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In selected patients, perfusion cardiac magnetic resonance imaging (MRI) is more sensitive in detecting ischemic coronary artery disease (CAD) than single-photon emission computed tomography (SPECT). However, cardiac MRI’s specificity lags behind that of SPECT, according to a large study published online March 4, 2012, ahead of print in the European Heart Journal.

For the multivendor MR-IMPACT II trial, investigators led by Juerg Schwitter, MD, of University Hospital Lausanne (Lausanne, Switzerland), compared the diagnostic performance of cardiac MRI and SPECT, using invasive angiography as the gold standard, in 533 patients with suspected or known CAD at 33 international centers. All patients were imaged with SPECT and invasive X-ray angiography within 4 weeks before or after cardiac MRI.

Intermediate CAD Prevalence

Of 465 patients with complete data from all 3 imaging modalities, 48.8% had coronary artery stenoses with greater than 75% area reduction, 15.7% had occlusions, and 27.7% had infarctions (5.4% with no significant stenoses on conventional angiography). The prevalence of CAD in those with no history of MI was 29%.

After exclusion of nonevaluable studies (5.6% for cardiac MRI and 3.7% for SPECT; P = 0.21), 425 patients were available for analysis. Using a CAD threshold of at least 75% area stenosis, cardiac MRI met the primary endpoint of noninferiority to SPECT with regard to sensitivity (0.67 vs. 0.59; P = 0.024), also showing superiority. On the other hand, cardiac MRI yielded a lower specificity (0.61 vs. 0.72; P = 0.038), indicating inferiority to SPECT.

The performance of cardiac MRI vs. SPECT also was assessed in the overall cohort, again showing higher sensitivity but lower specificity and no difference in positive or negative predictive value or accuracy (table 1).

Table 1. Diagnostic Performance of Cardiac MRI vs. SPECT

 

Among 515 patients who received contrast medium for pharmacological stress cardiac MRI, there were no deaths or other severe adverse events related to the drugs. In addition, no trends toward clinically significant changes in vital signs or ECG findings were seen. Only 12 patients who received contrast (2.3%) required treatment for side effects, including angina, headache, chest pain, and injection site bruising.

According to the authors, the findings regarding the accuracy of cardiac MRI are in accord with those of several earlier studies. They also confirm the strong safety profile of the modality, which avoids exposure to ionizing radiation.

In addition, Dr. Schwitter and colleagues note that the results support an important feature of MRI—the ability to correct for both cardiac and respiratory motion during the few seconds of contrast medium first-pass during hyperemia, thereby preserving the high spatial resolution and enabling detection of small perfusion deficits.

The relatively low specificity of cardiac MRI may be due to the fact that perfusion was compared with the macroscopic coronary anatomy imaged with angiography, which does not assess collateral flow on the microvascular level, the investigators write.

Testing ‘Real-World’ Performance

Dr. Schwitter and colleagues say the aim of the study was “to assess test performances in a realistic clinical environment across a substantial number of countries and not to repeat the results of a few leading high-performance centers.”

They also offer a couple of caveats in interpreting the results. First, gated SPECT was performed in just over half of patients, and attenuation correction algorithms were not applied. “These circumstances should be taken into account when considering the overall SPECT performance,” the investigators observe. Moreover, the results were obtained in a blinded fashion and thus may “underestimate test performance in the clinical setting where imaging information is generally interpreted in the context of additional patient information,” they say.

The authors also caution that with an intermediate prevalence of CAD in the study cohort, the results cannot be extrapolated to other populations, such as asymptomatic patients undergoing screening.

Moreover, CAD was defined primarily by anatomic assessment despite the fact that the presence of perfusion deficits depends not only on stenosis but on collateral flow and microcirculatory alterations, the authors point out. “For future comparative studies, invasive perfusion assessment by fractional flow reserve measurements would be desirable,” they add.

Study Details

Exclusion criteria included patients with acute MI, a history of CABG, unstable angina, decompensated heart failure, and severe arrhythmias considered to compromise the quality of MR imaging.

Using 1.5 Tesla scanners from various vendors, a breath-hold MR first-pass perfusion examination was performed to follow a bolus of 0.075 mmol/kg Gd-DPTA-BMA (Omniscan, GE Healthcare, United States) after a 3-minute adenosine infusion.

 

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Source:
Schwitter J, Wacker CM, Wilke N, et al. MR-IMPACT II: Magnetic Resonance Imaging for Myocardial Perfusion Assessment in Coronary artery disease Trial: Perfusion-cardiac magnetic resonance vs. single-photon emission computed tomography for the detection of coronary artery disease: A comparative multicenter, multivendor trial. Eur Heart J. 2012;Epub ahead of print.

 

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