Phase image artefacts
Lachlan Graham (Lachlan.Graham@dbce.csiro.au)
Tue, 22 Sep 1998 11:09:04 +1000
Dear Colleagues,
We have been trying to use the phase method of velocity imaging with
limited success. The pulse sequences are spin echo imaging sequences with
flow encoding gradient lobes added. The plan is to take images with the
flow on and with the flow off so that any background phase shifts can be
removed. The problem we are having is that we get phase images which have
large (linear) variations of phase across them (usually in the read
direction but sometimes both). These variations sometimes exceed +/- 180
deg so that image subtraction can be tricky. The other difficulty is that
they are not consistent and can vary from one image to the next even if
nothing is changed. I have tried increasing gradient stabilisation delays
subtantially and also the time between gradients but this did not work. I
have checked the phase cycling and also the effect of deliberately
de-shimming the sample to no avail. Magnitude images also seem quite
satisfactory. I would be grateful to hear from anyone with any suggestions
as to the cause of our dificulty. I admit that we have a low technology
set of unshielded gradients so maybe we are asking them to do what they
can't and we should stick to time of flight methods.
Thanks,
Lachlan Graham