BOLD contrast and fMRI L2.2

Question 1

what is BOLD contrast and what 3 processes is it dependent on?

Answer 1

BOLD is the signal measured in fMRI
- depends on the relationship between:
a) the increase in blood flow triggered by the increase in cellular activity
b) the magnetic properties of haemoglobin in the blood
c) The mismatch between the increase in blood flow and the increase in oxygen metabolism

Question 2

How does hemoglobin transport oxygen in red blood cells?

Answer 2

haemoglobin is a protein complex with 4 iron having heme groups to which oxygen attaches

Question 3

what are the magnetic properties of oxygenated and deoxygenated haemoglobin?

Answer 3

oxygenated haemoglobin is diamagnetic while deoxygenated haemoglobin is paramagnetic due to unpaired electrons with a strong magnetic moment

Question 4

how do the magnetic properties of deoxygenated hemoglobin affect the MR signal?

Answer 4

deoxygenated haemoglobin has a magnetic sensitivity of 20% more than fully oxygenated blood.
- this distorts the local homogeneity of the magnetic field leading to a spin de-phasing and signal loss

Question 5

why are gradients echoes sensitive to BOLD contrast?

Answer 5

gradient echoes are sensitive to BOLD contrast because they are very sensitive to static field inhomogeneities caused by the paramagnetic properties of deoxygenated hemoglobin

Question 6

explain the key findings of Ogawa’s study on BOLD contrast

Answer 6

• showed that BOLD contrast was greater in gradient-echo images compared to spin-echo images, showing the sensitivity of gradient echoes to blood oxygenation
• BOLD contrast was directly proportional to the depth of anaesthesia showing a link to oxygen metabolism.
• inhalation of 10% C02 caused the BOLD contrast to disappear. This is because increased blood flow in the absence of changes in oxygen metabolism raises the concentration of oxygenated hemoglobin and flushes out deoxygenated hemoglobin

Question 7

What did Fox and Raiches study reveal about changes in CBF,CMRgluc, and CMR02 in response to neuronal activity?

Answer 7

Their study showed that increases in neuronal activity during visual stimulation resulted in significant increases in CBF and CMRgluc but only a small increase in CMR02

Question 8

Explain the concept of reactive hyperemia in the context of BOLD contrast

Answer 8

Refers to the phenomenon where increased neuronal activity leads to a disproportionately larger increase in CBF compared to CMRO2. This results in a decrease in the concentration of deoxygenated haemoglobin in the venous space, leading to an increase in BOLD signal.

Question 9

what is the spatial extent of the BOLD effect relative to the area of neuronal activity?

Answer 9

The spatial extent of the BOLD effect is larger than the focus of increased activity and correlates with local field potentials

Question 10

why is the BOLD effect not instantaneous?

Answer 10

due to factors like:
- oxygen diffusion is limited by haemoglobin binding and resistance at the endothelium of brain capillaries
- time is needed for neuromuscular coupling to take effect and for changes in blood flow and oxygenation to occur

Question 11

what is the significance of the mismatch between CBF and CMR02 in BOLD contrast and why does the mismatch happen?

Answer 11

mismatch between the two is key for BOLD contrast because it results in a decrease in the concentration of deoxygenated haemoglobin leading to. a stronger MR signal, this mismatch happens because :
- if only the exact amount of required oxygen was delivered it would not be enough to keep the oxygen tension gradient for delivery to mitochondria, therefore CBF must increase disproportionately to ensure enough oxygen supply
- Reactive hyperemia may also be a response to keep brain tissue temperature

Question 12

define is GRE (gradient echo) and spin echo?

Answer 12

GRE:
- uses magnetic field gradients to make an echo without fully refocusing the spins, resulting in faster imaging but with high sensitivity to field variations

Spin echo:
- uses RF pulses to refocus spinning hydrogen nuclei and create a clear signal

Question 13

what is GRE sensitive to and why ?

Answer 13

static field gradients because they cant re-focus signals

Question 14

difference between T2 and T2* ( 3 points each )

Answer 14

T2:
1. time for transverse magnetisation decay due to spin-spin interactions
2. time is long
3. gives info on tissue health

T2*:
1. time for decay including both spin-spin interaction and magnetic field variations
2. time is shorter than T2
3. useful for BOLD fMRI due to its sensitivity to dHb

Question 15

spin vs gradient echo

Answer 15

spin echo:
1. uses RF pulses (90/180)
2. Less sensitive to variations
3.influenced by T2
4.uses: body imaging

Gradient echo:
1. uses magnetic field gradients
more sensitive to variations
2. signal decay is influenced by T2*
3. Uses : fMRI

Question 16

why is increasing the sensitivity to field variations a good and bad thing?

Answer 16

good:
- uses in BOLD fMRI where high sensitivity allows for the detection of little changes in blood oxygen levels in brain

bad:
- can lead to artefacts,distortions in image

Question 17

define Te ( time of echo) and formulas to back up answer

Answer 17

time from where RF pulse is sent to when we measure signal
signal = s0 x exp(-Te/T2*)
where s0 = signal of amplitude

Question 18

define oxy and deoxyhemoglobin

Answer 18

oxy= carries oxygen to active brain cells.
deoxy=Formed when oxygen is used by the cells and is more prevalent in areas with high metabolic activity.

Question 19

what happens to oxy/deoxygenated hemoglobin during neuronal activity?

Answer 19

• during neuronal activity more 02hb enters the area, diluting the dHb
• the presence of 02hb makes the environment less magnetic and thus allows the signal to decay more slowly