MR imaging-guided interstitial Nd:YAG laser phototherapy: dosimetry study of acute tissue damage in an in vivo model.

A dosimetry study of acute tissue damage induced by interstitial application of the neodymium-yttrium-aluminum-garnet (Nd:YAG) laser was performed with magnetic resonance (MR) imaging. The MR appearance of the lesion was correlated with gross and histopathologic findings. Seventy-six lesions were induced in rabbit muscle with laser power outputs of 5-20 W and exposure times of 20-600 seconds. MR imaging was performed immediately after laser exposure. T2-weighted spin-echo images clearly showed the acute thermal injuries caused by laser energy deposition and correlated best with histopathologic findings. These images showed three distinct layers, corresponding to central ablation, coagulative necrosis, and interstitial edema, respectively, in the pathologic findings. Lesion diameters measured on MR images showed a linear correlation with those in gross sections. Lesion volume increased not only with increasing total energy delivered but with increasing power output for a fixed total energy delivered. MR imaging is an accurate modality for dosimetry studies of laser-induced acute lesions.