Errors in the measurements of T₂ using multiple‐echo MRI techniques. II. Effects of static field inhomogeneity

The accurate estimation of the spin‐spin relaxation time T₂ is an important goal in magnetic resonance imaging particularly because it can be used for quantitative tissue characterization. The spin‐spin relaxation time T₂ may be estimated using multiecho pulse sequences, but the accuracy of the estimate is dependent on the fidelity of the spin‐echo amplitudes, which may be severely compromised by rf pulse and static field imper‐fections. In this paper, the effects of static field inhomogeneities are investigated. The propagation of the errors introduced by off‐resonance effects are analyzed through computer simulations and analytical solutions of the Bloch equations. A series of experiments performed on a simple tissue phantom using a whole‐body imaging system operating at 6.35 MHz corroborates the simulation and analytical results. For accurate measurements of T₂ using a whole‐body imaging system it is necessary to correct for these inhomogeneities. A correction scheme which would enable a more accurate estimate of T₂ is currently under investigation. © 1986 Academic Press, Inc.