Dear Amlish,
If you measure the brain volume in mm^3 (via segmentation) and then convert this to mL and then take a percentage of this (as given by the atrophy calculation) and then divide this value by the time interval (in years) then the result is atrophy in units of mL/year.
Our tools are developed to measure atrophy more accurately than you would get with a simple subtraction between segmentations, so I can imagine that the difference in segmentation values may not accurately equal your atrophy measurement. If you use the above result (using the baseline segmentation) then you will still get an accurate result, as any inaccuracy in the segmentation would end up as a second-order error (since it is multiplied by the atrophy percentage, which is small).
All the best,
Mark
On 2 Apr 2014, at 01:59, Amlish Munir <[log in to unmask]> wrote:
> Hello FSL users,
>
> Over the past few months, I have been employing FSL to acquire atrophy measurements on MR images. Once the script has ran, the atrophy rate is given to me as a percentage (%). My question is this- how can I convert the percentage atrophy measurements into a mL/year?
>
> Simply acquiring raw values for the segmented tissue in mm^3, converting it to mL and then using subtraction does not present as an accurate approach. It does not coincide with the atrophy % provided to me by the script and makes it difficult towards understanding the raw values that are being used to acquire atrophy measurements as given by the script.
>
> Thank you in advance,
>
> Amlish Munir
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