Gradient Rephasing - Video Lesson

Hey, everybody. Welcome back to today's lesson on pulse sequences. This lesson focuses on the process of gradient refacing in gradient echo sequences. Understanding how we use gradients to rephase the magnetic moments of the hydrogen nuclei allows for a better understanding of why we choose the parameters for a scan. During a pulse sequence, we use an excitation pulse to flip the net magnetization vector into the transverse plane. The size of the flip angle used determines how far into the transverse plane the NMV flips as well as how much longitudinal magnetization is lost. In spin echo sequences, we use a ninety degree flip angle which sends the NMV completely into the transverse plane and all longitudinal magnetization is lost. In gradient echo pulse sequences, we use a variable flip angle for our RF excitation pulse. This flip angle is less than ninety degrees, which means the NMV flips into the transverse plane, but not as far as in a spin echo sequence.