Magnetic Resonance physics question: RF pulse role and 'in-phase'

[Astrocyte]

Hi, after I majored in physics, I moved to the field of medical physics. However, I cannot understand some of their explanations. One of them that is bothering me a lot is RF pulse and in-phase.

What we measure in MRI is the magnetization of water particles. The magnetization comes from the sum of magnetic moments of protons in our body. Due to the Zeeman effect, the energy band of the proton splits into two in a magnetic field. Following statistical dynamics, it shows us a paramagnetic effect. That's how many protons in our body shape a magnetization.

In MRI, we often measure the horizontal component of the magnetization. Since the proton's magnetic moment precesses around the B0 magnetic field (horizontal magnetic field), the horizontal component of the magnetic moment can exist. Before hitting the RF pulse, the phase of the xy plane is different from proton by proton.

That is, protons' horizontal components are so random that they are canceled out in terms of magnetization. We call these various phases of protons' horizontal component of magnetic moment out-of-phase. However, after hitting the RF pulse, they are flipped in the opposite direction, and their vertical components of magnetic moment are canceled out in terms of magnetization.

This is because RF pulse has the same energy as the gap of the two split energy band. At the same time, their phase becomes the same. Therefore, the proton's horizontal components are not canceled out. Finally, we can observe the horizontal component of the magnetization of protons. For me, the change from out-of-phase to in-phase by RF pulse feels like magic because I cannot figure out an exact reason for this. Does Somebody know how can we explain the effect of RF turning out-of-phase to in-phase?

 

[Dale]

I will use the terms “longitudinal” and “transverse” instead of “vertical” and “horizontal”.

You appear to be missing the longitudinal magnetization. If you initially have fully relaxed magnetization, then after the RF excitation it is the longitudinal component (before) that produces the in phase transverse magnetization (after).

 

[marcusl]

Magnetization M in the absence of an RF field is longitudinal (parallel to the B0 field), as Dale mentioned. Application of an RF magnetic field B1 that is circularly polarized in the plane perpendicular to B0 cause the spin system to precess, thus generating transverse magnetization. When the frequency of B1 equals the Larmor resonance frequency, the transverse magnetization M is in phase with it. It is then convenient to view M in the rotating frame, a coordinate frame that rotates with B1. The magnitude of transverse magnetization varies sinusoidally during the time that B1 is applied, with the pulse length needed to generate maximum transverse magnetization called a "pi/2 pulse" because the net magnetization has rotated from longitudinal to transverse in the rotating frame.