MRI-Lecture 1 :

What is the magnetic field called?

B0

what happens when you go into the strong magnetic field inside the MRI scanner?

normally the magnetic moments of the hydrogen nuclei in your body are randomly orientated, cackling each other out, which is why you are not magnetised as you walk around.

When you go into the magnet in MRI scanner, the magnetic moments in hydrogen nuclei line up with main magnetic field.

State the 2 different states in which the hydrogen nuclei in your body can be in when in MRI scanner:

aligning in same direction or opposite direction(against magnetic field). Preference for the nuclei to be aligned spin up rather than spin down.

What factors determines whether the hydrogen nuclei align spin up?

1. how much energy the nuclei have e.g. if they have Low energy they are more likely to align with main magnetic field(spin up)

2. tendency to align spin up

What does bulk magnetisation mean?

vector sum of all the spin up and spin down moments.

preference for spins to align with the field compared to oppose the field means

that your body becomes very slightly magnetised

• when someone lies in the scanner, a small majority of their spins align with the B0, the main magnetic fields and that's what creates this bulk magnetisation = signal used to generate images

What does a Higher B0 mean?

higher B0 = more spin up alignments/higher M = higher signal = improves quality of images/spatial resolution

How is MRI signal described (vector ,amplitude + direction)?

magnetisation = vector

B0 = longitudinal axis ( Z direction)

transverse directions = X + Y

What is the most common magnetic field strength for clinical MRI scanner?

1.5T (Tesla)

Which two fundamental properties result in a particle having a magnetic dipole moment.

charge + spin

Explain what precession is:

when an object is spinning/has angular momentum, it will rotate around the direction of the applied force e.g. if an object is spinning and you push it away from direction of the magnetic field (up+down), it precesses.

Explain precession in MRI scanner:

• with the bulk magnetisation you align it with the main magnetic field to the MRI scanner, you then tip it slightly away because it has this property of angular momentum(to detect the MRI signal we need changing magnetic field so we tip the bulk magnetisation away) + it starts to precess around the main B0 field.

What is the equation for the precession frequency for MRI?

What does a higher gyromagnetic ration mean?

definition of faraday’s law of induction:

a changing magnetic field which is intersecting/adjacent to a coil of wire, induces a voltage/current in a nearby conducting wire.

Explain how detecting the MRI signal works:

After precession, there is Faraday’s Law of induction.

• you have this coil of wire just adjacent to this rotating magnet. The fact that you have a changing magnetic field, which is intersecting with this coil of wire by Faraday's law of induction,means that the change in magnetic field induces a voltage, and therefore a current in this conducting wire nearby.

• if you put an RF coil next to source of magnetisation to measure processional frequency, then you will have a current induced in that coil

• RF coils used to detect MRI signals

what is an array RF coil?