L2 - Neuroimaging

Question 1

Why is MRI good compared to X-rays and CAT scans?

Answer 1

Revolutionised clinical imaging by safely producing high resolution images of all tissues
Safe because it does not use ionising radiation
- Uses magnets to elicit a signal from protons contained in the water molecules of the body

Question 2

What is acceleration?

Answer 2

An accelerating charged particle emits electromagnetic waves

Acceleration - rate of change of velocity

Question 3

What is velocity?

Answer 3

Velocity - magnitude and direction

If magnitude or direction of a charged particle changes it is accelerating

Question 4

A particle spinning in its axis is?

Answer 4

Accelerating

Question 5

If you apply static external magnetic field to random proton domains?

Answer 5

Protons line up

Weak electromagnetic field produced by the tissue

Question 6

If you apply horizontal magnetic flux that switches on and off at radio frequencies to random proton domains?

Answer 6

Protons precess - wobble on their axons
Produces a varying detectable magnetic field
Wobble can be divided into horizontal and vertical components
MRI measure the changes in these two components as the protons respond
These changing signals are the basis of the image reconstruction

Question 7

Typical MRI sequencing method

Answer 7

1. Subject placed in a strong vertical magnetic field – protons line up
2. Super cooled, super conducting coil produces the static magnetic field
3. Horizontal radio frequency pulse is applied, tipping protons over so they rotate in synchrony in the horizontal plane
   - Head coil produces the RF pulse
4. RF pulse is turned off and protons begin to move out of phase – dephase
   - RF off horizontal field decays
5. Dephasing occurs quickly and leads to a loss of horizontal magnetism - T2
6. If horizontal radio frequency pulse remains off, the protons slowly realign themselves with the vertical magnetic field - restoration of vertical magnetisation
7. The time constant of recovery of longitudinal magnetisation - T1

Question 8

What is T2?

Answer 8

The time constant of loss of horizontal magnetism

Question 9

What is T1?

Answer 9

The time constant of recovery of longitudinal magnetisation

Question 10

What does T2 time constant curve look like?

Answer 10

Curved downwards

Question 11

What does T1 time constant curve look like?

Answer 11

Curved upwards

Question 12

Changes in T1 and T2 values are useful in getting information about?

Answer 12

The tissue protons are in

Question 13

What do he values of T1 and T2 depend on?

Answer 13

The surrounding matter and whether it causes dephasing or realignment to happen quickly or slowly

Question 14

How are the various T1 and T2 values recorded?

Answer 14

Computer attached to scanner assigns a brightness value to the various T1 and T2 being measured
Used to construct an image
Can look at T1 or T2 weighted images depending upon what you’re interested in visualising

Question 15

Different tissues have?

Answer 15

Different T1 and T2 values

Question 16

Is CSF dark or light in T1?

Answer 16

Dark - weak signal

Question 17

Is CSF dark or light in T2?

Answer 17

Light - strong signal

Question 18

What if fMRI good for?

Answer 18

Clinical application

Anatomical studies

Question 19

First fMRI method

Answer 19

The subject to be observed lay on a delicately balanced table
- When emotional or intellectual activity began in the subject the balance went down at the head-end - consequence of blood in his system

Question 20

Why does blood flow increase to metabolically active areas?

Answer 20

To supply the area working hard with nutrients & oxygen and flush away metabolic by-products

Question 21

What characteristic does deoxygenated blood have that oxygenated blood does not have?

Answer 21

Deoxygenated blood is paramagnetic

Question 22

What are the implication of paramagnetic deoxygenated blood?

Answer 22

Slight variation on MRI scanning

BOLD signal

Question 23

What is the BOLD signal?

Answer 23

Blood oxygenation level dependent signal

Question 24

What does the BOLD signal show?

Answer 24

Shows almost same anatomical detail plus indicates which area of brain is active - by assumption that more flow to an area means it must be active
Computer program can differentiate these differences and display image accordingly

Question 25

What is the Statistical Parametric Mapping programme?

Answer 25

Calculates probability that an area of brain has a different BOLD signal than the one next to it

Question 26

Statistical Parametric Mapping programme method

Answer 26

Compare a VOXEL with the voxel next to it - t-test
Gives a probability of increased blood flow to that area compared to those around it
All probabilities assigned colours which are overlaid on anatomical MRI – maps out activation

Question 27

Why do some people think the BOLD signal is wrong?

Answer 27

Think it

• Has nothing to do with nutrition since the increased blood flow exceeds the requirements of active neurones
• May be a non-specific cooling system

Question 28

Usefulness of fMRI

Answer 28

Patterns of activation can be different in different subjects performing same task
Comparisons in different people - non-linear registration applied on to a standard template
- Causes distortion of original image - fuzzy

Question 29

fMRI temporal and spatial resolution

Answer 29

Low temporal resolution

High spatial resolution

Question 30

fMRI is good at?

Answer 30

Telling where the brain is activated

Question 31

fMRI is bad at?

Answer 31

Telling you when its occurring
Takes several seconds for arterioles to dilate and allow increased blood flow which is detected in fMRI
- Therefore, BOLD signal takes several seconds to build up
- Bad for experiment in which timing of events is important

Question 32

If you’re investigating rapid cognitive tasks then?

Answer 32

fMRI may need to be coupled with another technique - electroencephalography


Question 33

Electroencephalography spatial and temporal resolution

Answer 33

High temporal resolution

Low spatial resolution