SAR-reduced spectroscopic FAST imaging with variable flip angles

Introduction A method to reduce the specific absorption rate (SAR) of the SSFP (Steady State Free Precession) based fast spectroscopic imaging (SI) technique spFAST [1] is presented. For spFAST the FID-like signal S1 is acquired while the echo-like signal S2 is suppressed. An optimal flip angle which depends on the relaxation times T1 and T2 and on the repetition time TR is used to maximize the signal-to-noise ratio (SNR) [2]. A better spectral resolution and a higher SNR can be achieved with increasing field strength. However, the specific absorption rate (SAR) limits the applicability of SSFP based SI methods for higher B0 fields. In the present study we show that variable flip angle schemes with a maximum flip angle lower than allows a considerable SAR reduction with only minor SNR loss. Furthermore, the oscillating signal intensities can be exploited for k-space weighting in one spatial phase encoding direction thus reducing the sidebands of the spatial pointspread function (PSF).