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Methods of Neuroscience > Task 5 fMRI > Flashcards

Task 5 fMRI Flashcards

1
Q

structural imaging

A

based on unique tissue properties

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2
Q

Blocked design:

A
  • recorded neural activity is integrated over “block” of time during which pp is presented a stimulus /performs certain cognitive task
  • recorded pattern is compared to other blocks recorded while doing same/different/no stimulus
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3
Q

Event-related design

A
  • across experimenta; trials, BOLD response is linked to specific events (e.g., stimulus presentation)
    –> can be used at great range of experiments
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4
Q

Temporal resolution

A

accuracy with which one can measure when a cognitive event is occurring

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5
Q

Spatial resolution

A

accuracy with which one can measure where a physiological change is occurring

  • unit: voxel (volume element) representing minimum unit of brain tissue sampled in each image
  • increased voxel size = lowered spatial resolution but increase in amount of active tissue detected
  • decreased voxel size = increased spatial resolution –> reduced sensitivity to BOLD effect but more spatially specific information
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6
Q

Measures to reach higher resolution

A
  • jittering: use of different delays between start of sampling of brain volume images rel. to start of stimulus presentation to subject
  • parallel imaging: spatial coding of signals from coil sensitivity profiles
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7
Q

Process of MRI

A
  1. protons are oriented randomly and spin at certain speed (precession)
  2. strong magnetic field (B0) is applied –> protons orient parallel or anti-parallel to B0
  3. brief radiofrequency pulse (RF) is applied –> knocks proton orientation to original orientation (“longitudinal magnetization”) and “inphase” precessions (“transverse magnetization”)
  4. recovery: protons are pulled back into magnetic field
    - time when 63% of longitudinal magnetization is recovered: T1 time
    - time when 63% of transverse magnetization is recovered: T2 time
    - T1 and T2 is unique to tissues
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8
Q

Time sequence of MRI

A
  1. 90 degree RF pulse –> rapidly decaying signal
  2. 180 degree refocusing pulse: synchrony due to differences in precession speed (T2 effects) –> energy is released (“echo”)
  3. echos are collected by scanner –> creates anatomical image
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9
Q

Advantages of MRI

A
  • completely safe and allows repeated measures
  • provides good spatial resolution: folds of individual gyri can be discerned and allows discrimination between white and grey matter
  • can be adapted to fMRI
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10
Q

Advantages of MRI compared to CT

A
  • no use of ionizing radiation
  • better spatial resolution
  • better discrimination between grey and white matter
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11
Q

Computerized tomography (CT)

A
  • constructed acc. to amount of x-ray absorption in different types of tissues
  • bone absorbs the most while CSF absorbs the least –> ventricles appear black, brain matter intermediate, skull appears white
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12
Q

CT applications

A
  • used to diagnose tumours
  • identify haemorrhages and other brain abnormalities
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13
Q

PET (Mechanism)

A
  • radioactive tracer is introduced into bloodstream
  • tracer isotopes are unstable -> rapidly decay into stable form by releasing positron from their nuclei
  • when positron collides with an electron: two gamma rays (photons) are released
  • radioation emitted from tracer is monitored by PET instrument: gamma ray detector

–> reconstruction of the image of distribution of blood flow: more blood flow = more radiation

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14
Q

PET (applications)

A
  • Pittsburgh compound B (PiB): biomarker for AD: binds to βA
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15
Q

Advantages of PET

A
  • radiolabelled pharmacological agents can be used to trace very specific pathways
  • less susceptible to signal distortion around air cavities
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16
Q

Limitations of PET

A
  • risk: use of radioactive tracer: exclusion of pregnant women, contraceptive coils, children, cochlear implants, pacemakers
  • poor temporal resolution: it takes 30 s for tracer to enter the brain and 30 s for radiation to peak –> critical window
  • poor spatial resolution: about 10 mm
  • constrained by rate of decay of radioactive agent (minutes)
  • difficult to interpret because of massive data sets
  • required blocked design
17
Q

Process of fMRI

A
  • radiowaves cause protons in hydrogen to oscillate
  • detector measures local energy fields that are emitted as protons return to orientation of magentic field
    (same as MRI)
  • fMRI focuses on magnetic properties of deoxygenated vs. oxygenated hemoglobin –> fMRI detectors measure ratio of oxygenated to deoxygenated hemoglobin: blood oxygen level-dependent (BOLD) effect
18
Q

Hemodynamic response function (HRF)

A
  1. initial dip: neurons consume oxygen –> rise in amount of deoxyhemoglobin –> reduction of BOLD signal
  2. overcompensation: blood flow in region increases due to increased consumption of oxygen –> BOLD signal increases significantly
  3. undershoot: blood flow and oxygen consumption dip before returning to baseline –> relaxation of venous system
19
Q

pre-processing

A
  1. Stereotactic normalisation: each brain is fitted to normal standard brain –> correction for individual differences
    - brains are divided up into voxels
    - each voxel is given a 3D spatial coordinates: Talairach coordinates
    - every coordinate can be mapped onto corresponding x y z coordinates on any other brain
  2. Smoothing
    - enhancing signal-to-noise ratio and increasing spatial extent of active regions
    - spreading raw activation level of any voxel to neighbouring voxels
    - small spatial distortion can produce spurious results
20
Q

Berman’s fMRI approaches

A

Localization: establishing correlations between brain and behaviour

Commonalities: examine overlapping/non-overlapping patterns of brain activity –> building up a view of shared and distinct processes among psychological tasks

Documenting individual differences: permits to understand consistencies and inconsistencies in human behaviour

Testing psychological models of behaviour

21
Q

Applications of fMRI

A
  • describe netwoks associated with particular cognitive operations and relationships among nodes within networks
22
Q

Advantages of fMRI over PET

A
  • less expensive
  • easier to maintain
  • no radioactive tracer: repeated testing becomes possible
  • spatial resolution is superior –> high resolution anatomical image obtained with MRI
23
Q

Signal to noise ratio

A
  • can be enhanced by smoothing: spreading activity across voxels –>

Methods of Neuroscience (10 decks)

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