fMRI/ basic physics of MRI: L7

Question 1

when did PET come about?
what did it involve?
-> fMRI has…

Answer 1

• prior to fMRI scans
• administering radioactive isotope to the patient, exposing them to a high amount of ionizing radiation
• > fMRI no radiation, no side effects

Question 2

fMRI

1. magnetic field
2. images
3. what CANNOT be brought into the room
4. participants sees

Answer 2

1. 1.5-9 Tesla
2. any body part
3. metal
4. a projection via mirrors mounted on the head coil

Question 3

what does the head coil do?

Answer 3

sends radio frequency pulses and also functions as a receiver

Question 4

1. MRI images?
2. % of water in brain
3. what are hydrogen atoms

Answer 4

1. structures of the brain
2. 70%
3. small bar magnets, “precessing” = spinning top about an axis

Question 5

• what do the hydrogen atoms do when magnetic field is applied?
• what does the precession frequency depend on?

Answer 5

• align pointing up or down
• not perfectly aligned & not static
• strength of magnetic field

Question 6

• in the scanner the brain becomes?

- how can we get a signal?

Answer 6

• a large magnet

- the magnetisation along the Z axis cannot be measured, we need to tilt the magnetic vector to get a signal

Question 7

• what is applied to the magnetic field to tilt it?

- what is its frequency

Answer 7

• a RF (radio frequency) applied perpendicular

- matches the precession frequency of the protons

Question 8

What does the RF create ( 2 effects)

Answer 8

1. tilts the magnetisation factor to the transversal plane

2. aligns the precession of the spins = protons are “in phase” or in synchrony

Question 9

what can the transversal plane now be?

what occurs after this?

Answer 9

• recorded as a signal: the head coil is used to send RF pulses but its also the receiver
• RF pulse is switched off and transversal magnetization decays = relaxation. Longitudinal magnetization is re-established

Question 10

during relaxation what is measured?

-> why?

Answer 10

• the effect of protons
• how long it takes for the signal to disappear (transversal magnetization decays with different speeds depending on the tissue
• > density of protons: they lose coherence influenced by other protons (bump into each other)

Question 11

how are brain images are reconstructed without exciting the entire brain at once?

Answer 11

• proton absorb energy from RF pulses ONLY when then frequency matches the protons precession
• therefore we cause the magnetic field to vary linearly -> causing the resonance frequency to vary throughout the brain

Question 12

how do we cause the magnetic field to vary linearly?

Answer 12

using gradients

-> a RF pulse of specific frequency will only excite a slice of brain

Question 13

What is the slice selecting gradient?

-> what coordinates?

Answer 13

vary field along the z-axis, different slices exposed to different field strengths,
gives us z-coordinate for all resulting signals

Question 14

what is the frequency encoding gradient?

-> what coordinates?

Answer 14

change the magnetic field within the slice using a second gradient,
gives us x-coordinates of the measured signal

Question 15

what is the phase encoding gradient?

-> what coordinates?

Answer 15

• a gradient along the y-axis causes protons to speed up their precession according to the strength of the magnetic field for a short time
• gradient is switched off & protons have the same precessing frequency but are “out of phase”
  gives us y-coordinates of the resulting signal

Question 16

what does Fourier transformation do?

Answer 16

3D reconstruct the entire space (brain)

Question 17

In what order can slices be measured & how long does it take?

Answer 17

• ascending, descending or interleaved

- 1 full 3D image = 1.5-2 seconds