Unit 4 - Imaging Flashcards
What is structural imaging?
Measures the spatial configuration of different types of tissue in the brain, as different types of tissues have different physical properties.
Produces detailed static maps of the physical structure of the brain.
What is functional imaging?
Measures temporary changes in brain physiology associated with cognitive processing, since neural activity produces local physiological changes in different regions.
What is CT? What does it measure? What can it not distinguish, and to what can it not be adapted? What is one risk of it?
Computerised Tomography, structural imaging technique
Measures the amount of X-ray absorption in different types of tissues, with the amount of absorption being related to density.
Struggles to distinguish between grey and white matter, and cannot be adapted for functional imaging purposes.
Radiation exposure
What is MRI? What are its advantages of CT? How does its spatial resolution compare to CT?
Magnetic Resonance Imaging
Can be used as a functional imaging method; Does not produce damaging radiation; Can discriminate better between grey and white matter
Better spatial resolution than CT
What is the physics behind MRI?
Most tissues contain lots of water molecules. These molecules have protons, which have a randomly oriented small, magnetic field. When a strong external magnetic field is applied, some of these protons’ magnetic fields align with the external field. Then, a radio pulse is sent, causing the proton to change orientation by 90 degrees. This change can be detected by the magnetic field, leading to the MR signal. Repeating this gets images for all slices.
What are the two types of MRI images?
T1-weighted images - based on variations in the rate at which protons return to their aligned state after the radiofrequency pulse (relaxation time)
-primarily used for structural imaging
T2-weighted images - measures the signal decay in the misaligned state (90 degrees turned) due to interactions with other molecules
-e.g., deoxyhemoglobin distorts
the T2 component
-basis for functional imaging
What is a hemodynamic method?
Techniques used to measure and analyse changes in blood flow and blood oxygenation levels within the brain or other tissues.
What does functional imaging often make use of? Why is this effective? Why is there a need for a baseline in such cases?
The bloodstream and blood oxygenation levels.
Oxygen and nutrient supply increased when neurons are active
Since neurons are always provided with blood, meaning that active regions need to be compared to their average blood flow and oxygenation levels.
What are two hemodynamic methods?
Positron Emission Tomography (PET), which measures blood flow to a region directly by injective a radioactive tracer into the blood.
Functional Magnetic Resonance Imaging (fMRI) and Functional Near-Infrared Spectroscopy (fNIRS), which measure blood oxygen concentration.
What is voxel-based morphometry (VBM)? What imaging technique uses it?
A technique for segregating and measuring differences in white and grey matter concentration.
Divides the brain into thousands of voxels and estimates the concentration of grey/white matter in each of them
Used by MRI
What is a voxel?
A volume-based unit (in 2D pixels) into which the brain is divided in imaging research
What is diffusion tensor imaging (DTI)? What imaging technique uses it?
Measures white matter connectivity in addition to just the amount.
Is based on water molecules trapped in axons that can only travel down the length of an axon but prevented from travelling out of the axon by the fatty membrane
When many such axons are arranged together, it is possible to quantify this effect.
Used by MRI
What is fractional anisotropy? What is it used by?
A measure of the extent to which diffusion takes place in some directions more than others.
Used by diffusion tensor imaging
What is Functional MRI? How does it work?
When neurons consume oxygen during neural activity, they produce a metabolic byproduct called deoxyhemoglobin. Deoxyhemoglobin is paramagnetic, meaning it distorts the local magnetic field. In the blood vessels near active neurons, the presence of deoxyhemoglobin causes a local magnetic field inhomogeneity. This magnetic field distortion leads to changes in the MR signal detected by the fMRI scanner. By measuring these changes in the MR signal, fMRI can infer areas of increased neural activity in the brain.
What is BOLD?
Blood oxygen-level-dependent contrast is the signal measured in fMRI that relates to the concentration of deoxyhemoglobin in the blood.
What is the Hemodynamic Response Function HRF?
Changes in the BOLD signal over time
What are the three phases of HRF? Which of the three stages is normally measured in fMRI and what does it represent?
Firstly, the initial dip. Neurons consume oxygen, so there is a rise in the amount of deoxyhemoglobin, leading to a reduction of the BOLD signal.
Then, there is the overcompensation. The increased consumption of oxygen leads to an increased blood flow in that region, and a BOLD signal increase because the increase in blood flow is greater than the increased consumption.
Lastly, the undershoot. Blood flow and oxygen consumption dip before returning to their original level, causing another temporary increase in deoxyhemoglobin.
The overcompensation is normally measured in fMRI, with the size of the peak indicative of the extent to which the region is active in the given task.
How is the temporal resolution of fMRI? How about its spatial resolution?
Poor temporal resolution
Very good spatial resolution
What is Functional Near Infrared Spectroscopy (fNIRS)?
Also measures the BOLD signal, but does not use magnetic fields.
Instead, it sends light of a particular wavelength to the brain. The signal passes freely through bone and skin but is more strongly scattered by oxyhemoglobin and deoxyhemoglobin. The extent to which signal is scattered is used to compute the BOLD response.
Larger BOLD response reflects more cognitive and neural activity.
