Shear-wave imaging guidance for high-intensity focused ultrasound therapy

Abstract

High-intensity focused ultrasound (HIFU) has attracted many research interests during the last three decades (since 1980s) for the precision of this treatment in the field of medical oncology and brain surgery. Even though the precision of HIFU application/exposure is particularly high, the overall quality of HIFU treatment is still under active development due to the need of better imaging guidance. Current technology for guiding HIFU is medical ultrasound imaging and magnetic resonance imaging, which mostly employ diagnostic imaging mode.

Most recently, magnetic resonance imaging has been adapted for real-time HIFU temperature monitoring (MRI thermometry). In contrast, medical ultrasound guidance technique for HIFU still relies mostly on B-image mode and the hyperechoic contrast of the HIFU lesion. Additionally, commercial MR thermometry is relatively expensive, and is not truly 2D real-time thermometry imaging at the time of writing (ExAblate, Insightec, Israel). In either case, the imaging guidance relies on the physical changes inside the tissue, which are beyond the normal physiology range, in order to effectively track and monitor the treatment area. This limitation causes unnecessary risk to healthy tissues during the targeting and monitoring in HIFU treatment.

HIFU-induced shear-wave (HiSW) imaging is proposed in this thesis as a new technique for tracking and monitoring the changes in the targeted area. The work includes HIFU transducer fabrication and characterization, in accordance with the FDA guidance. The transducers are then tested for the ability to induce shear-wave, using medical ultrasound imaging and high-resolution optical coherence tomography imaging.

Ultra-fast imaging technique in medical ultrasound is developed to detect small deformation, which is caused by the propagating HiSW. Due to physical limitations of ultrasound imaging, HiSW has to operate above the thermal and mechanical FDA limit. The parameters of acoustic intensity required for adequate HiSW detection are 200 W.cm-2 for ISPPA and 10 W.cm-2 for ISATA.

To avoid the low-resolution problem in medical ultrasound imaging, Optical Coherence Tomography is proposed for in-depth study of HiSW. Investigations on how HiSW behaves under different acoustic power output and pulse duration are conducted. The results indicate a linear relationship between HiSW displacement and acoustic power output, i.e. higher acoustic power output would give higher shear-wave displacement. In contrast, there is an optimum pulse duration, where HiSW displacement is highest for a given unit of acoustic energy.

Final work on beam steering explores the feasibility of using phased-array HIFU transducer to improve focused ultrasound treatment. The results show improved thermal elevation when the target is in constant motion, up to 20 oC in improvement. In future work, HiSW can give elasticity information of the surrounding area by the help of beam steering.

Date of Award	2019
Original language	English
Awarding Institution	University of Dundee
Supervisor	Zhihong Huang (Supervisor) & Ghulam Nabi (Supervisor)