Multiplanar MR temperature-sensitive imaging of cerebral thermal treatment using interstitial ultrasound applicators in a canine model

Purpose: To study the feasibility of an interleaved gradient-echo, echo-planar imaging (iGE-EPI) sequence for multiplanar magnetic resonance temperature imaging (MRTI) to monitor intracerebral thermal treatment three-dimensionally using multielement ultrasound applicators. Materials and Methods: Transmissible venereal tumor (TVT) fragments were injected into the right cerebral hemisphere of five dogs. Guided by MRI, an interstitial ultrasound applicator was inserted into the tumor or normal brain tissue. The iGE-EP1 sequence was used to estimate temperature changes by computing the complex phase-difference induced by temperature-dependent shifts in the proton resonance frequency of water. The thermal dose maps were updated every 6-8 seconds for five to seven image planes during treatment. The results of MRTI were compared with those of post-treatment MRI and histologic analysis. Results: The multiplanar MRTI monitored temperature and thermal dose distributions in tumor and normal brain tissue over the entire user-defined treatment volume. The ultrasound applicators produced contiguous areas of coagulative necrosis, resulting in 1.5-4.0 cm³ volumes of tissue necrosis. MRTI-based assessments of thermal-dose distributions were consistent with the results of post-treatment MRI and histologic analysis. Conclusion: Multiplanar MRTI is feasible for measuring necrosing thermal doses during intracerebral thermal delivery by interstitial ultrasound applicators.