  This article describes a novel quasi-real time system for quantitative and
volumetric measurement of tissue elasticity in the prostate. Tissue elasticity
is computed by using a local frequency estimator to measure the three
dimensional local wavelengths of a steady-state shear wave within the prostate
gland. The shear wave is created using a mechanical voice coil shaker which
transmits multi-frequency vibrations transperineally. Radio frequency data is
streamed directly from a BK Medical 8848 trans-rectal ultrasound transducer to
an external computer where tissue displacement due to the excitation is
measured using a speckle tracking algorithm. Bandpass sampling is used that
eliminates the need for an ultra fast frame rate to track the tissue motion and
allows for accurate reconstruction at a sampling frequency that is below the
Nyquist rate. A roll motor with computer control is used to rotate the sagittal
array of the transducer and obtain the 3D data. Two CIRS phantoms were used to
validate both the accuracy of the elasticity measurement as well as the
functional feasibility of using the system for in vivo prostate imaging. The
system has been used in two separate clinical studies as a method for cancer
identification. The results, presented here, show numerical and visual
correlations between our stiffness measurements and cancer likelihood as
determined from pathology results. Initial published results using this system
include an area under the receiver operating characteristic curve of
0.82+/-0.01 with regards to prostate cancer identification in the peripheral
zone.
