Introduction: Thermal ablation modalities for neurosurgical use currently include laser interstitial thermal therapy and transcranial MR guided focused ultrasound. There is mounting evidence that focused ultrasound (FUS) can also be used to modulate rather than ablate nervous tissue. In contrast to ablative lesions, which tend to require temperatures of above 60°C, the thermal changes needed for neural modulation have not been defined. Currently, MR thermal imaging lacks the sensitivity to measure the subtle temperature changes required for neuromodulation. In this preclinical study, we demonstrate the feasibility of using a thermode to evaluate temperatures associated with FUS neuromodulation.
Methods: Seven-week-old male Sprague-Dawley rats were treated with vincristine (intraperitoneal injection of .01mg/kg) to induce chemotherapy induced neuropathic pain. FUS was applied to the left L5 dorsal root ganglia (DRG) guided by a thermode at the area. Responses of the left hindpaw to Vonfrey filament and the Randall Selitto test (RST) to evaluate innocuous and noxious mechanical stimuli respectively, were evaluated before and after FUS treatment.
Results: The mean rise in temperature was 3.2°C from a baseline of 32.4°C ± 0.21°C to 35.6°C ± 0.12°C. This 3°C temperature change resulted in decreased sensitivity of mechanical thresholds from baseline through day 5 (p=.019). RST also demonstrated significant differences post treatment (p=.032). A power amplitude of 2.5W for a duration of 3 min achieved these goals.
Conclusions: Here we demonstrate that a 3°C change in the DRG during FUS can modulate behavioral effects for 5 days post treatment. Our ablative work suggests that nervous tissue is more susceptible to thermal changes than muscle or liver due to its acoustic properties. Moving forward we will develop an external thermode to be used on our FUS delivery device that resembles a diagnostic ultrasound to offer a fully external option for treatment of pain.
Patient Care: This research explores pulsed focused ultrasound as a palliative measure for neuropathic pain.
Learning Objectives: To evaluate a thermode for FUS modulation