The physics of T1- and T2-weighted MRI

Protons in a magnetic field have a microscopic magnetization, and act like toy tops that wobble as they spin. The rate of the wobbling, or precession, is the resonance or Larmor frequency. In the magnetic field of an MRI scanner, there is approximately the same number of proton nuclei aligned with the main magnetic field B₀ as counter aligned. Still, the aligned position is slightly favored because the nucleus has a lower energy in this position. This results in a net macroscopic magnetization pointing in the direction of B₀.
The exposure of the nuclei to a RF radiation (the B₁ field) at the Larmor frequency causes the nuclei in the lower energy state to jump to the higher energy state. At the macroscopic level, this causes the net magnetization to spiral away from the B₀ field. In a field of reference rotating with the field, the net magnetization vector rotates from the longitudinal direction to a flip angle proportional to the duration of the RF pulse. After some time, the magnetization vector becomes perpendicular to the main field B₀. In this position, it can be detected by the MRI scanner. For angles different from 90^o, the perpendicular component of the magnetization vector can still be detected, but the signal is of course weaker.
MR Imaging is based on the observation of the relaxation that takes place after the RF pulse has stopped. The return of the excited nuclei from the high energy to the low energy state is associated with the loss of energy to the surrounding nuclei. Macroscopically, this spin-lattice or T1 relaxation is characterized by the longitudinal return of the net magnetization to its maximum length in the direction of the magnetic field. This return is an exponential process of the form of 1 - e^-t/T1 (Figure ). The T1 relaxation time is the time constant of this exponential, i.e. the time needed for the magnetization to return to $63\%$ of its original value.

  

Figure 11.1: T1 and T2 relaxation

Microscopically, T2 relaxation, or spin-spin relaxation, occurs when the spins in the high and low energy state exchange energy but do not loose energy to the surrounding lattice. Macroscopically, this results in a loss of transverse magnetization. Once again, T2 relaxation is a exponential process, in the form of e^-t/T2 (Figure ), and the T2 time is the time needed for the transverse magnetization to lose $63\%$ of its value. In pure water, the T2 and T1 times are approximately identical. For biological material, the T2 time is considerably shorter than the T1 time.

  

Figure 11.2: T1 and T2 MR images.

By varying imaging parameters such as TR (repetition time) and TE (echo time), it is possible to weight the IRM signal to produce T1-, T2- or PD-weighted (proton density) images. From a medical perspective, it means that MR Imaging can provide multiple channels to observe the same anatomy. For instance, Figure shows T1- and T2-weighted images of the same brain. Different tissues appear differently in both images. White matter appears in a light grey in T1 and a dark grey in T2. Grey matter appears grey in both images. The Cerebro-Spinal Fluid (CSF) appears black in T1 and white in T2. The background of the image (air) appears black in both images.