Percutaneous Spinal Ablation in a Sheep Model: Protective Capacity of an Intact Cortex, Correlation of Ablation Parameters with Ablation Zone Size, and Correlation of Postablation MRI and Pathologic Findings

Abstract

Background and purpose: Despite the growing use of percutaneous ablation therapy for the treatment of metastatic spine disease, several issues have yet to be fully addressed. Our aims were to determine whether the vertebral body cortex protects against ablation-induced spinal cord injury; correlate radiofrequency, cryo-, and microwave ablation parameters with resulting spinal ablation zone dimensions and describe normal spinal marrow postablation changes on MR imaging.

Materials and methods: Ten thoracolumbar vertebrae in 3 sheep were treated with radiofrequency ablation, cryoablation, or microwave ablation under fluoroscopic guidance. Technique parameters were chosen to produce ablation zones that exceeded the volume of the vertebral bodies in sheep 1 and were confined to the vertebrae in sheep 2 and 3. Expected ablation zone dimensions were based on data provided by the device manufacturers. Postablation MR imaging was performed at 48 hours (sheep 1) or 7 days (sheep 2 and 3).

Results: In sheep 1, cryoablation and microwave ablations extended into the spinal canal and caused histologically confirmed neurologic injury, but radiofrequency ablation did not. The mean difference between the lengths of the radiofrequency ablation zone dimensions measured on gross pathology compared with those expected was 9.6 ± 4.1 mm. The gross pathologic cryo- and microwave ablation zone dimensions were within 1 mm of those expected. All modalities produced a nonenhancing ablation zone with a rim of enhancement, corresponding histologically to marrow necrosis and hemorrhagic congestion.

Conclusions: An intact cortex appears to protect against radiofrequency ablation-induced spinal cord injury, but not against non-impedance-based modalities. Ablation dimensions produced by microwave and cryoablation are similar to those expected, while radiofrequency ablation dimensions are smaller. Ablation of normal marrow produces a rim of enhancement at the margin of the ablation zone on MR imaging.

Fig 1.

Fluoroscopic documentation of ablation probe placement in sheep. Anteroposterior (A, C, and E) and lateral (B, D, and F) fluoroscopic images show transpedicular placement of the cryoablation probe within the T14 vertebra (A and B), the radiofrequency ablation probe within the L3 vertebra (C and D), and the microwave ablation probe within the L6 vertebra (E and F).

Fig 2.

Anteroposterior (A) and lateral (B) CT images of a normal lumbar vertebra. The thick black line represents the unilateral transpedicular trajectory of the ablation probe. By definition, the transverse and anteroposterior dimensions of the ablation zone are parallel and perpendicular to the ablation probe in the axial plane, respectively, and the craniocaudal dimension of the ablation zone is perpendicular to the ablation probe in the sagittal plane.

Fig 3.

Sagittal postcontrast T1-weighted image with fat suppression (A) and a T2-weighted image (B) of the vertebral level treated with cryoablation show the ablation zone extending beyond the posterior vertebral body into the spinal canal. There is enhancement and intramedullary T2 signal hyperintensity in the ventral aspect of the edematous spinal cord (white arrowheads). There is also inflammation in the soft tissues ventral to the spine where the ablation zone extended beyond the anterior vertebral body cortex (white arrows). Sagittal postcontrast T1-weighted image (C) and axial T2-weighted image (D) of the vertebral level treated with microwave ablation similarly show extension of the ablation zone into the spinal canal (white arrowheads). E, Hematoxylin-eosin staining of the spinal cord at the level of the cryoablated vertebra at ×100 total magnification shows axonal necrosis and edema. Similar findings were seen at the microwave-ablated level (not shown). F, Sagittal postcontrast T1-weighted image with fat suppression of the radiofrequency-ablated vertebra shows the posterior margin of the ablation zone confined to the vertebral body.

Fig 4.

MR imaging findings 7 days after RFA of an L2 vertebral body in a sheep model. A, T1-weighted oblique sagittal MR imaging (left, anterior; right, posterior) shows T1 hyperintense soft tissue within the probe tract (black asterisk) outlined by a signal void. The surrounding ablation zone is isointense to normal marrow and outlined by a thin hyperintense rim (white arrowheads). B, T2-weighted oblique sagittal MR imaging with fat suppression shows the hyperintense probe tract (black asterisk) and slightly hypointense ablation zone surrounded by a hyperintense rim (white arrowheads). C, T1-weighted, postcontrast subtraction images show enhancement along the probe tract (black asterisk) and the nonenhancing ablation zone surrounded by a rim of enhancement (white arrowheads).

Fig 5.

A, Gross pathology of the vertebral body shown in Fig 2 cut along the plane of the radiofrequency ablation probe tract. The tract is filled with hemorrhagic debris and is surrounded by a pale, tan zone of necrosis. B, Hematoxylin-eosin staining of the margin of the ablation zone at ×40 total magnification shows a dense band of red blood cells (black arrows), which demarcate necrotic marrow on the left from the intact region on the right. C, At ×200 magnification, the ablation zone shows empty lacunae (black arrows), representing a loss of osteocytes, within an intact trabecula.