Human neuroimaging Flashcards
What is our understaning of the brain?
• Our understanding is highly dependent on the methods we use to measure and quantify brain structure and function
Slices of brain to understand on cellular level, single unit electrophysiology to see how single cells are active, use patch clamp to measure ions that traverse memb
• The study of the human brain is currently driven largely by noninvasive brain imaging techniques
• fMRI is the workhorse of noninvasive functional brain imaging techniques and drives a lot of the research
• Electrophysiological techniques (EEG, MEG) are also enormously important
- Both neurological (e.g. brain damage, stroke, epilepsy) illnesses and psychiatric (e.g. depression, schizophrenia) illnesses are based in the brain
- Which field uses neuroimaging more? Why?
Neurology because The symptoms are behavioural in psychiatry, it would be hard to attribute areas assocated to the diseases, the smptoms are diff, the pops affected are heterogeneous (dont have just 1 affected area in the brain), its fctional diffs instead of physical changes in the brain
Compare structural and functional imaging
Structural imaging techniques provide a picture of brain anatomy
Single shot in time of what the brain looks like
Functional imaging techniques provide a picture of brain activity
Idea of how brain fctions, electrical potential change, metabolism at diff time, how active the brain is and how much energy its using, connect the É with a behaviour (task)
What are 2 types of structural imaging?
• X – rays and Computed tomography (CT)
• based on differential absorption of x-rays by different tissues
(e.g., brain and skull)
• Magnetic resonance imaging (MRI)
• based on differential hydrogen (proton) densities in different
tissues
Get lots of contrast between the diff types of tissues
What is a CT scanner?
Different neural tissues absorb only slightly different amounts of x-rays; these differences are enhanced with computer algorithms
Take many xrays (image slices) and use computer to combine them all to make 3D image
Big advances have come with software (how we can reconstruct the images) so we have better resolution
What is a MRI scanner?
MRI measures “proton densities”
Because proton densities are quite different in different neural tissues, the MRI has excellent spatial resolution
Fixed magnets always on (superconductive magnets in liquid He) create static field to line up all the p+ (in orientat° with the mag field) the add gradient (RF mag field prod by the second laer of coils), the 2 felds add together, knocks specific p+ over and when stop the RF pulse, the p+ come back and emiit energy, antennas pick up the emitted É as the p+ go back to their resting state, p+ in diff tissues take diff amounts of time to go back to resting
The gradients allow us to localise diff slices in the brain, combine images to make 3D image of the brain
What are contrast agents?
Gadolinium enhancement of brain tumor (tumors are highly vasc)
Contrast MRI angiogram: employs an intravenous contrast agent (e.g. gadolinium) to increase the (proton density) contrast between brain and blood supply
What is diffusion tensor imaging?
Structural MRI technique based on differential ability of water
molecules to diffuse in different directions in white matter
Create map that descripes flow of water through brain, tensors describe which way water is going
Image tracks (comms between large bundles of axons)
Image connections between diff parts of the brain
Diff from fctional imaging that infers connect°s from activity
What is PET?
Functional imaging technique
― images uptake of radioactive isotopes in active neural tissue
― requires injection of a radiolabelled isotope (e.g. glucose, oxygen)
Inject a tracer (ex radioactively labelled O2 or glucose) that emit energy; positrons that hit free é = anihilation and the É emitted from the collision = photon and can locate them to see what areas activated
Ex labeled glucose and give vis task (V1 requires more glucose) and locate emissions in that area
Expensive, difficult, somewhat invasive, have to make the labels (need cyclotron to create isotopes), isotopes have reall small half life (few hours)
What is fMRI?
― detects changes in regional cerebral blood flow (rCBF)
― neural activity changes the local ratio of oxygenated and deoxegenated haemoglobin; this leads to a measurable change in the MR signal
― BOLD signal: Blood Oxygen Level Dependent
― The “workhorse of cognitive neuroscience”
Same principal of MRI but instead of focussing on p+, focus on Hb that is ferrous and measure ox vs deoxy Hb
Have ex task and record the hemodynamic response; change in oxy vs deoxy Hb between time where do nothing and time where do task
Takes 2-4 s so have significat limitations in terms of temporal resolution, is not prob with imaging technique, its because were imaging a bio process that takes time, if compare with imaging neurons firing, has very bad temporal resolution
Has very good spatial resolution because we’re really scanning whereas ex EEG records from a distance and cant be sure what region was activated
How do we measure electrical and mag fields?
• PET and fMRI are haemodynamic techniques, based on the measurements of changes of blood flow in active neural tissue
• EEG and MEG are electrophysiological techniques, based on measuring electromagnetic fields generated by ionic current flow in active neurons
When neuron is active, flows of ions create pot diff between int and ext of neuron, current is prod and can measure change of pot on scalp
With change uf current, mag field is created and passes through tissues and can be measured on scalp
What is electrocorticography?
ELECTRODES PLACED DIRECTLY ON CORTICAL SURFACE (lowers impedence)
Electrode grids are used for presurgical evaluation, to zero in on locations of abnormal tissue, and to ensure that areas serving critically important functions (e.g. language) are not cut out during surgery.
“Opportunistic” research studies
Hard to do with EEG bc the currents pass through path of least resistance so currents get all mixed up so use arrays of electrodes to better localise
How is MEG useful for motor imaging?
Record neuronal activity basically when it happens (good temporal resolution) and still maintain good spatial resolution, can localise activations
Used to see if ppl have ex locked in syndrome by having them imaging motor tasks
How do we do Statistical analyses of functional brain images?
- The size and shape of the human brain is highly variable; in real objective space in 3D, activations are in diff places in everyones brain bc brain diff shape and size
- In order to perform statistical analyses of functional images, individual brains must be normalised to a standard size and shape
- This is done by digital warping and rotating of the images
- 2 standard template brains:
- Talairach atlas – based on a single well-characterised brain
- Montreal Neurological Institute (MNI) atlas – based on average of several hundred MRI scans
What are prominent applications of neuroimaging in neurology?
Structural MRI, CT
• Lesions, tumors, brain damage
Functional MRI
• Clinical usage still lags research usage
• Patients more difficult to scan than healthy experimental subjects, lesions may affect blood flow
• Presurgical and radiotherapy planning (identification of language and other crucial centres)
MEG
• Localisation of epileptic foci, presurgical functional mapping
• Only about 1500 MEG scanners world-wide, about ½ in hospitals
