External Email - Use Caution
Giving cubic volumes of voxels is a bit confusing, but I assume what you meant
to say is that you have 1mm T1w images and 0.7mm T2w images. Although you can
certainly can register these and take the ratio (and FreeSurfer bbregister is
the recommended way to do this as it has the highest accuracy of methods we
have tested), keep in mind that your resolution of the T1w/T2w will only be as
good as your lower resolution image. A better study design is to match the
spatial resolution of your T1w and T2w images so that you don’t sacrifice
resolution or SNR and to have the spatial resolution of both T1w and T2w 0.8mm
isotropic or better. 1mm is too coarse to accurately measure the thinnest
cortical areas that are 1.6mm and T1w/T2w myelin mapping is especially
demanding of surface quality, as if either the white or pial surface is not in
the right place you will get wildly inaccurate values. I am also assuming your
study is at 3T and in vivo. If those aren’t the case, many other issues can
come into play.
Matt.
From: <freesurfer-boun...@nmr.mgh.harvard.edu> on behalf of Steve Petersen
<marfne...@gmail.com>
Reply-To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Date: Thursday, November 3, 2022 at 3:57 AM
To: "freesurfer@nmr.mgh.harvard.edu" <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] T1w/T2w ratio maps: acquisition parameters T1w and T2w
External Email - Use Caution
Dear FreeSurfer experts,
I would like to calculate myelin maps using the ratio between T1w and T2w
scans. However, the acquisition parameters of the T2w images are slightly
different (e.g. voxel size T1w = 1mm3 vs voxel size T2w = 0.35 mm3 ; T1w
matrix: 240 x 240 vs T2w matrix: 640 x 640). The first step of the protocol
would involve a robust registration between the two MRI images (e.g. bbregister
method) which would lead to a downsampling of the T2w images. My question is,
could I conduct a reliable analysis of T1w/T2w ratio maps despite these
differences in the acquisition parameters between both MRI sequences?.
Thank you very much in advance.
Best regards,
Steve.
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