Validation of an optical flow method for tag displacement estimation

We present a validation study of an optical-flow method for the rapid estim ation of myocardial displacement in magnetic resonance tagged cardiac image s. This registration and change visualization (RCV) software uses a hierarc hical estimation technique to compute the flow field that describes the war ping of an image of one cardiac phase into alignment with the next. This me thod overcomes the requirement of constant pixel intensity in standard opti cal-how methods by preprocessing the input images to reduce any intensity b ias which results from the reduction in stripe contrast throughout the card iac cycle. To validate the method, SPAMM-tagged images were acquired of a s ilicon gel phantom with simulated rotational motion. The pixel displacement was estimated with the RCV method and the error in pixel tracking was <4% 1000 ms after application of the tags, and after 30 degrees of rotation. An additional study was performed using a SPAMM-tagged multiphase slice of a canine left ventricle. The true displacement was determined using a previou sly validated active contour model (snakes). The error between methods was 6.7% at end systole. The RCV method has the advantage of tracking all pixel s in the image in a substantially shorter period than the snakes method.