What is the effect of a 90-degree flip angle on the NMV?

A 90-degree flip angle in MRI refers to the manipulation of the net magnetization vector (NMV) by radiofrequency (RF) pulses. When a 90-degree RF pulse is applied, the NMV of the hydrogen nuclei, which is normally aligned with the main magnetic field (B0), is rotated from the longitudinal plane (aligned with B0) into the transverse plane.

In the longitudinal plane, the NMV is aligned with the magnetic field and points along the z-axis. The application of a 90-degree pulse causes this vector to shift to the xy-plane, which is the transverse plane. This is a critical process in MRI, as the transverse component of magnetization is what generates the signal that is detected to produce an image.

Once the NMV is in the transverse plane, it begins to decay due to T2 relaxation, which also contributes to the signal loss over time as the spins in the transverse plane become dephased. This phenomenon is fundamental in MRI, enabling the distinction between different tissues based on their relaxation characteristics.

Good understanding of these principles is crucial as they relate directly to the formation of MRI images and the behavior of magnetization in different states after RF pulses.