On restoring motion‐induced signal loss in single‐voxel magnetic resonance spectra

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Abstract
Destructive interference from phase fluctuations caused by motion during 1H magnetic resonance spectroscopy (MRS) stimulated‐echo acquisition mode (STEAM) and point‐resolved spectroscopy (PRESS) acquisitions can significantly diminish the traditional √N‐gain in signal‐to‐noise ratio (SNR) afforded by averaging N signals, especially in the torso. The SNR loss is highly variable among individuals, even when identical acquisition protocols are used. This paper presents a theory for the SNR loss, assuming that the phase fluctuates randomly. It is shown that SNR in conventional averaging is reduced by the factor sinc(σϕ√3/π), where σϕ is the standard deviation (SD) of the phase. “Constructive averaging,” whereby each individual acquisition is phase‐corrected using the phase of a high‐SNR peak before averaging, reverses the SNR loss from motion‐induced dephasing, resulting in a {1/sinc(σϕ√3/π)}‐fold SNR improvement. It is also shown that basing phase corrections on an average of √N adjacent points both improves correction accuracy and effectively eliminates false signal artifacts when corrections are based on low‐SNR peaks. The theory is validated over a sevenfold range of variation in signal loss due to motion observed in 1H STEAM and PRESS data acquired from 17 human subjects (heart: N = 16; leg: N = 1). Constructive averaging should be incorporated as a routine tool for in vivo 1H MRS. Magn Reson Med, 2006.