A Demonstration of Imaging on an NMR Spectrometer L. A. Hull Union College, Schenectady, NY 12308 Magnetic resonance imaging (MRI) in medicine grew out of NMR spectroscopy, yet few chemistry students understand the intimate connection between the two techniques.' In the following is described a simple demonstration derived from the earliwork that illustrates some of those connections. For NMR snectroscoov ~- . the ideal is a homoeeneous maenetic field throughout the sample so that responding nuclei in identical chemical environments are subjected to identical fields and respond with signals of identical frequency as dictated hv the Larmour e a u a t i ~ nIn . ~ the equation u is the frequency"for resonance, y is the gyromagnetic ratio for the particular nucleus, and B is the field sensed hy the nucleus.
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Nuclei in different chemical environments mayor may not be anisotropic (show up a t different chemical shiftsj denendine on the extent of the difference in the environment $s it effects the field sensed by the nucleus. An ideal spectrum displays the signals due to ench of the chemically different and distinct chemiral shift tor . .-. ~. -nuclei - ~ ~at ~ a- differenr ~ frequency). The line width in most solution spectra invoivine nonexchaneeable atoms is then dictated hv the extent of the field homogeneity throughout the sampleand the relaxation time constants of the n u c l e u ~The . ~ sample is spun in the magnet to help achieve greater average homogeneity. Careful attention is oaid to tuning the spectrometer so that field homogeneity throughout thesampie is attained. The process of tuning involves adjustingvarious shim coils arrayed around the sample to correct for inhomogeneities in the hulk magnetic field due to the NMR magnet. In the ideal these shim coils superimpose a small ele&omagnetically generated field on the field of the NMR magnetto correct for inhomogeneities in that hulk field. The shim coils X, Y, and Z attempt to correct for inhomogeneities along the axes oriented ahout the sample (or person) with other shim coils used for correcting fields between these axes. In standard MRI the object is to use the signals from hydrogen (dominated by water signals) to find out where that hvdroeen is located spatiallv and also to get contrast in the image Letween the signals emitted by different tissues. Soatial localization (the aspect of MRI illustrated by this dkmonstration) is achieved by applying a gradient magnetic field that is switchable among the x , y, and z axes.' This gradient field is achieved by using shim coils, hut now the object is to reduce the homogeneity of the hulk magnet hy superimposing small electromagnetically generated field gradients. These are fields that increase as a function of distance from some reference point. In the Larmour equation B becomes a function of position in the magnet rather ~~~
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than position in the molecule, and the frequency of the signal encodes positional information rather than chemical information. ~ j t this h sort of description i t becomes apparent how one "converts" an NMR spectrometer into an "imaging" instrument. I t is simply a matter of not spinning the sample and detuning the probe along the axis from which vou wish to -eet snatial or imaee information. . The simplest demonsrration of this phenomenon is to take asamole of n Ocontaminated withahout 1 -10''o H?O (mostly in the form of HOD of course) and take a standard spectrum with evervthine for spectroscopy. Superim.. optimized . pose over this
