Magnetic Resonance Imaging (MRI)

Question 1

What is MRI

Answer 1

An imaging technique that uses strong but stable electromagnetic.

Question 2

What are the main components of an MRI machine and their function (outer to inner)

Answer 2

Magnet, Gradient Coil (slightly change the magnetic field on the sides of the brain -> “brain slicing”), Radio frequency (RF) coil (to impose radio frequency to tilt the protons

Question 3

Explain how MRI works

Answer 3

-> look at the protons in the brain
-> protons precess randomly. However, their movement is affected by the magnetic field.
-> After the magnetic field is switched on, the protons are aligned to the Z-axis (B0 field), which is the direction of the body entering the machine.
(Note that the frequency of the precession of the protons are determined by the strength of the magnetic field = Larmor frequency
-> However, although there are aligned to the B0 field, nothing can be detected still.
-> Therefore, radio frequency that matches the Larmor frequency is imposed from the head coil.
-> This tilts all the protons to the transverse plane (i.e., towards the head coils) (magnetisation vector).
-> Afterwards, turn off the radio frequency.

Question 4

What happens after switching off the RF for the first time

Answer 4

-> Protons will quickly return to the B0 field. Two relaxations are observed, and used to imaging the brain.
1. T1 recovery (longitudional relaxation): T1 recovery is a time constant of when protons in certain brain regions return to the z-axis, which every brain regions differ (e.g., protons in fat have a faster recovery rate than protons in cerebrospinal fluid).
2. T2 decay (transversal relaxation): T2 decay is a time constant which measures the decay of protons from the transversal plane (leaving the transversal plane), which every brain regions differ. (e.g., the T2 decay in fat is quicker than cerebrospinal fluid)

Question 5

why do the brain regions have different brightness in different photos?

Answer 5

depends on when the signal is captured after turning the rf off.

Question 6

Name the gradient

Answer 6

1st: slice-selecting gradient:
-> The gradient slightly strengthens the magnetic field of the location that wanted to be imaged.
-> With the magnetic field of that part slightly different, only protons in that part will react and enter the transversal plane by the RF.

2nd: phase encoding gradient:
-> slightly changes the precession frequency of the protons.
-> with the precession frequency of the protons changed, even if the protons have the same larmor frequency when the gradient is switched off, they still differ in phase.

3rd: frequency encoding gradient:
-> change the magnetic field in the selected slice.
-> imposed during the read-out stage.
-> to completely understand the location of the protons.

Question 7

Some of the factors that should be considered when conducting MRI

Answer 7

number of slice
thickness of slice
the amount of voxel in the size
etc