The effect of volume sub-sampling on motion estimation of joints via MR imaging

The technique of three-dimensional imaging is being explored as a means of better understanding the morphology and kinematics of the foot and ankle. To capture information about the dynamic nature of the joint, MRI pulse sequences are used to rapidly acquire single-slice kinematic images, which are then used to track the motion of the bone or other tissues. This approach cannot capture true 3D motion information. On the other hand, full 3D acquisitions are time consuming. A more time-efficient alternative to this method that may give accurate 3D motion information may be to use select MRI slices, instead of full resolution 3D models, that may be just enough to capture the vital information needed to track motion. This was tested by removing slices from already acquired full kinematic MRI datasets and progressively removing slices to determine up to what level data can be eliminated and still achieve accurate motion tracking. We evaluated the ability of the reduced data set in tracking motion in terms of both volume overlap and actual motion estimated, and compared these with the results from the full resolution data. We based our analysis of accuracy on the ability to transform the reduced images from one position of the foot to another. In tracking the motion of the bones of the tarsal joints, we were able to reduce the number of slices to about 25% of the full data set while maintaining an accurate representation of motion within about 0.5mm of translation and 0.5 degrees of rotation of the motion estimated from full data.