Hemi-spectrum substitution after water signal fitting (HESWAF): an improvement of the modulus post-processing of MR spectra

Abstract

By improving the SNR of the spectra and eliminating eventual contaminations, the new strategies proposed here confer an additional competitive advantage to the modulus post-processing technique.

Absolute choline tissue concentration mapping for prostate cancer localization and characterization using 3D 1H MRSI without water‐signal suppression

“…A key step in the pipeline of several of these postprocessing methods was the selection of the modulus signal to remove side band artifacts of the water signal. 10 , 13 , 14 , 36 In our postprocessing, we also made use of modulus signal selection for this purpose next to the use of wavelet transforms to select and smooth the water signal. 16 The used TE of 88 ms in semi‐LASER MRSI acquisition is optimal for the in‐phase detection of the citrate signal 34 ; in a non‐water version, this TE leads to attenuated and broader acoustic sidebands further away from the water signal than occurring at shorter TEs.…”

“…However, as shown in previous studies, there is little if any loss in SNR due to the noise distribution in the modulus signal and other corrective effects of the modulus operation such as on eddy current artifacts. 13 , 14 , 36 The modulus operation may cause mirroring of downfield signals around the water line, interfering with the metabolite signals of interest. However, 1 H‐MR spectra of the prostate at TE = 88 ms do not show resonances downfield of the water signal, and if such signals would appear at shorter TE, this mirroring problem can be mitigated by the procedure described in Le Fur and Cozzone.…”

“…This water signal can be used as an internal reference for various purposes such as metabolite line‐shape distortion correction, absolute metabolite quantification, multicoil signal corrections, and in the case of single voxel, to correct for motion‐induced phase fluctuations between individual scans. 36 Moreover, in a non‐water‐suppressed 1 H‐MRSI approach with SPICE, the water signal was used for susceptibility mapping. 38 In our study we used the water line to correct for metabolite line‐shape distortions and for absolute quantification of choline content in the prostate.…”

“…Alternatively, postprocessing methods have been proposed to remove the water signal and its artifacts from water‐unsuppressed MRS and MRSI data. A key step in the pipeline of several of these postprocessing methods was the selection of the modulus signal to remove side band artifacts of the water signal 10,13,14,36 . In our postprocessing, we also made use of modulus signal selection for this purpose next to the use of wavelet transforms to select and smooth the water signal 16 .…”

“…The modulus operation may cause mirroring of downfield signals around the water line, interfering with the metabolite signals of interest. However, 1 H‐MR spectra of the prostate at TE = 88 ms do not show resonances downfield of the water signal, and if such signals would appear at shorter TE, this mirroring problem can be mitigated by the procedure described in Le Fur and Cozzone 36 . Recently, the fast MRSI method SPICE has been presented in a version without water‐signal suppression, which requires acquisition of a training set of data for proper water‐signal removal and makes no provisions for removing acoustic sidebands of the water signal 37 …”