Introduction

When we talk about the nuclear magnetic resonance, it’s better to getting into the basic knowledge of how we can investigate the phenomenon of it. Before that, let me introduce the concept of Spin. Then we will later introduce concept of Magnetic Moment, finally Gyromagnetic Ratio.

Spin

Spin is actually the type of angular momentum that possessed by atomic particles. So, spin can be also called spin angular momentum. atomic particles include protons, neutrons, nuclei, and electrons.

property

But the funny point is that actually the particles themselves are not actually spinning. it’s just the natural property that are not produced by other things. So particles can be illustrated as the tiny ball that have the I angular momentum.

interaction

Another characteristic of spin is that this property can be interacted with magnetic field (B0), just like the angular momentum L to the gravity field.

Quantitative

Magnitude of spin is quantized, it can be only measured with full or half integer (0, 1/2, 1, 3/2, 2, …). Electrons has only one spin value, which is S (1/2 always). And in MRI case, 1H is commonly used for measurement. And it has the I value of 1/2.

Non-zero spin imaging

Only the nuclei with non-zero spin can absorb and emit electromagnetic radiation. So as to resonate with the applied magnetic field (B0).

And 1H only consists of just a single proton. So, the common MRI signal usually comes from protons only due to the usage of 1H. But it should be avoid to use proton to explain the imaging of MRI when using nuclei other than hydrogen.

Magnetic Moment

It is a vector to measure the tendency of an object to interact with external magnetic field. The objet in this case can be refered to molecules, atoms, nucleus, or some subatomic particles. And magnetic moment of these objects can be also called as Magnetic dipole moment (μ) due to the bar shape magnet visualization that has the north and south poles.

μ will seek to align with the applied magnetic field(B0). The process will need to overcome the force called torque. coming from μ X B0. After alignment, μ will be lowest energy state with no torque; On the other hand, μ will be the highest energy state when pointing opposite to B0 direction. The formula of the energy of magnetic moment is E = - μ·B0

Gyromagnetic Ratio

The gyromagnetic ratio (γ) is a constant used to represent the reelationship between the spin (I) and the Magnetic dipole moment (μ). Here is the formula: μ = γI.

γ is different from various atomic species. Most of the nuclei has the positive value of it. But some nuclei and electrons are negative value. When γ > 0, it means that spin and magnetic dipole moment are at the same direction. γ < 0 means opposite directions, but still colinear.

Further Info

Here is the basic understanding of the terms called spin, magnetic moment, and gyromagnetic ratio. For more specific information. Please refer to here.