MRI-guided focused ultrasound ablation of lumbar medial branch nerve: Feasibility and safety study in a swine model

Abstract

Purpose: About 10-40% of chronic low back pain cases involve facet joints, which are commonly treated with lumbar medial branch (MB) radiofrequency neurotomy. Magnetic resonance imaging-guided focused ultrasound (MRgFUS), a non-invasive, non-ionising ablation modality used to treat tumours, neuropathic pain and painful bone metastasis can also be used to disrupt nerve conduction. This work's purpose was to study the feasibility and safety of direct MRgFUS ablation of the lumbar MB nerve in acute and subacute swine models.

Materials and methods: In vivo MRgFUS ablation was performed in six swine (three acute and three subacute) using a clinical MRgFUS system and a 3-T MRI scanner combination. Behavioural assessment was performed, and imaging and histology were used to assess the treatment.

Results and conclusions: Histological analysis of the in vivo studies confirmed thermal necrosis of the MB nerve could be achieved without damaging the spinal cord or adjacent nerve roots. MRgFUS did not cause changes in the animals' behaviour or ambulation.

Keywords: FUS; Facet joint; HIFU; MRI-guided; nerve ablation; swine model.

Figures

Figure 1

A. Schematic of facet joint FUS ablation, showing the FUS beam (blue) aimed at the surface of the facet joint, and the far field of the beam aimed away from the spinal cord. B. Schematic of the proposed FUS ablation approach with direct targeting of the MB nerve and far field area and spinal canal (black line) partially overlapping.

Figure 2

A. Schematic of a FUS treatment setup showing the axial MR image of the setup with a swine positioned on top of the gel pad and the FUS beam focus (green rectangle) placed between the articular and transverse processes (labelled as ap and tp). The FUS beam is aimed directly at the MB nerve. B. Example of region-of-treatment contour (solid magenta contour) drawn by the operator during the planning phase. Dashed green rectangles represent the focal regions of the individual sonications automatically prescribed by the planning software. C. Example of planning where no region of treatment was prescribed and instead each sonication’s focal region (dashed green rectangle) was individually placed by the operator.

Figure 3

MB nerve in Yorkshire swine was found to pass adjacent to the bone at the junction of the transverse and articular processes (labeled tp and ap). On histology (A), the nerve is seen surrounded by a layer adipose tissue (arrow). Scale bar is 1 cm. On MRI (B), a small hyper intense region surrounded by a hypo intense rim (arrow) corresponds to the position of the nerve determined from histology. Location of spinal cord is labeled as sc.

Figure 4

Follow-up contrast-enhanced cropped MR and CT images from subacute experiment 1, showing the changes in contrast uptake from ablation of the vertebral level target at L4 using 9 sonications. Location of the axial slice (A) is shown with white dashed line on the oblique coronal image (C), and the axial slice shown in (B) is positioned adjacent and caudal to slice in A. Thermal lesions appear as non-enhancing areas (white arrows) due to the lack of perfusion resulting from thermal damage. The dashed double-sided arrow in D shows the measurement of the extent of the lesion along the hypotenuse between the transverse and articular processes as reported in Table 1. The solid double-sided arrow shows the measurement of the thickness of the pedicle bone. Scale bar is 2 cm.

Figure 5

A–C: Example of heating of transverse process in acute experiment 2. A. Cropped magnitude image of the MR thermometry sequence with an overlay of the focal spot and the beam path, as displayed by the planning software. B. Peak temperature that resulted from a sonication planned in A with acoustic energy 646 J (acoustic power of 32 W and sonication duration of 20 s). Highest temperature was 63 C. Contrast-enhanced MR image showing a hypo intense region at the edge of the transverse process (arrowhead) corresponding to the location of heating in B. D. Follow-up contrast-enhanced MR image from acute experiment 3, showing the lesion, non-enhancing region labeled with arrowhead, which shifted away from the MB nerve due to the shift of the animal position

Figure 6

Examples of gross pathology from the acute experiment 2 and subacute experiment 1. Ablation in muscle tissue (black arrow) appears as a paler area in the acute case (A-B) and the area surrounded by the brown rim in the subacute case (C–D). The brown rim of hemorrhage is apparent in the superficial region of the bone (asterisk) in the subacute case (C). The nerve roots, superior to the ablations, appear normal on gross pathology (B, D). The dashed box shows the region selected for histological analysis. Subacute gross pathology image corresponds to the follow-up images in Fig 4.

Figure 7

Low magnification images of H&E histology from the acute (A) and subacute (B) studies. The dark red rim of hemorrhage (short arrows) visible at the surface of the bone between the articular process (ap) and transverse process (tp) made thermal lesions readily visible in subacute cases, compared to the acute cases. Changes in bone do not reach the spinal canal (sc). The nerve is shown with a long arrow. Dashed line delineates the interface between the bone and muscle. 1 mm scale bar.

Figure 8

H&E histology of the targeted nerve and adjacent muscle. Compared to normal muscle (A), ablated muscle showed swelling of muscle fibres, sarcoplasmic hypereosinophilia and fragmentation, and nuclear shrinkage (B) with macrophages present at the boundary of the lesion in subacute cases (arrow, C). 50 µm scale bar. Compared to the control nerve (D), the ablated nerve exhibited at least one, and often a combination, of the following changes: hyalinization of epineurial collagen (asterisk, E), nuclear shrinkage (arrow, E, F), hyperemia of endoneurial vessels (long arrow, F), and loss of axons associated with dilation of the myelin sheath (arrowhead, F). 20 µm scale bar.

Figure 9

H&E histology showing the cellular changes in the bone adjacent to the ablated MB nerve. Compared to control tissue, ablated bone had the following changes: shrinkage of osteoblasts (arrow, D, E) and increased eosinophilia of the extracellular spaces (asterisk) and hemorrhage (arrow) in bone marrow (F). 50 µm scale bar.