Task 5 - PET fMRI Flashcards
Structural imagining
- Different types of tissue have different physical properties
- Static maps
- CT, MRI
Functional imaging
- Neural activity produces local physiological changes
- Dynamic maps
Preprocessing
-Correcting for head movement
-Stereotactic normalization
®Smoothing
-Optional steps: spatial/temporal filtering, re-sampling, re-ordering of data
How does PET function?
-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
PiB
- 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
PET advantages
- 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
PET disadvantages
- 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
how does MRI work
-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
How does MRI work?
-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
How does fMRI work?
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
BOLD signal
-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%
Resolution fMRI
Spatial resolution: 1mm
Temporal: several seconds
advantages fMRI
- 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
disadvantages fMRI
- 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
Temporal resolution and experiment duration
- 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
Spatial resolution and brain coverage
- 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
Possible solution for temporal and spatial resolution – Jittering
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
Correction for head movement
-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
Stereotactic normalization
- 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
Smoothing
- 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
Studies of localization
- 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
Studies of commonalities in brain activation
- 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
Studies of distinctiveness in brain activation
- 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
Documenting individual differences
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
