What does the 90 degree RF pulse do in a spin echo sequence?

In a spin echo sequence, the 90-degree RF pulse is crucial for manipulating the net magnetization vector (LMv) of the system. When the RF pulse is applied at 90 degrees, it effectively tips the LMv from the longitudinal plane (where it is aligned along the main magnetic field) into the transverse plane, thereby creating a state where the magnetization is perpendicular to the magnetic field.

This transverse component is essential because it is in this plane that the signal is generated. As the protons precess in the transverse plane, they induce a measurable voltage in the receiver coils, leading to the formation of the MRI signal. This signal strength relies on the density of the excited spins and the characteristics of their relaxation processes after the RF pulse. Therefore, the primary function of the 90-degree RF pulse is to prepare the tissue's magnetization for signal acquisition by placing it into the transverse plane.