Fast frequency mapping with balanced SSFP: theory and application to proton-resonance frequency shift thermometry.

A method is presented for the rapid acquisition of frequency maps based on multiecho balanced steady-state free precession (balanced SSFP, fast imaging with steady precession (True FISP), fast imaging employing steady-state excitation (FIESTA), or balanced fast field echo (FFE)). This technique was applied to measure temperature changes within a gel phantom based on the temperature-sensitive water proton-resonance frequency. The frequency was determined as the slope of a linear fit of the phases measured at different TEs along the echo train. The signal-to-noise ratio (SNR) of multiecho SSFP was analyzed for different parameters, such as relaxation times and flip angle, as well as for different local field inhomogeneities. The theoretical and experimental results were compared with results from the established multiecho fast low-angle shot (FLASH) method. Depending on the given tissue parameters, a significant increase in the accuracy of the frequency/temperature shift estimation compared to FLASH was observed. Furthermore, balanced SSFP can be used with very short TRs without generating a saturation-related signal loss, which is essential for real-time temperature mapping.