Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

P3 Physical > Physiology & Contrast Mechanisms for mapping brain activity > Flashcards

Physiology & Contrast Mechanisms for mapping brain activity Flashcards

1
Q

What are examples of measuring brain activity?

A
  1. EEG/MEG
  2. Electrophysiology
  3. FDG PET
  4. Autoradiography
  5. 13C MRI
  6. BOLD fMRI
  7. ASL/VASO/CA
  8. 150 PET
  9. NIRS
  10. Optical imaging
  11. Doppler US
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are examples of neuroelectric activity?

A
  1. Action potential
  2. Postsynaptic activity
  3. Excitatory/inhibitory
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is metabolic consequence?

A
  1. Oxygen consumption

2. Glucose consumption

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is an example of vascular response?

A
  1. Blood flow
  2. Blood volume
  3. Blood oxygenation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is fNIR?

A

A non-invasive imaging method involving the quantification ofchromophoreconcentration resolved from the measurement of near infrared (NIR)lightattenuation or temporal or phasic changes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What does NIR spectrum light take advantage of?

A

Optical windowin which skin, tissue, and bone are mostly transparent to NIR light in the spectrum of 700-900nm, whilehemoglobin(Hb) and deoxygenated-hemoglobin (deoxy-Hb) are stronger absorbers of light.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What do differences in the absorption spectra of deoxy-Hb and oxy-Hb allow the measurement of?

A

Relative changes in haemoglobin concentration through the use of light attenuation at multiple wavelengths

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the brief introduction to BOLD imaging?

A
  1. Ogawa et al 1990 - BOLD effect in rat brain

2. Kwong et al, 1992 - BOLD FMRI in human

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the functional MRI in brief?

A
  1. Stimulus
  2. Neuronal activity
  3. Neurovascular coupling
  4. BOLD contrast
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is a stimulus?

A

Carefully designed to elicit a specific response from some cognitive system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What does neuronal activity require?

A

Energy leading to an increase in oxygen demands

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is neurovascular coupling?

A
  1. Vessels dilate
  2. Blood flow increases
  3. The supply of oxygen exceeds demand
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is BOLD contrast?

A

The net increase in oxygenated haemoglobin in the vasculature leads to an increase in signal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What does BOLD magnetic resonance signal used in functional imaging of the brain reflect?

A

Loss of oxygen from haemoglobin causing its iron to become paramagnetic, which influences the magnetic field experienced by protons in the surrounding water molecules

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What happens during neuronal activity?

A
  1. An increase of oxygen usage
  2. Larger fractional increase in blood flow
  3. Increase in blood volume
  4. Net decrease of the amount of deoxygenated haemoglobin present
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what does PET imaging detect?

A

Activity-induced increases in blood flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the relationship first proposed 100 years ago?

A

Understanding the increased blood flow associated with neural activity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Where has brain energy usage been attributed mainly to?

A

Activity in the presynaptic terminal or to the energy needed to take up neurotransmitter glutamate and convert it to glutamine in astrocytes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is dominated by excitatory postsynaptic currents?

A

BOLD signal from primates did directly reflect energy consumption

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What happens during rest period?

A
  • Haemoglobin carries oxygen
  • Neuron is taking oxygen from the haemoglobin  oxyhaemoglobin
  • Deoxyhaemoglobin in the absence of oxygen
  • In the venous part, there are some oxyhaemoglobin or deoxyhaemoglobin – this is rest period
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What happens during activity?

A
  • Needs more oxygen
  • Take more oxygen and there will be less oxyhaemoglobin in the venous part and more deoxyhaemoglobin
  • Blood flow will increase to provide more oxygen to the neuron
  • The supply will increase the demand by a factor of 2
  • There is more oxygen coming than it is consumed by the neuron
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Deoxy-haemoglobin as endogenous contrast agent?

A
  1. oxygen metabolism increase
  2. Deoxyhaemoglobin increase
  3. Blood flow inceease
    deoxyhaemoglobin decrease
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is the impact of magnetic susceptibility?

A

MRI signal = amplitude of magnetization vector M
M = the sum of all the magnetization vectors of each proton in one voxel
Every small magnetization vector is rotating about the z axis
The precessional frequency is proportional to the amplitude of the magnetic field
Even inside a voxel they may not experience exactly the same magnetic field
So they are dephasing
The sum of non-parallel vectors is smaller than the sum of parallel vectors.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is T2 decay?

A

The signal loss cannot be recovered

25
Q

What is T2* decay?

A

The additional signal loss can be recovered via a spin-echo

26
Q

What is iron-containing Heme group?

A

Makes deoxygenated haemoglobin (Hb)

  1. Paramagnetic (attracted and form induced magnetic field in the direction of the applied magnetic field)
  2. Different from tissue (H20)
27
Q

What does the bound oxygen shield?

A

Heme group making oxygenated Hb

  1. Diamagnetic (repelled and form induced magnetic field in the opposite direction of the applied magnetic field
  2. Same as tissue (H20)
28
Q

What does vessel cause?

A

A distortion in the magnetic field as it has different magnetic susceptibility

29
Q

What does the effect of distortion of magnetic field depend on?

A

Amplitude of applied magnetic field

30
Q

Vessel at 0.5T

A

No distortion of the magnetic field

31
Q

What happens when you increase B0?

A

The amplitude of the magnetic field is different around the vessels that create dipole

32
Q

What does paramagnetic deoxyhameoglobin cause?

A

A local field distortion in and around the vessel: intra and extra-vascular components
The distortions are amplified at higher field

33
Q

What is found in the middle of phantom?

A

Another tissue with different magnetic susceptibility

34
Q

Where can you see the dipole?

A

On the phase image that is proportional to amplitude of the B0

35
Q

What is found in the resting and active state?

A

Resting state - deoxyenated Hb

Active state - oxygenated Hb

36
Q

Why does vectors get dephased?

A

Normal T2* decay

37
Q

What are vessels full of?

A

Deoxygenated Hb that creates a dipole - distortion of magnetic field

38
Q

Why does deoxygenated Hb get more dephased?

A

because of the additional source of inhomogeneity due to the vessels of deoxygenated Hb – more inhomogeneity of the magnetic field, and more difference of precessional frequency between protons and more dephasing

39
Q

What happens at TE of 40 ms?

A

the signal will be higher in the oxyhaemoglobin [in the active state]
• Parameter: T2*
• The T2* will be longer for the activity state

40
Q

What is R2*?

A

inverse of the T2* - the activity state – more oxygen, slower decay, longer T2* - smaller R2*

41
Q

What happens in the resting state?

A 
More deoxyhaemoglobin
more B0 inhomogeneity
More rapid dephasing
More rapid decay of signal
Shorter T2*
longer R2*
42
Q

Why must TE must be sufficient long to?

A

Capture the BOLD effect but not too long to have enough MR signal

43
Q

What is bold sensitivity proportional to?

A

TE and signal at the time TE

44
Q

Where is super-sensitivity found?

A

Between active and resting state

45
Q

What is T2* at 3T?

A

30ms

But if you have a scanner of 7T, the T2* will be shorter about 20ms

46
Q

What does haemodynamic response function (HRF) represent?

A

The amplitude of signal if there is neuronal activity at time 0

47
Q

What happens if there is a neuronal activity at time 0?

A
  • Decrease in oxygen, decrease in signal because of the increase in oxygen consumption, but the blood flow didn’t have time to re-apply quickly
  • There is an exceed of supply compared to demand – there is an increase in signal, and it takes longer
48
Q

What is the maximum amplitude that the signal can achieve

A

About 5-6 seconds?

49
Q

How long will it last at the end of the total change in signal?

A

10-20 seconds

50
Q

What are the key findings in the experimental characterisation of the haemodynamic response?

A
  1. CBF increases much more than CMRO2 with brain activation, producing a reduction of E and the total deoxyhemoglobin present in an image voxel
  2. The CBF and BOLD responses to even a very brief stimulus are delayed by 1–2 s and have a temporal width on the order of 4–6 s.For a sustained stimulus of 20 s or longer, the response typically reaches a plateau value, although there can be substantial variation
  3. A post-stimulus undershoot of the BOLD signal is common and may last for 30 s or more.
  4. with longer duration stimuli tending to have longer post-undershoots. The CBF response typically shows only a shorter and weaker post-stimulus undershoot, or none at all
  5. Some investigators have reported an initial dip of the BOLD signal lasting 1–2 s before the standard BOLD signal increase
  6. The BOLD response typically exhibits a temporal nonlinearity such that an appropriately shifted and added response to a brief stimulus over-predicts the true response to an extended stimulus This temporal nonlinearity is most pronounced when the brief stimulus is less than about 4 s and the extended stimulus is longer than 6 s. Comparing short and long duration stimuli that are both longer than about 4 s, the temporal nonlinearity is reduced.
  7. Nonlinearity has also been reported as a “refractory period”, such that two identical stimuli presented close together in time produce a net response with less than twice the integrated response of a single stimulus alone

8There is a growing body of evidence suggesting that the baseline CBF can have a strong effect on the magnitude of the BOLD response to the same stimulus

51
Q

What is dynamic of the BOLD response an interplay of?

A
  1. CBF (cerebral blood flow)
  2. CMRO2 (consumption rate of O2)
  3. CBV (cerebral blood volume)
52
Q

What are the sequence for dynamic of the BOLD response?

A
  1. Initial dip
  2. Positive response (most fMRI studies)
  3. Undershoot
53
Q

What is the Balloon model?

A
  1. CBF increase by arterioles reducing resistance
  2. Pressure raised in venules as consequence
  3. increased pressure increases CBV in venules by about 15%
  4. Elastic properties of venous bed may delay compliance and induce transient mismatches between CBV and CBF
54
Q

Dynamic of the BOLD response

A
  1. Increase in CMR02 preceding CBF increase
    –> signal change
    <0.5% up to 1s
  2. Reduced oxygen extraction fraction, reduced dHb concentration
    - -> ca. 1-5% signal change after ca. 6s
  3. Undershoot due to delayed return of CBV to baseline
    –> ca. 2% signal change for up to 30s
    very controversial
55
Q

What are the factors that affect the BOLD signal?

A
  1. The strength field
  2. Size of the vessel
  3. MR pulse sequence used
56
Q

What is the strength field?

A

At high field, the BOLD signal doesn’t depend on the intra-vascular component if the TE is set to the T2* ( At TE-T2* (GM) the signal from the blood has already vanished

57
Q

What is the size of the vessel?

A
  1. Active vs Rest –> change in T2* but also in T2
  2. The bigger the magnetic field is the change in T2* is
  3. The effect on T2 is maximal for vessel radius of 5um but the effect on T2* increases when the vessel radius increases
58
Q

What is MR pulse sequence used?

A
  1. A spin-echo sequence is able to refocus the protons in that case: spin-echo can suppress the extra-vascular signal of large vessels

P3 Physical (21 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions