Neuroimaging techniques Flashcards
brain system
is a collection of structures in the brain that work together to perform a common function
brain network
the structure of circuits that connect areas of neurons together
systems neuroscience
an analysis of how systems bring about change in behaviour
what neuroimaging method uses X-ray radiation
CT
what neuroimaging method uses radio waves?
MRI
what neuroimaging method uses radiolabelling?
PET
Computed tomography (CT)
tomography is imaging by creating slices using any kind of penetrating wave. this forms a 3D picture.
x-rays
are absorbed to different degrees by tissues of different densities - dense tissues absorb the most x-rays so the photographic film would be minimally exposed.
CT machinery
CT is composed of radiation of a source of x-rays, and a detector separated 180 degrees from the sources. X-ray images of very thin slices are integrated with a computer to form an image
CT hierarchy
Air (darkest=least dense) < fat < CSF < white matter < grey matter < blood from haemorrhage < bone (brightest=most dense).
advantages of CT
good for showing acute bleeding or fracture of the skull
relatively quick
cheap
less intimidating for people
disadvantages of CT
structure only, not function
not good for detail in the brain because of minimal contrast between areas
dose of radiation
Positron Emission Tomography (PET) method
radiation is emitted from a radioisotope injected intravenously is registered by external detectors. positrons from isotopes travel short distances, combine with electrons, and annihilation results in energy release picked up by the detector.
what can PET be used for?
to highlight areas of increased metabolism of glucose
advantages of PET
functional
can identify and characterise tumors and benign/melignant
disadvantages of PET
poor resolution of brain tissue
area localized is only approximate
requires radiation dose
four general principles of MRI
- magnetic field aligns hydrogen atoms
- radiofrequency pulse is applied and moves the hydrogen atoms out of alignment
- measure the time it takes for the hydrogen atoms to recover their alignment and spin after the pulse is turned off
- the recovery times are mapped to form an anatomical image.
what is the idea behind MRI?
hydrogen atoms in different tissues take different amounts of time to recover
MRI tissue hierarchy
Fat (fastest recovery=brightest) → white matter → grey matter → CSF (slowest recovery=darkest)
units of MRI images
voxel - volume pixel
how do CT and MRI compare when it comes to scanning method?
CT - quick and less scary
MRI - takes much longer, confining/noisy
how do CT and MRI compare when it comes to quality?
CT - inferior for brain detail
MRI - superior for brain detail
how do CT and MRI compare when it comes to bone and blood?
CT - preferable for rapid assessment if bleeding; good for identifying skull fractures
MRI - inferior
T1 MRI
recovery time for the magnetic vector of the hydrogen atoms to return to resting alignment after the radiofrequency pulse
what is T1 useful for?
highlighting fat in the brain
T2 MRI
recovery time for the axial spin of the hydrogen atoms to return to the resting state after the radiofrequency pulse - can produce almost opposite effects to T1
what is T2 useful for?
investigate water in the brain
T2-flair
free-flowing fluid (CSF) is dark while non-free-flowing fluid is bright
what does flair stand for?
fluid attenuation inversion recovery
what is T2-flair used for?
distinguish between CSF and edema fluid
Water is bright in what type of scan?
T2 MRI
diffusion MRI
local magnetic fields applied to the head at different angles. high vision atoms move in different directions, visualizing the movement of water along tracts
anisotropy
the degree to which hydrogen ions travel along white fibre bundles
cohesiveness or integrity are used to measure…
white fibre bundles
isotropic
when hydrogen atoms are not constrained, anisotropy = 0
anisotropic
when hydrogen atoms are constrained, anisotropy = 1
fractional anisotropy
has a range between 0 and 1.
it is the practical assessment of the degree of white matter integrity
diffusion tractography
the 3D representation of fractional anisotropy: build and identify white matter tracts based on the different directionality FA.
what is diffusion tractography useful for?
visualizing the difference between long and short projection fibers, as well as long and short association fibers
Functional MRI
T2*-weighted scan. the adjusted scan parameters are sensitive to deoxygenated hemoglobin. rapidly repeated scans measure BOLD signals. regions where there is increased blood flow and reduced conc of dHb are the active regions.
BOLD
blood oxygen level-dependent
task-functional MRI
when participants perform a task while in the scanner
resting-functional MRI
when participants rest in the scanner without thinking about anything for 10 mins
diffusion tensor imaging
maps the functional anisotropy of a fibre bundle.
when during the lifetime does FA increase?
during adolescence
the smaller the FA…
the less organized the white matter
advantages of MRI
Detailed anatomical structure
Detect pathology with anatomical precision
Measure white matter integrity
Ability to assess brain function and functional connectivity at rest and during tasks
Does Not involve radiation, is relatively safe and individuals can be repeatedly scanned
Advantages of MRI compared with CT
CT is anatomically poor contrast, cannot measure white matter integrity, cannot give a functional measure
Advantages of MRI compared with PET
PET is anatomically poor, less available; MRI does not need a radioactive tracer injected
Disadvantages of MRI
Susceptibility to magnetic interference - can result in poor signal quality and can be a patient safety issue
Requires significant technical expertise due to the physics, computation and statistical processing
Diffusion and functional imaging are approximate, but are not a direct measure of white matter/ functional brain activity
Disadvantages of MRI compared with CT
cannot visualise bone or acute tissue bleeding; relatively expensive, longer procedure, can be frightening due to confinement
Disadvantages of MRI compared with PET
much more difficult to accurately quantify metabolic or molecular processes
