Gradient log direction and relative start acquisition for Philips scanner

[Jothi-Ni]

Dear Lars,

I am using a 3T Philips scanner with the wireless physiological sensors (sampling rate=496Hz). My fMRI sequence has a TR=2.9s, Nslices=53, Nscans=152. I am using the PhysIO toolbox and I have a few questions for scan/physlog time synchronization.

Based on the explanation in the FAQ I selected gradient_log.grad_direction to z, as gradient z is showing the highest peaks in my case (see below). Is that correct? I chose the following values for gradient_log.zero=0.48 and gradient_log.slice=0.54 based on the recommendation from the FAQ. I left gradient_log.vol and gradient_log.vol_spacing empty.

This is how the raw gradient time courses look like when I zoom in.

Zooming in even more:

Regarding a comment you once raised on relative_start_acquisition:
"I would suggest that you just leave this field at 0 and let PhysIO try to figure out the scan synchronization from the gradient timecourses."

So you mean setting it to 0 (and not empty) which would mean simultaneous start of physlog file and scan start, which is not the case for my study as the physlog seems to start some seconds before the actual scan starts. If I leave this field empty, however, I run into an error message (Failed 'TAPAS PhysIO Toolbox'
Error using + Matrix dimensions must agree.). If I set it to 0, I at least don't get an error message and PhysIO runs without errors. Does the scan synchronization from the gradient time courses in this case overwrite/ignore the relative_start acquisition time?

Can the number of found volume events be seen as a check to verify whether PhysIO runs correctly on my dataset? In my case it found 152 volumes and that is indeed the number of scan volumes which I specified for this fMRI sequence.

Eventually the generated multiple_regressors.txt file can then be used as an additional regressor in my first level model in SPM for example, correct?

Thank you,

Jothini