HC 8 Flashcards
What is NIRS?
Near infrared spectroscopy.
The principle of optical tomography is that an object can be reconstructed by gathering light transmitted through it.
The requirement is that the object transmits at least partially light. The use of near infrared light is between 650 and 1000nm.
Skin, tissue and bone are largely transparent to near infrared light.
It is a not so massive and cheaper machine than MRI.
How does NIRS work?
Light on the skull and then measure the reflection of light. Reflectance provides information about the activity in the brain. Light is directed to the skull by a set of small photo emitters. Reflectance is picked up by a set of photo detectors.
Penetration of the depth of light is proportional to source-detector distance.
Oxyhemoglobin (HbO2) and deoxyhemoglobin (HbR) are strong absorbers of light, but differ in their absorption spectra. Smaller than 800nm, HbR absorbs better and more than 800nm, HbO2 absorbs better. Use light with different wavelengths.
What is fNIRS?
Functional near infrared spectroscopy.
Concentration of HbO2 and HbR change due to neural activity. This causes the reflection of light to change.
How does fNIRS work?
Reconstruct the object with the light that shines through it. Different light absorbtion depending on how far the detectors are from the source. It is cheaper than MRI.
How does fNIRS relate to fMRI?
It picks up the same BOLD contrast, oxyhemoglobin and HRF that is the basis of fMRI.
How can fNIRS be applied?
It can be used with infants and children, since it is less sensitive to motion.
How does fNIRS compare to fMRI?
Advantage:
+Portable, cheaper, silent, more practical and less intrusive. Good tolerance to motion artefacts.
+Measures both oxygenated and deoxygenated hemoglobin levels
+Better temporal resolution. Higher sampling rate
Disadvantage:
-No fine spatial localization. The main cause is because bones scatter light (both the transmitted and reflected.)
-Measures only superficial activity. No reflectance from within sulci or deeper structures
What is PET?
Positron Emission Tomography. It is an imaging technique that helps visualize metabolic processes in the body, providing valuable information for diagnosing and managing various diseases. It is much more expensive than MRS.
It is an unique contribution relative to fMRI, measuring metabolism and can detect biomarkers and neurotransmitter concentrations.
Positron emission involves injection of radioactive tracers. It is not an injection of the isolated isotope, but attached to a molecule with specific biological action. Molecule and site injection determines spread of the tracer.
What are the principles of PET?
- Radiotracer injection: a small amount of a radioactive substance called a radiotracer is injected into the patient’s bloodstream.
- Radiotracer uptake: the radiotracer circulates through the body and accumulates in tissues with high metabolic activity.
- Positron Emission: The radioactive isotope in the tracer decays by emitting a positron which is the antimatter counterpart of an electron.
- Annihilation event: when a positron encounters an electron in the body, they annihilate each other.
- Detection: the PET scannen consists of a ring of detectors that surround the patient. They pick up the gamma photons produced by annihilation events.
- Image reconstruction: the PET system uses the timing and location of the detected gamma photons to reconstruct a three-dimensional image of the tracer distribution in the body. This process involves complex algorithms that compute likelihood of where annihilation events occured.
What are the steps of a PET-Scan procedure?
- Preparation: The patient may be asked to fast to ensure low blood sugar levels, which helps accuracy of the test.
- Radiotracer administration: the radiotracer is injected and the patient typically waits to allow the tracer to distribute and accumulate in the target tissues.
- Scanning: the patient lies on a table that moves through the PET scannen, which captures the gamma photons emitted from the body.
- Data processing: The collected data is processed by a computer to generate detailed images of the tracer concentration in different tissues.
- Interpretation: interpret images to identify areas of abnormal metabolic activity.
What are coincidence events?
Fundamental to the operation of PET scannes, enabling the accurate localization of metabolic activity within the body and generation of high-resolution images.
What are half-lifes in PET?
The half-life of a radioisotope in PET is a key parameter that influences the timing, logistics and clinical utility of the imaging procedure. It determines how long the radiotracer remains active in the body, affecting both the quality of the imaging data and the radiation exposure to the patient.
Because they are short, conditions in blocks also need to be short. Different transmitters have different half-lifes.
How is oxygen-15 used to measure neural activity?
Short half-life of 2 minutes. The distribution is a linear relationshop to incoming blood volume. The total amount of oxygen in a brain region is an indication of local neural activity, because of the over-supply of oxygenated blood following neural acitivty.
How does a typical PET experiment look like?
-Low number of conditions (4-8)
-Conditions are typically tested in blocks of around 1 minute
-Often only 2 blocks per condition
-In between blocks short waiting period with new injection
What are some unique contributions of PET?
Measures metabolism. The tracer fluorine-18 is attached to glucose. This is called fluerodeoxyglucose (FDG). This can be used to diagnose cancer and brain diseases.
PET targets specific neurotransmitter systems. Tracer is attached to a molecule with concentration related to activity of one specific neurotransmitter.
