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MR Imaging–guided Radio-frequency Thermal Ablation of the Lumbar Vertebrae in Porcine Models¹

¹ From the Depts of Radiology (S.G.N., A.J.A., I.C.S.M., J.L.D., J.S.L.), Pathology (S.N.E.), and Biomedical Engineering (J.L.D.), Univ Hospitals of Cleveland/Case Western Reserve Univ School of Medicine, 11100 Euclid Ave, Cleveland, OH 44106-5056. Received Jul 24, 2001; revision requested Sep 12; revision received Dec 21; accepted Feb 25, 2002. Univ Hospitals of Cleveland/Case Western Reserve Univ Interventional MR Program supported in part through research collaborations with Siemens Medical Systems and Radionics. Also supported by grants from Whitaker Foundation, American Cancer Society, and NIH 1R01 CA81431-01A1 and 1R01-CA84433-01. Address correspondence to J.S.L. (e-mail: lewin@uhrad.com).

View larger version (79K): [in a new window]   Figure 1. A, Transverse and B, sagittal fast SE T1-weighted (680/24; echo train length, five; number of signals acquired, two) images obtained to confirm RF electrode position prior to ablation. The electrode is seen as a linear signal void (arrows) traversing the right pedicle into the vertebral body. The sagittal image plane allows the electrode tip (arrowhead in B) to be directed inferiorly to reach the middle or lower part of the vertebral body, thereby providing rapid confirmation of the electrode position in all three dimensions. The electrode diameter appears larger than its real diameter, since we applied frequency encoding in a perpendicular direction to the electrode shaft to increase the electrode conspicuity.

View larger version (106K): [in a new window]   Figure 2. RF ablation within the central part of L3 vertebral body. Transverse A, contrast-enhanced SE T1-weighted (528/26; flip angle, 90°; number of signals acquired, three), B, fast SE T2-weighted (2,600/96; echo train length, seven; number of signals acquired, seven), and C, fast SE STIR (2,700/48; echo train length, seven; number of signals acquired, five) MR images acquired in vivo on day 14 after ablation demonstrate a well-circumscribed oval thermal lesion located totally within the confinement of the vertebral body. The thermal lesion displays signal hypointensity in all pulse sequences (*, A-C), with a hyperintense rim on T2-weighted and STIR images (arrowheads, B and C) that enhances on the contrast-enhanced image (arrowheads, A). D, Transverse multiplanar reformatted CT image of the same lesion acquired after sacrifice shows reactive bone marrow sclerosis (arrowhead) marginating the lesion and surrounding the position of the electrode tip (arrow). E, Gross pathologic specimen obtained at the same level as the images shows the dark red electrode track (arrow), surrounded by a pale ovoid area of tissue necrosis with a thin, dark erythematous rim (arrowheads) outlining the periphery of the necrotic region. F, Histologic section (hematoxylin-eosin stain; original magnification, x250) obtained from the thermal lesion shows the hemorrhagic needle tract (*), surrounded by coagulative necrosis of hematopoietic cells (white arrowhead) and the osteoblasts of bony trabeculae (black arrowhead). Within the necrotic zone that occupies most of the field, the nuclei of viable infiltrating monocytes and/or macrophages are visible, scattered within necrotic debris. Viable trabecular bone and marrow hematopoietic cells (white arrow) and adipocytes (black arrow) are evident outside the necrotic zone along the left margin of the field.

View larger version (60K): [in a new window]   Figure 3. A, Transverse STIR (2,700/48; echo train length, seven; number of signals acquired, five) MR image and B, corresponding gross pathologic section show a successful ablation performed within the posterior part of the L4 vertebral body. Although the nearest point of the RF electrode track (curved arrows) is located 2 mm from the spinal canal, this pig did not develop any neurologic deficit after the procedure, and neurohistologic examination revealed C, a completely normal spinal cord with intact nerve cells in the gray matter, seen preserving their normal nuclei (arrows, C) (hematoxylin-eosin stain; original magnification, x125). Arrowheads in A indicate the margins of the induced thermal lesion within the vertebral body. The difference between this ablation and that shown in Figure 5 is that the RF electrode is not placed entirely over the posterior vertebral cortex, but it approximates it at a more localized area. Also note the epidural vessel (B, straight arrow) located just at the point where the electrode is closest to the spinal canal. The blood flow within this vessel, coupled with the cerebrospinal fluid pulsations, might have contributed to some heat dissipation effect that protected the spinal cord.

View larger version (124K): [in a new window]   Figure 4. Two ablation procedures performed within the right pedicles of the L5 (top) and L6 (bottom) vertebrae. A, D, Transverse fast SE T2-weighted (2,600/96; echo train length, seven; number of signals acquired, seven) MR images acquired in vivo on the the 7th day after ablation show the hypointense thermal lesions in the right vertebral pedicles, surrounded by hyperintense rims (arrowheads, A and D) and traversed by linear areas of hyperintensity representing the RF electrode tracks (arrows, A and D). Note that the right exiting nerve root on A lies within the extent of the hyperintense margin of the lesion, while on D it lies within the periphery of the hypointense lesion itself. B, E, Gross pathologic sections from both lesions show the pale necrotic lesions involving the right pedicles (*, B) and bounded by the hypointense erythematous margins (arrowheads, B and E). The RF electrode track is shown in E (curved arrow). Note the crumpling and adhesion of the exiting nerve root (straight arrow) on the side of the lesion in E. C, Histologic section of the right nerve root of the lesion depicted on A and B (hematoxylin-eosin stain; original magnification, x250) demonstrates mild axonal edema (arrowheads), and F, histologic section (hematoxylin-eosin stain; original magnification, x125) of the right nerve root of the lesion depicted on D and E shows adhesion (between arrows) of the nerve root (N) to the perineurium (dura mater, DM), along with an inflammatory monocyte and glial infiltrate (arrowheads) within the nerve root adjacent to the site of adhesion. Examination results of the adjacent spinal cord segments were normal for both cases.

View larger version (106K): [in a new window]   Figure 5. Ablation performed in the most posterior part of the L3 vertebral body with the RF electrode placed directly over the posterior cortex of the vertebral body. This animal developed immediate paraplegia following ablation and was sacrificed after 48 hours. Transverse A, T2-weighted and B, STIR MR images obtained just prior to sacrifice show a less-defined hypointense area (arrowheads, A and B) ventral to the RF electrode track (straight arrows, A and B), with hyperintensity (curved arrows, A and B) involving the spinal cord denoting associated myelopathy (better seen on the STIR image). C, Sagittal STIR MR image of the same ablation demonstrates that the area of myelopathy (arrowheads) extends well above and below the level of the thermal lesion. The thermal lesion is seen as a round area of hypointensity (arrow) that is better defined than on transverse images, with the electrode track seen on end as a bright dot in the center of the lesion. (A-C, Imaging parameters are the same as those in Figure 2). D, Transverse multiplanar reformatted CT image acquired after sacrifice documents the complete integrity of the posterior vertebral body cortex (arrowhead) adjacent to the RF electrode track (arrow). E, Gross pathologic section from the same pig shows a pale area of bone necrosis (*) at the posterior vertebral body. The margins of the lesion are not very well defined in this 2-day-old lesion when compared with the previous 7- and 14-day-old lesions. The arrow indicates the site of the RF electrode track. F, Histologic section (hematoxylin-eosin stain; original magnification, x125) of the spinal cord at the assumed area of myelopathy on MR images demonstrates diffuse gray matter necrosis with loss of the nuclei of nerve cells (arrows).

View larger version (37K): [in a new window]   Figure 6. Plot of the mean RF current and tissue impedance as functions of time during the 10-minute ablations. Deposited energy, represented by the RF current, reached a maximum during the 1st minute of ablation, then continued to decline slowly as the ablation progressed. Tissue impedance decreased during the 1st minute to maintain a constant value throughout the entire ablation procedure.

View larger version (120K): [in a new window]   Figure 7. Transverse contrast-enhanced SE T1-weighted (528/26; flip angle, 90°; number of signals acquired, three) MR image clearly demonstrates the nonprotective property of the cortical bones during in vivo RF thermal ablation. Despite placement of the RF electrode within the pedicle of this vertebra, the induced thermal lesion continued to grow outside the vertebra to develop an even larger intramuscular component (white arrows), with the intact cortical bone (black arrow) conspicuous as a linear area of hypointensity within the less hypointense coagulated area. Arrowheads represent the intravertebral component of the thermal lesion.

View larger version (29K): [in a new window]   Figure 8. Flowchart highlights the proposed role of RF thermal ablation among the major treatment options currently available for patients with spinal metastases.