Dear Haakon,
one additional remark to your third point:
>> Third: Suppose I run multiple experiments during one scanning session,
>> in which the acquisition parameters(no. of slices, voxel size, slice
gap,
>> TR, TE) vary between experiment. Is it necessary to acquire a
>> fieldmap matched to each experiment?
If you change the number of voxels or echo spacing you have to calculate
new VDMs for each EPI data set with different total EPI readout time.
But this can be done from one acquired field map, if you ignore the
different shims (see Chloe's comment).
Best wishes
Michael.
Chloe Hutton schrieb:
> Dear Haakon
>
>> First: I've yet to see any concrete recommendations as to optimal
>> settings for B0 maps. I'm currently gathering field maps using a factory
>> optimized sequence which leaves me with a phase and magnitude image that
>> the Fieldmap toolbox accepts readily. However, the specific parameters of
>> the field map, such as voxel size, and slice number is seldom in
>> correspondence with the functional data I collect. Am I right in assuming
>> that this is suboptimal, and that we SHOULD acquire field maps that are in
>> exact correspondence to the functional images with regard to 1: voxel size
>> 2: number of slices and 3: acquisition orientation. Am I forgetting
>> something?
>
> Assuming that you are acquiring gradient echo fieldmaps, the FieldMap
> toolbox is designed to accept input for which 1) voxel size, 2) number of
> slices and 3) acquisition orientation do NOT have to be the same as the
> EPI. We standardly acquire fieldmaps of 3mm^3 voxel size regardless of the
> EPI voxel resolution (not all of our EPI sequences have 3mm^3 voxels).
> This voxel size is a good trade-off between acquisition time and the
> spatial scale of field changes that can be corrected for.
>
> Importantly, we recommend to acquire 64 transverse slices so that the
> whole of the head is covered. This means that field information will be
> acquired beyond the EPI slice coverage and avoids any unnecessary
> truncation of the EPI data when the distortion correction is applied.
>
> It is also important that the shim values/currents are the same for both
> the EPI and the fieldmap data. On Siemens scanners it is possible to do
> this by selecting 'Copy Parameters', from the EPI volume then selecting
> 'Adjust Volume' when prescribing the field map acquisition. This step
> copies the information about the volume over which the shim is adjusted
> for the EPI data into the prescription of the field map, ensuring that the
> field was the same over this region. I'm not sure how this can be done
> with Philips or GE scanners.
>
>> Second: Suppose I have a rather large amount of data with fieldmaps
>> for in which voxel size and number of slices is not in correspondence with
>> the functional data. Is it still possible to perform fieldmap based
>> unwarping?
>
> Ideally, as described above, the field map should be acquired with the
> same shim values as is acquired for the EPI data. The actual number of
> slices acquired and voxel size is not important as the FieldMap toolbox
> can take care of this using the positioning information encoded in the
> image headers.
>
>> Third: Suppose I run multiple experiments during one scanning session,
>> in which the acquisition parameters(no. of slices, voxel size, slice gap,
>> TR, TE) vary between experiment. Is it necessary to acquire a
>> fieldmap matched to each experiment?
>
> Again, the most important thing is the actual field information that is
> acquired for the field map rather than the geometry.
>
> Best wishes
> Chloe
>
>> I have a sneaking suspicion that the reason why I can't find these
>> questions answered anywhere is because the answers are rather obvious to
>> those well-versed in these matters, but I hope you will find time to
>> answer me in spite of this.
>>
>> Best wishes,
>> Haakon,
>> University of Oslo
>>
>>
>
>
--
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Dr. Michael Erb
Sektion f. experimentelle Kernspinresonanz des ZNS
Abteilung Neuroradiologie, Universitaetsklinikum
Hoppe-Seyler_Str. 3, D-72076 Tuebingen
Tel.: +49(0)7071/2987753 priv. +49(0)7071/61559
Fax.: +49(0)7071/294371
e-mail: <[log in to unmask]>
www: http://www.medizin.uni-tuebingen.de/nrad/sektion/
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
|