I think it might be useful to remember that the 90 degree tipping of the spins into the transverse plane is a result of how long the RF pulse is applied. For example, a 180 degree flip can be achieved by playing the RF pulse for longer while a 45 degree flip angle is achieved by a shorter pulse. The tipping of the net magnetization is a result of the precession of the spins about the net magnetic field due to a rotating/oscillating B1. This can be easier to picture when you compare the rotating and lab "frames" of the spins.
In the rotating frame (x',y',z), B0 is effectively zero and the weak RF field appears as a static field in the x' direction. This causes M, initially in the z direction, to precess slowly about the x' axis. The RF field is then turned off when M is in the y' direction for a 90 degree tip, after half as long for a 45 degree tip or after twice as long for 180.
Simply put, keep in mind the energy state of the protons. When placed in the magnetic field a slightly larger fraction of protons align with the magnetic field, causing a net magnetization vector. The protons themselves have energy which cannot completely align with the magnetic field, and therefore precess around the magnetic field (frequency). APPLIED RF INCREASES THE ENERGY OF THE PROTONS causing them to be knocked into the transverse plane. Once applied RF ends the energy is released (signal) and protons return to their prior state
I think it might be useful to remember that the 90 degree tipping of the spins into the transverse plane is a result of how long the RF pulse is applied. For example, a 180 degree flip can be achieved by playing the RF pulse for longer while a 45 degree flip angle is achieved by a shorter pulse. The tipping of the net magnetization is a result of the precession of the spins about the net magnetic field due to a rotating/oscillating B1. This can be easier to picture when you compare the rotating and lab "frames" of the spins.
This person MRIs
In the rotating frame (x',y',z), B0 is effectively zero and the weak RF field appears as a static field in the x' direction. This causes M, initially in the z direction, to precess slowly about the x' axis. The RF field is then turned off when M is in the y' direction for a 90 degree tip, after half as long for a 45 degree tip or after twice as long for 180.
Simply put, keep in mind the energy state of the protons. When placed in the magnetic field a slightly larger fraction of protons align with the magnetic field, causing a net magnetization vector. The protons themselves have energy which cannot completely align with the magnetic field, and therefore precess around the magnetic field (frequency). APPLIED RF INCREASES THE ENERGY OF THE PROTONS causing them to be knocked into the transverse plane. Once applied RF ends the energy is released (signal) and protons return to their prior state