Magnetic resonance imaging of interstitial laser photocoagulation in brain.

Magnetic resonance (MR) imaging can be used to monitor the development of thermal lesions induced in tissue using interstitial laser photocoagulation (ILP). A potential application for ILP is the treatment of surgically inaccessible brain tumors. For the successful clinical application of MR-monitored ILP, it is necessary to relate MR images of ILP lesions to the actual induced lesions. In this preliminary study we performed ILP in the normal brains of anesthetized cats by delivering interstitially 1.0, 1.5, and 2.0 W of continuous-wave Nd:YAG laser energy (1,064 nm) for 1,000 s via a plane-cut 400 microns core optical fiber. At 48 h post-irradiation the lesions consisted of four sharply demarcated concentric zones of thermal damage. Lesion diameter increased linearly with delivered power. T2-weighted proton spin-echo images acquired during ILP showed a region of complete or near signal loss that underestimated the actual lesion at 48 h. Gadolinium-enhanced T1-weighted spin-echo images acquired immediately post-irradiation showed the actual lesion precisely.