Simultaneous proton resonance frequency shift thermometry and T measurements using a single reference variable flip angle T method.

Purpose: Implement simultaneous proton resonance frequency (PRF) shift and T measurements with equivalent temporal resolution using a single reference variable flip angle method. This novel method allows for simultaneous thermometry in both aqueous and fatty tissue.

Methods: This method acquires a single reference image at the lower flip angle and all dynamic images at the higher angle. T is calculated using a single reference variable flip angle method, which accounts for the reference image temperature remaining constant. Monte Carlo simulations determined the optimal dynamic flip angle for combined PRF and T measurements. This method was evaluated in MR-guided focused ultrasound heating experiments using a gelatin phantom and human cadaver breasts. In vivo measurement precision was demonstrated in healthy female volunteers under nonheating conditions.

Results: Temperature rise during MR-guided focused ultrasound heating was measured in aqueous tissue with both PRF and T . Both measures show good qualitative agreement in both space and time in aqueous tissue. The T change due to temperature increase was measured in fat, demonstrating the expected temporal response. The dynamic flip angle that produces optimal SNR for PRF measurements is lower than the optimal angle for T measurements, necessitating the selection of a compromise angle.

Conclusion: The single reference variable flip angle method provides a reliable way to simultaneously measure PRF temperature and T change and overcomes PRF's inability to simultaneously monitor temperature in aqueous and adipose tissues. Future work will calibrate T change to temperature, enabling real-time temperature in fat and increasing patient safety and treatment efficacy during thermal interventional treatments.