To the right are images of a brain tumor with intrinsically long T1 and T2 values having opposite intensities on T1- and T2-weighted images. To understand this “paradox”, you must realize that a pixel’s “brightness” or “darkness” on an MR image is directly related to the magnitude of the detected MR signal. The magnitude of the MR signal after an RF-pulse is in turn, dependent on two factors:
- The size of Mz, the z-component of tissue magnetization (M) before the RF-pulse.
- The size of Mxy, the transverse components of M after the RF-pulse (when the signal is recorded).
T1 reflects the length of time it takes for regrowth of Mz back toward its initial maximum value (Mo). Tissues with short T1’s recover more quickly than those with long T1’s. Their Mz values are larger, producing a stronger signal and brighter spot on the MR image.
T2 reflects the length of time it takes for the MR signal to decay in the transverse plane. A short T2 means that the signal decays very rapidly. So substances with short T2’s have smaller signals and appear darker than substances with longer T2 values.