Lecture 8 - Neuroimaging Flashcards
Brain Imaging: Anatomy
CAT
Photograph
PET
MRI
Structural Brain Imaging – CT scans
- Computerized Axial Tomography (CAT scan or CT).
- X-rays passed through the head at many different angles.
- Different tissues (brain, CSF, white and grey matter) have different densities – therefore a different proportion of the X-ray is absorbed and an image can be formed.
Details about the Structural Brain Imaging – CT scans
- Spatial resolution not so good. (lil fuzzy)
- Lesions often take some time (hours) to become evident on CT.
Good at picking up:
- tumour
- brain injury
- easily accessible
Structural Brain Imaging – MRI scans
- Spatial resolution very good. (much stronger)
- Immediate imaging of lesions.
small lesion would pop out right away
CT vs. MRI
MRI is clearier
Hydrogen protons and MRI
huge electromagnetic
- magnetizes your entire body
- Hydrogen protons most readily available atoms in human body.
- Protons spin around a given axis.
spin in same direction –> magnetized the image
Radio Frequency and Resonant Frequency
- Protons precess around the main magnetic axis (spinning top analogy).
- The frequency of their precession is dependent on the strength of the magnet.
- A RADIO FREQUENCY (RF) pulse is used in MRI to push protons out of alignment with the magnetic field.
Protons pushed out of alignment with the magnetic field (left)
– longitudinal relaxation refers to the time it takes for them to come back into alignment with the magnetic field (right).
- Align protons
- Add energy
- Recording how that energy comes off of them & travels through the brain & diff. tissues & creating an image out of that diff. in energy field
A T1 weighted image
Different types of tissue approach equilibrium at different rates allowing us to differentiate things like white and grey matter.
Angiogram
- measures the structure of the cerebral blood supply.
- used to image aneurysms (balloon - potential brain burst) and other vascular defects (e.g., arteriovenous malformations).
- twists/knots in capillary blood supply
- X-ray in combination with contrast agents.
- made blood look black
Anatomy vs. Function
Brain anatomy
CT & MRI
Brain function
EEG, PET, fMRI
Functional Neuroimaging
Electrical activity
Metabolism
Electrical activity (v. direct)
- Event-related potentials (ERP), visual evoked potentials (VEP) all derivative from electroencephalogram (EEG).
Metabolism (indirect measures of brain activation)
- Positron Emission Tomography (PET) and Blood Oxygenated Level Dependent (BOLD) functional MRI (fMRI).
Electroencephalogram (EEG)
- Large populations of neurons firing produce electrical potentials that can be measured at the SCALP.
- Brain, skull and scalp passively conduct signals that can be amplified and measured.
- Important tool for diagnosis in epilepsy and sleep disorders.
ERPs and VEPs
- EEG tends to record global brain activity. (spatial resolution isn’t the greatest)
- ERPs (and VEPs) are a special case of EEG.
- average EEG trace from a large number of trials.
- align signal to onset of a stimulus or response – hence event-related potential (ERP).
Pros and cons of ERPs
Pros
- Good temporal resolution. (how fast can I detect a change?)
- Good at linking specific physiological markers to cognitive processes. (how long after do you see a spike after seeing a face)
Cons
- Poor spatial resolution.
- Difficult to get at some brain regions (e.g. temporal cortex).
The First “Brain Imaging Experiment”
Angelo Mosso
Italian physiologist
(1846-1910)
“[In Mosso’s experiments] the subject to be observed lay on a delicately balanced table which could tip downward either at the head or at the foot if the weight of either end were increased. The moment emotional or intellectual activity began in the subject, down went the balance at the head-end, in consequence of the redistribution of blood in his system.”
*more blood went rushing to head when they were doing complex math problems
Positron Emission Tomography (PET)
- Measures local changes in CEREBRAL BLOOD FLOW (CBF) by utilizing radioactive TRACERS that rapidly decay and emit positrons.
- When the positrons collide with electrons, two photons travel in opposite directions allowing the location of the collision to be determined.
PET and subtraction
Run two conditions – stimulation (e.g., look at visual images) vs. control (e.g., look at blank screen).
Measure the difference in activation between the two images (i.e., subtract control from stimulation).
This provides a picture of regional cerebral blood flow relative to visual stimulation.
Motion vs. colour
Subject views coloured screen (left) vs. moving random black and white dots (right).
Both task activate early visual areas (V1 (primary visual cortex) and V2).
Subtracting the two images reveals different brain areas for colour (V4) vs. motion (V5) processing.
The Rise of fMRI
746 papers (2001)
44, 497 papers (2020)
that utilize fMRI
fMRI
popular with scientists & marketing (to see if they like a companies logo or something - it’s questionable)
The Big Magnet
Very strong 1 Tesla (T) = 10,000 Gauss Earth’s magnetic field = 0.5 Gauss 3 Tesla = 3 x 10,000 divided 0.5 = 60,000X Earth’s magnetic field (v. strong magnetics)
Continuously on
Main field = B0
x 60,000 =
3T magnet
Magnet safety
very strong magnetic fields – even large and heavy objects can ‘fly’ into the magnet bore.
Screen subjects carefully.
Develop strategies for screening yourself every time you enter the magnet.
Man the torpedoes.
MRI vs. fMRI
MRI - high resolution (1mm)
- 1 image
fMRI - low resolution (~3 mm but can be better)
- many images (e.g., every 2 sec for 5 mins)
fMRI
Blood Oxygenation Level Dependent (BOLD) signal
indirect measure of neural activity
↑ neural activity → ↑ blood oxygen → ↑ fMRI signal
Functional MRI (fMRI)
Blood Oxygenation Level Dependent (BOLD) contrast
At Rest “Baseline”
oxyhemoglobin/deoxyhemoglobin <
During Task “Activated” oxyhemoglobin/deoxyhemoglobin
fMRI doesn’t like deoxy
fMRI
Functional images are subtracted from one another.
The differences in the two images are then superimposed on the anatomical image.
fMRI spatial resolution
images can be co-registered to the subject’s own brain (not an average brain as in PET)
PET vs. fMRI
PET allows you to track multiple metabolic processes so long as the emitted photon can be detected – allows imaging of some neurotransmitters.
PET is invasive – radioactive isotopes can only be administered (at experimental levels) every 4 – 5 years.
fMRI has much greater spatial resolution (≈ mms).
fMRI has greater temporal resolution – can detect activation to stimuli appearing for less than a second (PET is limited by the half life of the isotope used).
DTI
Diffusion tensor imaging (DTI) allows us to measure microscopic movement of water in the brain. This makes it possible to visualize the location, orientation, and direction of the brain’s white matter tracts.
Transcranial Magnetic Stimulation (TMS)
TMS applies a magnetic pulse to a certain brain region to TEMPORARILY MODULATE the function of that region.
Pros and Cons of TMS
Pros
- Good temporal resolution. (has effect right away)
- Can presumably disrupt individual processes within a task. (disrupts speech for ex, during talking)
- Potential combination with other imaging techniques.
Cons
- Poor spatial localization – how focal is the stimulation?
- Can’t stimulate certain areas (e.g., temporal lobe – orbitofrontal cortex) and can only stimulate cortical surface.
- Distance effects – changed interactions due to stimulation.
- Can induce seizures (particularly rTMS).
Combining TMS and other imaging techniques
Areas can be identified functionally and then used to position a TMS coil.
Or, they can be used at the same time.
TMS coil inside the fMRI head coil.
Using fMRI to position a TMS coil
