Adjusting Shims

Shim coils produce small magnetic fields used to cancel out inhomogeneity in the static field. In shimming, the current in shim coils is adjusted to make the magnetic field as homogeneous as possible. Computer-controlled digital-to-analog converters (DAC's) regulate the room-temperature shim coil currents. Every time a new sample is introduced into the magnet or probe is changed, it is necessary to readjust the shims. The FID's below show a good and poorly shimed sample.

• Loading Shim Values
• Loading a Shim File
• Saving a Shim File
• Shim Gradients
• Automated Shimming on the Lock
• Which Shims to Use on a Routine Basis
• Shimming Different Sample Geometries

Loading Shim Values
1. Click on the Lock page in the Start panel.
2. Click on the Load Shims into Hardware button
This is the equivalent of the command line instructions: load=’y’ su. Shim values stored in the current experiment are loaded (this may not be suitable for the current sample).

Loading a Shim File
Load a shim set from the Locator to the shim buttons area of the Shim page as follows:
1. Click the Locator Statements button (magnifying glass icon).
2. Select Sort Shimsets. List the shimsets by probe or filename.
3. Select a shim set and drag-and-drop it onto the graphics canvas or shim buttons area of the Shim page.
Saving a Shim File
Save the shim values to a file as follows:
1. Enter a file name in the field next to the Save Shims button, and press Return.
2. Click the Save Shims button.

Shim Gradients
The shims are actually printed coils wrapped round a cylindrical form that encloses the NMR probe. A coil (or sum of coils) whose field is aligned along the axis of the magnet is called a Z axial shim gradient (Z1, Z2, Z3, etc.). Coils whose fields are aligned along the other two orthogonal axes are called X and Y radial shim gradients (X1, XY, X2Y2, Y1, YZ, etc.). The field offset coil Z0 (pronounced “zee-zero”) alters the total magnetic field. Each shim gradient is controlled by its own parameter; for example, the X1 shim gradient is controlled by a parameter named x1. Depending on the value of the shimset parameter, shim values range from –2047 to +2047 or from –32767 to +32767, with a value of zero producing no current.

Automated Shimming on the Lock
Like locking, shimming can be done manually using the controls on the Shim page. Automated shimming is often preferred, however. It can be set up from the Standard page of the Start tab:

Shim on Lock options menu next to When
• Not used — disables automatic shimming.
• Every sample — Shims before the start of data acquisition for each new sample.
• Every Experiment— Shims before the start of data acquisition for each new experiment.

• Every FID— Shims before the start of data acquisition for each FID. Shim on Lock options menu next to Shim method.
• z1z2
• Low non-spins
• All z’s
• Hi-res z’s
• Fine z1z2
• Fine z1-z3

Autoshim is controlled by the selection made from the Shim method menu. It is a complete background Autoshim method that provides no interaction with the operator. The type of automatic shimming to be done during routine sample changes depends on the level of homogeneity required on any particular sample, the change in sample height, and the maximum time desired for shimming.
• For average homogeneity needs with samples which are either long or all of identical height, Z1Z2 shimming is usually sufficient.
• If sample height might vary, the option all z’s has been found to be the most reliable, at the expense of greater time spent in shimming. This method shims first Z1, Z2, and Z4, then Z1, Z2, and Z3, and finally Z1 and Z2.

Selecting Shims to Optimize

Which Shims to Use on a Routine Basis
The following suggestions assist in routine shimming, especially on shim systems with a larger number of shim channels:
• Establish and maintain lineshape – Use Z to Z5, possibly Z6, X, Y, ZX, ZY, and possibly Z2X and Z2Y. The effects of Z7 and Z8 (and realistically Z6) are too small to see with the lineshape sample.
• Shim a new lineshape sample of different geometry – Use Z to Z5, possibly Z6, X, Y, ZX, ZY, and possibly Z2X and Z2Y.
• Shim a new sample of the same geometry – Use Z, Z2, and maybe Z3.
• Shim a new sample of different geometry – Use Z to Z4 and possibly Z5, X, Y, ZX, ZY.
• Shim for water suppression – Start with a shim set that produces a good lineshape for the same sample geometry. Next, tweak Z and Z2, and then vary Z5 and Z7 to minimize the width of the base of the water (Z and Z2 may need to be tweaked if Z5 changes by more than 100 to 200 coarse units). About 80 to 90 percent of the odd-order axial-gradient induced water width is probably dominated by Z5, with Z7 and perhaps some Z3 providing the rest. The even-order axial shims (Z2, Z4, Z6, and Z8) affect the asymmetry of the residual water line (using presaturation). All four of these even-order axial shims can affect the final water linewidth, with Z2 and Z4 being at about the 5 mM solute level and above, Z6 being at about the 1 mM solute level, and Z8 being at about the 0.3 mM solute level. The even-order axial shims will perform as expected unless the sample is less than 40 mm in length, in which case the shims still control the water linewidth but much less responsively. Beware of the use of Z4 to narrow an asymmetric residual water line of a sample shorter than about 40 mm. One is probably destroying the base of the standard lineshape faster than the residual water signal is being narrowed. This is because the residual water resonance width is affected more by magnetic susceptibility interfaces as the sample gets shorter. For samples under 40 mm, the iterative use of Z5-Z7 with Z6-Z8-Z4-Z2 can narrow the residual water line, but the results obtained may be hard to reproduce on subsequent samples due to an increased sensitivity to slight changes in sample geometry.
Shimming Different Sample Geometries
Some suggestions when moving the sample:
• Moving the same sample up – Z, Z3, and Z5 need to become more positive.
• Shortening and centering (moving up) the sample – Z2 and Z4 need to become much more positive. The trends for Z and Z3 are mixed and more complex, but they tend to become a little more negative. It appears as if Z and Z3 are driven positive as the sample is pulled up, but they are driven negative faster as the sample shortens. When shimming a lineshape sample, plan on the following changes (starting from lineshape shims for a 700 μL sample at a depth 67-68 mm): 700 μL to 600 μL: move Z2 +50 DAC units and move Z4 +250 units. 700 μL to 500 μL: move Z2 +200 units and Z4 +600 units.
The Z2 and Z4 changes track well with sample volume, but are relatively independent of tube depth. It is therefore easiest when changing sample geometries to make the appropriate Z2 and Z4 corrections, then adjust the more complex Z1-Z3-Z5 interactions as needed.
Shimming on the Lock Signal Manually

When shimming on the lock, the intensity of the lock signal is monitored as the shim settings are adjusted. Each shim setting controls the current through shim coils that control magnetic field gradients in different directions. It is important to know that the Z direction is parallel to the vertical direction of the probe and it is for this reason that the height of the sample in the NMR tube affects the Z shim settings rather dramatically.

• Routine Shimming
• Setting Low-Order (Routine) Shims
• Removing Spinning Sidebands (Non-Routine)
• Setting the High-Order Axial Shims (Non-Routine)
• Setting High-Order Radial Shims (Non-Routine)

Routine Shimming
1. If the shim settings are way off the mark (e.g., if the temperature has changed), load the shim settings that have been most recently established for the starting point probe.
2. Click Setup Hardware.

3. Make sure the probe has a sample, that it is spinning at the correct speed, and that the system is locked onto the deuterium resonance from the lock solvent.
4. Check that the lock signal is not saturated. The signal is considered saturated if changing the lock power by 6 units (6 dB) produces a change in lock level changes more than a factor of two. Adjust the lock gain if necessary.
5. Open the Shim page.
Try a change of +10 or - 10 in the setting for Z1. If the lock level goes up with one of these, continue in that direction until the level is maximized (it no longer increases, but instead begins to fall).
6. Change the setting for Z2C by +10 or - 10 and continue in that direction until the level is maximized.
7. Adjust Z1 for maximized lock level; then adjust Z2 for the same. Continue this iterative process until the lock level goes no higher. If the lock level increases to 100, decrease lock gain and then continue to adjust Z1 and Z2. Lock power can be adjusted as needed.
In most cases, this concludes the shimming; however, some times it is necessary to shim the other Z controls and the non-spin shims. This must not be undertaken in the same way as the procedure above suggests. Go through Z1, Z2, Z3, and Z4 iteratively until the lock signal is maximized this may result in a degraded signal shape. Hence, the following procedure is suggested for a second level of shimming:
1. After Z1 and Z2 have been adjusted for maximum lock signal, write down the lock level, adjust Z3 in one direction, say by +10, and then reoptimize Z1 and Z2 (iteratively) until the lock signal is at a maximum. Note this level of the lock signal. If the lock signal is higher than it was before (when it was first recorded), continue changing Z3 in the same direction. Every change in Z3 must be followed by optimization of Z1 and Z2 until the lock level is at a maximum.
2. Repeat step 1 with Z4. That is, change Z4 in one direction, then optimize Z1 and Z2. If the lock level does not go up, change Z4 in the opposite direction and optimize Z1 and Z2. Continue until the highest possible lock level is obtained.
3. Repeat steps 1 and 2 iteratively until the highest possible lock level is obtained.

4. Turn the spinner off and go through the non-spin shims, one at a time, maximizing the lock level for each one. Then return and go through each again. Continue through all until the lock level is as high as possible. If lock is lost, increase the lock gain.
5. Turn the spinner on and optimize Z1 and Z2 as described above, return to the nonspins (turn the spinner off) and reoptimize these. Continue until the highest lock level is obtained. For an ultimate check, insert the lineshape sample (CHCl₃ in deuteroacetone for ¹H and dioxane in deuterobenzene for ¹³C) and examine the line shape to make certain the original specifications and meet, especially for the line shape at 0.55% and 0.11% of the total peak height. Also examine the height of the spinning sidebands.
Setting Low-Order (Routine) Shims
The following procedure describes how to set the low-order, or routine, shims. Reset Z0 and lock phase if very large changes in the room temperature shims have been made. With this procedure, concentrates on improving the symmetry of the main resonance as well as the half-height resonance and line shape.
1. Adjust the lock level to about 80 if possible. Maximize lock level with Z1. Maximize lock level with Z1 and Z2. Do this by making a change in Z2 followed by maximizing with Z1 again. Continue to iterate in this manner until the lock level no longer increases.
2. Acquire the spectrum.If the sample is properly shimmed, the lines should be symmetric.
3. If the lines are not symmetric or unusually broad at the base, adjust. Normally adjust Z3, Z4, or the non-spins for most routine samples.

4. When there is a need to adjust Z3, do so by interactively shimming Z1 and Z3 in the manner described in step 3 for Z1 and Z2. Changes in Z3 may affect Z2 so after shimming Z3 maximize Z1 and Z2 again.
Removing Spinning Sidebands (Non-Routine)
If the spinning sidebands are not within specification, use this procedure to remove them.
1. Write down the lock level, set SPIN to off, and write down the lock level.
2. Adjust lock to about 80 if possible.
3. Maximize lock level with X.
4. Maximize lock level with Y.
5. Maximize lock level with X and Y. Do this by making a change in Y followed by maximizing with X again. Continue to iterate in this manner until there is no longer an increase the lock level.

6. Maximize lock level with X and ZX. Do this by making a change in ZX followed by maximizing with X again. Continue to iterate in this manner until there is no longer an increase the lock level.
7. Maximize lock level with Y and ZY. Do this by making a change in ZY followed by maximizing with Y again. Continue to iterate in this manner until there is no longer an increase the lock level.
8. Repeat step 4 above.
9. Maximize lock level with XY and ZXY (ZXY not available on 13 or 14 channel shim systems).
10. Repeat step 4, step 5, and step 6.
11. Set SPIN to on and acquire a spectrum. If the sample is properly shimmed, the lines should be symmetric.
12. If the lines are not symmetric or unusually broad at the base adjust. For most routine samples, do not adjust Z3, Z4, or the non-spins.

13. If necessary adjust Z3, by interactively shimming Z1 and Z3 in the manner described in step 3 in the previous procedure. Changes in Z3 may affect Z2 so after shimming Z3 maximize Z1 and Z2 again.
Setting the High-Order Axial Shims (Non-Routine)

1. If Z4 needs to be adjusted, look at which side of the peak the asymmetry appears—low field to the left and high field to the right.
2. Determine which direction to move Z4. If the asymmetry is large (Z4 is far off), change Z4 by a considerable amount to try to push the asymmetry to the other side of the peak. This provides two important pieces of information:
• Confirms that Z4 is the problem if the asymmetry moves.
• Indicates what the actual value of Z4 should be when Z4 is changed. The values that caused it to be on either side of the peak, the correct value must be between the two extremes.
3. Set Z4 to the value that produces neither a high-field nor low-field asymmetry. Z4 affects all the shims below it, so repeat the in the “Setting Low-Order (Routine) Shims” procedure.
4. Maximize the lock level with Z5.
5. Repeat step 3 and step 4 until no further increase is obtained.

Setting High-Order Radial Shims (Non-Routine)
Note that Z2X, Z2Y, ZX2–ZY2, Z3X, Z3Y, and Z5 are not available on 13-channel shim systems.
1. Set SPIN to off and write down the new lock level.
2. Set the lock level to about 80.
3. Maximize the lock level by shimming Z2X against ZX.

4. Maximize the lock level by shimming Z2Y against ZY.
5. Repeat the “Removing Spinning Sidebands (Non Routine)” procedure.
6. Maximize the lock level by shimming ZXY against XY.
7. Maximize the lock level by shimming ZX2–ZY2 against (X2-Y2).
8. Set SPIN to on and adjust the lock level to 80.
9. Maximize the lock level by shimming Z1, Z2, Z4, and then Z1, Z2, Z3.
10. Repeat step 1.
11. Maximize the lock level by shimming X3 against Y3
12. Maximize the lock level by shimming Z3X against Z3Y if available. Refer the Oxford magnet manual for approximate Z3X and Z3Y values. Be aware that the signs may be reversed in the Oxford manual, experiment to determine the correct sign.

13. Look at the spectrum and decide where to concentrate effort:
• For a broad base, adjust Z4 and Z5.
• For spinning sidebands, adjust the proper order radial shims.
As Z4 and Z5 are optimized, the contribution of Z3 to the breadth of the base becomes more clear, as well as any contribution from the high-order radial shims. Several cycles of shimming are required.
In some cases, local maxima will be encountered, causing the greatest problems. A local maxima may be indicated if a high-order shim continues to increase and eventually reaches the maximum output of the shim supply, without having reached the optimal lock level. In such a case, carefully reexamine the lower-order shims by making large excursions (systematically), beginning with the lowest-order shim and working up. This is a particularly difficult issue when dealing with the high-order radial shims such as X3, Y3, Z3X, and Z3Y, because their perturbation of the lock level is small relative to the change in the shim current. At the same time their perturbation of the spectrum is significant in experiments such as water suppression, but their effects can go unnoticed or may not be importantin some routine 1D spectra, where large solvent peaks are not encountered.

Shimming PFG (Pulsed Field Gradients) Systems
These procedures apply to the Performa I system. Once in operation, leave the amplifier on while using the gradient system, to allow the amplifier to reach a long-term equilibrium.
Performa I
1. Open the System settings window (Edit->System settings).
2. Click Setup Hardware.

Introduction to Gradient Shimming

Gradient autoshimming provides rapid, automatic adjustment of axial room-temperature shims. It is a very reliable way to set high-order shims, eliminating many hours previously spent on shimming. Typical gradient autoshimming time is only a few minutes, and all steps are done with a few clicks of a mouse button. Gradient autoshimming is implemented for use with the axial gradients (Z-gradients). A PFG amplifier and probe is installed. Gradient autoshimming can be performed using the homospoil gradient (Z1 room temperature shim coil). For aqueous samples, water protons provide sufficient signal for shimming. For deuterated solvents, gradient shimming can be performed if there is sufficient deuterium signal. Deuterium gradient shimming is feasible on most samples where the lock solvent is a single, strong resonance, which includes the majority of solvents of interest for routine NMR use. Homospoil gradient shimming is available on all systems, with or without PFG. The Automated Deuterium Gradient Shimming module is required for deuterium gradient shimming with PFG or homospoil.
Deuterium Gradient Shimming

Deuterium gradient shimming is feasible for most deuterated solvents for which lock solvent has a single, strong deuterium resonance with sufficient signal. Automated deuterium gradient shimming automatically holds the lock at its current value and switches the transmitter cable to pulse the lock coil when an experiment is run with tn='lk'.
The system administrator must make a shimmap on deuterium before deuterium gradient shimming can be used. Follow the procedure using the deuterium signal for all steps. The transmitter power (tpwr) should be kept low to avoid probe arcing, with a 90° pulse greater than about 200 μs.
Homospoil Gradient Shimming

VnmrJ allows the use of homospoil (room temperature Z1 shim coil) as a general gradient type. It does not require the use of a pulsed field gradient module and thus is available on systems without PFG. When homospoil is switched on in a pulse sequence, the shim current is set to maximum for a given period of time.
• To use homospoil as a quick homogeneity spoil in a pulse sequence, use hsdelay. This is the traditional homospoil method, and is usually done at the beginning of a relaxation recovery delay (e.g., hsdelay(d1)). The parameter gradtype is ignored.
• To use homospoil as a general gradient type in a pulse sequence, i.e. for gradient shimming during automation or gradient shimming in general:
1. Login as the system hardware administrator, typically this is the user vnmr1.
2. Select Utilties from the main menu.

3. Select System settings.
4. Click on the System tab.
5. Click on the System config button.
6. Click on the Z Axis Gradient menu.
7. Select Homospoil (this sets gradtype='nnh').
Using config and setting gradtype of ‘nnh’ writes the setting of gradtype to the disk and it is read as system global parameter. The parameter pfgon is ignored, since a separate gradient amplifier is not needed. Homospoil is then triggered by gradient statements such as rgradient('z',gzlvl1). If the value of gzlvl1 is non-zero, homospoil is switched on; if the value of gzlvl1 is zero, homospoil is switched off. Only one sign and strength of gradient current is available during a pulse sequence, and is set by hardware. Homospoil gradients may be switched on only for a limited period of time, usually 20 ms. This time limit is determined by hardware in spectrometer systems. Check the pulse sequences to ensure this time limit is not exceeded. The behavior of homospoil gradients is quite different from that of a pulsed field gradient. The gradient strength is much weaker than the traditional PFG, and the recovery time is much longer because of eddy currents. The strength and recovery of the gradient depends on the shim coils and system hardware. Typically, these gradients are suitable only for profile-type experiments and unsuitable for gradient coherence-selection experiments such as GCOSY and GNOESY. For most gradient experiments, pulsed field gradients are preferred if available. Homospoil gradients are suitable for 1H and 2H gradient shimming on some systems.
Homospoil Gradient Shimming for 1H or 2H
To use homospoil gradient shimming, first configure the homospoil gradients. Then, follow the procedure in "Mapping Shims and Gradient Shimming. To use homospoil deuterium gradient shimming with different solvents, use Find z0 before gradient shimming.
Configuring Gradients and Hardware Control

1. Confirm that PFG (pulsed field gradients )or homospoil gradients are installed. A PFG probe is required.
2. Confirm that the gradients are active by checking that gradtype and pfgon are set correctly. Use config to change gradtype if necessary. Use System Settings to set pfgon if necessary.

Recommended Samples for Gradient Shimming
The following samples are recommended for gradient shimming.
Calibrating the 90° Pulse for 1H and 2H
1. Insert a sample and find lock.
2. Disable sample changer control (loc='n').
3. Adjust lock power, lock gain, and lock phase. Make coarse shim adjustments on Z1, Z2, X1, and Y1.
4. Select the Proton protocol.
5. Click the Acquire tab.
6. Select the Channels page.
7. Do one of the following procedures:
Lock (2H) 90º pulse calibration
a. Set the Observe Nucleus to lk
b. Set 90 Degree Pwr to 42 (to avoid arcing).
c. On the Acquisition page, set the Observe Pulse to 200.
d. Continue with step 8
Proton (1H) 90º pulse calibration
a. Verify that the observe nucleus is set to H1.
b. Continue with step 8
8. Click the Acquire button or enter ga and wait for acquisition to finish. Only a single line will be visible.
9. Click the Process tab and select the Cursors/Line List page.

10. Click the Transform button.
11. Place the cursor near the peak and click the Place on Nearest Line button.
12. Click the Move Transmitter button.
13. Select the Acquire tab, then the Acquisitions page, and click the Arrays button.
14. Enter pw for the Param Name, Array size 20
15. Set the following:

1H shimming	2H shimming
First value =4	First value =100
Increment =4	Increment =100

16. Select Acquisition -> Acquire and WFT, (or enter ga) and wait for acquisition to complete.
17. On the Acquisition page set pw90 to the value of pulse width corresponding to first maximum.
18. Click Arrays and click UnArray in the Array window.
19. Set the observe pulse to the pw90 value.
20. Enter pw90 and tpwr in the probe file, if desired.

Mapping the Shims

1. Stop sample spinning.
2. Disable sample changer control (loc='n').
3. Adjust lock power, lock gain, and lock phase. Make coarse shim adjustments on Z1, Z2, X1, and Y1.

4. Click on Tools.
5. Select Standard Calibration Experiments.
6. Select Set up Gradient Shimming.
Standard parameters are retrieved from gmapz.par the first time Set Up Gradient Shimming is clicked, or if a shimmap was previously made, parameters are retrieved from the current shimmap.

7. Click the Acquire tab.
8. Select the Gradient Shim page.

9. Click one of the following buttons under Set Acquisition Parameters to retrieve the parameters from the probe file (if available) and setup gradient shimming. When usingHomospoil gradient shimming parameters, make sure that homospoil gradients are enabled.
10. Click the Acquisition page.
11. Set up the acquisition parameters. Typical parameters for different solvents for deuterium shimming are listed below.

Solvent	Scans	Relaxation delay, sec.
Deuterochloroform	8-32	2
DMSO-d6	4-16	2
D2O	1-4	2
Deuterobenzene	1-4	2
deuteroacetone	1-4	6-12

Actual parameters might vary, depending on solvent concentration, probe, and system hardware.
12. Set pw as follows:
• For PFG, set pw to the 90-degree pulse or less.
• For homospoil, set pw to the 90-degree pulse and p1 to 180-degree pulse.
13. Click the Gradient Shim page
14. Click Acquire Trial Spectra on the Gradient Shim page to test the parameters. Two top-hat profile spectra should appear if the parameters are correctly set. If these spectra do not appear, check the following:

• Gradients are active
• Acquisition parameters, pw, tpwr, nt, and gain are correct.
• Adjust parameters to see good signal-to-noise with no ADC overflow.
15. Enter a map name for the shimmap in the Current mapname field (any string valid for a file name). Or click the Set by date button.
16. Click Automake Shimmap on the Gradient Shim page.
The number of shims used to make the map is determined by the software.
If the mapname already exists, there is a prompted for a new mapname, and whether to overwrite the current mapname.
17. After acquisition is finished, click on Set mapname into probe file button on the Gradient Shim page.
Starting Gradient Shimming
Click Gradient Autoshim on Z on the Gradient Shim page to start shimming as a system administrator. This button starts gradient shimming using current parameters, and displays the curve fit and shim adjustments for each iteration.
Quitting Gradient Shimming
Click the Quit Gradient Autoshim button to quit gradient shimming and exit the gradient shimming set up panel. This also retrieves the previous parameter set and data, including any data processing done on the previous data set.
Gradient Shim Commands and Parameters
The following commands and parameters are useful for performing special functions or can be set manually. Refer to the VnmrJ Command and Parameter Reference for full descriptions

Commands
gmapshim<('files'|'quit')>       Run gradient autoshimming, quit.
gmapsys*                                 Enter Gradient Shimming setup panels, make shimmap.
gmapz<(mapname)>               Get parameters/files for gmapz pulse sequence.

____________
          * gmapsys<'shimmap'<,'auto'|'manual'|'overwrite'|mapname>

Parameters
d2                                             Incremented delay for 1st indirectly detected dimension.
d3                                             Incremented delay for 2nd indirectly detected dimension; arrayed to 2 values
gradtype*                                 Gradients for x, y, and z axes
gzlvl {DAC value}                      Pulsed field gradient strength
gzsize {integer,1 to 8}              Number of z-axis shims used by gradient shimming
gzwin {0 to 100}                        Percentage of spectral window used by gradient shimming
p1                                             First pulse width—If > 0, it is used between the gradient pulses as a 180                                                  refocusing pulse, and the gradients have the same sign.
                                                 pfgon{'nny' if on} PFG amplifiers on/off control
pw                                            Pulse width; it can be <90° if p1=0.
solvent                                     Lock solvent
vtcomplvl                                 Variable temperature compensation for gradient shimming
gmapspin                                Enable or disable spinning during gradient shimming
gmap_z1z4                              Gradient shim initially on z1-z4
* gradtype {3-char string from 'c','d','n','w','l','p','q','s','t','u','h'}

How Automated Shimming Works
If a shim goes out of range, the shim is set to maximum and shimming continues with the
remaining shims. If convergence is then reached, shimming is tried once more with all Z shims and continues unless a shim goes out of range again. An experiment with the shims arrayed is run to map the shims and processed to make the shimmap. Coarse shims are used if present. The parameters and data for the shimmap are stored in the file userdir + '/gshimlib/ shimmaps/' + mapname + '.fid'. These parameters are retrieved the next time gradient shimming parameters are retrieved.