroglycerin and adverse outcomes.”

They conclude that the study by Terashima et al “open a new door for noninvasive imaging and can be viewed as a harbinger of things to come: an era in which noninvasive evaluation of coronary function, using repeated imaging before and after perturbations, comes to the fore.”