MR-IMAGING OF HO-YAG LASER DISKECTOMY WITH HISTOLOGIC CORRELATION

Twelve cadaveric vertebral specimens were imaged after holmium yttrium aluminum garnet (Ho:YAG) laser diskectomy to determine the usefulness of magnetic resonance (MR) imaging in evaluating treatment outcome. T he Ho:YAG laser was operated at 1.0-2.0 J per pulse, 5 Hz, and 250-mus ec pulse width. The total energy varied between 600 and 2,700 J. Two d istinct patterns emerged on MR images. Tissue ablation at higher power (1.5 and 2.0 J per pulse) produced discrete signal voids that correla ted with areas of laser-induced ablation identified at gross inspectio n. More subtle changes, characterized by a high-signal-intensity ring, were seen in the specimens lased at 1.0 J per pulse. The latter appea rance corresponded to incomplete vaporization of diskal tissue, a broa der zone of minimal thermal injury, and sparing of adjacent vertebral endplates. Total mass loss did not appear to be affected by the choice of power setting (1.0 vs 1.5 J per pulse), with total laser energy he ld constant.