Shimming Simulation (SM)
(Starting with version 990110)
This command enters a subroutine which allows the user to practice adjusting the on-axis (Z) shims under ideal conditions. The "sample" consists of a single peak. By default, the peak is exactly on resonance, but this can be changed by adjusting the Z0 shim. The value of each shim gradient is displayed in the upper right portion of the screen. A value for the current lock level is displayed at the upper left, along with the lock level corresponding to the previous set of shims, so the user can easily gauge the effect of a shim adjustment. Perfect shims will give a lock level of 1000.
Each shim value is changed using the number keys on the keyboard — each time a number between 0 and 8 is pressed, the corresponding shim is changed by a set increment. By default, the increment is 5 units. To change to larger step sizes, use the greater than key (>) to increase the Sensitivity, which applies a factor to the increment size. (It is also possible to change the increment; see below). The number keys apply a positive increment. Holding down the shift key while typing a number applies a negative increment. These and other subcommands are described below.
Several parameters can be adjusted by selecting Edit Parameters from the Edit menu, which displays the screen shown below.
The lineshape is calculated as the superposition of "mini-samples" located along the length of the detector coil. In the case of a very broad resonance due to poor shims, the peak shape can become distorted because of this "discrete" approximation. A series of small, individual peaks can be seen when the signal should really be a continuous curve. This can be corrected by ssing a larger number of minisamples, but this will slow the calculation.
Number of points refers to the number of data points used in the Fourier Transform.
Randomize level refers to the highest order magnet gradient which is randomized by the ? command for practice shimming. By default, all gradients up to and including Z6 are randomized, but this can be changed to make the shimming problem easier (lower value) or harder (higher value).
A specific value for any shim can be entered directly, as an alternative to adjustment by increments. Choosing Set Gradients from the Set menu displays the screen below.
Test yourself with the Shimming Game
To simulate an imperfect magnet, whose gradients must be corrected using shims, select Randomize from the Set menu (or type ?). The resulting lineshape will be poor, and the lock level will be low. The goal is to obtain a narrow, symmetrical peak and a lock level of 1000, corresponding to a perfectly shimmed magnet. The values of the magnet gradients can be displayed by selecting Toggle Answer from the Display menu (or type :). The value for each shim needed to compensate perfectly for these gradients is the same absolute value as the corresponding magnet gradient, but the opposite sign. When the answer is displayed, a score is also shown, calculated from the elapsed time and percent lock level recovery, which are also displayed. Maximum possible score is 100,000.
A very useful tool for shimming under these conditions is the Z1 profile.
- # (0-8) Increment corresponding shim, Z0 – Z8
shift-# Decrement corresponding shim, Z0 – Z8
? Randomize magnet gradients
: Display magnet gradient values
F Display FID
S Display spectrum
E Edit parameters
G Set shim gradients
I Set increments for each shim adjustment
Z Zero magnet gradients
[ Apply negative Z1 gradient
] Apply positive Z1 gradient
= Remove Z1 gradient
> Increase sensitivity — increases multiplication factor for shim increments
< Decrease sensitivity — decreases multiplication factor for shim increments
<ENTER> Exit shimming subroutine
The Acorn NMR web site includes a description of real-world factors affecting shimming and a systematic shimming procedure. See Shimming Explained.
Last updated: 1/10/99.