On the use of a modified optical flow algorithm for the correction of axial strain estimates in ultrasonic elastography in medical diagnosis

Abstract

Tumors of the prostate exhibit a significant mechanical hardness in comparison to the surrounding tissue. However, they often can not be reliably detected in the B-mode image. For this reason, digital palpation of the prostate is still a major tool in tumor diagnostics. Ultrasonic elastography estimates the tissue elasticity from an image series recorded under increasing axial tissue compression applied by the transrectal probe. As a new imaging modality, it promises to support conventional prostate screening methods. Usually, time delays from two corresponding A-lines are estimated by cross-correlation methods applied to the acquired rf data set assuming a uniform axial compression. However, in clinical in vivo data, especially in data which were acquired with a sector scanner, significant lateral motion of the tissue which deteriorates the elasticity estimate was observed. A method is presented which uses a modified ‘‘optical flow’’ algorithm to determine the global two-dimensional motion of the imaged object. This information is used to correct the corresponding A-lines according to the encountered lateral deformation. It is shown that this method is able to improve the strain estimates on data with low signal-to-noise conditions. Images of phantoms and clinical in vivo data are presented. [Work supported by DFG Grant ER 94/20-1.]