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-bounces@nmr.mgh.harvard.edu on behalf of Steve Petersen marfneuro@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.

________________________________ The materials in this message are private and may contain Protected Healthcare Information or other information of a sensitive nature. If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail.