Calibration of RF pulses

After shimming, the RF pulses have to be calibrated.

For this purpose, a number of 1D-¹H pulses are recorded because the ¹H pulse length is very sensitive to the composition of the sample. Calibration of the ¹³C and ¹⁵N pulses is done less frequently.

The goal is to find the ideal pulse length that will rotate the magnetization by 90 degrees, i.e. from z- to y-axis. A perfect 90-degree pulse will give the strongest signal.

Because it is hard to determine the maximum of the signal, a 360-degree pulse is applied to rotate the magnetization from z- to z-axis. This results in no signal if the pulse length is optimal. The ideal 90-degree pulse is then calculated from the 360-degree pulse.