Reconstruction of elastic modulus distribution from envelope detected B-mode data

A variety of ultrasonic methods have been shown to be capable of imaging the stiffness of soft tissue but most require specialised equipment and non-standard approaches to ultrasonic examination. In breast imaging by Elastography, for example, the elastic modulus distribution (EMD) is estimated by using 1-D cross correlation tracking of radiofrequency (RF) echo signals to image the distribution of strain induced by a controlled static deformation: Our work aims to explore the possibilities for employing standard ultrasound scanning methods and equipment, and simple processing techniques, to image the EMD of soft tissues. A simple reconstruction algorithm was developed and its convergence was studied at different elastic contrasts (EC) using 'ideal' strain data that was derived using the finite element method. EMD reconstruction of hard inclusions improved substantially according to a Chi-squared statistic, but soft inclusions showed little change after the first iteration. For example Chi-squared for a + 20 dB EC inclusion reduced from 10 at the start to 0.0001 after 5 iterations, whilst it improved from 3 to only 1.6 for a - 20 dB inclusion. The performance of a 1-D cross correlation tracking algorithm was studied with RF and envelope detected data when external strains of 2% and 5% were applied to the surface of a simulated phantom. The algorithm accurately estimated displacement less than or equal to 1.5 lambda with both RF and ED data when the applied strain was 2%. However better precision and signal to noise ratio (SNR) was obtained using RF as oppose to ED. At an applied strain of 5% more accurate estimates, better precision and SNR was obtain using ED. We conclude that there is no great disadvantage to using the envelope detected signal for estimating internal tissue displacements that are likely to be encountered in standard clinical scanning practice.