Magnetic Resonance Imaging (MRI) Practice Test

What happens to precessing protons immediately after the application of the 90-degree RF pulse?

1. They stop precessing

2. They begin to precess in phase and tip into the transverse plane

3. They move into the longitudinal plane

4. They scatter randomly

The correct answer is: They begin to precess in phase and tip into the transverse plane

The application of a 90-degree RF (radiofrequency) pulse has a significant effect on the precessing protons within a magnetic field. Prior to the pulse, these protons are aligned with the direction of the magnetic field and are precessing around it. When the 90-degree RF pulse is applied, it energizes the protons, causing them to shift from their alignment along the longitudinal plane (the direction of the magnetic field) to the transverse plane (perpendicular to the magnetic field). This RF pulse effectively tips the magnetization vector of the protons by 90 degrees. Consequently, after the pulse, the protons begin to precess in phase within the transverse plane. Being in phase means that their precession is synchronized, leading to a coherent signal that can be detected, which is crucial for generating MRI images. The transverse magnetization produced after the pulse is what contributes to the formation of the MR signal that is then used for imaging. The other options relate to misunderstandings of the behavior of protons in an MRI environment after the RF pulse. The protons do not stop precessing; instead, they continue to precess in the transverse plane. Moving into the longitudinal plane would denote a return to equilibrium rather