W4L1 - fMRI Physics

Question 1

What is the first method for brain imaging. Why is it used less now

Answer 1

Positron Emission Tomography (PET).

Exposes patient to ionizing radiation whereas fMRI does not

Question 2

What is the Telsa field for fMRI

Answer 2

1.5 - 9 Tesla (3T for most)

Question 3

fMRI magnetic field vs Earth

Answer 3

3T = 60000x Earth’s 65 microtesla

Question 4

What are the tools required for fMRI. Include Steps

Answer 4

1. ) Participant is placed on the bed and moved into the magnet (No metals - main danger!)
2. ) Participants can see projection via. mirror mounted on head coil

Question 5

How is the experiment controlled from outside the scanner room

Answer 5

Responses can be given via scanner-compatible keys, joystick, touchpad, etc.

Question 6

What is head coil used for

Answer 6

(1) Send RF frequency pulses
(2) Receiver for RF frequency pulse
(3) Fixed to avoid movement

Question 7

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

Answer 7

> 70% of brain consists of h20.

H+ can be thought as small bar magnets, “precessing” like a spinning top about an axis

Question 8

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

What are their precession properties

Answer 8

• Random spins are aligned parallel or anti-parallel in MRI scanner
  
  • Not perfectly aligned
• Not static
  
  • Still precessing randomly
  • Precession frequency of proton depends on the strength of the magnetic field (Individual photon recessing at same frequency but different points in space)

Question 9

The axis along which the magnetization is build up in the scanner is called …

Answer 9

The axis along which the magnetization is build up in the scanner is called the Z-axis (Magnetization Vector)

Question 10

Why can’t we use the Z-axis. What should we do then?

Answer 10

• Magnetisation along the Z-axis can’t be measured
• Tilt the magnetisation vector by a RF pulse perpendicular to magnetic field via head coil
  
  • RF Pulse must match precession frequency of protons

Question 11

What is the amplitude of the RF pulse sent?

Answer 11

Matches the precession frequency of protons

Question 12

What does the RF pulse do to protons. 3 Effects

Answer 12

1. ) Protons to absorb energy.
2. ) Tilts magnetization vector (Z) to the transversal plane (X and Y)
3. ) Align precession of spins; Protons’ rotations phase-coherent (each proton doing same thing)

Question 13

What can the transversely rotating magnetization vector do?

Answer 13

The transversely rotating magnetization vector can then be recorded as a signal.

The head coil is used to send the RF pulses, but it is also the receiver (Just a signal now)

Question 14

What happens after we got the transversely rotating magnetization vector and the signal recording. What happens then to the protons?

Answer 14

Off the RF pulse. Transversal magnetization decays and summed effect can be measured.

RELAXATION

1. ) Protons emit excess energy
2. ) Transversal magneitzation (X and Y) disappear and longitudinal mangeitzation reestablished
3. ) Protons loses phase coherence

Question 15

Why do protons decay at different speeds.

Answer 15

Transversal magnetization decays with different speeds depending on the tissue

• Different densities of protons
• Lose coherence because other protons in environment influence influence them
• Signal from different protons get out of phase with each other and cancel each other out.
  
  • Structural brain image depends on WHEN signal is recorded during this process

Question 16

How do we reconstruct brain image from signals. What is the premise

Answer 16

Premise:

• Protons will absorb energy from RF pulses only when RF pulse frequency = precession frequency (i.e. Resonance Frequency)
• Cause magnetic field to vary linearly by causing resonance frequency to vary linearly (Gradients)
• RF pulse of a specific frequency will only excite one slice at a time when resonance frequency of protons match frequency of RF pulse

Question 17

How do we reconstruct z-axis

Answer 17

“Slice selecting gradient”

• Vary gradient field along the z-axis
• Expose different slices to different field strengths
  
  • 1 slice excited at a time using a specific RF pulse, because precession frequency will not be matched for the others

Question 18

How do we reconstruct x-axis

Answer 18

“Frequency Encoding Gradient”

• Vary gradient along the y-axis
  
  • Change the magnetic field within the slice found by z-axis
• Protons within each slice also have different precession frequencies

Question 19

How do we reconstruct y-axis

Answer 19

“Phase encoding gradient”

• Using a gradient along the y-axis causes protons to “speed up” their precession according to the strength of the magnetic field for a very short time
• When switched off, all protons are all back to the same precessing frequency, but they are “out of phase” with each other
• By measuring the phase signal, we now also get the y-coordinate of the resulting signal

Question 20

Knowing x-y-z axis, how do we do it.

Answer 20

Fourier transformation to reconstruct the entire space.

Takes about 1-3 seconds