TASK 5 - fMRI Flashcards
structural imaging
= different types of tissue have different physical properties, used to construct static maps of physical structure of the brain
CT
= computerised tomography = amount of X-ray absorption in different types of tissue
- amount of absoption is related to tissue density
- exposure to small amount of radiation
- typically used in clinical settings
- cannot distinguish between grey matter and white matter + cannot be adapted for functional imaging
MRI
= magnetic resonance imaging = create images of soft tissue of the body; amount of water in each type of tissue varies
- different types of tissue behave differently when stimulated
- construct 3D image of the layout of tissues
- strong magnetic field is applied across part of body being scanned –> sends radiofrequency in (aligns protons) and collects output radiofrequency
Tesla
= strength of magnetic field of scanner
- the higher the Tesla (= the stronger the magnetic field of machine) –> the finer detailed the image
MRI
- physical basis
= uses the spinning protons of the hydrogen nuclei in water
- -> spinning: spin around north-south axis and they are oriented according to some direction
- hydrogen nuclei has only one proton with 1 water molecule on each side
- water is to some extent present in all tissue types
MRI
- precesssion
= what we use for measurement
- radiofrequency = speed of precession spins
- speed of precession is proportional to strength of magnetic field
MRI
- Lamar equation
= precession frequency (how fast the protons spin) is dependent on strength of magnetic field
- important to understand how images are formed
MRI
- alignment
= when radiofrequency pulse administered, the protons align and their magnetic fields add up
- all protons are synchronised
1. aligned to B0 (magnetic field of scanner) 2. aligned horizontally - pulse must fit to magnetic field, according to Lamar equation
- magnetic field stays in horizontal position for some time
MRI
- free induction decay
= FID = after each pulse, protons realign themselves with the magnetic field of the person
- produces an electromagnetic echo
- how fast a signal decays/dephases depends on homogeneity of magnetic field in the nighbourhood
relaxation times
- T1
= longitudinal relaxation time = time constant which determines the rate at which excited protons return to equilibrium
- measure of time taken for spinning protons to realign with external magnetic field (B0)
- -> both relaxation times (T1 + T2) depend on tissue type
relaxation times
- T2
= transverse relaxation time = time constant which determine the rate at which excited protons reach equilibrium/go out of phase
- measure of time taken for spinning protons to lose phase coherence among nuclei that are spinning perpendicular to main field
- -> both relaxation times (T1 + T2) depend on tissue type
MRI
- spatial encoding
= how we create an image, determine strength of signal at each frequency (= position)
- apply gradient in x direction: spins/precession frequency depend on their position along the gradient
- spatial info is then frequency-encoded; assign amount of signal to spatial locations - time domain signal sums all frequencies
- Fourier analysis: decomposes signal and shows amount of signal for each frequency
advantages of MRI over CT
- does not use ionising radiation (can be scanned multiple times)
- better spatial resolution: discrimination of individual gyri
- better discrimination between white and grey matter: enables early diagnosis of some pathologies
- can be adapted to functional imaging (fMRI)
functional imaging
= neural activity produces local physiological changes in that region, produce dynamic maps of the moment-to-moment activity
hemodynamic response methods
= hemodynamic response = when the activity of neurones increases, the blood supply to that region increases relatively to others (providing it with more glucose + oxygen)
- -> PET measures the change in blood flow directly (and the supply of different molecules to that region)
- -> fMRI measures the concentration of oxygen in the blood
PET
= positron emission tomography = measure local variations in cerebral blood flow that are correlated with mental activity –> increased blood flow to the brain regions that have heightened neural activity
- indirect measure
PET
- method
- radioactive substance is introduced/injected into the bloodstream (= tracer)
- possible to use radio-labelled neurotransmitters to investigate particular pathways + effects of drugs - radiation emitted from this tracer is monitored by the PET instrument
- radioactive isotopes within the injected substance decay by emitting a positron from their atomic nuclei
- positron collides with electron –> two photons (gamma rays) are created
- gamma rays move in opposite directions at speed of light - PET scanner (= gamma ray detector) determines where the collision took place
fMRI
= functional magnetic resonance imaging = measure the ratio of oxygenated to deoxygenated hemoglobin (= BOLD signal)
- similarity MRI: imaging focused on the magnetic properties of the deoxygenated form of hemoglobin
- allows to image time course of brain activity
- indirect measure: based on neuronal metabolic processes
fMRI
- physical properties
- when area becomes more activated/stimulated, the area consumes more energy (sources: oxygen, glucose)
- -> more active = more energy required (food) - neuronal tissue is receiving energy from oxygenated haemoglobin in the blood
- -> haemoglobin has the oxygen attached to it (in a bag) and brings you the food/energy - when neurones absorb the energy (=oxygen), the haemoglobin becomes deoxygenated
- -> you eat all the food, and haemoglobin leaves without oxygen attached to it - concentration of oxygenated haemoglobin in the blood increases in the active area, because the area requested it
- -> haemoglobin/blood delivers too much oxygenated haemoglobin (too much food)
fMRI
- BOLD
= blood oxygen-level-dependent contrast = paramagnetic properties of deoxygenated haemoglobin introduce distortions in the magnetic field –> distortions indicate the concentration of deoxygenated haemoglobin in the blood
- which indicate that energy has been consumed?
- -> if there is low concentration of deoxygenated haemoglobin in the blood, the area needed a lot of energy (= was active)
fMRI
- deoxygenated + oxygenated haemoglobin
- deoxygenated haemoglobin (Hb)
- is paramagnetic = distorts the magnetic field
- when area inactive, Hb concentration is high –> disturbs homogenous magnetic field of scanner –> fast dephasing –> low (f)MRI signal - oxygenated haemoglobin (HbO2)
- is demanded when neurone becomes active (energy) –> HbO2 concentration is high (less Hb) –> less distortion; more homogenous magnetic field –> dephasing takes longer –> stronger signal over time
- -> precessing frequencies differ
hemodynamic response function
= HRF = changes in the BOLD signal over time; way that the BOLD signal evolves over time in response to an increase in neural activity
- initial dip: neurones consume oxygen –> small rise in amount of deoxygenated haemoglobin (that was already in the region) –> reduction of BOLD signal
- will always limit our temporal resolution
- -> you are so hungry, that you aren’t hungry anymore - overcompensation: increased consumption of oxygen, blood flow to the region increases –> increase in blood flow is greater than the increased consumption = BOLD signal increases
- -> you eat - undershoot: blood flow and oxygen consumption dip before returning to their original levels –> temporal increase in deoxygenated haemoglobin (oversupply: area does not require that much energy anymore)
- may reflect relaxation of blood system
- -> you are shortly tired after eating a lot
- HRF relatively stable across sessions with same participant in same region; variable across different regions within same individual and more variable between individuals
- different HRF can be superimposed on each other
data analysis
- preprocessing
= needed to improve signal-to-noise ratio
preprocessing
- motion correction
= compensate for small head movements
- motion can cause voxels to locate differently
- after recording, before data analysis
a) detection: if we detect misplaced area and we know how much it changed, we can rotate it along x, y, z axes
b) correction: undoing detected motion; spatial interpolation is necessary