Characterization and correction of modulation sidebands in 1H MRS without water suppression by spatiotemporal field monitoring

Introduction Highly efficient water suppression schemes such as WET or VAPOR [1] are widely considered a prerequisite for state-of-the art in-vivo H magnetic resonance spectroscopy (MRS). However, there are multiple motivations for H MRS without water suppression: (1) the water signal needed for absolute quantification based on the internal water reference method as well as eddy current and phase corrections can be acquired without additional scan time; (2) efficient water suppression schemes have been shown to cause saturation transfer effects especially for resonance lines appearing downfield of water; (3) resonance lines that appear close to the water resonance such as glucose or lactate might be observed since they are not suppressed and (4) a time consuming iterative water suppression optimization is not necessary. The feasibility of in-vivo H MRS without water suppression has been demonstrated with regard to the dynamic range of analog-to-digital conversion. However, modulation sidebands, which are introduced as a pair of antiphase peaks placed equidistantly to the right and left of the water resonance by gradient-vibration-induced B0 field oscillations, hamper metabolite quantification. To overcome this problem, a number of post-processing approaches, gradient inversion schemes [2] and subtraction schemes based on frequency selective inversion pulses [3] have been devised. All robust approaches require either modifications to the existing spectroscopy sequence or cannot be combined with MRS imaging. In this work, it is therefore proposed to characterize modulation side bands that are caused by gradient vibrations with the help of spatiotemporal field monitoring [4]. More specifically it is investigated whether calibration data acquired with a recently proposed field camera [5] permit the prediction and direct correction of modulation sidebands in H MRS data acquired without water suppression.