MULTIRESOLUTION IMAGING IN ELASTOGRAPHY

The range of strains that can be imaged by any practical elastographic imaging system is inherently limited, and a performance measure is va luable to evaluate these systems from the signal and noise properties of their output images. Such a measure was previously formulated for s ystems employing cross-correlation based time-delay estimators through the strain filter. While the strain filter predicts the signal-to-noi se ratio (SNRe) for each tissue strain in the elastogram and provides valuable insights into the nature of image noise, it understated the e ffects of image resolution (axial resolution, as determined by the cro ss-correlation window length) on the noise. In this work, the strain f ilter is modified to study the strain noise at multiple resolutions. T he effects of finite window length on signal decorrelation and on the variance of the strain estimator are investigated. Long-duration windo ws are preferred for improved sensitivity, dynamic range, and SNRe. Ho wever, in this limit the elastogram is degraded due to poor resolution . The results indicate that for nonzero strain, a window length exists at which the variance of strain estimator attains its minima, and con sequently the elastographic sensitivity, dynamic range and SNRe are st rongly affected by the selected window length. Simulation results corr oborate the theoretical results, illustrating the presence of a window length where the strain estimation variance is minimized for a given strain value. Multiresolution elastography, where the strain estimate with the highest SNRe obtained by processing the pre-and post-compress ion waveforms at different window lengths is used to generate a compos ite elastogram and is proposed to improve elastograms. All the objecti ve elastogram parameters (namely: SNRe, dynamic range, sensitivity and the average elastographic resolution-defined as the crosscorrelation window length) are improved with multiresolution elastography when com pared to the traditional method of utilizing a single window length to generate the elastogram. Experimental results using a phantom with a hard inclusion illustrates the improvement in elastogram obtained usin g multiresolution analysis.