What does T1 measure in the context of MRI signal?

T1 represents the longitudinal relaxation time, which is the time it takes for the nuclear spins to realign with the magnetic field after a perturbation, such as a radiofrequency pulse. In the context of MRI, T1 is crucial because it defines the regrowth of the magnetization vector back to its equilibrium state along the direction of the magnetic field. This regrowth time is particularly important in determining the contrast of images produced during MRI scans, as different tissues have varying T1 times, which can lead to varying image intensities.

Understanding T1 is essential for optimizing imaging sequences and selecting appropriate parameters for different diagnostic requirements. The regrowth to equilibrium influences how tissues appear in T1-weighted images, providing vital information for diagnosis and assessment.