Introduction to Magnetic Resonance Imaging (MRI)

What are CT scans?

Tomographic images in 3D created from many 2D projection images using ionizing radiation

— series of projections

What do CT scans need in order to make an image?

Measurement needs transformation

What are MRIs?

Images using three types of magnetic fields created from the transmission of RF from nuclear magnetic moments (of protons)

What is a magnetic field?

A vector field that is direction AND magnitude expressing spatial influence of magnetism

What do magnetic fields describe?

Invisible forces due to

1. magnets

2. Electric currents

What are the units of magnetic fields?

1 Tesla = 10,000 Gauss

Examples of magnetic fields

1. Earth’s magnetic field: ~0.3 G = 30 uT

2. Clinical MRI “𝐁𝟎” field: 65 mT→7T

3. Most common clinical MRI 1.5 and 3 T

Name the two types of magnets

Permanent magnet and electromagnet

What is a permanent magnet?

1. Electron configurations aligned by external field

2. Ferromagnetic objects can become magnetized

What is an electro-magnet?

Magnetic field from current through a wire (ampere’s law)

MRI fields are mostly made this way

What is the static magnetic field? (B₀)

It points along the z-axis, and it is very uniform in the middle. The higher the current you have, the higher the field that goes through this

(Nuclear) magnetic moments and net magnetization

Atoms with an odd number of protons or nucleons (protons + neutrons( have intrinsic magnetic moment

What is you dont have an MRI B₀ field?

The nuclei (protons) orient randomly

What do you see when you apply B₀ field?

Protons with mag. mom. point slightly towards B₀- there is net magnetization

This wants to precess about magnetic fields

Protons/magnetization

In individual spins want to

Precess around B₀

The rate depends on the

Nucleus

The net magnetization precesses at the same

Special frequency

What is precession frequency?

Hydrogen nucleus (in water) is most common for MRI