The Free Induction Decay (FID) Signal - Video Lesson

Hey, everybody. Welcome to today's lesson on the basic principles of MRI. This lesson focuses on the free induction decay or FID signal and how relaxation and pulse sequences play a role in MR signal generation. It is important to understand the FID as it provides the initial signal in an MRI pulse sequence. The free induction decay or FID signal is the signal that is released after the initial radio frequency or RF excitation pulse is switched off. Remember, after a patient is placed in an MRI scanner, their hydrogen nuclei align parallel or antiparallel to the magnetic field b sub zero. The difference between the number of parallel and antiparallel hydrogen nuclei is the net magnetization vector or NMV. An RF excitation pulse is emitted that knocks the NMV out of the longitudinal plane into the transverse plane and causes them to precess in phase with one another.