INTRAVASCULAR SUSCEPTIBILITY CONTRAST MECHANISMS IN TISSUES

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.