Theory: MR Phase

As mentioned in the introduction, water protons resonate at different frequencies because of the static magnetic field is perturbed by the presence of of objects or tissues with different magnetic susceptibility. The frequency difference in brain tissues can be detected as the difference in phase accumulation over time ( $phase = frequency \times time$ ). Therefore, the phase measurement of the MRI signal allows us to map the effect of the magnetic susceptibility of brain tissues.

It should be noted that the phase can only be measured in the range of [-180, 180] degrees (or [ $-\pi, \pi$ ] radian).

Back to Exercise 1.