Hi,
Just to point to another possible aspect for your problem. You wrote
that the movement occurs when you apply pressure. Is this pressure
realized with some mass displacement within the magnetic field? If so,
this displacement might cause a variation of the B0 field, which results
in apparent motion primarily in phase encode direction. If this might
be a reason why you get stimulus correlated motion artefacts, you might
want to discuss with your MR physicist solutions to reduce this problem
already for the data acquisition.
good luck,
wolf
On 27/04/11 10:00, Jesper Andersson wrote:
> Dear jeongchan,
>
>> i'm doing human fmri study with stimulation on the subject.
>>
>> mostly whenever we applied pressure, subjects' head move aroud 0.8 sometimes ~1.8 (mm? voxel? shift parameter in the *.par file from mcflirt-fsl). i know i'd better change the design might not cause much motions, but it'll be critical for me if i change.
>>
>> probably, the raw data in itself have a distorted slice timing and/or too much motion so that the data will be interpolated alot.
>>
>>
>> here's my question...
>>
>> is there any way to use priori information about the movement(shift mostly) so that i can shift each volume without any interpolation before i run mcflirt-fsl, as if the 4D data collected without much motion?
>> the reason that i'm asking is that i don't want the data to be interploated too much, so it would be better if i can change the initial location before motion correction.
>> (i've looked through the mcflirt options '-init' but don't know in detail.)
>>
>> my idea: put some markers on the subject's head during the scan then detect the marker so that i can quantifiy the movement (using mcflirt only based on the marker not the brain) during the run. then shift the each volume using the shift-information from my marker then perform motion correction. but don't know more better way.
> the interpolation errors increase up to a half of a voxel, after which they decrease again and become zero/negligible at one voxel. They do not get biger and bigger as the distance/movement increases. Therefore I do not think that your strategy would buy you anything.
>
> There are other movement related affects that do increase approximately linearly with movement (specifically with rotation around an axis non-parallel to the main magnetic field) but those cannot be corrected by the strategy you suggest.
>
> Good luck Jesper
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