The shim process during AutoPreScan (APS) is acquiring 3 orthogonal shim maps in the center of the prescribed VOI. For the region that is used to optimize shim the intersections of those 3 slices and a sphere around the VOI are used.
So the shim displayed by APS represents shim of the prescribed VOI.
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and as the last question, why does GE spectroscopy consider Cr peak for SNR evaluation instead of NAA
Thank you for your kind attention
This decision dates back to the introduction of PROBE in the 90s. My assumption is, as we used Cr as reference we also use it for SNR measurement. As you have the NAA/Cr ratio you can easily calculate the SNR based on NAA.
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Thank you very much for your reply. Itâs said that Cr is variable in different lesions and different grades of tumorâŚthatâs why I asked this questionâŚ
Best,
FWHM estimation should be fairly consistent as long as thereâs a substantial peak that it can be estimated from (although Iâm not aware of any literature having looked into that in detail).
NAA and Cr are usually the strongest singlets (if you donât count the unsuppressed water), which is why theyâre used for FWHM/SNR estimation in the first place. Obviously, as you correctly say, that may not necessarily be true in lesions/tumor.
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Thank you very much 
Do we have access to raw spectrum on GE scanner?
The raw data is stored in âpfileâ format in /usr/g/mrraw. The numbers in file name are incremented and can be found in the dicom of the series as ârun numberâ. Easiest is to just check /usr/g/mrraw after acquisition. The most recent pfile corresponds to the last series.
pfile naming convention: Pxxxxx.7 where xxxxx is a 5 digit number starting with 00000 and increments by 512.
According to MRS minimum reporting standards, the raw spectrum not the fitted data alone must be shown as fitted spectrum does not reflect SNR or potential systematic artifacts
Does this sentence mean that the raw spectrum must be accessible to operators for visual inspection as quickly as the fitted one?
Ok. My misunderstanding.
The spectrum we display as dicom image is the raw, not a fitted spectrum (for single voxel or CSI with ReadyView).
I understand this as a statement not to look at the fitting result (only). Some tools either just show the fitted spectrum or the fitting result as a thick, coloured line on top of the spectrum so inspection of data quality isnât possible or at least difficult.
Thank you very much.
But Ralph, I think it means spectrum without processing steps that are for improving visual inspection like apodization that improves SNRâŚthatâs why it calls it RAW SPECTRUMâŚRAW SPECTRUM here doesnât mean fitted spectrum without fitted resultsâŚAm I right?
Hi
Which is routinely implemented on systems? Autoshim or manual shim?
and which order
Thank you
Auto Prescan (APS) is done by default if either APS is executed explicitly or if scan is pressed without any previous prescan (auto, manual, spectroscopy).
The user should realise if manual Prescan is done as the name says this requires manual interaction.
Manual prescan is sometimes performed by users and immediately stopped to run a scan with same prescan settings as the previous scan.
@noeskera
so what is the benefit of pushing auto prescan button which is routinely done by operators?
Raw spectrum does mean spectrum without apodization. The idea is to give readers a chance to assess the signal to noise, and quality of the spectrum. It is not a truly raw spectrum, as some pre-processing steps (coil combination, phase correction, frequency correction, eddy current correction, etc) may have been done.
This allows to check the APS results, mainly line width before starting the scan. In case of a high line width the user can either repeat APS and see if second iteration improves result or slightly shift voxel.
Thanks you very much
what about if we turn the automatic water suppression optimization off? then any difference between default APS and manual?
and what is the type of autoshim ? Is it iterative?
Manual prescan is not doing anything. It is just showing you the current settings and you can manually change them. Only APS runs the steps to set receive gains, transmit gain, shim, ⌠correctly. AWS is usually switched off by default and is just one extra step within APS.
Autoshim is measuring B0 maps in 3 orthogonal slices in the center of the prescribed VOI and then optimising B0 field within the sphere around the VOI. So it is a 1-step (B0-mapping) approach.
I meant pushing APS button by manuallyâŚso the difference is just displaying th information above and if AWS is on the water FWHM and suppression is addedâŚright?
Thank you very much Ralph