MRI (Basics) / W4L1

Question 1

What is the most common method for studying brain activity?

Answer 1

Functional Magnetic Resonance Imaging (fMIR)

Question 2

What is a ‘head coil’ used for?

Answer 2

To send radio frequency (RF) pulses and functions as a receiver for the incoming signal

Question 3

What is the role of proton in MRI. What is it thought of

Answer 3

The brain consists of 70% water, which contains hydrogen atoms (H+ protons). The small spins of the protons emits a small magnetic charge which can be directed by MRIs. Once directed, an MRI machine will allow the protons to drift back into their natural orientation which when captured can leave a structural image of the brain.

Question 4

What happens when protons are put into very strong magnetic field in MRI scanner

Answer 4

• They align parallel or anti-parallel
• The random spin direction of the protons becomes becomes oriented into the direction of Z-axis
• The precession frequency of the protons depends on the strength of the magnetic field

Question 5

What is the Lamor frequency?

Answer 5

The Larmor frequency describes the angular frequency of the precession of a charged particle’s magnetic moment in a magnetic field

Question 6

How do we measure a signal once protons are aligned with the z-axis?

Answer 6

We cannot. First we need to orintate them from the z-axis to teh transveral plane.

Question 7

How do we measure a signal once protons are aligned with the z-axis?

Answer 7

We cannot. First we need to orintate them from the z-axis to teh transveral plane.

Question 8

If an MRI magnet allows for protons to point in the same direction (z-axis), what does an RF pulse do?

Answer 8

The radio frequency (RF) pulse matches the Lamour frequency and causes all the protons to presess in synchrony.

Question 9

After protons are oriented and precessing the same way, what is the next step?

Answer 9

Turn off the RF frequency.

Question 10

What happens when one turns off the RF frequency?

Answer 10

• Protons align again with magnetic field (longitudinal relaxation or T1 recovery)
• T2 decay (transversal relaxation). Protons falling out of precession

Question 11

What is T1 recovery?

Answer 11

T1 is a time constant function and refers to how long it takes protons to realign with z-axis (straight up). T1 is different for different tissue types.

Question 12

What is T2 decay?

Or transversal relaxation

Answer 12

T2 decay refers to the time it takes for protons fall out of pression. The more protons in one area the faster this occurs.

Like a dancers when the music stops.

Question 13

How are T1 and T2 the same and how are they different?

Answer 13

Both are time functions and both are dependent on different types of tissues. However T2 is much faster than T1 recovery.

Therefore different types of sequences (T1-weighted, or T2-weighted) capture different signals due to this recovery process

Question 14

What is the role of gradient coils in the MRI machine?

Answer 14

Gradient coils allow us to choose where we get our signal from

By controlling this gradient one can match the frequency of the RF pulse to the area of interest (also called ‘resonance’). Allowing for a particular brain slice to be chosen

Question 15

What is phase encoding gradient ?

Answer 15

• Phase encoding gives us the x-axis coordinate of the resulting signal
• This is possible because the pression frequency of each slice will reflect the gradient. Therefore we can match the slice frequency with our RF pulse

Question 16

What is frequency encoding gradient ?

Answer 16

• Change the magnetic field within this slice found in z-axis (vary the gradient along the y-axis)
• Protons in each slice also have different precession frequencies