Non-Invasive Targeted Peripheral Nerve Ablation Using 3D MR Neurography and MRI-Guided High-Intensity Focused Ultrasound (MR-HIFU) : Pilot Study in a Swine Model

PURPOSE: Ultrasound (US)-guided high intensity focused ultrasound (HIFU) has been proposed for noninvasive treatment of neuropathic pain and has been investigated in in-vivo studies. However, ultrasound has important limitations regarding treatment guidance and temperature monitoring. Magnetic resonance (MR)-imaging guidance may overcome these limitations and MR-guided HIFU (MR-HIFU) has been used successfully for other clinical indications. The primary purpose of this study was to evaluate the feasibility of utilizing 3D MR neurography to identify and guide ablation of peripheral nerves using a clinical MR-HIFU system. METHODS: Volumetric MR-HIFU was used to induce lesions in the peripheral nerves of the lower limbs in three pigs. Diffusion-prep MR neurography and T1-weighted images were utilized to identify the target, plan treatment and immediate post-treatment evaluation. For each treatment, one 8 or 12 mm diameter treatment cell was used (sonication duration 20 s and 36 s, power 160-300 W). Peripheral nerves were extracted < 3 hours after treatment. Ablation dimensions were calculated from thermal maps, post-contrast MRI and macroscopy. Histological analysis included standard H&E staining, Masson's trichrome and toluidine blue staining. RESULTS: All targeted peripheral nerves were identifiable on MR neurography and T1-weighted images and could be accurately ablated with a single exposure of focused ultrasound, with peak temperatures of 60.3 to 85.7°C. The lesion dimensions as measured on MR neurography were similar to the lesion dimensions as measured on CE-T1, thermal dose maps, and macroscopy. Histology indicated major hyperacute peripheral nerve damage, mostly confined to the location targeted for ablation. CONCLUSION: Our preliminary results indicate that targeted peripheral nerve ablation is feasible with MR-HIFU. Diffusion-prep 3D MR neurography has potential for guiding therapy procedures where either nerve targeting or avoidance is desired, and may also have potential for post-treatment verification of thermal lesions without contrast injection

MR thermometry guidance of HIFU thermal ablation of peripheral nerves.

<p>Temperature maps (top) indicate peak temperature of 82.5°C. Thermal dose maps (bottom) predict lesion size of 12 x 29 mm.</p

Light microscopy images of nerves, H&E staining.

<p><b>A</b> Untreated peripheral nerve at 5x magnification. <b>B</b> Treated peripheral nerve (6 mm distal to center of ablation) at 5x magnification showing epineurial collagen homogenization (a), vascular congestion (b), widening of the subperineural space (arrow). <b>C</b> Untreated peripheral nerve 40X detail of axons and Schwann cells. <b>D</b> Treated peripheral nerve 40x detail showing regressive nuclear changes karyolysis (dashed arrow) and pyknosis (arrow) and vacuolization (arrowhead).</p

MR neurography of pig sciatic nerve before (top) and after (bottom) MR-HIFU.

<p>SHINKEI images reconstructed as thick-slab (15 mm) maximum intensity projections in sagittal, axial, and coronal orientations.</p

MRI evaluation of thermal lesions in peripheral nerve.

<p>Hyperintense region on 3D diffusion-prepared MR neurography (top) corresponds with non-enhancing region on T1 post-contrast image.</p

Targeting of peripheral nerve using MR-HIFU treatment planning system.

<p>The sciatic nerve is hyper-intense on MR neurography and iso-intense on T1 where dual-bundle structures are seen against fat. HIFU beam overlay (white) indicates 12 mm diameter target region.</p

Light microscopy images of peripheral nerves at 40x magnification, Masson’s trichrome staining.

<p><b>A</b> Normal appearance of myelin on Masson’s trichrome; myelin sheaths stain purple-red (black arrow). <b>B</b> Section of treated peripheral nerve at the center of the lesion showing absence of purple-red staining (arrow) indicating acute damage to the myelin sheaths. Asterisk (*) indicates subperineural edema.</p