Evaluation of MRI protocols for the assessment of lumbar facet joints after MR-guided focused ultrasound treatment

Abstract

Background: MR-guided focused ultrasound (MRgFUS) might be a very safe and effective minimally invasive technique to treat facet joint pain caused by arthritis and other degenerative changes. However, there are still safety concerns for this treatment and challenges regarding MR imaging and temperature mapping due to susceptibility effects between the bone and soft tissue near the joint, which has resulted in poor MR image quality. The goal of this research was to evaluate multiple magnetic resonance imaging (MRI) pulse sequences for characterizing ablated lumbar facet joint lesions created by high-intensity focused ultrasound (FUS) and compare the findings to histological tissue assessment. In particular, we investigated the use of T2-weighted MRI to assess treatment effects without contrast administration.

Methods: An IACUC approved study (n = 6 pigs) was performed using a 3T widebore MRI system equipped with an MRgFUS system. Facet joints of the lumbar vertebra were ablated using 1-MHz frequency and multiple sonication energies (300-800 J). In addition to T2-weighted MRI for treatment planning, T1-, T2-, and T2*-weighted and perfusion MRI sequences were applied. Signal intensity ratios of the lesions were determined. Histopathology was used to characterize cellular changes.

Results: Ablation of the facet joint, using MRgFUS, was successful in all animals. T2-weighted images showed high signal intensity in the edematous facet joint and adjacent muscle, while delayed contrast-enhanced T1-weighted images showed an enhanced ring surrounding the target volume. T2*-weighted GRE images revealed inconsistent lesion visualization. Histopathology confirmed the presence of cellular coagulation (shrinkage), extracellular expansion (edema), and hemorrhage in the bone marrow.

Conclusions: MRgFUS provided sufficient precision and image quality for visualization and characterization of ablated facet joints directly after ablation. MRI may help in monitoring the efficacy of FUS ablation without contrast after treating patients with back pain.

Keywords: Delayed contrast-enhanced MRI; Facet joint; MRgFUS; T2-weighted FSE.

Figures

Fig. 1

T2-weighted planning images are shown with the focal spot on the facet joint (left image). The beam orientation was chosen to best protect the spinal nerve roots, the spinal canal, and the spinal process as outlined in green. After HIFU treatment, the area of ablation is shown in blue (dose overlay) on the right image

Fig. 2

Coronal T1-FSE images. a Baseline. b After FUS ablation. c After both FUS and contrast injection. The lesions were delineated, as hypoenhanced zone, after administration of 0.15 mmol/kg Gd-DTPA, suggesting lack of delivery of the contrast media due to the damage of microvessels

Fig. 3

Coronal T2-FSE (left) and DCE T1-FSE (right) after FUS treatment. The T2-FSE image shows a positive contrast of the lesions. The DCE T1-FSE image shows the hyperenhancement of the border zone, but not the core of ablated lesion. The following energies were applied (from top to bottom): 650, 300, 500, and 800 J, and the arrows indicate ablated lesions. A positive relationship between energy and lesion size can be appreciated in both images

Fig. 4

Axial T2-FSE (left) and DCE T1-FSE (right) after FUS treatment. The T2-FSE image shows a positive contrast of the lesions. The DCE T1-FSE image shows the hyperenhancement of the border zone, but not the core of ablated lesion

Fig. 5

Axial T2-weighted images. a Acquired before FUS. b After FUS treatment. The lesion is clearly visible as hyperintense signal, suggesting the presence of edema. The sonication energy used for the depicted joint was 800 J. Smaller lesion sizes were generated with smaller sonication energies

Fig. 6

Sagittal MR images of the paraventricular muscles obtained on T2-FSE (left) and DCE T1-FSE. T2-FSE image shows the hyperintense edematous ablated lesion, while T1-FSE image shows the donut pattern enhancement after administration of Gd-DTPA

Fig. 7

Representative axial T2*-weighted images acquired before (a) and after (b) FUS ablation. The slice location corresponds to the slices depicted in Fig. 4. A small hyperintense lesion is visible after ablation (arrow)

Fig. 8

Sagittal image and histopathological section through the paravertebral muscles show the wedge shape-ablated lesion on DCE (left) and microscopy (right). Arrows denote the necrotic core with damaged tissue. Note the border edematous hyperenhanced zone (arrowheads) surrounding the ablated core on DCE (scale shows the SI range in arbitrary units). The border edematous zone is also seen on microscopy (arrowheads, calibration bar = 100 μm)

Fig. 9

Microscopic sections of dorsal root ganglion after 800-J sonication showing no evidence of injury in neural cells or axons. Magnifications are ×10, ×40, and ×100

Fig. 10

Histological sections of facet joint. a Control side with zoom ×2. b Control side with zoom ×40. c Ablated side with ×2. d Ablated side with ×40. Evidence of hemorrhage is clearly demonstrated in the treated facet joint. There was no change in the architecture of the osteoblast, most likely due to the short period of exam after ablation