Simultaneous T1 measurements and proton resonance frequency shift based thermometry using variable flip angles

A method is presented which allows precise temperature and longitudinal (T1) relaxation time measurements with high spatial and temporal resolution. This is achieved by combining dynamic variable flip angle based T1 relaxation mapping with proton resonance frequency shift based thermometry. Herein, dynamic T1 mapping is either used as a complementary measure of temperature or for the detection of T1 contrast agent release. For the first application, the temperature evolution during a high-intensity focused ultrasound tissue ablation experiment was measured in both, porcine fat and muscle, simultaneously. In this application, temperature accuracies of 2.5 K for T1-based thermometry in fat and 1.2 K for proton resonance frequency shift-based thermometry in muscle were observed. The second application relates to MR-guidance of high-intensity focused ultrasound-induced local drug delivery by means of thermo-sensitive liposomes labeled with a T1 contrast agent (Gd-HPDO3A). When the measured temperature exceeded the phase transition temperature of the liposomes, T1 was observed to decrease with a good temporal and spatial correlation due to the release of Gd-HPDO3A. The presented results demonstrate the feasibility of the proposed method for two important applications in MR-guided noninvasive therapy. It offers a high temporal resolution when compared with interleaved Look-Locker based T1 mapping techniques and thus represents an interesting candidate for simultaneous real-time monitoring of T1 and temperature changes.