THE NONSTATIONARY STRAIN FILTER IN ELASTOGRAPHY .2. LATERAL AND ELEVATIONAL DECORRELATION

The nonstationary evolution of the strain filter due to lateral and el evational motion of the tissue scatterers across the ultrasound beam i s analyzed for the 1-D cross-correlation-based strain estimator. The e ffective correlation coefficient that includes the contributions due t o lateral and elevational signal decorrelation is used to derate the u pper bound of the signal-to-noise ratio in the elastogram (SNRe) predi cted by the ideal strain filter, In the case of an elastically homogen eous target, if the transducer is on the axis of symmetry of such targ et in the elevational direction, the motion of the scatterers out the imaging plane is minimized, In addition, the ultrasound beam along the elevational direction is broader, allowing scatterers to stay longer within the beam during tissue compression, Under these conditions, lat eral signal decorrelation becomes the primary contributor to the nonst ationary behavior of the strain filter, Both the elastographic SNRe an d the dynamic range are reduced, with an increase in lateral decorrela tion, Finite element simulations and phantom experiments are presented in this paper to corroborate the theoretical strain filter, The nonst ationary behavior of the strain filter is reduced by confining the tis sue in the lateral direction (minimizing motion of tissue scatterers), thereby improving the quality of the elastogram. (C) 1997 World Feder ation for Ultrasound in Medicine & Biology.