Tracking cardiac twist in gated PET imagery

Traditional clinical techniques for imaging heart function, like gated SPECT and PET, have shown that the motion of the left ventricle during the cardiac cycle can be described by relatively global parameters such as ejection fraction, longitudinal shortening, radial contraction and wall thickening. Despite the poor spatial and temporal resolution of these datasets, a number of techniques are now available which can accurately measure these global parameters. Recently, phase contrast and tagged MRI techniques have shown that in addition to the global components, the left ventricle also exhibits considerable local twist during the cardiac cycle. We present a validation study to investigate whether this torsion can be detected and tracked in gated cardiac PET imagery. Data were acquired with 40 msec gating intervals from six normal subjects using retrospectively gated list mode PET and tagged MRI. The MRI data were analyzed using the Findtags software package and custom software to determine the ground truth motion fields. A four-dimensional deformable motion algorithm based on optical flow was then used to estimate the motion from the PET dataset. Results indicate that though the estimation algorithm can quite accurately determine the component of motion normal to the ventricular surfaces, the torsion component is considerably more difficult to track.