Hi Annelene,
The units along the X-axis are the signal frequency. You can use the
-bptf option to fslmaths to apply a band-pass filter. You need to
specify the high- and low-pass frequencies as sigma values in volumes.
You can convert from frequency to sigma (in volumes) using the
following formula:
sigma_vols = 1.0 / (2.0 * freq * TR)
So if, for example, your data has a TR of 2 seconds, and you want to
filter out frequences in the range 0.09-0.11Hz, you would use the
following sigma values:
sigma_lp = 1.0 / (2.0 * 0.09 * 2.0) = 2.7778
sigma_hp = 1.0 / (2.0 * 0.11 * 2.0) = 2.2727
i.e.
fslmaths <input> -bptf 2.2727 2.7778 <output>
However, the low-pass filter has a very slow roll-off, so you may need
to experiment a little with the low-pass sigma to get the results you
want. Also note that you may wish to de-/re-mean your data
before/after applying the filter.
See these posts for more information:
- https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=fsl;9ffadc97.1109
- https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=FSL;36bcf0ce.1709
- https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=FSL;c65efaab.1312
Cheers,
Paul
On 30/12/2018, Annelene Dahl <[log in to unmask]> wrote:
> Hi!
>
> I have discovered an artifact that keeps appearing in the resting state raw
> data in all my subjects, and using FSL eyes, it seems from the 'Power
> Spectra 3' window that it has a very specific frequency, which extends over
> multiple components seen with ICA. I am attaching a screenshot of the
> FSL/Melodic view to make it clearer.
>
> I would like to try to filter out this frequency, and in doing so I have two
> questions:
>
> 1) What are the units on the x-axis in 'Power Spectra 3' window - Hz? MHz?
> 2) How would one go about removing a specific band of frequencies? Is there
> a fslmaths command to remove a particular frequency in the middle?
>
> Thanks for your help, and happy new year!
>
> Annelene
>
>
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