fNIRS PET-SPECT Flashcards
Near Infrared Spectroscopy NIRS
Principle optical tomography = an object can be reconstructed by gathering light transmitted through it
Requirement: object transmits at least partically light
- use of near infrared light = 650 - 1000 nanomill.
- skin, tissue and bone are largely transparent to near infrared light
NIRS
- not som massive and cheaper than MRI
- light on skull, measure reflection of light
- reflectance provides info about brain activity
- small photo emitters direct the light to the skull
- reflectance is picked up by photo detectors
Banana shape
Penetration depth of light is proportional to source-detector distance and the light will go in a banana shape
How does NIRS say something about neural activity
Oxyhemoglobin and deoxyhemoglobin are strong absorbers of light, but differ in their absorption spectra
- <800 nm: HbR > HbO2
- >800 nm: HbO2 > HbR
–> hemodynamic method
Functional near infrared spectroscopy
Concentration HbO2 and HbR change due to neural activity –> reflection of light changes
- picks up the same BOLD contrast and HRF that is the basis of fMRI
NIRS system
- single-channel system and multi-trial stimulation
- multi-channel system and single-trial stimulation
- high-density system and single-trial stimulation
fNIRS and fMRI
There is a high correspondance fNIRS-fMRI at group level
- no fine spatial localization with light, because bone scatters light (in and then again when out)
- fNIRS measures only superficial activity, no deeper structures
- fNIRS is portable, cheaper, silent, more practical and less intrusive. It also has good tolerance to motion artefacts
- fNIRS measures both oxygen levels separately
- fNIRS better temporal resolution (hemodynamics stay the same, so how is it better –> higher sampling rate)
Applications of fNIRS
Neurodevelopment
- brain function in infants and children
- atypical development
- neurodevelopment disorders
–> specific for children because you can reach deeper brain regions, since the brain is smaller
–> less light scatter, because of the flexibility of the skull
–> don’t have to stay still - ADHD
Cortical pertubations in psychiatric conditions
Motor tasks/walking
- neurorehabilitation tool
- BCI (motor disabilities)
Positron Emission Tomography PET
Dominant hemodynamic imaging method in the 80s
- unique contribution relative to fMRI (measuring metabolism and detection of biomarkers and neutrotransmitter concentrations
- positron emission
Positron emission
Involves injection of radioactive tracers
- injection not of isolated isotrope, but attached to a molecule with specific biological action
- molecule and site of injection determines spread of the tracer
- radionuclides: short half-life = positron emission decay
PET procedure
Tracer production and injection
- radioisotope decay, positron emission adn annihilation (interaction positron and electron) => pair of photons released in opposite direction
- detection of 2 such photons and localization original position annihilation by PET camera and 3D construction based on coincidence events
PET to measure neural activity
Oxygen-15
- short-half life of 2 minutes
- distribution: linear relationship to incoming blood volume
- total amount of oxygen in a bran region: indication of local neural activity
Typical PET experiment
- low number of conditions (4-8)
- conditions are typically tested in blocks of around 1 minutes
- often only 2 blocks per condition
- in between blocks: short waiting period with new injection
Unique contributions of PET
Metabolism: tracer fluoride-18 (half life of 110 minutes) attached to glucose = fluorodeoxyglucose (FDG)
- cancer diagnosis
- brain disease diagnosis (alzheimer or plaques/tangles for example.)
Target-specific neurotransmitter systems
- tracer attached to molecule with concentration related to activity of one specific neurotransmitter
Important neurotransmitters
Amino acids: GABA, glutamate, glycine
Monoamines: dopamine, noradrenaline, serotonin
- norepinephrine: arousal, vigilance, attention, processing of salient events and memory formation
- serotonin: mood, emotion, reward processing and impulsivity
- Dopamine: reward processing, cognitive control and memory formation
Transmitter-specific PET
- radio-labeled ligands (tracers)
- most common: carbon (C) and fluorine (F)
–> tracers compete with transmitters to bind to receptors
–> level of tracer binding inversely proportional to level of transmitter binding: higher tracer binding => lower transmitter binding
Single Photon Emission Computed Tomography SPECT
Simplet form of PET
- uses readily available, stable radioglands (no cyclotron required)
- gamma photon is emitted by radioisotope and detected by simple gamma camera
- half life is a lot longer though
Main difference SPECT and PET
Longer tracer half life
- images are processed over longer time
- limit on number of test conditions, but production and transport advantage
Lower spatial resolution (small set of sensors instead of ring)
–> picture less precise and lower clarity
Considerations in SPECT vs PET
Cost: PET> SPECT
Availability: SPECT > PET
Spatial resolution: PET > SPECT
Temporal resolution: PET > SPECT
Sensitivity: PET > SPECT
Signal to Noise ration: PET > SPECT
- Variety of ligands: PET > SPECT
PET and SPECT vs fMRI
+ PET and SPECT target synaptic neurotransmission directly
+ ability to measure blood volume quantitatively
+ confronted with less unknown parameters
- temporal and spatial resolutions are poorer
- more invasive (radioactivity)
- expensive
- limited to specific regions or unspecific in the distribution
