OBJECT: High quality, artifact free, ¹H MRS require a homogeneous magnetic field over the volume of interest (VOI) and an effective global water suppression (WS). However, the shim coils act globally and, hence, affect the resonance frequency over the entire object, making the frequency selective WS spatially selective. Unsuppressed water signal may cause spurious echo artifacts. AIM: Study and visualise the effect localised shimming has on WS. MATERIALS AND METHODS: A water suppression imaging (WSI) scan for visualising regions unaffected by the WS was designed and applied in vivo for two different VOI localisations, at two field strengths. Two shim optimisation methods, and first and second order shims were compared. In addition, shim settings for six VOI localisations were retrospectively investigated. RESULTS: The WSI-scan effectively visualised the spatial extent of the WS. The coverage decreased when the shim was optimised on a smaller VOI. Second order shims accentuated the problem, resulting, on average, in a WS coverage of only 35% of the head volume. CONCLUSION: Localised shimming can result in large regions of unsuppressed water, which can lead to spurious echo artifacts in the spectrum. To help overcome these problems globally optimised shims can be used during WS.
OBJECT: High quality, artifact free, ¹H MRS require a homogeneous magnetic field over the volume of interest (VOI) and an effective global water suppression (WS). However, the shim coils act globally and, hence, affect the resonance frequency over the entire object, making the frequency selective WS spatially selective. Unsuppressed water signal may cause spurious echo artifacts. AIM: Study and visualise the effect localised shimming has on WS. MATERIALS AND METHODS: A water suppression imaging (WSI) scan for visualising regions unaffected by the WS was designed and applied in vivo for two different VOI localisations, at two field strengths. Two shim optimisation methods, and first and second order shims were compared. In addition, shim settings for six VOI localisations were retrospectively investigated. RESULTS: The WSI-scan effectively visualised the spatial extent of the WS. The coverage decreased when the shim was optimised on a smaller VOI. Second order shims accentuated the problem, resulting, on average, in a WS coverage of only 35% of the head volume. CONCLUSION: Localised shimming can result in large regions of unsuppressed water, which can lead to spurious echo artifacts in the spectrum. To help overcome these problems globally optimised shims can be used during WS.
Authors: Kevin M Koch; Laura I Sacolick; Terence W Nixon; Scott McIntyre; Douglas L Rothman; Robin A de Graaf Journal: Magn Reson Med Date: 2007-03 Impact factor: 4.668
Authors: Göran Starck; Maria Ljungberg; Marie Nilsson; Lars Jönsson; Stefan Lundberg; Tord Ivarsson; Susanne Ribbelin; Sven Ekholm; Arvid Carlsson; Eva Forssell-Aronsson; Maria L Carlsson Journal: J Neural Transm (Vienna) Date: 2008-06-05 Impact factor: 3.575
Authors: Ivan Tkáč; Dinesh Deelchand; Wolfgang Dreher; Hoby Hetherington; Roland Kreis; Chathura Kumaragamage; Michal Považan; Daniel M Spielman; Bernhard Strasser; Robin A de Graaf Journal: NMR Biomed Date: 2020-12-16 Impact factor: 4.478
Authors: Adam Berrington; Natalie L Voets; Puneet Plaha; Sarah J Larkin; James Mccullagh; Richard Stacey; Muhammed Yildirim; Christopher J Schofield; Peter Jezzard; Tom Cadoux-Hudson; Olaf Ansorge; Uzay E Emir Journal: Tomography Date: 2016-06
Authors: Vincent O Boer; Mads Andersen; Anna Lind; Nam Gyun Lee; Anouk Marsman; Esben T Petersen Journal: Magn Reson Med Date: 2020-02-14 Impact factor: 4.668