Dear All,
Once again, this group proved to be wonderful team of colleagues! Thank you
so much to all who have responded. I give bellow a summary of their input.
Most likely it is indeed the z1 cryoshim drifting, and this can be fixed only
by putting more current in the cryoshim z1 coil. On our Inova console, we
can buy more time by adding on the Z0/Z1 board a 10 ohm resistor in parallel
to the existing one. This will allow more current on the rt z1, without heating
the shim stack too much.
It is most likely, a bad/compromised joint, but it could also possibly be the
switch itself. And by going in and resetting the cryo-shim current, activating
and cycling the switch MIGHT get this settled down.
It could also be a problem with the gradient amplifier's bias adjustment.
Unplug the gradient amp and see how much Z1 is needed to return to the same
lock level.
You can "decouple" cryoshims from RT shims by running a proton spectrum with
all RT shims set to zero. If you monitor this peak (call it the cryolineshape)
over time and it changes you know you have a cryoshim drifting. I would look
at your cryolineshape (with RT shims all zero) twice; once with (your shim
power supply) on and again with it off. If there is a difference in these
two spectra it's a hint there is a electrical problem with RT shims.
Thank you all again,
Ion
Received on Thu May 20 2021 - 06:45:58 MST