Jonathan Ophir

Power spectral strain estimators in elastography

Elastography can produce quality strain images in vitro and in vivo. Standard elastography uses a coherent cross-correlation technique to estimate tissue displacement and tissue strain using a subsequent gradient operator. Although coherent estimation methods generally have the advantage of being highly accurate and precise, even relatively small undesired motions are likely to cause enough signal decorrelation to produce significant degradation of the elastogram. For elastography to become more universally practical in such applications as hand-held, intravascular and abdominal imaging, the limitations associated with coherent strain estimation methods that require tissue and system stability, must be overcome. In this paper, we propose the use of a spectral-shift method that uses a centroid shift estimate to measure local strain directly. Furthermore, we also show theoretically that a spectral bandwidth method can also provide a direct strain estimation. We demonstrate that strain estimation using the spectral-shift technique is moderately less precise, but far more robust than the cross-correlation method. A theoretical analysis, simulations and experimental results are used to illustrate the properties associated with this method.

Pulse echo method and apparatus for sound velocity estimation in vivo

Sound velocity along a straight segment is estimated by measuring ultrasound pulse travel time as ultrasound pulses are directed along the straight segment from successively different transmitting or receiving positions. A sequence of data pairs comprising travel times and distances traveled is measured and plotted on a graph. Estimated sound velocity is derived from the slope of affine equation fitted to the plotted points.

Method for enhancing the accuracy of in vivo sound velocity estimation

A method of enhancing the accuracy of in vivo sound velocity estimation by identifying segments of different sound velocity along a tracked ultrasound beam. The effects of refraction on the tracked beam at the interface between tissue regions are estimated. Also disclosed is a technique for estimating the refractive effects of naturally occurring and artificially introduced acoustical contrast fluids.

Cardiac elastography-a feasibility study

Early detection of cardiovascular diseases has been a very active research area in the medical imaging field. Assessment of the local and global mechanical functions is one of the major goals for accurate diagnosis. In this paper, we studied the use of elastography for estimation and imaging of the local cardiac muscle displacement and strain in vivo. In its noninvasive applications, elastography has been typically used to determine local tissue strain through the use of an externally applied compression. For our study, we utilized the cardiac muscle motion, i.e., contraction and relaxation, as the mechanical stimulus, and acquired successive RF data frames over a few cardiac cycles in parasternal and apical views. Best quality cine-loop elastograms were obtained at higher frame rates due to the relatively lower decorrelation noise while the expected tradeoff between signal-to-noise ratio and spatial resolution was also observed. Furthermore, the strain contrast was higher in the parasternal case, when solely the posterior wall was imaged while strain estimation was more robust in the apical case. High repeatability of the results was observed through elastographic measurements over several cardiac cycles. Finally, an M-mode version of elastography was used in order to follow a particular segment of the image in the course of two cardiac cycles. Not only do these preliminary results show that elastography is feasible in cardiac applications in vivo, but also that it can provide new information regarding the cardiac motion and mechanical function. Future prospects include the assessment of the role of elastography in the detection of ischemia as well as infarction.

Three-dimensional motion estimation in elastography

In elastography we are capable of estimating the two in-plane principal strain components following an applied compression, namely the axial and lateral components, along and perpendicular to the compressor/transducer axis, respectively. However, the motion resulting from the compression is three-dimensional. Therefore, in order fully describe the resulting three-dimensional motion we need to also estimate the third strain, or elevational (out-of-plane) component. In this paper, we describe a method that estimates motion, i.e., displacement and strain, in the elevational direction. In a similar way as in the lateral motion estimation, and by assuming that ultrasonic frames are available in more than one parallel elevational planes, we used methods of interpolation and cross-correlation between elevationally displaced RF echo segments. We were thus able to obtain high resolution elevational displacement and strain estimates. An iterative method corrected for motion in the axial and lateral directions. As a result, together with the corrected axial and lateral strain estimates, we obtained the full 3D normal strain tensor resulting from a uniform compression. Results of this method from three-dimensional finite-element simulations are shown.

Methods for dynamic range expansion and enhancement of the signal-to-noise ratio in elastography

Two methods are discussed to increase the dynamic range and enhance the signal-to-noise ratio in elastography (SNR/sub e/). One method uses variable applied strains to expand the elastographic dynamic range by selecting the strain estimates with the highest SNR/sub e/ out of a multitude of strain estimates. The second one is a completely new estimator that estimates the strain by an iterative temporal stretching algorithm of the windowed postcompression RF echo signal. The authors examine these methods using 2D finite element simulations and resulting experimental strain filters. Finally, the combination of these two methods is shown to produce an elastogram with higher dynamic range as well as SNR/sub e/.

Improvement in elastograms using multiresolution elastography

The authors analyze the performance of multiresolution elastography, where the strain estimate with the highest elastographic signal-to-noise ratio (SNR/sub e/) obtained by processing the pre- and post-compression waveforms at different window lengths. All the objective elastographic parameters, namely: SNR/sub e/, sensitivity dynamic range and the average resolution (defined as the cross-correlation window length) are improved with multiresolution elastography when compared to the traditional method of utilizing a single window length to generate the elastogram. Multiresolution elastography is based on the presence of an optimal window length for each strain value where the strain variance is minimized. Long-duration windows are preferred for small strains, while large strains require a small window length. The sensitivity, dynamic range and SNR, improve with an increase in the window length. However, with an increase in the window length the elastogram is degraded due to poor resolution. Experimental results using a phantom with a hard inclusion illustrate the improvement in the elastogram obtained using multiresolution analysis.