New MR perfusion test compares well to cardiac cath

MR has gained an edge in its tug of war with CT to serve as a noninvasive alternative to cardiac cath. Researchers at Massachusetts General Hospital and Harvard Medical School in Boston, working with colleagues at Beneficencia Portuguesa Hospital in São Paulo, Brazil, have fine-tuned an MR perfusion technique to assess blockages of the coronaries, achieving 88% accuracy compared to the invasive gold standard, cardiac catheterization.

"We have shown that cardiac MRI can be used reliably as an alternative to other, more invasive, detection techniques due to its high diagnostic accuracy; its comprehensive evaluation of cardiac function, perfusion, and viability; and the lack of radiation exposure," said Dr. Ricardo C. Cury, MGH director of clinical cardiac MRI.

Results from the study, featured in the July issue of Radiology, detail the new technique, called stress first-pass perfusion MRI. It involves delayed contrast-enhancement methodology unlike the MR sequences typically used to examine the heart. The goal was to depict clinically significant coronary artery stenosis (defined as a 70% stenosis) in patients suspected of having or known to have coronary artery disease (CAD).

Utilizing stress first-pass perfusion MRI, the researchers injected a contrast medium, then performed MR at timed intervals, using pharmacologically induced stress, and looked for ischemia or necrosis using a delayed contrast-enhancement technique. Studies were conducted on a GE Healthcare 1.5T Signa CV/i equipped with a four-element phased-array cardiac coil. Exam times averaged about 50 minutes. No severe complications were reported, although two patients had minor events: one dyspnea, the other chest pain.

The researchers enrolled 47 patients with chest pain in the dual-site study. (One was later excluded due to poor-quality MR images). All had been scheduled to undergo coronary angiography. They were divided into two groups, one composed of 32 patients suspected of CAD, the other of 14 patients with prior history of heart attack and suspected new arterial lesions.

Reporting in the July Radiology article, entitled "Diagnostic performance of stress perfusion and delayed-enhancement MR imaging in patients with coronary artery disease," lead author Cury and colleagues described the technique. They used pulse sequences to localize the heart, then induced vasodilatation using dipyridamole delivered intravenously over four minutes. Approximately two minutes later, breath-hold first-pass perfusion MR imaging was performed using a hybrid gradient echo-planar imaging pulse sequence during a bolus injection of Magnevist.

After first-pass perfusion MRI, aminophylline was administrated to antagonize the dipyridamole vasodilator effect. As the contrast medium was washing out of the myocardium, left ventricular function was assessed with cine images and a steady-state free precession technique implemented to evaluate global and regional wall motion.

Ten minutes after stress perfusion, a second bolus of Magnevist was injected intravenously to allow the acquisition of rest perfusion images using the same pulse

sequence as for stress perfusion. About 10 minutes later, delayed-enhancement MR imaging was performed using an inversion-recovery prepared gated fast gradient-echo

pulse sequence. These images were acquired and displayed to highlight normal myocardium, which appeared as dark areas, and regions of delayed-enhancement

myocardium, which appeared as bright areas.

Data were acquired using multiple sequences with varying inversion times. Images were selected for the most appropriate inversion time, according to the authors. Coronary angiography was performed after MR.

Traditional angiography demonstrated significant CAD in 65% of the patients. Of these 30 (of 46) patients, MRI demonstrated CAD with 88% accuracy. In patients with only one diseased vessel, the accuracy of MRI increased to 96%. In patients who had previously undergone bypass graft surgery, the MR accuracy was 90%.

The accuracy of the new technique is enough to establish its potential to help in deciding whether to move a patient to cardiac catheterization or coronary artery bypass surgery, according to Cury.

"In addition to diagnostic accuracy, cardiac MRI is safe," he said. "It can provide information about the anatomy, function, blood flow, and damage that the heart has sustained."