Thursday, March 20, 2008
Gradient Calibration - 1D MRI
When people think about magnetic resonance imaging (MRI), they often think about the huge whole body imagers in hospitals. NMR spectroscopists use one dimensional MRI in a specially prepared sample to calibrate the Z gradient strength of their spectrometers. The sample consists of a plastic disk with a precisely known thickness immersed in a column of water (see the figure below). 1D MRI is conceptually quite simple - a linear field gradient is applied during the collection of an FID. Since the magnetic field varies across the sample and the NMR frequency is proportional to the magnetic field strength, the resulting NMR spectrum represents a one dimensional image of the sample. The spectrum has a "notch" missing as a result of the plastic disk. The width of the "notch" is proportional to the applied gradient strength and the thickness of the plastic disk. If the thickness of the plastic disk is precisely known, then the strength of the applied gradient can be calculated. The spectrum is rounded at the edges as a result of the gradient strength falling off away from the center of the sample. A modified version of this experiment using an echo is used as a routine method of calibrating gradient strengths.