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Degraded water suppression in small volume ¹H MRS due to localised shimming
Asa Carlsson1, Maria Ljungberg, Göran Starck
1Radiation Physics, University of Gothenburg, Gothenburg, Sweden. asa.carlsson@vgregion.se
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.
