Neuroimaging Flashcards
history of brain imaging techniques
- X-ray (2d visualization)
- Cerebral angiography: used iodine as a contrast agent to increase contrast between vasculature (bloodflow) and tissue to see arteries travelling into brain (helpful for looking for effects of strokes; hemorrhages)
3 methods of structural (anatomical) brain imaging
- CT (previously CAT)
- MRI
- DTI (within MRI)
Computed Topography (CT)
- type of structural imaging
- Used for strokes, hemorrhages, tumors
- Form of x-ray that rotates around the head (3d visualization)
- Limitation: radiation exposure
Magnetic Resonance Imaging (MRI)
- type of structural imaging
- Put head in strong magnetic field (3T), hydrogen atoms line up along the poles of magnetic field
- Big magnetic fields not always better (largest is ~24T)
- Can do overlay plot and Diffusion Tensor Imaging
overlay plot
- can be done through MRI
- Different patients with different lesions -> what’s responsible?
- Overlaying various images indicates common damage amongst individuals responsible for behavioural changes
Diffusion Tensor Imaging
- type of MRI
- Just looking at white matter in the brain
- Indicates lesions (missing parts of white matter); differences in white matter tracts between different populations (ie. Those with psychopathy); tumours
How does an MRI machine work?
- 1: align all protons with large magnetic field
- 2: momentarily perturb that alignment with a second varying magnetic field (send a pulse through; will knock protons out of alignment)
- 3: measure radiofrequency (RF) signal producing during realignment with the large magnetic field (“relaxation”/going back to normal)
3 kids of functional (activity-based) brain imaging
- EEG
- PET
- fMRI
- These are all non-invasive, indirect measures
Electroencephalography (EEG)
- Cap covered in electrodes, gel used to get better signal
- Shows waves/changes in voltage; looking for overall changes in activity (action potentials)
- Amplitude (height of waves; volts) & frequency (length between waves; cycles per second/Hz)
- Waves: Gamma (high Hz) -> beta -> alpha -> mu -> theta -> delta (low Hz); high brain activity (ie. Doing math problems) means higher Hz
Positron Emission Tomography (PET)
- Give injection of radioactive substance (ie. Radioactive glucose), watch where it goes
- Diaschisis: ability to detect functional problems at would be hidden in structural scans
Functional Magnetic Resonance Imaging (fMRI)
- Looks for changes in bloodflow
- Ogawa: recognized difference in magnetic qualities between oxygenated and deoxygenated blood
- Hemodynamic response: area that was active 6 seconds ago gets lots of oxygenated blood
- Blood-Oxygen Level Dependent (BOLD) Response: area “lights up” with lots of blood; indication that it’s active
- Functional hyperemia: sends more blood there, strongly driven by glutamate release, causes dilation of blood vessels
what can functional brian imaging measure?
- Changes in bloodflow
- Glucose consumption
- Oxygen
paired image subtraction
- Functional imaging technique
- Subtracting experimental condition from control condition when looking at brain activity (displayed as clusters of “voxels”)
- Assumptions: basic processes that all come together (constituent cognitive processes)
- Quality of results depends on control condition
mean difference images
- Functional imaging technique
- Average activity across all individuals
- Potential problem: doesn’t look like any 1 person’s activity
PET scans and comas
- PET scans revealed brain activity of people in comas in response to voices/image
- however, initial tests showed this was automatic and not indicative of higher-level processing/awareness
fMRI scans and comas
- used to see if people in comas have awareness/higher-level processing
- Came up with technique: asking people to mentally tour your home or play imaginary tennis (lights up different brain areas in fMRI)
- Used this to get vegetative patients to answer yes or no questions (play tennis for yes, tour home for no)
degrees of conciousness
locked-in syndrome -> minimally conscious -> vegetative -> coma -> brain death
neuroimaging and strokes
- Neuroimaging rehabilitation may translate to better outcomes after stroke
- includes structural imaging, functional imaging, and combined measures
neuroimaging and strokes: structural imaging
- Structural imaging (ex. CT, MRI, diffusion tensor imaging) allows for lesion identification and location
- Limitation: provides no info about function
- PLORAS system predicts ability to re-gain speech by comparing brain scans of patient to brain scans of similar patients
neuroimaging and strokes: functional imaging
- Functional imaging measured BOLD signals from brain which may indicate neural activity
- BOLD levels post-stroke indicate impairment
neuroimaging and strokes: combined measures
- May be best to combine neuroimaging with clinical or neurophysiological measures to predict response to therapy
- Meausres of structure & function may be useful in predicting which patients are more likely to benefit from intervention
- Limitation: studies re: this have only been conducted with patients at chronic stage of stroke recovery
- PREP: algorithm combining clinical, neurophysiological, and neuroimaging measures to predict recovery of upper limb function
problems with fMRI studies
- spatial averaging
- spatial resolution
- temporal resolution
- “not necessarily necessity”
- focus on increase in activity
- regional hemodynamics (in diff. regions of brain)
- confound: anxiety (scanner causes it)
- drugs (ex. having coffee before scan)
- reliability
- statistics
problems with fMRI studies: spatial averaging
averaging doesn’t really work; doesn’t give you true location
problems with fMRI studies: spatial resolution
spatial resolution not good for talking about specific brain regions
problems with fMRI studies: temporal resolution
activity can be measured across seconds whereas activity happens using miliseconds
problems with fMRI studies: “not necessarily necessity”
although brain activity correlates with behaviour, it doesn’t mean that the activity of that part of the brain causes the behaviour
problems with fMRI studies: focuses on increases in activity
sometimes inhibition of activity can be just as important as increase in activity
problems with fMRI studies: anticipatory hemodynamics
brain predicts activity and preps for it by shunting oxygen there before you start the task
problems with fMRI studies: statistics
dead salmon showing brain activity shows problem of false positives; if you don’t correct/control your data you could have a huge amount of false positives
Default Mode Network
- The brain’s activity at resting state (seen with fMRI)
- Shows that even at passive state, brain is quite active
- Includes medial prefrontal cortex, posterior cingulate, and lateral parietal cortex
voxel
3d equivalent of a pixel
