Improving precision of tissue shear modulus quantification within the region of acoustic radiation force excitation with compounded displacement estimates

The time-to-peak (TTP) displacement within the region of acoustic radiation force excitation (ROE) is directly related to the tissue shear modulus. It has previously been shown that the ROE TTP can be directly converted to shear modulus using a look-up table (LUT) for a given radiation force excitation focal configuration. While this method has the advantages of requiring less input data for stiffness reconstruction, ease of experimental implementation, and high spatial resolution, it suffers from lack of precision due to jitter in ultrasonically tracked TTP displacement estimates. To reduce the variance of TTP measurements, compounding of TTP values obtained using different receive apertures was investigated. A previously validated 3D FEM model was used to calculate tissue displacement from impulsive radiation force excitation using the Siemens CH4-1 transducer and a fixed focal configuration (focal depth 49 mm, f/2, 2.2 MHz). Tissue was modeled as a linear elastic isotropic material with shear modulus from 1.5-20 kPa. Ultrasonic tracking of this displacement field in the ROE was simulated in FIELD II using a receive spatial compounding beamforming technique. A single f/2 transmit aperture using 52 elements with a focal depth of 49 mm and 3.1 MHz center frequency was used. A total of 96 elements were used for receive. These were grouped to form 9 overlapping receive apertures each of 32 elements. The magnitude of TTP jitter after averaging TTP values from all 9 receive apertures was 0.25±0.02 ms for a simulated shear modulus of 1.5 kPa and 0.12 mm displacement tracking kernel. In comparison, the TTP jitter without compounding using all 96 elements in a single receive aperture was 0.56±0.07 ms. By using these compounded TTP values, the average ROE TTP stiffness reconstruction error was 0.3 kPa, compared to 1.0 kPa without compounding. Compounding TTP values from the different receive apertures achieved TTP jitter reduction despite the fact that 1) speckle observed by the different apertures is not fully decorrelated, and 2) receive aperture locations offset from the ROE axis have inherently higher TTP jitter. Moreover, this technique does not compromise spatial resolution, and can be experimentally implemented on a scanner capable of parallel beamforming without reduction of pulse repetition frequency (PRF).