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MRI Features After Radiofrequency Ablation of Osteoid Osteoma with Cooled Probes and Impedance-Control Energy Delivery

¹ Department of Radiology, Mater Misericordiae University Hospital, Eccles St., Dublin 7, Ireland.
² Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.

OBJECTIVE. The purposes of our study were to determine the temporal changes in MR signal in bone after radiofrequency ablation of osteoid osteoma and the size of the zone of marrow signal change produced by the radiofrequency technique and to compare the size of the zone with published data for radiofrequency ablation with manual-control protocols.

MATERIALS AND METHODS. Radiofrequency ablation was performed in 10 patients with a clinical and radiologic diagnosis of osteoid osteoma. A cooled radiofrequency probe was inserted in the nidus. Twelve minutes of radiofrequency energy was applied from a 200-W radiofrequency generator in an impedance-control setting. MRI with multiplanar turbo spin-echo T1-weighted and STIR sequences was performed at 1, 7, and 28 days after the procedure in seven patients. The three remaining patients had follow-up imaging at 28 days only. The images were reviewed by two radiologists who categorized the imaging features and measured the marrow zone of signal alteration when visible. The size of the zone of marrow signal change produced by the radiofrequency technique was compared with published data for radiofrequency ablation with manual-control protocols.

RESULTS. A 1-mm band of homogeneous altered marrow signal distributed symmetrically parallel to the entire probe tract was seen earliest, at 1 day, in the femoral neck lesion treated with the 2-cm probe. The band was low signal on the T1 sequence and high signal on the STIR sequence, and the diameter of the zone was 27 mm. By 7 days, five of the seven treated bones showed a band of marrow signal alteration. By 28 days, all 10 treated bones had a band of marrow signal alteration. The interband distance at 90° to the probe measured on STIR images at 28 days was a mean of 20.9 mm (confidence interval, 16.1-25.7 mm [p < 0.05]; range ± measurement error, 10.5-35 ± 1.64 mm) with a 1-cm probe and 30.5 mm (measurement error, ± 0.78 mm) on T1 images without contrast material when a 2-cm exposed-tip probe was used. Higher-output generators with impedance-control software and internally cooled radiofrequency probes with longer exposed tips produce larger zones of marrow signal change than expected with manual-control protocols.

CONCLUSION. MRI allows detection of temporal marrow signal change after radiofrequency ablation. The marrow signal change with a high-energy delivery protocol is larger than manual-control protocols.

Keywords: ablation • bone • bone marrow • MRI • osteoma • radiofrequency ablation

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