NMR Q: Why an RF pulse can rotate Mz so fast

[CarlosJv]

Learning about Nuclear Magnetic Resonance for a while.

I have read than when the main field B0 is applied along the Z axis, it takes a time (T1) in the order of seconds for the M0 field to be created in the body we are studying.

Yet we can apply a short (maybe in the order of one wave length) RF pulse and get all that M0 rotated into the X-Y plane, in no time.

I don't understand the mechanism why this is possible.

If M0 were to rotate into the X-Y plane because of the variable B created by the RF pulse, then it would take also around T1 time.

Yet another question: the B of the RF field is variable, how can it rotate M0 into the X-Y axis.

Thanks,

Carlos

NMR Q: Why an RF pulse can rotate Mz so fast

 

[LightHero]

?Answer: The reason why an RF pulse can rotate Mz so quickly is because of the way the magnetic field of the RF pulse interacts with the molecules in the sample. When the RF pulse is applied, it causes a rapid oscillation of the magnetic field which disrupts the alignment of the nuclei’s magnetic moments. This disruption causes the magnetic moments to precess (or rotate) around the direction of the RF pulse, and as they precess they become aligned with the X and Y axes of the external magnetic field. This alignment happens very quickly, allowing for fast imaging of the sample.

 

[astrokreng]

?

There are several factors that contribute to the fast rotation of Mz by an RF pulse in NMR.

Firstly, the RF pulse is tuned to the resonant frequency of the nuclei in the sample, which is typically in the radiofrequency range. This means that the RF pulse is able to interact with and manipulate the nuclear spins in a very efficient manner.

Secondly, the RF pulse is applied for a very short duration, typically in the order of microseconds. This short duration allows for a rapid change in the magnetic field, causing a quick rotation of the nuclear spins.

Additionally, the RF pulse is applied at the same time as the main magnetic field (B0) is present. This creates a combined magnetic field that is able to rotate the nuclear spins at a much faster rate than if the RF pulse were applied alone.

Furthermore, the nuclear spins are constantly interacting with their surrounding environment, causing them to lose their alignment with the main magnetic field (Mz). The RF pulse is able to overcome these interactions and quickly rotate the nuclear spins into the X-Y plane.

In summary, the combination of a resonant frequency, short duration, and simultaneous application with the main magnetic field allows for the fast rotation of Mz by an RF pulse in NMR. I hope this helps to clarify the mechanism behind this phenomenon.