An incoherent HIFU transducer for treatment of the medial branch nerve: Numerical study and in vivo validation

Abstract

Background: Chronic back pain due to facet related degenerative changes affects 4-6 million patients a year in the United States. Patients refractory to conservative therapy may warrant targeted injections of steroids into the joint or percutaneous medial branch nerve denervation with radiofrequency ablation. We numerically tested a novel noninvasive high intensity focused ultrasound transducer to optimize nerve ablation near a bone-soft tissue interface. Methods: A transducer with 4 elements operating in an incoherent mode was modeled numerically and tested pre-clinically under fluoroscopic guidance. After 6 lumbar medial branch nerve ablations were performed in 2 pigs, they were followed clinically for 1 week and then sacrificed for pathological evaluation. Results: Simulations show that the acoustic spot size in water at 6 dB was 14mm axial x 1.6mm lateral and 52mm axial x 1.6mm lateral for coherent and incoherent modes, respectively. We measured the size of N = 6 lesions induced in vivo in a pig model and compared them to the size of the simulated thermal dose. The best match between the simulated and measured lesion size was found with a maximum absorption coefficient in the cortical bone adjusted to 30 dB/cm/MHz. This absorption was used to simulate clinical scenarios in humans to generate lesions with no potential side effects at 1000 and 1500 J. Conclusion: The elongated spot obtained with the incoherent mode facilitates the targeting during fluoroscopic-guided medial branch nerve ablation.

Fluoroscopy-Guided High-Intensity Focused Ultrasound Neurotomy of the Lumbar Zygapophyseal Joints: A Clinical Pilot Study

“…Prior technical validation analyses and in vivo preclinical studies in a large animal model in which this fluoroscopy-guided HIFU system was used provided evidence for the safe and effective ablation of the MB in the present study [ 12 , 14 ].…”

Numerical study of high‐intensity focused ultrasound (HIFU) in fat reduction

“…w b and Q are the blood perfusion rate and external heat source, respectively. Q represents the volumetric heat generation due to the absorption of acoustic intensity in the tissue domain and can be calculated by the following equation: $$$$\begin{equation}Q\ = \ 2\alpha \frac{{{P}_{rms}^2}}{{\rho c}}\end{equation}$$$$ $$$$\begin{equation}p_{rms}^2 = \frac{1}{{\ {t}_f - {t}_i}}\mathop \smallint \limits_{{t}_i}^{{t}_f} {p}^2dt\ \end{equation}$$$$where α and $${P}_{rms}$$ are the absorption coefficient of tissue and root mean square (rms) of acoustical pressure 27,28 . t i and t f are the start and end times of integration.…”

“…where α and P rms are the absorption coefficient of tissue and root mean square (rms) of acoustical pressure. 27,28 t i and t f are the start and end times of integration. This interval should be in the part of the solution, in which the pressure behaves in an orderly manner.…”

Fluoroscopy-guided high-intensity focused ultrasound neurotomy of the lumbar zygapophyseal joints: a prospective, open-label study

“…HIFU energy was delivered with the Neurolyser XR annulararray transducer, which converges multiple ultrasound beams in an incoherent mode 18 onto a target location. Each beam passes through tissue with little effect, but at the focal point where the beams converge, a significant increase of acoustic intensity is achieved.…”