Assessing Temperature Changes in Cortical Bone Using Variable Flip-Angle Ultrashort Echo-Time MRI

摘要

MR-guided high-intensity focused ultrasound ablation is a promising, noninvasive method for treatment of bone tumors and palliation of pain. During thermal therapy, temperature mapping is necessary to ensure proper heat deposition in targeted tumors as well as to prevent unnecessary heating in surrounding tissues. Conventional MR thermometry exploits the proton resonant frequency shift of water protons, which normally requires a long echo time; therefore, this method is not appropriate for cortical bone due to its short T_2~* relaxation time. This work demonstrates that ultrashort echo-time MRI can characterize T_1 changes in cortical bone caused by temperature changes. Ex vivo experiments were performed to heat diaphysis segments of bovine femurs with an interstitial ultrasound applicator. The T_1 increase in the heated parts of cortical bone was observed. The temerature dependence of T_1 in cortical born was also assessed by heating bovine bone samples in a temperature-controlled water bath. T_1 mapping of cortical bone enabled by ultrashort echo-time MRI might allow for more accurate characterization of thermal dose during treatment of bone tumors.