Hybrid proton resonance frequency/T1 technique for simultaneous temperature monitoring in adipose and aqueous tissues.

Thermal therapy procedures being carried out under MR guidance would be safer if temperature changes could be accurately monitored in both water-based and fat-based tissues. To this end, we present a hybrid proton resonance frequency (PRF)/T(1) approach for simultaneously measuring PRF shift temperatures in water-based tissues and T(1) changes in fat-based tissues. The hybrid PRF/T(1) sequence is a standard radiofrequency spoiled gradient echo sequence executed in a dynamic mode with two flip angles alternating every time frame. The PRF information is extracted every time frame using the image phase in the standard approach, and the T(1) information is extracted every two time frames using a variable flip angle approach. Simulation studies, ex vivo high intensity focused ultrasound heating experiments, and in vivo stability experiments were performed to test the feasibility of the approach. The results indicate that the hybrid PRF/T(1) approach provides PRF temperature maps of the same quality as those obtained by traditional PRF methods while simultaneously being able to track T(1) changes in fat-based tissues. Although several potential error sources exist for the T(1) measurements, the approach is a promising start toward realizing quantitative temperature measurements in both water-based and fat-based tissues.