HC8 fNIRS, PET-SPECT Flashcards
What is Near-Infrared Spectroscopy (NIRS)?
An optical tomography method that reconstructs objects by gathering transmitted light, used to measure brain activity through light absorbance.
What light range does NIRS use?
Near-infrared light from 650 to 1000 nm.
How does NIRS measure brain activity?
It shines light on the skull, and photo detectors measure the reflected light. The absorbance of light indicates levels of oxy-hemoglobin (HbO2) and deoxy-hemoglobin (HbR).
Why is NIRS more practical than MRI?
- It is cheaper
- More portable
- Silent
- Has better tolerance to motion artifacts compared to MRI.
What determines the penetration depth of NIRS light?
The source-detector distance—the longer the distance, the deeper the light penetrates the brain, following a banana-shaped path.
How do hemoglobin types absorb light differently?
< 800 nm: HbR absorbs more light than HbO2.
> 800 nm: HbO2 absorbs more light than HbR.
What is functional Near-Infrared Spectroscopy (fNIRS)?
Measures the change in concentration HbO2 and HbR due to neural activity. The reflection of light changes. It picks up the same BOLD contrast and HRF which is the basis of fMRI. It can say something about causality compared to fMRI.
What are the steps of fNIRS hemodynamics?
Neural activation → metabolic demand → increased blood flow → increase in HbO2 → washout of HbR.
How does fNIRS compare to fMRI?
Similarities: Both measure BOLD contrast and blood flow changes.
Differences: fNIRS is cheaper, portable, and silent but has lower spatial resolution and only measures superficial activity.
Why does fNIRS have lower spatial resolution than fMRI?
The skull scatters transmitted and reflected light, reducing fine spatial localization.
Can fNIRS measure deeper brain structures?
No, it only measures superficial activity and cannot capture signals from deep brain structures like sulci.
Does fNIRS have better temporal resolution than fMRI?
Some claim it does due to a higher sampling rate, making it easier to estimate HRF onset, but actual temporal resolution remains similar.
What are common applications of fNIRS?
- Neurodevelopment: Studying infant/child brain function.
- Psychiatric conditions: Cortical perturbations in disorders like anorexia.
- Motor tasks: Walking and movement studies.
Why is fNIRS useful for studying children?
Their skulls are thinner, reducing light scattering, and fNIRS is quieter than MRI.
What did an fNIRS study reveal about emotional face processing?
Happy faces: Higher OxyHb in the left temporal cortex.
Angry faces: Higher OxyHb in the right temporal cortex.
How do younger and older people differ in brain activation in motor tasks?
- Younger people: More OxyHb in PMC.
- Older people: More OxyHb in SFG
What is Positron Emission Tomography (PET)?
Hemodynamic imaging method. It is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in metabolic processes, and in other physiological activities including blood flow, regional chemical composition, and absorption.
What makes PET unique compared to fMRI?
PET can directly measure metabolism and neurotransmitter concentrations, whereas fMRI only measures blood oxygenation (BOLD contrast).
Positron emission decay
The term “positron emission decay” refers to the decay process of the radionuclide, where it emits a positron (a particle with a positive charge) as part of this rapid decay. The emission of the positron eventually leads to the annihilation of the positron with an electron, producing two photons. These photons are detected in imaging techniques like PET to visualize metabolic processes or activity in the brain or other parts of the body.
What is the procedure for Positron Emission Tomography (PET)?
In PET, tracers are injected into the body. To create the tracer, a cyclotron is used to attach a radioactive isotope to a molecule, making it radioactive (unstable). This leads to radioisotope decay where the molecule emits a positron. The positron then annihilates with an electron, generating a pair of photons. These photons are emitted in opposite directions and are detected by a camera to create a PET image.
What is a coincidence event?
When two photons are detected at the same time in different channels, it’s called a coincidence event. This helps to localize where the annihilation took place. If no coincidence event is detected, it means there was no annihilation, which is also valuable information.
Why is Oxygen-15 often used in PET?
- It has a short half time of only 2 minutes. -
- This is a tracer which has a distribution which is linear to the relationship to incoming blood volume. The total amount of oxygen is a particular brain region is therefore an indication of the local neural activity.
What is the experimental design of PET?
- Block-design: Short testing blocks (1 min) followed by a waiting period.
- Requires multiple injections since PET tracers have a short half-life.
- Does not allow event-related designs.
How is PET used in diagnosing diseases?
PET is mainly used for measuring metabolism. For example, the tracer fluorine-18 attaches to glucose to form fluorodexoyglucose. This is used to diagnose cancer and brain diseases. Tumours use a lot of glucose, so there will be a lot of fluorodeoxyglucose there. Hypometabolism is an indication for Alzheimer’s disease.
Important neurotransmitters
Amino acids: GABA, glutamate, glycine. -
Monoamines: dopamine, noradrenaline, serotonine.
o Dopamine: for reward processing, cognitive control and memory formation.
o Noradrenaline: for arousal, vigilance, attention, the processing of salient events and memory formation.
o Serotonine: for mood, emotion, reward processing and impulsivity.
How does tracer binding relate to neurotransmitter binding and metabolism in PET?
The level of tracer binding is inversely proportional to the level of transmitter binding in neurotransmitter systems. This means that the higher the tracer binding, the lower the transmitter binding.
This relationship only applies to neurotransmitter systems. For metabolism, the higher the number of tracers, the higher the metabolism.
In cases of executive processing, more dopamine binding occurs, which leads to lower tracer binding.
Why is PET not a treatment method?
PET only provides information about brain function, such as detecting neurofibrillary tangles in Alzheimer’s disease but does not treat it.
What is Single-Photon Emission Computed Tomography (SPECT)?
A simpler form of PET that measures blood flow using gamma photons instead of positrons.
How does SPECT compare to PET?
Advantages: Cheaper, easier availability, no cyclotron needed.
Disadvantages: Slower signal processing, lower spatial resolution, less precise images.
When would you choose SPECT over PET?
Only when availability is the main concern. PET is superior in all other aspects.
Advantages of PET/SPECT over fMRI?
- Directly targets neurotransmission.
- Measures blood volume quantitatively.
- Fewer unknown parameters linking signals to neural activity.
Disadvantages of PET/SPECT compared to fMRI?
- Lower temporal and spatial resolution.
- More invasive (requires injection & radiation exposure).
- Expensive and tracers are region-specific.
