One of our students is collecting gradient echo images with a
ringing artifact. The phantom is a uniform cylindrical agarose gel 5mM
CuSo4. And we are looking at the circular two dimensional slice image in
the center of the phantom. There is one big dark spot (or it can be a
big light dark spot depending on which slice you look or which value of
the refocusing slice gradient you put in) off center of the image with
alternating dark and bright rings around it. The rings are irregular.
This is not taking place on the corner of the image and the field of
view is about twice the image so I think that moire artifacts are rulled
out.
These artifacts come about when we change the standard set up which
yields good images. The changes that we make are: 1) NMR transiever is
different ( we use TECMAG on the changed setup as opposed to the
standard set up: Varian) , 2) amplifier, and preamps are chnaged. We
connect the outputs of our gradient signals (xyz) into the gradient
amplifier of the standard set up and disconnect the inputs from varian
system. Two implications are: 1) the xyz shims are now not valid. We can
correct this however on the TECMAG by addinf dc values of the gradient
during the gradient pulse 2) The eddy compensation is no longer valid.
We haven't done anything to remedy this problem.
I rulled out data clipping becasue the artifacts are still present when
the tip angle is small. I should mention that the amplifier we use now
can only output 10W. I don't think I'm saturating the power amplifier
because the amplifier has a red led indicating saturation should it ever
happen. The B1 field is uniform according to my RF measurements. Using
the same RF coil I get good images in the standard set up. The only
culprit seems to me could be the uncompensated eddy currents. However
does it make sense for images to have this ringing problem with eddy
currents not compensated for ? The ringing is very sensitive to the
value of the refocusing gradient that I use.
Any help would be greatly appreciated . ( could you please reply to :
b-behnia_at_mrel.beckman.uiuc.edu )
many thanks,
-babak behnia
Received on Thu Jun 29 2000 - 18:32:07 MST