Four-dimensional affine registration models for respiratory-gated PET

The heart position shifts considerably due to motion associated with the re spiratory cycle, and this motion can degrade the image quality of cardiac-g ated positron emission tomography (PET) studies. One method to combat this motion-induced blur is a respiratory-gated acquisition followed by recombin ation of registered image volumes using a rigid-body motion assumption; how ever, nonrigid deformation of the heart from respiratory motion may reduce the effectiveness of this procedure. We have investigated a 12-parameter gl obal affine motion model for registration of different respiratory gates in an end-diastolic cardiac PET sequence. To obtain robust estimates of motio n, a four-dimensional registration model was devised that encouraged smooth ly varying motion between adjacent respiratory time frames. Registration pa rameters were iteratively calculated using a cost function that combined a least squares voxel difference measure with a penalty obtained from a predi ction prior. The prior was calculated from adjacent time frames assuming co nstant velocity and an affine model. After registration, the principal exte nsion ratios were calculated to measure the degree of nonrigid motion. In d ata from ten subjects, extension ratios of over 5% were common, indicating that an affine model may provide better registrations and in turn, better m otion-corrected composite volumes than could a technique restricted to the six-parameter rigid body assumption.