What happens to SNR when increasing the flip angle in MRI up to the Ernst angle?

Increasing the flip angle in MRI up to the Ernst angle leads to an increase in the signal-to-noise ratio (SNR). The Ernst angle is defined as the angle that maximizes the signal generated during the MRI process given the specific relaxation times of the tissues being imaged. At this angle, the efficiency of the excitation pulse aligns with the tissue's longitudinal relaxation time, optimizing signal capture.

As the flip angle increases towards the Ernst angle, more of the net magnetization vector is rotated into the transverse plane where it can contribute to the signal detected by the coils. This results in a stronger signal being produced while maintaining the same level of noise, thus enhancing SNR. Beyond the Ernst angle, further increases in flip angle do not yield additional signal improvements and can even start to decrease SNR due to saturation effects, which is why it's crucial to remain within this optimal range.