Shimming on a Spectrum
By selecting Shim on Spectrum from the Scanning Page you are able to alter shim values using a visual interactive method.
Once selected, the panel shown in Figure 1 appears which displays the shimming in real-time.
Figure 1. The Vnmr Acqi Shim panel.
First go to the left of the panel and select the Fid and Envelope method and use the Up/Down and Expand/Compress buttons to alter the sizing of the FID now displayed in the window above.
Next move to the display on the right. Here you can Load, Save and Delete shim values in named files. You may want to select an existing shim file to provide starting values for your new subject. With the left-hand mouse, click on "Load" and then select your choice with the right-hand mouse and release. The shim values are now loaded into the boxes below the labelled Shim sets.
To change the shim values use the right or left-hand mouse buttons to click on the required shim. The right-hand button will increase the values and the left-hand mouse will decrease the values. The increments of the changes can be toggled between Coarse steps or Fine steps by clicking the middle mouse button of the individual shim button. To change the values of these increments select the Coarse Step/Fine Step button and type in the required increment to be used.
An alternative to using the mouse control is to manually type in the values in the boxes below each individual shim.
Saving a Shim File
If you want to save your shim values type in a file name at the Shim file prompt and press Save with the left-hand mouse.
Linewidth
To determine how good the shim is you will need to check the linewidth of water (measured as the FWHM value in Hertz). This linewidth should be a value as small as possible below 30Hz for any imaging sequence (and in particular for EPI).
When done, press Close to exit the shimming programme and from the Auto Prescan page select Auto Centre Frequency. A water spectrum will be acquired and displayed in the interactive graphical window. The linewidth, in Hertz, will be displayed in the Varian window at the top left of the scanning console.