MULTILEVEL AND MOTION MODEL-BASED ULTRASONIC SPECKLE TRACKING ALGORITHMS

A multilevel motion model-based approach to ultrasonic speckle trackin g has been developed that addresses the inherent trade-offs associated with traditional single-level block matching (SLBM) methods. The mult ilevel block matching (MLBM) algorithm uses variable matching block an d search window sizes in a coarse-to-fine scheme, preserving the relat ive immunity to noise associated with the use of a large matching bloc k while preserving the motion field detail associated with the use of a small matching block. To decrease further the sensitivity of the mul tilevel approach to noise, speckle decorrelation and false matches, a smooth motion model-based block matching (SMBM) algorithm has been imp lemented that takes into account the spatial inertia of soft tissue el ements. The new algorithms were compared to SLBM through a series of e xperiments involving manual translation of soft tissue phantoms, motio n field computer simulations of rotation, compression and shear deform ation, and an experiment involving contraction of human forearm muscle s. Measures of tracking accuracy included mean squared tracking error, peak signal-to-noise ratio (PSNR) and blinded observations of optical flow. Measures of tracking efficiency included the number of sum squa red difference calculations and the computation time. In the phantom t ranslation experiments, the SMBM algorithm successfully matched the ac curacy of SLBM using both large and small matching blocks while signif icantly reducing the number of computations and computation time when a large matching block was used. For the computer simulations, SMBM yi elded better tracking accuracies and spatial resolution when compared with SLBM using a large matching block. For the muscle experiment, SMB M outperformed SLBM both in terms of PSNR and observations of optical flow. We believe that the smooth motion model-based MLBM approach repr esents a meaningful development in ultrasonic soft tissue motion measu rement. (C) 1998 World Federation for Ultrasound in Medicine & Biology .