Dear Martina,
Thank you for the reply. I agree with you that the echo spacing does not change (should remain 0.46ms). I used the formula
Echo Spacing = 1 / [(0019,1028) * (0051,100b component #1)]
>>> where
>>> 0019 1028: BandwidthPerPixelPhaseEncode -- BW in PE direction (Hz)
>>> and
>>> 0051 100b: AcquisitionMatrixText (gives size of *reconstructed* matrix)
which is 1/(28.694*76)=0.46msec which matches up the siemens protocol sheet for echo spacing.
I guess my confusion for the total readout time came from the fact that we are only collecting 57 lines of k space (76*6/8=57) as opposed to the full 76 lines. I would think the distortion would have to be based on the actual readout time (57*.46ms) as opposed to the reconstructed image readout time (76*.46ms) since the distortion should not be accumulating for the 2/8 portion. So assuming the calculation above for echo spacing above is correct do you still think the readout time of 76*.46 is correct or should this be 57*.46, or is the echo spacing time i calculated above (0.46msec) incorrect?
Thanks,
Ajay
Dear Ajay,
In the Fieldmap toolbox the TOTAL_EPI_READOUT_TIME you should use is 76*0.46ms to have the correct effective time.
This is because, unlike accelerating using parallel imaging which halves the field of view at the time of acquisition by doubling the k-space interval, the use of partial Fourier in the acquisition does not change the echo spacing (or k-space interval).
Hope that helps,
Martina
