Effect of rf field inhomogeneities on spin-echo measurements

Abstract

The effect of radio frequency inhomogeneities on nuclear magnetic transverse relaxation time (T2) measurements has been investigated experimentally and numerically. Measurements using the Meiboom-Gill modification to the Carr-Purcell sequence have been emphasized; however, some other methods have been considered. The calculations have been performed assuming that the magnetization obeys (a) the Bloch equations and (b) the modified Bloch equations. In case (a) the errors resulting from rf field inhomogeneities were found to be small to moderate, and a method is presented whereby the upper limits to these errors can be determined experimentally. Furthermore, the errors are decreased considerably in a homogeneous main magnetic field adjusted to resonance and go to zero when T1 = T2. In case (b) the errors were considerably larger than in case (a) and the presence of a homogeneous main magnetic field did not necessarily decrease these errors. Even quite small rf field inhomogeneities were found to produce significant errors in the exchange times, chemical shift differences, and activation parameters derived from the transverse relaxation times.