Topic 4 (Investigating the brain) Lecture 2 (Types of brain scanning) Flashcards
What does CT scan stand for?
Computerised tomography
Describe CT scanning
- The patient is placed in a tube
- The tube both transmits and detects Xray signals containing ionising radiation
- The x ray signals pass through the brain and what is not absorbed is detected by the detector
- Different tissues are able to absorb different amounts of the signal
Describe MRI scanning
- A person is moved into a strong magnetic field
- Hydrogen atoms in H20 the body react to this magnetic field by aligning with it and moving in the same frequency in the magnetic field
- Radio waves also move at the same frequency of the magnetic field
- Low energy H20 molecules do not automatically move with the magnetic field
- Therefore radio waves are used to make the low energy H20 molecules move alongside the magnetic field
- When the machine stops emmiting radio waves the low energy H20 molecules release and go back to their original position. This releases RF waves which are measured and interpreted by a computer
What are 4 uses of an MRI scan?
- Diagnosis
- Staging of a condition/ monitering progression
- Guiding brain surgery
- Superimposing with other types of imaging to gain understanding
What are 3 discomforts/ limitations of an MRI
- The patient has to lie very still and it can be claustrophobic
- It is very noisy in the scanner
- People with medical or metal implants e.g cochlear implants may not be able to have a scan
Describe Diffusion Tensor imaging
- DTI allows the mapping out of white matter tracts
This relys on the idea that water molecules do not diffuse at random in the brain.
Instead they take the path of least resistance which is along the neuronal pathways rather than across them
This is because myelination would make it difficult for water molecules to cross neurons
DTI uses the techniques of an MRI scanner to measure the diffusion of water and map out the likely pathways in the brain
What is the difference between direct and indirect techniques of measuring brain functions?
- Direct measures of brain function must be measuring neural activity
- Indirect measures of brain function measure a correlate of neural activity. The correlate used is metabolic activity.
How can direct measures of brain function work?
- Direct measures of brain function measure neuronal acitivity. This can be done by:
Measuring electrical signals from neurons
Measuring the magnetic fields induced by the electrical signals.
How does the indirect measure of brain function by measuring metabolic activity work?
- When neural activity increases the neurons require greater amounts of oxygen and glucose
- This is carried to them by the blood
- Therefore blood flow should increase when neural activity is higher
- Therefore measuring blood flow reveals areas of incresed neural activity.
What does EEG stand for?
electroencephalography
Describe how EEG’s work
Electrical signals from within the brain seep through sutures in the skull and can be detected by electrodes on the head
What are strengths and weaknesses of EEG’s
Strength:
- The process is very quick so the technique has excellent temporal resolution
- It is a cheap and portable technique with minimal discomfort
Weaknesses:
- The electrical signals take the path of least resistance through the skull so it can be hard to tell where they came from. Therefore there is poor spatial resolution
Describe how EEG signals are interpreted
- The EEG signal provides a waveform for each electrode
- Each waveform will have its own frequency which can be associated with different functions or levels of arousal
- Event related action potentials can be recorded by looking at the EEG wave immediatly after a stimulus or event
What does MEG stand for?
Magnetoencephalography
How do MEG’S work?
- MEG’s are a direct measure of brain function
- They measure the magnetic fields induced by neural activity
- They do this using detectors containing SQUIDS
- SQUIDS refers to superconducting quantum unit interference devices
What are the strengths and weaknesses of MEG’s
STRENGTHS
- The magnetic fields are induced simultaneously with the electrical activity so there is excellent temporal resolution
- For superficial signals e.g the cortex the spatial resolution is very good as it is easy to know where they have come from as the magnetic signals travel to the detectors directly from the sources,
WEAKNESSES
- For deeper signals, the magnetic signals degrade very quickly so spatial resolution can be very poor
- It is expensive and not portabe
What is a voxel when used in neuroscience
A voxel is just an area of space that contains a certain number of thousands of neurons. It is used to represent specific dimensions in the brain.
What is the Haemodynamic response?
- A neuron fires
- 7 seconds later, blood flows to the area carrying oxygen and glucose
- for 15 seconds after the neurons stop firing, the levels of oxygen and glucose drops below baseline
- By 20 seconds after the neurons stop firing the resting state of activity has resumed
What does fMRI stand for?
Functional magnetic resonance imaging
What is the basis of fMRI scans
- The haemodynamic response results in different levels of oxygenated and deoxygenated blood in brain regions
- This creates the BOLD (blood oxygen level dependent) signal
- This allows us to see what areas of the brain are most active
- fMRI scans use the same techniques of MRI scans however fMRI scans include the BOLD signal
What are the strengths and weaknesses of fMRI scans?
STRENGTHS
- It is based on MRI and so has excellent spatial resolution
WEAKNESSES
- The haemodynamic response is slow, therefore fMRI’s have poor temporal resolution
- It has the same weaknesses as MRI e.g noisy and claustrophobic
What does PET stand for?
Positron emission tomography
How do PET scans work?
- PET scans also rely on the haemodynamic response
- The patient recieves a radioactive version of oxygen or glucose by inhalation or injection
- This radioactive substance travels in the blood
- When the haemodynamic response causes blood flow to certain areas to be increased, the radioactive signal from this area will also increase
- The radioactive signal is detected by gamma ray detectors. Gamma rays are formed when a positron from the decaying tracer collides with a nearby electron.
What are 3 limitations of PET scans?
- The slow haemodynamic process combined with radioactive decay means there is very poor temporal resolution
- Because it is gamma rays from the annihilation of positrons that are detected, the signal does not derive from the exact location of the tracer therefore spatial resolution is limited
- The scanning is expensive due to the radioactive substances
- The radioactive substances mean that there is some level of risk
How is PET used to examine specific neurotransmitters?
- Radioactive substances are used which bind to the same neurotransmitter receptors, but with less affinity than the actual neurotransmitter
- This means that when the neurotransmitter it will displace the tracer and we will see a change of signal
When talking about resolution, are smaller numbers or bigger better or worse?
Smaller numbers indicate better resolution
What are the spatial resolutions (mm) of EEG, MEG, fMRI and PET scanS
EEG: 20
MEG: 15
fMRI: 5
PET: 8
What are the temporal resolutions (ms) of EEG, MEG, fMRI and PET scans?
EEG: 1
MEG: 1
fMRI: 1000
PET: 50000
