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Peripheral Nerve Focused Ultrasound Lesioning-Visualization and Assessment Using Diffusion Weighted Imaging.

ABSTRACT: Objectives: Magnetic resonance-guided focused ultrasound (MRgFUS) is a non-invasive targeted tissue ablation technique that can be applied to the nervous system. Diffusion weighted imaging (DWI) can visualize and evaluate nervous system microstructure. Tractography algorithms can reconstruct fiber bundles which can be used for treatment navigation and diffusion tensor imaging (DTI) metrics permit the quantitative assessment of nerve microstructure in vivo. There is a need for imaging tools to aid in the visualization and quantitative assessment of treatment-related nerve changes in MRgFUS. We present a method of peripheral nerve tract reconstruction and use DTI metrics to evaluate the MRgFUS treatment effect. Materials and Methods: MRgFUS was applied bilaterally to the sciatic nerves in 6 piglets (12 nerves total). T1-weighted and diffusion images were acquired before and after treatment. Tensor-based and constrained spherical deconvolution (CSD) tractography algorithms were used to reconstruct the nerves. DTI metrics of fractional anisotropy (FA), and mean (MD), axial (AD), and radial diffusivities (RD) were measured to assess acute (<1-2 h) treatment effects. Temperature was measured in vivo via MR thermometry. Histological data was collected for lesion assessment. Results: The sciatic nerves were successfully reconstructed in all subjects. Tract disruption was observed after treatment using both CSD and tensor models. DTI metrics in the targeted nerve segments showed significantly decreased FA and increased MD, AD, and RD. Transducer output power was positively correlated with lesion volume and temperature and negatively correlated with MD, AD, and RD. No correlations were observed between FA and other measured parameters. Conclusions: DWI and tractography are effective tools for visualizing peripheral nerve segments for targeting in non-invasive surgical methods and for assessing the microstructural changes that occur following MRgFUS treatment.

SUBMITTER: Walker MR

PROVIDER: S-EPMC8299784 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Objectives: The application of magnetic resonance-guided focused ultrasound (MRgFUS) for the treatment of neurological conditions has been of increasing interest. Conventional MR imaging can provide structural information about the effect of MRgFUS, where differences in ablated tissue can be seen, but it lacks information about the status of the cellular environment or neural microstructure. We investigate in vivo acute changes in water diffusion and white matter tracts in the brain of a piglet model after MRgFUS treatment using diffusion-weighted imaging (DWI) with histological verification of treatment-related changes. Methods: MRgFUS was used to treat the anterior body of the fornix in four piglets. T1 and diffusion-weighted images were collected before and after treatment. Mean diffusion-weighted imaging (MDWI) images were generated to measure lesion volumes via signal intensity thresholds. Histological data were collected for volume comparison and assessment of treatment effect. DWI metric maps of fractional anisotropy (FA), apparent diffusion coefficient (ADC), axial diffusivity (AD), radial diffusivity (RD), and mean diffusivity (MD) were generated for quantitative assessment. Fornix-related fiber tracts were generated before and after treatment for qualitative assessment. Results: The volume of treated tissue measured via MDWI did not differ significantly from histological measurements, and both were significantly larger than the treatment cell volume. Diffusion metrics in the treatment region were significantly decreased following MRgFUS treatment, with the peak change seen at the lesion core and decreasing radially. Histological analysis confirmed an area of coagulative necrosis in the targeted region with sharp demarcation zone with surrounding brain. Tractography from the lesion core and the fornix revealed fiber disruptions following treatment. Conclusions: Diffusion maps and fiber tractography are an effective method for assessing lesion volumes and microstructural changes in vivo following MRgFUS treatment. This study demonstrates that DWI has the potential to advance MRgFUS by providing convenient in vivo microstructural lesion and fiber tractography assessment after treatment.

Project description:BackgroundModerate-to-severe acute pain is prevalent in many healthcare settings and associated with adverse outcomes. Peripheral nerve blockade using traditional needle-based and local anesthetic-based techniques improves pain outcomes for some patient populations but has shortcomings limiting use. These limitations include its invasiveness, potential for local anesthetic systemic toxicity, risk of infection with an indwelling catheter, and relatively short duration of blockade compared with the period of pain after major injuries. Focused ultrasound is capable of inhibiting the peripheral nervous system and has potential as a pain management tool. However, investigations of its effect on peripheral nerve nociceptive fibers in animal models of acute pain are lacking. In an in vivo acute pain model, we investigated focused ultrasound's effects on behavior and peripheral nerve structure.MethodsFocused ultrasound was applied directly to the sciatic nerve of rats just prior to a hindpaw incision; three control groups (focused ultrasound sham only, hindpaw incision only, focused ultrasound sham+hindpaw incision) were also included. For all four groups (intervention and controls), behavioral testing (thermal and mechanical hyperalgesia, hindpaw extension and flexion) took place for 4 weeks. Structural changes to peripheral nerves of non-focused ultrasound controls and after focused ultrasound application were assessed on days 0 and 14 using light microscopy and transmission electron microscopy.ResultsCompared with controls, after focused ultrasound application, animals had (1) increased mechanical nociceptive thresholds for 2 weeks; (2) sustained increase in thermal nociceptive thresholds for ≥4 weeks; (3) a decrease in hindpaw motor response for 0.5 weeks; and (4) a decrease in hindpaw plantar sensation for 2 weeks. At 14 days after focused ultrasound application, alterations to myelin sheaths and nerve fiber ultrastructure were observed both by light and electron microscopy.DiscussionFocused ultrasound, using a distinct parameter set, reversibly inhibits A-delta peripheral nerve nociceptive, motor, and non-nociceptive sensory fiber-mediated behaviors, has a prolonged effect on C nociceptive fiber-mediated behavior, and alters nerve structure. Focused ultrasound may have potential as a peripheral nerve blockade technique for acute pain management. However, further investigation is required to determine C fiber inhibition duration and the significance of nerve structural changes.

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Peripheral Nerve Focused Ultrasound Lesioning-Visualization and Assessment Using Diffusion Weighted Imaging.

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