CT, MR equivalent in ED assessment of myocardial infarct

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Cardiac imaging for myocardial infarct size in an emergency department (ED) is constrained by the difficulty of conducting MR or nuclear medicine exams on unstable patients. However, using more widely available CT technology shows promise as an alternative imaging technique that may provide similar information.

A multinational study by cardiologists and radiologists at the VA Medical Center in San Francisco and the University Claude Bernard in Lyon, France, found that delayed enhanced CT allows accurate visualization of early myocardial contrast uptake compared with delayed enhanced MRI and does not require an additional contrast injection after PCI (AJR 2008;191:441-447)

“The size of the infarct is one of the most important predictors of long-term left ventricular function in patients with an acute myocardial infarction,” said lead author Loic Boussel, MD. “Imaging of myocardial infarct size is difficult in the emergency setting as the current ‘gold standard’ methods of delayed enhanced MRI and nuclear medicine techniques are difficult to perform in unstable patients. CT is very easy and quick to perform even at the acute phase so we wanted to find out if it can do the same job as MRI and potentially provide further information.”

Comparison between delayed enhanced MDCT (A and C) and delayed enhanced MRI (B and D) in short-axis views (A and B) and two-chamber views (C and D). Transmural extent of enhancement of inferior left ventricular wall (arrows) visualized with delayed enhanced MDCT matches that of delayed enhanced MRI findings. Partial involvement of right ventricle (arrowhead, A and B) is also visible. Image and caption by permission of the American Roentgen Ray Society.

The team’s study included 19 patients with acute MI who underwent delayed enhanced CT immediately after emergency PCI and underwent delayed enhanced MRI within eight days of the procedure.

Only patients with a thrombolysis in myocardial infarction (TIMI) score of 0 or 1 of the culprit coronary artery before endovascular angioplasty and TIMI score of 2 or 3 after angioplasty were selected.

The investigators performed CT scans on a Philips Healthcare Brilliance 40 scanner and evaluated images on a Brilliance 3D workstation.

Reconstruction parameters for axial slices were a 2-mm effective section thickness, 1-mm increment, standard intermediate reconstruction filter (kernel CB), and adapted field of view. Retrospective ECG-gated reconstruction in the mid-diastolic phase (75 percent of the R-R interval) was performed.

The MR scans were performed on a 1.5-Tesla MR system, either a Philips Intera or a Siemens Medical Solutions’ Avento, followed by image evaluation on a GE Healthcare Advantage workstation.

Consensus analysis on image quality was performed by three CT observers and two MRI observers. Sensitivity, specificity and predictive values of delayed enhanced MDCT were calculated using MRI as the gold standard, according to the researchers.

“The sensitivity, specificity, positive predictive value, and negative predictive value of CT were, respectively, 90.1 percent, 96.7 percent, 93.5 percent and 94.9 percent for the classification of involved versus healthy segments, and 87.6 percent, 97.7 percent, 95 percent and 93.9 percent for the classification of transmural extent,” the authors wrote.

Boussel and colleagues noted that the sensitivity of CT was slightly lower than the specificity in their results, which they believe indicates that some segments showing delayed enhancement on MRI were missed on CT. They also found that in some cases, CT overestimated infarct size, particularly with regard to the extent of transmural enhancement.

However, the results overall demonstrate the feasibility of using delayed contrast-enhanced CT immediately after emergency PCI without the need for an additional contrast injection.

“In all cases, CT allowed adequate visualization of myocardial contrast uptake in the same territory as the occluded coronary artery,” the authors wrote. “All CT examinations showed good image quality in addition to good contrast between healthy and infarcted myocardium. Agreement, in terms of the number of segments involved, transmural extension, and myocardial infarct size, was found to be very good with delayed enhanced MRI.”

Researchers used a CT scanning protocol specifically designed for post-infarct imaging, which lowered radiation exposure to less than 5.1 mSv. It involved an ECG-triggered scan of the entire heart at 80 kV and 600 mAs per slice. Pitch was set at 0.2. An additional reduction of the dose should be achievable using prospective gating instead of retrospective gating, according to the authors.