Simplification of free-running cardiac magnetic resonance by respiratory phase using principal component analysis

Cardiac magnetic resonance imaging (CMR) provides many cardiac functional insights. The reliance of standard cine CMR upon breath holds is not feasible for some patients. Its process of combining multiple heartbeats is unsuited to arrhythmias. Real-time cine methods sidestep these problems but can introduce respiratory displacement of the heart. To aid CMR acquisitions during breathing, we developed post-processing software to diminish the effects of respiratory displacement of the heart. It uses principal component analysis to resolve respiratory motions from cardiac cycles in the dynamic image. The software groups heartbeats from expiration and inspiration to decrease the appearance of respiratory motion. The effects of respiratory motion and such motion correction were evaluated on short-axis views (acquired with compressed sensing) of 11 healthy subjects and 8 cardiac patients. The smallest correlation coefficients between end-systolic frames of the original dynamic scans averaged 0.79. After segregation of cardiac cycles by respiratory phase, the mean correlation coefficients between cardiac cycles were 0.94 {+/-} 0.03 at end-expiration and 0.90 {+/-} 0.08 at end-inspiration. The improvements in correlation coefficients were significant in paired t-tests, i.e., P [≤] 0.01 for healthy subjects and P [≤] 0.001 for heart patients at end-expiration. Two expert cardiothoracic radiologists, blinded to the processing, assessed the dynamic images in terms of blood-myocardial contrast, endocardial interface definition, and motion artifacts. Clinical assessment preferred cardiac cycles during end-expiration, which maintained or enhanced scores in 90% of healthy subjects and 83% of the heart patients. Performance remained high in a case of arrhythmia and irregular breathing. Heartbeats collected from end-expiration reliably mitigated respiratory motion when the new software was applied to DICOM files from real-time acquisitions.