MR-Guided Radiofrequency (RF) Thermal Ablation of the Lumbar Vertebrae in a Porcine Model

Purpose:

To test the hypotheses that: (1) MR-guided radiofrequency thermal ablation (RTA) of the vertebrae is feasible in a porcine model; (2) Safety of procedure depends upon location of ablation within the vertebra; and (3) MRI allows accurate monitoring of induced thermal lesion size and shape.

Materials and Methods:

Ten percutaneous MR-guided RTAs were randomized over various lumbar vertebral levels and locations in seven pigs. Animals were followed up for 2,7,or 14 days before sacrifice. Thermal lesion size and shape on immediate and follow-up MRIs were compared to gross pathology. Mean absolute differences between lesion diameters on pathology and MRIs were evaluated using paired t-test, as were differences between lesion-to-vertebra CNR on each sequence. Clinical and imaging data were correlated to histopathology.

Results:

Successful RF electrode placement into targeted part of vertebra was achieved in all procedures. Ablations away from neural elements were safe to perform. Pedicular ablations resulted in radiculopathy, whereas direct ablations over posterior cortex resulted in paraplegia.

Lesion sizes from T2WI were closest to gross pathology (MAD = 0.722mm ± 0.83) followed by CE-T1WI (1.27mm ± 0.83), and STIR (1.5mm ± 1.84). Size measurements on T2WI were significantly better than CE-T1WI (p= 0.013) but not different from STIR (p= 0.27). CNR was significantly higher for CE-T1WI than for T2WI (p<0.0001) or STIR (p<0.0001).

Conclusion:

MR-guided RTA of the vertebrae is feasible in a porcine model, and safety of procedure depends upon location of ablation within the vertebra. MRI allows accurate monitoring of thermal lesion size and shape.

Figure 1: Axial (A) and sagittal (B) TSE T1-weighted images(TR/TE/ETL/NSA: 680/24/5/2) obtained to confirm the RF-electrode position prior to ablation. The electrode is seen as a linear signal void (arrows, A and B) traversing the right pedicle into the vertebral body. As shown on (B), the sagittal scan plane allows the electrode tip (arrowhead) to be directed inferiorly to reach the mid- or lower part of the vertebral body, thereby providing rapid confirmation of the electrode position in all 3 dimensions. The electrode diameter appears larger than its real diameter as we apply frequency encoding in a perpendicular direction to the electrode shaft to increase the electrode conspicuity.

Figure 2: (A) Axial STIR MR image and (B) corresponding gross pathologic section showing an example of a successful ablation performed within the posterior part of L4 vertebral body. Although the nearest point of the RF-electrode track (curved arrows, A and B) is located at 2mm distance from the spinal canal, this pig did not develop any neurological deficit after the procedure, and the neurohistologic examination revealed a completely normal spinal cord (C) with intact nerve cells in the gray matter seen preserving their normal nuclei (arrows, C) (125X magnification). Arrowheads in (A) indicate the margins of the induced thermal lesion within the vertebral body. Note the epidural vessel (B, arrow) located just at the point where the electrode is closest to the spinal canal. The blood flow within this vessel coupled with the CSF pulsations might have contributed to some heat dissipation effect that protected the spinal cord.

Figure 3: Flowchart highlighting the proposed role of radiofrequency thermal ablation among the major treatment options currently available for patients with spinal metastases.