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ShimIt User Manual


CHAPTER 1: Introduction

The ShimIt auto-shimming system was written in 1990 as a side-product of the NMRanalyst software. With further improvements in user interface and documentation as well as patent protection1 ShimIt is now ready for the first public release of the software.

Running ShimIt is similar to manual shimming based upon the shape of a displayed NMR signal. ShimIt runs "inside" the spectrometer software (here: VNMR) so all commands and displays of the spectrometer software remain user accessible. For the interactive shimming on a displayed lineshape a spectroscopist acquires a spectrum, changes the shim values, and repeats this cycle until a satisfactory homogeneity is obtained. In contrast, ShimIt only needs to be told which shim values to optimize and then loops through changing these shim values and acquiring new spectra without the need for further user interaction. The difference between manual and ShimIt optimization of the lineshape is that ShimIt evaluates the lineshape in a way which has been mathematically proven to lead to the optimal homogeneity without the risk of getting trapped with sub-optimal shim values.

There are two competing approaches for automated shimming. Automated lock-level optimization is based on special lock hardware and is quite fast and effective for "touching up" low-order z shims. However, like the manual lineshape optimization the lock-level optimization is plagued by local optima in the optimization criterion. So ShimIt should be used for every major homogeneity adjustment. However, for minor low-order z shim adjustments, such as between sample changes, the lock-level optimization currently has a speed advantage over ShimIt.2 The second class of competing auto-shimming methods involves the determination of the actual three-dimensional field strength through imaging or special Nuclear Magnetic Resonance (NMR) mapping probes. The problem of such techniques is that they require the use of special probes and have sensitivity problems, limiting their applications to proton (normally water) samples. ShimIt has none of these limitations. In fact, knowledge of the three-dimensional field strength is not needed for shimming and in the time needed to acquire and reconstruct a single image, ShimIt can acquire and analyze several Free Induction Decays (FIDs). Hence, after integration of ShimIt into a spectrometer system the software could favorably compete even in cases where imaging techniques are applicable.

Another strength of ShimIt is that the method is independent of the spectrometer and magnet used. In practice however, ShimIt has to cooperate with spectrometer software and know at least what shims are available on the magnet used. To reflect this complexity ShimIt is currently implemented as a server process incorporating the general shimming method and additional client software incorporating the spectrometer and magnet specific information. The strength of the current server-client implementation of ShimIt is the flexibility to adjust the software to different spectrometer systems and to experiment with different user interfaces.

Since shimming is such a fundamental problem in NMR, each spectrometer system should be delivered with a reliable auto-shimming system. Due to the cost of spectrometer time ShimIt should be ultimately integrated into each spectrometer system to achieve maximum shimming speed. Currently the only ShimIt client available is for Varian VNMR 4.3 or newer systems. Development of client software for other spectrometer systems is planned for future releases of ShimIt. The VNMR client recognizes all seven "shim sets" understood by VNMR 4.3. Despite extensive testing, the ShimIt client software probably still has deficiencies. Please report all problems and suggestions for improvement to me at the contact information given earlier. Please note that this software is protected by copyright and the underlying shimming method is patented.

1U.S. Patent No. 5,218,299 and 5,572,125.

2With minor hardware changes it would be possible to acquired complete FIDs through the lock channel. These FIDs could then be used for the ShimIt optimization of the magnet homogeneity.



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