Shear wave dispersion ultrasonic vibrometry for measuring prostate shear stiffness and viscosity: an in vitro pilot study

Abstract

This paper reports shear stiffness and viscosity "virtual biopsy" measurements of the three excised noncancerous human prostates using a new tool known as shear wave dispersion ultrasound vibrometry (SDUV) in vitro. Improved methods for prostate guided-biopsy are required to effectively guide needle biopsy to the suspected site. In addition, tissue stiffness measurement helps in identifying a suspected site to perform biopsy because stiffness has been shown to correlate with pathologies, such as cancerous tissue. More importantly, early detection of prostate cancer may guide minimally invasive therapy and eliminate insidious procedures. In this paper, "virtual biopsies" were taken in multiple locations in three excised prostates; SDUV shear elasticity and viscosity measurements were performed at the selected "suspicious" locations within the prostates. SDUV measurements of prostate elasticity and viscosity are generally in agreement with preliminary values previously reported in the literature. It is, however, important to emphasize here that the obtained viscoelastic parameters values are local, and not a mean value for the whole prostate.

Fig. 1

A graphic describing the principle of transrectal-SDUV for the measurement of prostate viscoelastic parameters. A harmonic shear wave is produced by a “Push” ultrasound beam, and its propagation is monitored by a separate “Detect” ultrasound beam at two positions using the same probe. The shear wave speed is calculated from its phase φ₁ and φ₂ measured at 2 locations (separated by a distance Δr) along its traveling path.

Fig. 2

Prostate imaging with two modalities: (a) corresponds to an X-ray fluoroscopic image in which clusters of calcifications appear as bright spots. The urethra is also shown as a black dot at the center of the image. (b) corresponds to a conventional B-mode ultrasound image in which 5 regions were selected and shown as colored circles. SDUV excitation points are shown as white dots on the figure. The image size is (HxW≈) 4.5×5 cm².

Fig. 3

Experimental setup for SDUV measurements. The excised prostate was embedded in a gelatin phantom. The push and detect transducers were co-focused before the experiment. The push transducer creates a propagating shear wave and the detect transducer measures the motion at several locations.

Fig. 4

Example of an SDUV stiffness and viscosity measurement of an excised human prostate within one single region. (a) & (b): amplitude of displacement and phase of vibration records detected at 3 different locations, 1 mm apart. (c): shear wave speed vs. frequency. The stiffness and viscosity were μ₁ = 1.80 kPa and μ₂ = 1.09 Pa.s, respectively (d): Shear wave speeds calculated from 5 acquisitions to check the repeatability of (c). The mean stiffness and viscosity were μ₁ = 1.78 kPa and μ₂ = 1.11 Pa.s, respectively.