105. FMRI: MECHANISM

Question 1

1. What are the main differences between MRI and FMRI?

Answer 1

MRI:
- it studies the brain anatomy
- it produces one high resolution image

FMRI:
- it studies the brain function
- it makes use of many low resolution images

Question 2

2. Define: FMRI.

Answer 2

• this is a rapid MR imaging technique
• it takes images of the brain during an activity or
  stimulus test
• it also takes images of the brain at rest

NB:
- different images of the different faces of the brain are
obtained

Question 3

3. What do we do with the two images?

IMAGE 1= the brain during an activity (stimulus test)
IMAGE 2= the brain at rest

Answer 3

• they are subtracted from one another
• this demonstrates the functional brain activity

Question 4

4. What is the functional brain activity a result of?

Answer 4

• it is a result of the increased blood flow to the activated
  cortex

Question 5

5. Define: BOLD.

Answer 5

• this stands for Blood Oxygen Level Dependence
• it is the most physiologically important effect
• it produces MR signal intensity changes
• it does this between the stimulus and the rest

Question 6

6. What does the BOLD technique make use of?

Answer 6

• it uses the differences in the magnetic
  susceptibility of Oxyhaemoglobin and
  Deoxyhaemoglobin
• this helps to obtain different intensities

Question 7

7. Define: Haemoglobin.

Answer 7

• this is a molecule
• it contains Iron
• it transports Oxygen in the Vascular system
• this is because the oxygen binds directly to the iron

Question 8

8. Define: Oxyhaemoglobin.

Answer 8

• this is a diamagnetic molecule
• the magnetic properties of iron are suppressed
• it produces a weak magnetic response

Question 9

9. Define: Deoxyhaemoglobin.

Answer 9

• this is a paramagnetic molecule
• it creates an inhomogeneous Magnetic field
• it does this immediately where is it found
• this increases the T2* Decay

Question 10

10. What is the advantage of looking at the Oxyhaemoglobin and the Deoxyhaemoglobin?

Answer 10

• it allows us to maintain images using the FMRI
  technique

Question 11

11. What do the body tissues do at rest?

Answer 11

• they use a substantial fractionation of the blood
• this blood is flowing through the capillaries

Question 12

12. What is found in Venous Blood?

Answer 12

• it contains an almost equal mix
• of Oxyhaemoglobin and Deoxyhaemoglobin

AN IMBALANCE:
- in the deoxyhaemoglobin and the oxyhaemoglobin
- results in a more bold FMRI signal being produced

Question 13

13. What happens when an individual exercises?

Answer 13

• the metabolism is increased
• more oxygen is needed
• more oxygen is taken from the capillaries

Question 14

14. What is the brain very sensitive to?

Answer 14

• it is very sensitive to low concentration of
  oxyhaemoglobin

THIS SENSITIVITY RESULTS IN:
- the Cerebral Vascular System increasing its blood flow
- this blood flow goes to the Activated Area
- this causes a drop in the Deoxyhaemoglobin

Question 15

15. What does a drop in the Deoxyhaemoglobin result in?

Answer 15

• dephasing
• this corresponds to an increase in the signal intensity

Question 16

16. What happens to the Blood Oxygenation during Brain activity?

Answer 16

• it is increased
• the specific location of the Cerebral Cortex are
  activated
• this happens during specific tasks

Question 17

17. What activates the Visual Cortex?

Answer 17

• seeing
• this checks Brain Activity

Question 18

18. What activates the Auditory Cortex?

Answer 18

• hearing
• this checks Brain Activity

Question 19

19. What activates the Motor Cortex?

Answer 19

• finger tapping
• this checks the brain activity

Question 20

20. What is another set of activities that can stimulate the Brain Cortices?

Answer 20

• more sophisticated tasks
• these include Maze Paradigms
• they include thought-provoking tasks

Question 21

21. What can be said about the effects of Blood Oxygen Level Dependence (BOLD)?

Answer 21

• they are short lived
• they require extremely fast sequences

SUCH AS:
- Echo Planar Imaging
- Fast Gradient Echos

Question 22

22. How do we acquire Blood Oxygen Level Dependence images?

Answer 22

• we use a long Echo Time
• it is usually between 40 - 70 ms
• we use Echo Time Shifting Techniques
• the Brain task is modulated on and off

Question 23

23. What happens with the “off” and “on” images?

Answer 23

• the “off” images are subtracted from the “on” images
• a more sophisticated statistical analysis is performed

Question 24

24. What do we do with images that were taken of regions that were activated above some threshold level?

Answer 24

• these images are overlaid onto anatomical images

Question 25

25. What Clinical Application do we use FMRI images for?

Answer 25

WE USE THEM TO: - understand brain function - evaluate a stroke - understand epilepsy - understand pain - understand behavioural problems

Question 26

26. What is the Biological relevance of MR Signals when it comes to T1 and T2 Contrast?

Answer 26

- it helps us to understand the Tissue Content

Question 27

27. What is the Biological relevance of MR Signals when it comes to T2* Blood Oxygen Level Dependence (BOLD) Contrast?

Answer 27

- it helps us to understand the Tissue Function

Question 28

28. What is the Biological relevance of MR Signals when it comes to Diffusion Contrast?

Answer 28

- it helps us to understand the Tissue Micro-Structure