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Method of Cognitive Neuroscience > Task 5 - PET fMRI > Flashcards

Task 5 - PET fMRI Flashcards

1
Q

Structural imagining

A
  • Different types of tissue have different physical properties
  • Static maps
  • CT, MRI
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2
Q

Functional imaging

A
  • Neural activity produces local physiological changes

- Dynamic maps

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3
Q

Preprocessing

A

-Correcting for head movement
-Stereotactic normalization
®Smoothing
-Optional steps: spatial/temporal filtering, re-sampling, re-ordering of data

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4
Q

How does PET function?

A

-radioactive substance introduced into bloodstream
-radiation emitted from the ‘tracer’ is monitored
Radioactive decay happens:
-radioactive isotopes (eine Atomart) in the substance emit a positron from their atomic nuclei

-tracer is most where more activity is -> at the more active sites, more positrons are emitted -> then positron and electron can collide -> more gamma rays at active sites

> positron collides with an electron -> 2 photons/gamma rays are created

  • 2 photons move in opposite directions at the speed of light -> pass through brain tissue, skull and scalp
  • scanner (gamma ray detector) determines where the collision took place
  • more blood flow -> more radiation
  • > Measures photons that are produced during decay of the tracer
  • > Measures change in blood flow to a region directly
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5
Q

PiB

A
  • radioactive agent Pittsburgh Compound B
  • protein-specific carbon-labeled dye that could be used as a PET tracer
  • binds to beta-amyloid
  • beta amyloid: Alzheimers may be caused by the decay of production of amyloid -> leads to characteristic plaques
  • PET can be used to measure beta-amyloid plaques
  • tool for diagnosing Alzheimer’s
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6
Q

PET advantages

A
  • less susceptible to signal distortion around the air cavities (sinuses, oral cavity)
  • with radiolabeled neurotransmitters: possible to investigate neural pathways to study effects of drugs on the brain
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7
Q

PET disadvantages

A
  • Block design experiments must be used
  • data sets are massive -> comparison of conductions produces many differences
  • difficult to make inferences about each area’s functional contribution from neuroimaging data
  • temporal resolution of 30s
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8
Q

how does MRI work

A

-radio waves cause protons in hydrogen atoms to oscillate
-detector measures local energy field that are emitted as protons return to the orientation of the magnetic field created by MRI
Magnetic: nuclear magnetic spins
Resonance: matching of frequency between radio frequency pulse and the precession of the spins
Imaging: signal measled by the MRI scanner is spatially encoded and the algorithm produces the images

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9
Q

How does MRI work?

A

-Strong magnetic field is applied
-Protons in water molecules in the body (hydrogen nuclei in H2O) have weak magnetic fields
-Fields will be oriented randomly -> strong external field applied -> small fraction will align with this
Once protons are aligned:
-Brief radio frequency pulse is applied -> orientation of aligned protons by 90 degrees to original orientation
-As the protons spin in this new state, they produce detectable change
-Will be pulled back automatically to original alignment

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10
Q

How does fMRI work?

A

no direct measure of neural events

  • measure metabolic changes correlated with neural activity
  • when neurons consume oxygen, they convert oxyhemoglobin to deoxyhemoglobin
  • deoxygenated hemoglobin is paramagnetic (weakly magnetic in the presence of a magnetic field) -> introduces distortions in local magnetic field
  • oxygenated hemoglobin is not paramagnetic
  • detectors measure ratio of oxygenated to deoxygenated hemoglobin => blood oxygen level-dependent BOLD effect
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11
Q

BOLD signal

A

-more blood in active areas
-> measures concentration of oxygen in blood
-areas with high concentration of oxyhemoglobin give a higher signal (a bright image)
-high BOLD signal if ratio between oxy/deoxy-hemoglobin tissue concentration increase
-BOLD sensitivity proportional to magnetic field strength:
Magnetic field of 1.5T: signal changes of 1-5%
3T: changes of 2-10%

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12
Q

Resolution fMRI

A

Spatial resolution: 1mm

Temporal: several seconds

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13
Q

advantages fMRI

A
  • less expensive and easier to maintain than PET
  • no radioactive tracers -> therefore same individual can be tested repeatedly, either in a single session or over multiple sessions
  • spatial resolution is superior to PET
  • functional connectivity can be studied
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14
Q

disadvantages fMRI

A
  • poor temporal resolution
  • dependent on hemodynamic changes
  • massive data sets -> comparison of experimental and control conditions produces many differences
  • difficult to make inferences about each area’s functional contribution from neuroimaging data
  • BOLD signal primarily driven by neuronal input rather than output
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15
Q

Temporal resolution and experiment duration

A
  • time to repetition (TR): time between 2 excitation pulses = time to collect one brain volume (composed of many slices)
  • > determines temporal resolution (sampling rate)
  • the shorter the TR -> the lesser slices -> limited brain coverage

-One should get max of useful info per time unit + min. time in scanner per subject unit

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16
Q

Spatial resolution and brain coverage

A
  • ideally: smallest voxel size + acquisition of whole encephalic tissue available in a subject
  • resolution can be increased (through reduced voxel size, which also reduces susceptibility artifacts) at expense of signal to noise and time
  • small voxel size -> negative for signal to noise ratio -> reducing sensitivity to BOLD  BUT more spatially specific info
  • spatial resolution: voxel that represents min. unit of brain tissue sampled in each image
  • if increase in spatial resolution + temporal resolution fixed: amount of brain tissue sampled (number of image slices) has to be reduced
  • if spatial increased and brain coverage is maintained, temporal has to be less
17
Q

Possible solution for temporal and spatial resolution – Jittering

A

Jittering = use of different delays between start of sampling of brain volume images relative to start of stimulus presentation

-if all images collected with same delay from stimulus presentation: time-lock strategy -> all brain regions sampled at same time points

  • if one jitters (offsets=verschieben) stimulus presentation time to image acquisition, different time points would be sampled at each stimulus presentation
  • > requires more trials
  • > advisable if full brain coverage is needed, as well as temporal resolution
18
Q

Correction for head movement

A

-good spatial resolution -> small spatial distortions can produce spurious results
-if person moves head, the position of any active region will also move around
THEN:
-> either region is harder to detect or a false-positive result could be obtained

19
Q

Stereotactic normalization

A
  • each brain is divided into thousands of small volumes (voxels)
  • each voxel can be given 3 dimensional spatial coordinates (x,y,z)
  • the template of each brain is squashed/stretched to fit into standard space
20
Q

Smoothing

A
  • spreads some of the raw activation level of a given voxel to neighboring voxels
  • the closer the neighbor, the more activation it gets
  • enhances signal to noise ratio
  • signal = corresponds to larger cluster of activity
  • noise = isolated voxel
  • > increases spatial extent of active regions -> when averaging activity across individuals: greater chance of finding common regions of activity
  • reduces spatial resolution
21
Q

Studies of localization

A
  • localizing psychological functions to brain regions
  • identify brain behavior correlations
  • issue: how modular are brain regions? Is there one-to-one mapping of functions onto brain regions?
  • pattern of activation over regions may be critical
  • not individual, encapsulated brain areas are activated selectively for different stimuli
  • research concerned with localization need not be restricted to identify one-to-one brain-to-behavior mappings
  • rest on consistent mapping of brain and behavior that is found across individuals
22
Q

Studies of commonalities in brain activation

A
  • if 2 tasks lead to activation of common brain areas, then those 2 tasks/behaviors are likely to share some process(es)
  • fMRI can be used to infer cognitive processes involved in one task by showing similarities in brain activation to a better understood task
  • rest on consistent mapping of brain and behavior that is found across individuals
23
Q

Studies of distinctiveness in brain activation

A
  • seek to discover distinct activations between 2 tasks
  • discovering such dissociations permits the inference that 2 tasks have different cognitive processes mediating them
  • most findings of distinctive activations yield results of partial overlap in activations -> distinctiveness may be quantitative rather than qualitative
  • rest on consistent mapping of brain and behavior that is found across individuals
24
Q

Documenting individual differences

A

Documenting individual differences -identification of differences across individuals

  • e.g.: variability in activation when viewing happy expressions was predicted by measuring participant’s scores on an extraversion scale
  • studies on individual differences in brain activation can play a role with behavioral data in accounting for both consistent and inconsistent behavior across tasks

Method of Cognitive Neuroscience (9 decks)

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