Quantification of non–water‐suppressed MR spectra with correction for motion‐induced signal reduction

Abstract

Intrascan subject movement in clinical MR spectroscopic examinations may result in inconsistent water suppression that distorts the metabolite signals, frame-to-frame variations in spectral phase and frequency, and consequent reductions in the signal-to-noise ratio due to destructive averaging. Frameto-frame phase/frequency corrections, although reported to be successful in achieving constructive averaging, rely on consistent water suppression, which may be difficult in the presence of intrascan motion. In this study, motion correction using nonwater-suppressed data acquisition is proposed to overcome the above difficulties. The time-domain matrix-pencil postprocessing method was used to extract water signals from the non-water-suppressed spectroscopic data, followed by phase and frequency Inevitable subject motions during successive data acquisition in magnetic resonance spectroscopy (MRS) may cause signal loss because of two major mechanisms (1-4). First, motion-induced magnetic field drifts may lead to frequency shift over multiple MRS acquisition frames. Second, subject movements in the presence of the spoiler gradients may result in inconsistent phase variations among acquisition frames. In conventional MRS studies, data from all acquisition frames are simply averaged, and therefore the destructively averaged spectrum would be affected by signal-to-noise ratio (SNR) attenuation and spectral shape distortion because of frame-to-frame phase/ frequency variations. It has been shown that, by performing frame-by-frame corrections for the intrascan phase/ frequency variations, the constructively averaged MRS data exhibit significantly improved SNR and spectral quality (1-8). The search for an effective intrascan phase/frequency correction method is thus an active field of research for in vivo MRS.Over the past decades, several methods have been proposed to measure the intrascan phase/frequency variations in MRS scans in order to perform constructive averaging (1,2,4-6). In one of these methods, the non-water-suppressed (NWS) free induction decay and water-suppressed (WS) MRS data were acquired in an interleaved manner (2). Intrascan phase/frequency variations estimated from the NWS free induction decay were used to correct the WS MRS data. In this approach, however, the phase variations in WS MRS data are corrected by values estimated at different time points. Therefore, the effectiveness of the phase correction might be suboptimal, particularly in the presence of abrupt intrascan movements.Alternatively, the phase variations induced by intrascan motion could be measured on a frame-by-frame basis directly from the residual water signals of WS MRS data (1,4). However, even though the information estimated directly from the WS MRS data better reflects the instant phase variation in comparison to the interleaved free induction decay-based measurements, this approach has two main limitations: First, the residual water signals in WS MRS data have low SNR that limits the accuracy of frameby-frame phase estimation (1). ...

Proton MRS and MRSI of the brain without water suppression

“…All these will have adverse effects on the spectral fitting and, in turn, on the quantification of metabolites. A strategy to overcome this disadvantage for single voxel MRS is to save the signals shot-by-shot during data acquisition and perform phase correction and frequency shifts prior to the coherent combination of signal in the postprocessing [60,121,122]. One way to do this is by using as a reference the residual water signal from individual shots in the water-suppressed MRS.…”

“…One way to do this is by using as a reference the residual water signal from individual shots in the water-suppressed MRS. But the use of residual water as a reference has some limitations [60]. The major limitation is that the residual water signal may be distorted by the WS pulses, especially in brain regions with different tissue susceptibilities and when the subject moves.…”

“…Thus, the phase and frequency shifts of the water may not be consistent with those of metabolites [123]. These limitations can be avoided when the unsuppressed water signal is used as a reference to correct the phase and frequency shifts related to interscan motions [60]. To this end, the phases (u n = u 0 + Du n ) and frequencies (f n = f w + Df n ) of water signals in each individual scan are determined.…”

“…The number of averages usually ranges from 64 to 256, and the scan lasts several minutes or more. In these procedures, intrascan subject motion may occur, causing phase and frequency shifts of the signals [60]: …”

“…(3) In MRSI, the water signal can be used to correct voxel-to-voxel frequency shifts caused by the inhomogeneity of the main magnetic field B 0 [49,59]. (4) In single voxel MRS with a large number (usually of signal averaging to increase SNR, water signal can be used to correct lineshape distortions and to eliminate artifacts caused by subject motion [60]. (5) In MRSI using multichannel coils, the water signal can be used as a better reference than a metabolite signal for spectral alignment [59,[61][62][63] and for optimal combination of multichannel spectra to increase the signal-tonoise ratio (SNR) [62][63][64].…”