We will talk about Magnetic Resonance Phenomenon in this week.
But what's signal source of MRI?
People say nuclear magnetic resonance (NMR) for which are concerned
about the charge and the angular momentum of the nuclei of atoms,
but not about the radioactivity.
So, a nucleus with either an odd atomic number,
or an odd mass number has an angular momentum phi, called the spin.
Okay. The spin is a pure quantum mechanical quantity with a notated classical analog.
So, we mentioned this briefly in the last week.
Let's consider a proton or a hydrogen nucleus,
the spin will give this nucleus a spin angular momentum phi,
and then magnetic moment mu.
So, which are related through a proportionality constant gamma in the following equation.
So, mu equals gamma phi.
So, both of them are vector quantities which means they have magnitude and direction.
And as shown here, those pins rotate which generate the angular momentum,
so which generate magnetic movement,
so it can be considered as a small magnet as we mentioned in the last week.
Here the gamma is called the gyromagnetic ratio
and it's in the unit of a 10 to six radiance,
per second, per Tesla for proton,
and is often denoted as a gamma over two pi.
So, which can represent the over hertz per Tesla.
A nucleus or proton and C13 and florine 19 and sodium
23 and phosphorus 31 are important metals
in MRI for imaging and most anatomical MRI is about proton,
as I mentioned in the last weeks,
and more than 90 percent is about the proton.
So, because, the reason is the hydrogen nucleus consisting of
a single proton can be considered as a simple magnet.
So, and it's naturally abundant in
our body because the human bodies are made of 70 percent of water.
And water molecule consists of two hydrogen atoms and one oxygen atom as shown here.
So, there are a huge number of protons in our body so,
18 and they are,
these magnetic moments are randomly distributed and randomly oriented in nature.
And then, what would be the MR signal?
And the MR signals are generated by manipulating these magnetic moment mu,
through magnetic resonance, and
very similar analogy is electromagnetic induction from a bar magnet.
If we have a circuit as shown here and this is a coil,
and then if a magnet is near this coil,
and then when this magnet moves and then the current can be induced in this coil.
And that's the basic mechanism of detecting MR signal.
So, there are huge number of protons in
our body which can be considered as a small magnet.
And then, when they move,
and we can make them move and then we can detect the induced signal as shown here.
So, that's the basic source of MR signals.
Sunghong ParkAssociate Professor
