Water removal in MR spectroscopic imaging with L2 regularization

Abstract

Purpose An L2‐regularization based postprocessing method is proposed and tested for removal of residual or unsuppressed water signals in proton MR spectroscopic imaging (MRSI) data recorded from the human brain at 3T. Methods Water signals are removed by implementation of the L2 regularization using a synthesized water‐basis matrix that is orthogonal to metabolite signals of interest in the spectral dimension. Simulated spectra with variable water amplitude and in vivo brain MRSI datasets were used to demonstrate the proposed method. Results were compared with two commonly‐used postprocessing methods for removing water signals. Results The L2 method yielded metabolite signals that were close to true values for the simulated spectra. Residual/unsuppressed water signals in human brain short‐ and long‐echo time MRSI datasets were efficiently removed by the proposed method allowing good quality metabolite maps to be reconstructed with minimized contamination from water signals. Significant differences of the creatine signal were observed between brain long‐echo time MRSI without and with water saturation, attributable to the previously described magnetization transfer effect. Conclusions With usage of a synthesized water matrix generated based on reasonable prior knowledge about water and metabolite resonances, the L2 method is shown to be an effective way to remove water signals from MRSI of the human brain.

Accelerated J‐resolved 1H‐MRSI with limited and sparse sampling of (‐space

“…Although lipid contamination can be adequately reduced by the semi‐LASER excitation pulses, we still need to handle the residual water signals. Significant work has been done to address the water removal problem in conventional MRSI experiments 48‐50 . We used a modified strategy to remove the residual water signals from our J‐resolved MRSI data.…”

“…Significant work has been done to address the water removal problem in conventional MRSI experiments. [48][49][50] We used a modified strategy to remove the residual water signals from our J-resolved MRSI data. More specifically, because the water signal for the first TE (40 ms) had good SNR, we simply applied a subspace-based method for its removal, which has been described in detail in several publications.…”

Test-retest reproducibility of human brain multi-slice 1H FID-MRSI data at 9.4 T after optimization of lipid regularization, macromolecular model and spline baseline stiffness

“…Although, lipid suppression by L2 is widely used, there are only few publications beside the original article by Bilgic et al 49 (one paper combining water and lipid removal by Lin et al 66 and an abstract from Hangel et al 67 ) investigating the lipid removal by L2-regularization in more detail. All other MRSI publications at ultra-high fields using L2 (or rarely L1) give no hints on how they found the optimal regularization parameter and rarely report the value at all.…”

Magnetic resonance spectroscopic imaging of downfield proton resonances in the human brain at 3 T

“…Although conventional upfield MRSI is typically not performed using spectral-spatial pulses, there are some potential advantages of using this approach; lack of saturation of water results in spectra that are free of chemical exchange or magnetization-transfer effects from water (as has been previously observed in particular for total creatine), 7,12,36 and the pulse sequence is shortened by the omission of the water-suppression module. Furthermore, the excitation and refocusing profiles could also be tuned to minimize lipid resonances, perhaps reducing the need for lipid suppression modules in the pulse sequence.…”