Advanced MRI-3

Question 1

What is fMRI used for?

Answer 1

-A technique used for measuring brain activity

Question 2

What does fMRI work?

Answer 2

-Detects changes in blood oxygenation and flow that occur in response to neural activity

Question 3

What is observed in brain areas that are more active?

Answer 3

• Consume more oxygen to meet increased demand in blood flow

- So blood flow to these areas is increased

Question 4

What type of maps do fMRI produce?

Answer 4

-Used to produce activation maps showing brain regions in a particular mental process

Question 5

What are 3 advantages of fMRI?

Answer 5

• Non-invasive (NO RADIATION)
• High spatial (3mm3)
• Good temporal resolution (~2secs)

Question 6

Why is fMRI an attractive imaging technique?

Answer 6

• Popular tool for imaging normal brain function

- Provided new insight to brain functions- brain regions associated with memory, language, pain, learning and emotion

Question 7

How is oxygen delivered to neurone by haemoglobin in capillary?

Answer 7

-By haemoglobin in capillary red blood cells

Question 8

What is observed when neuronal activity increases?

Answer 8

• Increased demand for oxygen

- Local increase in blood flow to regions of increased neural activity

Question 9

How can the haemoglobin be described when it is oxygenated and deoxygenated?

Answer 9

• Haemoglobin is diamagnetic when oxygenated

- Paramagnetic when deoxygenated

Question 10

Why are Small differences in MR signal observed?

Answer 10

-Depending on degree of oxygenation

Question 11

Describe the BOLD response

Answer 11

1. Momentary decrease in blood oxygenation immediately after neural activity increases, known as “initial dip” in haemodynamic response.
2. Period where blood flow increases -(overcompensates demand). This means blood oxygenation actually decreases following neural activation.
3. Blood flow peaks after ~6s and falls back to baseline
4. Baseline recovered after 20-30s

Question 12

Why do we use Gradient echo (GRE) images?

Answer 12

• They are sensitive to tissue T2* relaxation

- T2* decreases as magnetic field uniformity decreases

Question 13

What does Deoxygenated blood contain and what does it do?

Answer 13

• deoxyhæmoglobin (dHb)

- Is paramagnetic, and distorts the magnetic field

Question 14

What happens as the blood becomes deoxygenated and what effect does this have on the GRE image?

Answer 14

• dHb increases and T2* decreases

- GRE image gets darker

Question 15

What happens as the blood becomes oxygenated and what effect does this have on the GRE image?

Answer 15

• dHb decreases and T2* increases

- GRE image gets brighter

Question 16

Describe the magnetic field and the length of T2* in arterial blood?

Answer 16

• Uniform magnetic field

- Long T2*

Question 17

Describe how the haemoglobin changes as it passes from artier to venous blood

Answer 17

• Oxyhemoglobin arrives and transfers oxygen for tissue metabolism
• The deoxyhemoglobin becomes paramagnetic and interacts with magnetic field
• This shortens the T2*

Question 18

Why is performing an fMRI useful on tumour?

Answer 18

• Allows you to see activations around text tumour
• Allows you to see which regions to avoid when cutting out tumour
• Allows greater improvements to be seen post op

Question 19

What are the limitations of fMRI?

Answer 19

• Measures secondary physiological correlates of neural activity, it is not a direct measure
• When comparing fMRI response between individuals it is impossible to say whether the differences are neural or physiological in origin
• fMRI should not be used to localize brain function alone

Question 20

According to big man Barrick, what should fMRI used for instead?

Answer 20

-To map the functional parts of the brain that act in different combinations for different tasks, or via functional networks.