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PHYS4010: Functional Medical Imaging > fMRI > Flashcards

fMRI Flashcards

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USMLE® Step 1 flashcards
1
Q

Give 4 examples of medical scans that can be used for functional (physiological) imaging

A
  • fMRI
  • PET
  • EEG
  • MEG
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2
Q

fMRI can provide ____ spatial resolution but ______ temporal resolution.

A

Good
Limited

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3
Q

PET can provide ____ sensitivity but ________ ___ spatial resolution because it is limited to measuring blood flow with tracers.

A

High
Relatively low

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4
Q

EEG and MEG can provide ____ _____ temporal resolution but _______ spatial information.

A

Very high
Limited

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5
Q

Why can brain activity be mapped?

A

Because brain activity is relatively localised

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6
Q

What happens when the brain responds to a stimulus?

A

Metabolism increases which leads to increased oxygen consumption. This causes an increase in blood flow, and paradoxically this overcompensates leading to an INCREASE in local brain oxygenation in active brain regions.

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7
Q

What is the MR signal sensitive to in fMRI?

A

The different magnetic susceptibilities of oxygenated and deoxygenated blood.

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8
Q

Oxygenated blood is ________, similar to tissue.

A

Diamagnetic

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9
Q

Deoxygenated blood is ________ compared to tissue.

A

Paramagnetic

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10
Q

What is oxygen bound to in blood?

A

Haemoglobin (Hb)

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11
Q

How many O₂ molecules can Hb carry?

A

4

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12
Q

What does the amount of oxygen in a red blood cell depend on?

A

The partial pressure, pO₂

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13
Q

What is deoxyhaemoglobin?

A

Hb that has no bound oxygen

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14
Q

What is oxyhaemoglobin?

A

Hb that has bound oxygen

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15
Q

What is the Hb dissociation curve? What does it determine?

A

A curve showing the amount of oxygen bound to a haemoglobin molecule.

It determines whether the Hb has a preferential release/hold of oxygen.

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16
Q

Deoxygenated haemoglobin (dHb) is strongly _________. Upon oxygenation, one of the electrons is transferred to bound O₂ molecules and so the ____ spin state dHB changes to a ___ spin state and becomes __________ which is ________.

A

Paramagnetic
High
Low
Oxyhaemoglobin
Diamagnetic

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17
Q

What type of contrast agent is haemoglobin? Why?

A

Endogenous because it is confined to red blood cells so does not have to be injected into the body to act as a contrast agent.

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18
Q

Why does deoxygenated blood have a short T₂* decay time?

A

There is a difference in susceptibility between dHB and tissue, leading to the generation of local magnetic field gradients (inhomogeneous field) around vessels and a shorter T₂* decay.

∆B = χB₀

19
Q

What does BOLD contrast stand for?

A

Blood
Oxygenation
Level
Dependent
Contrast

20
Q

Define BOLD contrast

A

The variation in image intensity arising from the changes in T₂* associated with the concentration of dHb (and hence the oxygen content of blood).

21
Q

How do regional blood flow, blood volume and oxygen consumption change in active regions of the brain?

A

Regional blood flow and blood volume increase substantially.
Oxygen consumption only increases slightly.

22
Q

What is the net result of brain activity on oxygenation?

A

There is an increase in venous oxygenation due to a decrease in dHb, hence, T₂* also increases and R₂* (= 1/T₂*) decreases. This also means that there is an increase in signal around the veins in ACTIVE brain areas.

23
Q

What is the percentage increase of blood flow in active brain regions?

A

70%

24
Q

What is the percentage increase of blood volume in active brain regions?

A

30%

25
Q

What is the percentage increase of oxygenation in active brain regions?

A

From 60% to 75%

26
Q

Define oxygen extraction fraction (OEF)

A

The difference in the blood saturation between arterial inflow and venous outflow.

27
Q

Give the equation for oxygen extraction fraction (OEF)

A
28
Q

What does the extraction of oxygen from the blood depend on?

A

The oxygen gradient across the vessel wall.

29
Q

What is the haemodynamic response function (hrf)?

A

The time course of the BOLD signal change in response to stimulation.

30
Q

Give the three key features of the haemodynamic response function

A
  1. An initial ‘deoxy’ dip
  2. Positive peak (haemodynamic delay)
  3. Prolonged undershoot
31
Q

Describe the initial ‘deoxy’ dip of the haemodynamic response function (hrf)

A

A dip due to the drop in venous oxygenation before an increase in blood flow.

32
Q

Describe the positive peak of the haemodynamic response function (hrf)

A

A delay term at ~8s that occurs when oxygenated blood has been pushed into the venules.

33
Q

Describe the prolonged undershoot of the haemodynamic response function (hrf)

A

The venous system balloons and there is uncoupling between blood flow and volume, leading to this undershoot.

34
Q

Give 7 design features that have to be implemented for fMRI to work

A
  1. Use a high field as fMRI is field strength dependent.
  2. The subject’s head must be immobilised (e.g. by using foam padding).
  3. To select the correct slices for functional imaging, high-resolution anatomical images must be taken prior to fMRI.
  4. Stimuli should be presented whilst the patient is in the scanner and should follow one of 3 measurement classes.
  5. EPI is used to record images as it has a high T₂* weighting.
  6. Use an echo time for EPI such that TE equals the T₂* of the tissue as this provides the optimal contrast-to-noise ratio to detect brain activation.
  7. Record images at a high frame rate using multi-slice echo-planar imaging.
35
Q

Describe the 3 stimulus classes for fMRI design

A
  1. BLocked design: stimuli are clustered together in blocks of length 15-60 seconds to allow for the appearance of continuous activation.
  2. Event-related design: the responses to individual events are imaged.
  3. Resting state: no stimuli are shown and variations in signal across the brain are recorded.
36
Q

State the 2 types of data analysis that can be completed for fMRI images

A
  1. Image realignment (motion correction)
  2. Statistical analysis
37
Q

Why is image realignment used for fMRI? How is it done?

A

To correct for any movement occurring between volume datasets.

Adjustments are made to minimise the sum of squared differences between the images. The whole dataset is referenced against an anatomical image or a ‘template brain’.

38
Q

Give the equation for image realignment in fMRI

A

H = squared differences between the images
U = reference image
V = image to be registered

39
Q

How is statistical analysis used for fMRI?

A

The change in the BOLD signal between on and off periods is calculated.

40
Q

Give the equation for the % change in BOLD signal

A

S_on = signal during on period
S_off = signal during off period

41
Q

What are the 3 types of statistical analysis used in fMRI?

A
  1. Correction analysis
  2. GLM analysis
  3. Statistical mapping
42
Q

State 5 limitations of fMRI

A
  • Only measures the secondary blood flow, not neural activity.
  • Isn’t quantitative.
  • Low temporal resolution.
  • Only detects focal activity.
  • In early stages for clinical use.
43
Q

Give the equation for the Ernst angle of a sample

A

α = Ernst angle
TR = Repetition time
T1 = longitudinal relaxation time

PHYS4010: Functional Medical Imaging (8 decks)

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