[Paper Review] Quasi-Real Time Multi-Frequency 3D Shear Wave Absolute Vibro-Elastography (S-WAVE) System for Prostate

Tajwar Abrar Aleef, Julio Lobo+8 more|arXiv (Cornell University)|May 9, 2022

Ultrasound Imaging and Elastography4 citations

TL;DR

This paper presents a quasi-real-time, 3D multi-frequency shear wave elastography system for prostate cancer detection using transperineal mechanical excitation and radiofrequency ultrasound data. By employing bandpass sampling and speckle tracking, it enables accurate, low-frame-rate tissue motion tracking and absolute elasticity quantification, achieving an AUC of 0.82 ± 0.01 in classifying cancer in the peripheral zone via histopathology correlation.

ABSTRACT

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.

Motivation & Objective

• To develop a quasi-real-time, 3D, quantitative elastography system for prostate tissue stiffness mapping to improve cancer detection.
• To overcome limitations of conventional shear wave elastography, such as low signal-to-noise ratio and tissue heating from high-intensity acoustic pulses.
• To enable volumetric, absolute elasticity measurements using multi-frequency, steady-state mechanical excitation via a voice coil shaker.
• To validate the system’s accuracy and feasibility using phantoms and two clinical studies with histopathology correlation.
• To demonstrate the clinical utility of absolute elasticity values in identifying cancerous regions in the prostate peripheral zone.

Proposed method

• A mechanical voice coil shaker delivers multi-frequency transperineal vibrations (75–80 Hz) to induce steady-state shear waves in the prostate.
• Radiofrequency (RF) ultrasound data is streamed from a BK Medical 8848 trans-rectal probe to an external computer for processing.
• Speckle tracking is used to measure tissue displacement from the shear wave motion with sub-micrometer accuracy.
• Bandpass sampling is applied to reconstruct high-frequency tissue motion at sampling rates below the Nyquist rate, eliminating the need for ultra-fast frame rates.
• A computer-controlled roll motor rotates the transducer array to acquire 3D volumetric data across multiple sagittal planes.
• Tissue elasticity is computed from the local wavelength of the shear wave using a local frequency estimator, with results averaged across multiple frequencies for robustness.

Experimental results

Research questions

• RQ1 Can a multi-frequency, steady-state shear wave elastography system achieve accurate, 3D, absolute elasticity measurements in the prostate at quasi-real-time frame rates?
• RQ2 Does bandpass sampling enable reliable tracking of high-frequency tissue motion without requiring ultra-high frame rate ultrasound systems?
• RQ3 Can absolute elasticity values derived from this system correlate numerically and visually with histopathologically confirmed cancerous tissue in the prostate?
• RQ4 How does the system perform in clinical settings, particularly in distinguishing cancerous from benign tissue in the peripheral zone?
• RQ5 What are the sources of false positives or discrepancies in elasticity measurements, and how do biological factors like calcification or edema affect results?

Key findings

• The system achieved an area under the receiver operating characteristic curve (AUC) of 0.82 ± 0.01 for classifying prostate cancer in the peripheral zone using a multi-parametric classifier combining elasticity and phasor data.
• Strong numerical and visual correlation was observed between regions of elevated stiffness on absolute elasticity maps and histopathologically confirmed cancerous tissue in both the prostatectomy and focal therapy studies.
• Phantom validation showed good agreement between the system’s elasticity measurements and those obtained via magnetic resonance elastography (MRE).
• The use of bandpass sampling enabled accurate reconstruction of high-frequency tissue motion at frame rates significantly below the Nyquist rate, reducing hardware and computational demands.
• False positive stiffness elevations were observed in some regions, potentially due to artifacts such as calcification or edema, highlighting the need for improved modeling of viscoelastic and wave pattern effects.
• The system demonstrated feasibility for in vivo 3D volumetric elastography in the prostate without exceeding FDA safety limits for tissue heating.