Computer simulation of magnetic field inhomogeneity effects on nuclear magnetic resonance measurements.

摘要

There are a number of possible applications of pulsed nuclear magnetic resonance (NMR) techniques; for example, it has been proposed that NMR be used to sort fruits and vegetables, to measure the amount of saturated fat in vegetable oils, and to detect osteoporosis in humans early. These applications use pulsed NMR to measure relaxation times and other characteristic properties of a sample. By subjecting a sample to pulsed RF magnetic fields, the sample's equilibrium magnetization is disturbed, and measurements can be taken from the induction signal due to the changing magnetization. Magnetic field inhomogeneities in a NMR system, however, break the phase coherence of magnetic moments that compose the magnetization, thereby affecting measurements.; In this work a computer model, based on Bloch's equation with diffusion term, is developed to assess the effects of magnetic field inhomogeneity on NMR spin echo measurements. Simulations indicate that dephasing of magnetization across a sample by more than 1 rad during 180{dollar}spcirc{dollar} RF pulses result in significant deviations of echo ratio value from Carr-Purcell theory predictions. Echo ratio values are affected to a lesser degree by higher order inhomogeneity distributions. With unbalanced pulsed field gradient (PFG) experiments, spin echo measurements deviate significantly from Stejskal-Tanner theory predictions, if the second gradient pulse has a smaller pulse width or pulse height than the first gradient pulse.; This work is useful in the design of low cost NMR systems. A significant portion of the development costs for a NMR system goes into making the magnetic fields more uniform. By obtaining NMR measurements under less homogeneous magnetic fields, NMR techniques will offer a practical alternative to other measurement methods.