Combined ARFI and Shear Wave Imaging of Prostate Cancer: Optimizing Beam Sequences and Parameter Reconstruction Approaches

Abstract

This study demonstrates the implementation of a shear wave reconstruction algorithm that enables concurrent acoustic radiation force impulse (ARFI) imaging and shear wave elasticity imaging (SWEI) of prostate cancer and zonal anatomy. The combined ARFI/SWEI sequence uses closely spaced push beams across the lateral field of view and simultaneously tracks both on-axis (within the region of excitation) and off-axis (laterally offset from the excitation) after each push beam. Using a large number of push beams across the lateral field of view enables the collection of higher signal-to-noise ratio (SNR) shear wave data to reconstruct the SWEI volume than is typically acquired. The shear wave arrival times were determined with cross-correlation of shear wave velocity signals in two dimensions after 3-D directional filtering to remove reflection artifacts. To combine data from serially interrogated lateral push locations, arrival times from different pushes were aligned by estimating the shear wave propagation time between push locations. Shear wave data acquired in an elasticity lesion phantom and reconstructed using this algorithm demonstrate benefits to contrast-to-noise ratio (CNR) with increased push beam density and 3-D directional filtering. Increasing the push beam spacing from 0.3 to 11.6 mm (typical for commercial SWEI systems) resulted in a 53% decrease in CNR. In human in vivo data, this imaging approach enabled high CNR (1.61-1.86) imaging of histologically-confirmed prostate cancer. The in vivo images had improved spatial resolution and CNR and fewer reflection artifacts as a result of the high push beam density, the high shear wave SNR, the use of multidimensional directional filtering, and the combination of shear wave data from different push beams.

Keywords: acoustic radiation force; prostate cancer; shear wave imaging; ultrasound.

Fig. 1:

Diagram of the location of the ARFI and SWEI track beams relative to the acoustic radiation force push excitation, for the 12L4 transducer. For each push, four ARFI track beams were centered about the push excitation and spaced 0.17 mm apart. The off-axis track beams (6 left and 6 right) were located 2.01 mm from the push excitation, with 0.78 mm track spacing between these SWEI track beams.

Fig. 2:

(a) Lateral position and time of peak velocity values computed from tracked data from two consecutive pushes. (b) The shear wave propagation time between the two adjacent push locations is estimated. Each estimate is represented by a black line. (c) The velocity signals are aligned by temporally offsetting the signals obtained at the tracking locations from the second push by the average of the five estimated propagation times. (d) Aligned shear wave space-time trajectory over the entire 50 mm lateral field of view at an axial depth centered in a lesion in the CIRS elasticity phantom. The lesion, which is associated with a steeper slope indicating a higher shear wave speed, is delineated by the dotted white lines.

Fig. 3:

Shear wave elasticity images reconstructed with downsampled push beam locations with spacing of (a) 0.3 mm, (b) 4.2 mm, (c) 8.3 mm, and (d) 11.6 mm. The blue pixels near the bottom of the images indicate that a shear wave speed could not be reliably estimated due to decorrelation in those regions.

Fig. 4:

Effects of push beam spacing and push beam transmit voltage on target contrast-to-noise ratio in the phantom dataset with track spacing of 0.2 mm. Data are shown for push beam transmit voltages of 50 V (blue) and 30 V (green). Shaded error bars represent the standard deviation over six speckle realizations.

Fig. 5:

Effects of push beam spacing and directional filtering on target contrast-to-noise ratio in the phantom dataset with track spacing of 0.2 mm. Data are shown for 3-D directional filtering (blue) and no directional filtering (orange) applied. Shaded error bars represent the standard deviation over six speckle realizations.

Fig. 6:

Contrast-to-noise ratio versus lateral track spacing in the phantom dataset with a push spacing of 0.3 mm. The data were linearly fit (blue dotted line).

Fig. 7:

Edge resolution versus lateral track spacing in the phantom dataset with a push spacing of 0.3 mm. The data were linearly fit (blue dotted line).

Fig. 8:

Matched in vivo B-mode, SWEI, and ARFI prostate images in axial (top row) and coronal (bottom row) views, with a Gleason Grade Group 1 (Gleason score 3 + 3 = 6) cancerous lesion (green arrows) that was confirmed in whole-mount histology (green region). The stiff central zone of the prostate (white arrows) is distinguished from the softer surrounding peripheral zone. The contrast-to-noise ratio, computed from segmented cancerous and non-cancerous regions, was 0.93 for B-mode, 1.61 for SWEI, and 1.74 for ARFI.

Fig. 9:

Matched in vivo prostate images in axial (top row) and coronal (bottom row) views, with a large Gleason Grade Group 2 (Gleason score 3 + 4 = 7) cancerous lesion (green arrows) that was confirmed in whole-mount histology (purple region is Gleason 4, green region is Gleason 3). White arrows indicate the stiff central zone of the prostate. The urethra (cyan arrows) appears soft in both SWEI and ARFI. The contrast-to-noise ratio, computed from segmented cancerous and non-cancerous regions, was 0.81 for B-mode, 1.86 for SWEI, and 1.70 for ARFI.