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Does anyone know for sure if a shim map is off center slightly left in a negative direction...and I assume it is a centering issue do you whether it would be probe up/down? |
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Hi Cal, The shim map will change if the probe is not at the center of shim stack. Besides centering the probe you can also check whether your shim stack is centered in the magnet. In our "Oxford" magnet the stack is attached by a screw at the bottom. If the screw loosens, the stack can move down. As for predicting the direction of change in the map when the probe moves - you can try it once and find out what the correlation is on your system. I've been doing some service on the magnet today and recorded shim maps at two positions of the probe (all maps at 1 scan on deuterium, D2O sample in "regular" 5 mm tube): (1) - normal, (2) - about 7 mm lower. Here is what the maps looked like: Normal shim map:
Moved the probe down:
Another map after re-shimming in the lowered position:
The Z1 profile (as well as others) is clearly shifted from the center of the map. |
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Thanks Evgeny, The pics did it. I've been running up/shimming an Oxford 600 where I didn't believe the shim spacing and this sort of answers my question. I suspected the system was too low and my map had shims skewed to negative side as well and looks identical to yours. Thanks again. Cal Carpenter |
Good question, Cal. It will depend on the polarity of the gradients being used. Since there is no standardized 'direction' for shims, you will need to find your answer empirically, IMO. - Jerry Hirschinger (Oct 19 '10 at 10:57)