Topic 4 (Investigating the brain) Lecture 2 (Types of brain scanning)

Question 1

What does CT scan stand for?

Answer 1

Computerised tomography

Question 2

Describe CT scanning

Answer 2

• 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

Question 3

Describe MRI scanning

Answer 3

• 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

Question 4

What are 4 uses of an MRI scan?

Answer 4

• Diagnosis
• Staging of a condition/ monitering progression
• Guiding brain surgery
• Superimposing with other types of imaging to gain understanding

Question 5

What are 3 discomforts/ limitations of an MRI

Answer 5

• 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

Question 6

Describe Diffusion Tensor imaging

Answer 6

• 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

Question 7

What is the difference between direct and indirect techniques of measuring brain functions?

Answer 7

• 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.

Question 8

How can direct measures of brain function work?

Answer 8

• 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.

Question 9

How does the indirect measure of brain function by measuring metabolic activity work?

Answer 9

• 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.

Question 10

What does EEG stand for?

Answer 10

electroencephalography

Question 11

Describe how EEG’s work

Answer 11

Electrical signals from within the brain seep through sutures in the skull and can be detected by electrodes on the head

Question 12

What are strengths and weaknesses of EEG’s

Answer 12

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

Question 13

Describe how EEG signals are interpreted

Answer 13

• 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

Question 14

What does MEG stand for?

Answer 14

Magnetoencephalography

Question 15

How do MEG’S work?

Answer 15

• 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

Question 16

What are the strengths and weaknesses of MEG’s

Answer 16

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

Question 17

What is a voxel when used in neuroscience

Answer 17

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.

Question 18

What is the Haemodynamic response?

Answer 18

1. A neuron fires
2. 7 seconds later, blood flows to the area carrying oxygen and glucose
3. for 15 seconds after the neurons stop firing, the levels of oxygen and glucose drops below baseline
4. By 20 seconds after the neurons stop firing the resting state of activity has resumed

Question 19

What does fMRI stand for?

Answer 19

Functional magnetic resonance imaging

Question 20

What is the basis of fMRI scans

Answer 20

• 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

Question 21

What are the strengths and weaknesses of fMRI scans?

Answer 21

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

Question 22

What does PET stand for?

Answer 22

Positron emission tomography

Question 23

How do PET scans work?

Answer 23

• 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.

Question 24

What are 3 limitations of PET scans?

Answer 24

• 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

Question 26

How is PET used to examine specific neurotransmitters?

Answer 26

• 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

Question 27

When talking about resolution, are smaller numbers or bigger better or worse?

Answer 27

Smaller numbers indicate better resolution

Question 28

What are the spatial resolutions (mm) of EEG, MEG, fMRI and PET scanS

Answer 28

EEG: 20

MEG: 15

fMRI: 5

PET: 8

Question 29

What are the temporal resolutions (ms) of EEG, MEG, fMRI and PET scans?

Answer 29

EEG: 1

MEG: 1

fMRI: 1000

PET: 50000