The factors affecting the rate of loss of transverse magnetization in
gradient echo and spin-echo pulse sequences have been quantified using
computer modeling for media containing arrays of susceptibility varia
tions. The results are particularly relevant for describing the signal
losses that occur in tissues containing capillaries of altered intrin
sic susceptibility from the administration of exogenous contrast agent
s or arising from changes in blood oxygenation. The precise magnitudes
and relationship of gradient echo and spin-echo decay rates depend on
geometrical factors such as the sizes and spacings of the inhomogenei
ties, the rate of water diffusion, field strength, and echo times. The
conventional separation of contributions to transverse decay rates ar
ising from so-called static field effects and diffusion is shown to be
inappropriate for many situations of practical interest because diffu
sion introduces a motional averaging of the static field even in gradi
ent echo sequences. The result of diffusion in some regimes is to redu
ce the decay rate from field inhomogeneities in gradient echo sequence
s, so that T-2 is longer in media such as tissue where water diffuses
reasonably rapidly, than would be the case for stationary nuclei. The
effects of different types of contrast agent and the implications for
functional imaging based on the effects of deoxyhemoglobin in brain t
issue are considered.