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Biomedical Imaging and Instrumentation

Biomedical Imaging and Instrumentation - Poster Session E

Poster R13 - Prostate Ablation Surgery Planning using High Intensity Focused Ultrasound-based Elastography Mapping

Saturday, October 26, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Gerald L. Lee, BS Biomedical Engineering (he/him/his)

Ph.D. Student
Center for Industrial and Medical Ultrasound, Applied Physics Lab, Department of Bioengineering, UW Seattle
Seattle, Washington, United States

Co-Author(s)

MS

Minho Song

Ph.D.
Center for Industrial and Medical Ultrasound, Applied Physics Lab, Department of Mechanical Engineering, Department of Gastroenterology, UW Seattle, United States
OS

Oleg A. Sapozhnikov

Senior Principal Engineer
Center for Industrial and Medical Ultrasound, Applied Physics Lab, Department of Gastroenterology, UW Seattle, United States
IP

Ivan M. Pelivanov

Research Associate Professor
Department of Bioengineering, UW Seattle, United States
GS

George R. Schade

Urologist, Assistant Professor
Department of Urology, UW Seattle, United States

Primary Investigator(s)

TK

Tatiana D. Khokhlova

Research Associate Professor
Center for Industrial and Medical Ultrasound, Applied Physics Lab, Department of Gastroenterology, UW Seattle, United States

Introduction: Prostate Cancer (PCa) ablation surgery using endorectal high-intensity focused ultrasound (HIFU) relies on defining accurate margins of the tumor before treatment. Shear wave elastography (SWE) through transrectal ultrasound (US) has been a reliable tool to outline PCa, but is restricted by push capabilities to depths less than 4 cm. Additionally, clinical HIFU therapy systems lack built-in arrangements to verify positional accuracy following SWE planning.

To perform SWE at larger depths and navigate limitations in system arrangements, a HIFU array integrated with an in-line US imaging probe was developed. The HIFU array can produce stronger acoustic radiation force based push than transrectal US systems. The in-line US imaging probe can then be used to observe the resulting tissue displacement, providing a completely integrated method to perform tissue elasticity mapping immediately prior to HIFU-based treatment. We aim to validate this HIFU-SWE method in tissue-mimicking phantoms and apply these findings to precisely define PCa during HIFU-based therapy planning.

Materials and

Methods: Tissue displacement patterns resulting from numerical simulation of a localized HIFU push reveal two shear wave types: “classic” shear waves propagating perpendicularly to the acoustic axis, and “longitudinal” shear waves that propagate along the axis (Figure 1). Tracking tissue displacement along the acoustic axis over time after the HIFU push can provide the longitudinal shear wave velocity, from which the elastic modulus can be calculated.

A 1.5 MHz HIFU transducer (75 millimeter aperture, F-number 0.75) was built in-house with a coaxial US imaging probe (ATL P7-4) controlled by a Verasonics V1 system (Figure 2A). A variable HIFU push was delivered into a homogenous tissue-mimicking phantom, and the resulting displacement along the acoustic axis was measured by the US imaging probe through speckle tracking over a 50 millisecond time window (Figure 2B). The longitudinal shear wave group velocity is then determined using a linear regression over different points on the acoustic axis, identified by the arrival time of maximum displacement (Figure 3A).

Tissue-mimicking phantoms (6% polyvinyl alcohol, 3% Al2O3 powder) were fabricated to model different elastic moduli by changing the number of freeze-thaw cycles (FC) from 1 to 3, with mean elastic moduli calibrated as 7.5, 30, and 60 kPa respectively using a clinical SWE imaging system (SSI Aixplorer). These elasticities are consistent with suggested elasticity ranges used to discriminate between benign (25-30 kPa) and cancerous (55-80 kPa) prostate tissue in humans.

Results, Conclusions, and Discussions: Observed maximum displacement ranged from 1-50 microns, increasing with a larger push duration and decreasing with larger target stiffness. Two longitudinal shear waves – one traveling towards the transducer, and the other away – were observed in each recorded propagation pattern. Only the shear wave traveling towards the transducer was considered due to potential thermal lens effects at the HIFU focus affecting the speed of the other shear wave.

The HIFU-SWE method produced mean elastic moduli of 5, 29, and 71 kPa for the three phantoms (FC 1-3) respectively. As seen in Figure 3B, those values are within the measurement uncertainty of those yielded by the clinical SWE system. These findings demonstrate that the HIFU-SWE method is feasible and allows for accurate determination of the elastic modulus distribution along the HIFU axis, which makes it a promising tool for mapping targeted tissue elasticity immediately before HIFU treatment. The use of a multi-element HIFU array with electronic focus steering would allow to perform this mapping without mechanical translation of the HIFU transducer, and is planned in the near future.

Acknowledgements (Optional):