Re: NMR question: how an RF pulse can rotate M0 so fast into X-Y


carloshav...@xxxxxxxxx wrote:
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.

Why is T1 so long, this is a good question. To flip a spin you need
changing magnetic fields at the resonant frequency of the nuclear
moment. The required fields only occur rarley in the sample so it
takes a long time.


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.

This is can most easily be understoood in a reference frame that is
rotating at the resonance frequency. In that frame the RF magnetic
field is constant and supplies a torque u X B.

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.

The applied RF field is at the resonance frequency and is very
strong.
The natural fields in the sample are weak and spread over a large
region of frequency space.

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

Ahh, the larger the RF B field the shorter the time needed to cause a
90 degree spin flip. You need to adjust the RF pulse lenght for the
desired amount of spin rotation.

George
.