Optical Imaging (NIRS), Nuclear Medicine (PET/SPECT) & Invasive Methods

NIRS (near infrared spectroscopy) and the principle behind it

principle optical tomography: an object can be reconstructed by gathering light transmitted through it. Skin, tissue and bone are largely transparent to near infrared light. Light of skull, measure reflection of light. Reflectance provides info about brain activity (3-4 cm, penetration depth of light is proportional to source-detector distance in a banana shape) Light is directed to skull by set of small photo emitters. Reflectance is picked up by set of photo detectors

What can NIRS differentiate?

Oxyhemoglobin and deoxyhemoglobin are strong absorbers of light but differ in their absorption spectra that’s why they use light with different wavelengths.. 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.

FNIRS usage

Neurodevelopment: brain function in infants and children, atypical development, neurodevelopment disorders or motor tasks/walking e.g. correlating of speech intelligibility in 6-13 years old children

PET (positron emission tomography)

Measuring metabolism, detection of bio markers and neurotransmitter concentration. Aka isotope emits positron which in their place collide with electron emitting 2 photons/gamma rays in opposite direction

Coincidence events

Pair of photons released in opposite direction. Detection of 2 such photons and localization original position annihilation (along straight line) by PET camera and 3D image construction

Typical PET experiment

Low number of conditions (4-8), 2 blocks per condition, in between blocks is a short waiting period with new injection

What isotope in PET is used for neural activity

oxygen-15 (short half-life of 2 minutes), attached to blood

What isotope in PET is used for metabolism

fluorine-18 (half-life 110 min) attached to glucose (fluorodeoxyglucose DFG). Used to diagnose cancer, brain diseases. Or hypo metabolism of temporoparietal region in Alzheimer’s. Other compounds as biomarkers for plaques and tangles

Target-specific neurotransmitter systems

tracer attached to molecule with concentration related to activity of one specific neurotransmitter

Neurotransmitters used specific PET

DA: reward processing, cognitive control, memory formation, NE: arousal, attention, processing of salient events, memory formation and SE: mood, emotion, reward processing, impulsivity

Problem only with (neuro)transmitter tracers PET

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

SPECT (single-photon emission computed tomography)

Simpler form of PET. Readily available, no cyclotron required. Gamma photon is emitted by radioisotope and detected by simple gamma camera only capturing 1 photon

Invasive electrophysiology

choice of electrode depends on clinical need and anatomical objective. Especially in epilepsy patients not responding to medical treatment. Electrodes removed prior to surgery.

Electrocorticgraphy (ECoG)

use of electrodes placed directly on the exposed surface of the brain to record electrical activity from the cortex

Stereo-electroencephalography (SEEG)

recording electrocephalographic signals via depth electrodes

Micro stimulation (causal)

Electrode inside neural tissue (only performed when electrodes are crucial for clinical purposes). Small electrical current influences the activity of neurons near the tip, the larger the current, the wider the affected area and the stronger the influence on nearby neurons. Too strong results in damage. Strong activated action potentials but weak changes membrane potential

DBS (Deep brain stimulation)

is micro stimulation where electrodes in deep structures of brain, more permanently implanted (e.g. in Parkinson’s, severe depression and OCD)

Neural pacemaker

repetitive and chronic stimulation across long period of time. As a treatment through various mechanisms (enhancing/inhibiting/synchronizing activity)