Signal decay reflects two processes occurring after radiofrequency excitation: irreversible T2 relaxation and additional loss of phase coherence caused by local magnetic-field variations. Tissue with stronger microscopic susceptibility differences develops greater dephasing, so its signal decreases more rapidly. This relationship allows T2* measurements to reveal composition-related differences and field disturbances within biological tissue.
T2* includes both irreversible T2 relaxation and extra dephasing produced by microscopic magnetic-field inhomogeneities. A conventional T2-related measurement does not represent this combined effect in the same way. Consequently, T2* is especially sensitive to susceptibility-related changes, making it useful when iron-containing compounds, blood oxygenation, hemorrhage, or vascular alterations influence local magnetic fields.
Susceptibility differences alter the magnetic field experienced by nearby spins. Those spins then lose phase coherence at different rates, producing localized signal loss rather than uniform decay across the tissue. Because the strength of this effect varies with tissue composition and vascular conditions, the resulting contrast can connect microscopic or molecular changes with broader patterns of tissue function.
T2* contrast provides sensitivity to blood oxygenation, allowing blood-oxygen-level-dependent functional MRI to detect changes associated with tissue activity. The method translates oxygenation-related magnetic-field effects into differences in signal decay. In biology, this supports investigations that relate changing vascular or oxygenation states to functional behavior in tissue.
T2* measurements can identify signal changes associated with iron-containing compounds and hemorrhage because these conditions influence local magnetic susceptibility and therefore spin dephasing. The resulting maps help researchers assess where such magnetic-field disturbances occur within tissue. This information supports the study of composition changes and disease-related alterations without treating signal loss as a nonspecific finding.
By mapping susceptibility-sensitive signal loss, T2* imaging provides information about vascular structure and blood oxygenation alongside tissue composition. Researchers can compare these patterns with biological function to examine how cellular or molecular properties relate to tissue organization. The same approach also helps characterize alterations associated with hemorrhage or other disease-related changes in vascular tissue.