Dear Sourena,
> Thank you for explaining this issue.
> As I have seen in HCP protocol the common practice is a pair of AP/PA
> spin echo EPI with matched BW/FOV/matrix. Is it advisable to acquire
> extra volumes?
Acquiring 2-3 volumes per blip-direction is very quick and offers some protection against something going one with one volume.
> or even some BOLD volumes with an opposing
> phase-encoding direction?
BOLD (ge-epi) is worse for this since we then have signal dropout in addition to the distortions. If you acquire your ge- and se-EPIs with the same BW/FOV/matrix you can still use the field from the se-EPI for the ge-EPI data.
Jesper
>
> Yes I was thinking about the gradual heating of the gradients. I
> wonder if disturbance in the active shim is something serious to
> avoid.
> Sincerely yours,
> Sourena
>
> On 2/11/14, Jesper Andersson <[log in to unmask]> wrote:
>> Dear Sourena,
>>
>>> Should the topup approach be considered superior to fieldmap
>>> correction regarding (single-band) fMRI?
>>
>> if you are only getting a single pair for calculating a fieldmap I don't
>> think it is superior to a "traditional" fieldmap. It is mainly just another
>> way of obtaining the field.
>>
>>> How serious is the effect of gradient thermal state on fMRI/DTI
>>> signals specially when the scanner has just started its daily job?
>>> Does there exist some guideline to adhere?
>>
>> I am not sure what you mean by "gradient thermal state". Are you talking
>> about the gradual heating of the gradients during a long run? If so, I don't
>> think it is a problem for fMRI. Anything slow and gradual tend to be
>> regressed out anyway.
>>
>> For DTI I don't know. Maybe someone else has some experience?
>>
>> Jesper
>>
>>>
>>> Thanks in advance,
>>> Sourena
>>
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