Delivering Focused Ultrasound to Intervertebral Discs Using Time-Reversal

Abstract

Chronic low back pain causes more disability worldwide than any other condition, and is thought to arise in part through loss of biomechanical function of degenerate intervertebral discs (IVD). Current treatments can involve replacing part or all of the degenerate IVD by invasive surgery. Our vision is to develop a minimally invasive approach in which high intensity focused ultrasound (HIFU) is used to mechanically fractionating degenerate tissue in an IVD; a fine needle is then used to first remove the fractionated tissue and then inject a biomaterial able to restore normal physiological function. The goal of this manuscript is to demonstrate the feasibility of trans-spinal HIFU delivery using simulations of three-dimensional ultrasound propagation in models derived from patient CT scans. The CT data were segmented in to bone, fat and other soft tissue for three patients. Ultrasound arrays were placed around the waist of each patient model and time-reversal was used to determine the source signals necessary to create a focus in the centre of the disc. The simulations showed that for 0.5 MHz ultrasound a focus could be created in most of the lumbar IVDs, with the pressure focal gain ranging from 3.2 to 13.7. In conclusion, it is shown that, with patient specific planning, focusing ultrasound into an IVD is possible in the majority of cases despite the complex acoustic path introduced by the bony structures of the spine.

In silico feasibility assessment of extracorporeal delivery of low-intensity pulsed ultrasound to intervertebral discs within the lumbar spine

“…This is likely due to the larger posterior window between spinous processes at the L5-S1 level. Greatest localization capabilities in L2-L3 and L3-L4 agrees with the findings of Qiao et al (2019), and is likely due to the minimized shadowing effects of hip and rib bone structures at these levels. The peak temperature elevations corresponding to a representative LIPUS target exposure of 100 mW cm −2 I SPTA , as shown in figure 10, were <1 • C and thereby below thermal toxicity thresholds for all investigated target point and anatomical model combinations.…”

“…Numerical modeling approaches have incorporated time-reversal in order to calculate necessary driving parameters (amplitude and phase settings) that can be applied to phased arrays in order to mitigate beam distortion, and have been experimentally validated to restore high quality focusing capabilities in complex anatomical environments (Aubry et al 2003, Marquet et al 2009. Qiao et al applied time-reversal modeling techniques to investigate the feasibility of generating high acoustic pressures and intensities within the nucleus of lumbar IVDs in order to generate localized inertial cavitation and fractionate the nucleus to facilitate partial disc replacement interventions (Qiao et al 2019). This study investigated a custom bilateral phased array with curvilinear geometry surrounding posterolateral aspects of the body, designed to produce a tight acoustic focus at the very center of the IVD.…”

Oxford’s clinical experience in the development of high intensity focused ultrasound therapy

“…The aim of the TARDOX trial [29][30][31][32] was to assess the safety and feasibility of delivering doxorubicin to solid tumours via thermosensitive liposomes. This was a phase I trial conducted at Churchill Hospital.…”

Strategies and safety simulations for ultrasonic cervical spinal cord neuromodulation

“…Ultrasonic approaches to lumbar spine have been investigated using simulation to estimate pressure distributions of ultrasound focused to intervertebral discs (Qiao et al 2019). Ultrasonic approaches to the human cervical spinal cord have not been addressed, and the morphology of the cervical spine offers several challenges and opportunities for ultrasound delivery.…”