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Hi Bruce,

Thank you very much for your response. I would like to calculate this distance error for specific regions on the flatmap after the transformation, so I can formulate cuts to minimize it for the region I am interested in. Do you have any suggestions on how I can do it?

From reading your paper : *Fischl B, Sereno MI, Dale AM. Cortical surface-based analysis. II: Inflation, flattening, and a surface-based coordinate system. Neuroimage. 1999 Feb;9(2):195-207. doi: 10.1006/nimg.1998.0396. PMID: 9931269, *I understand that distances are calculated using the corrected *Dijkstra **algorithm* as an estimator of geodesic distances.

In your response, you said that the distance error is calculated with:

*100 * (mean(abs(distance error)) / mean(original_distances))*

Are the *original_distances = the geodesic distances (gd) calculated (using the Dijkstra algorithm) for each vertex on the inflated surface*?

Is the *distance error = (gd on the flatmap) - (gd on the inflated surface)?*

Do you know how I can do this in matlab for just *one region using the DKTaltas*? for example, the superior frontal?

Any help would be greatly appreciated!

Kind regards, Sarah

On Wed, 9 Nov 2022 at 17:02, freesurfer-request@nmr.mgh.harvard.edu wrote:

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4. Re: volumes/surfaces FS6.0 vs FS7.2/3.2 (Nolan, Jackson)
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Message: 1 Date: Tue, 8 Nov 2022 17:20:22 +0000 From: Sarah Lavelle lavells1@tcd.ie Subject: [Freesurfer] Calculating Distortion in mris_flatten() To: freesurfer@nmr.mgh.harvard.edu Message-ID: < CABRNrkfJFpcKP0jhBX4Fmv6OKr1KamrNr5Tap_5bM3H5v--eyQ@mail.gmail.com> Content-Type: text/plain; charset="utf-8"

    External Email - Use Caution

Hi freesurfers,

Does anyone know how to measure the distortion of your inflated surface -> flatmap transformation (using mris_flatten)?

When being run, the function outputs "final distance error ...%" in the terminal. What exactly does this mean? and how is this calculated?

I would like to research optimal cuts for flatmaps, depending on the region of interest (ROI), so we can minimize the distortion of the ROI itself on the 2D map. If anyone has any ideas on how this can be done or is interested in working with me on this, please contact me via < lavells1@tcd.ie>

Kind regards, Sarah