Session Three (Neuro-Imaging)

Question 1

What are the two most commonly used forms of neuro-imaging?

Answer 1

SMRI (Structural MRI) and FMRI (Functional MRI).

These are the best methods we have for looking at the structure and the function of the brain, respectively.

Question 2

What is MR Spectroscopy and how is it used?

Answer 2

• Form of neuro-imaging that looks at what chemicals are present in different parts of the brain.
• Compounds studied are usually metabolites.
• Normally compare a suspect region (e.g. a tumour) to a control region (healthy brain).
• The differing levels of compounds, as well as the presence of others, can aid in diagnoses (e.g. of cancer) as well as research.

Question 3

What is Arterial Spin Labelling?

Answer 3

A form of neuro-imaging that provides very accurate details on brain perfusion.

Question 4

What is Diffusion Tensor Imaging?

Answer 4

A form of MRI that looks into how different chemicals, normally water, diffuse across a tissue.

Question 5

What is Tractography?

Answer 5

A form of MRI that allows us to see white matter, more specifically the nerve tracts white matter ranges itself into.

Question 6

What is the difference between fMRI and sMRI practically?

Answer 6

• Same machine, just used in a different way.
• Different programmes initiate different pulse sequences, generating the different results.
• S takes about 3-5 minutes and produces a super high quality image of the brain’s anatomy, from which diagnoses can be made.
• F is taken very quickly, normally among to produce a different image every second. In this way it can tell us more about what is happening in the brain at a given moment.
• F sacrifices anatomical accuracy for info on what the brain was doing that second, the reverse is true for S.

Question 7

Is there a middle ground between sMRI and fMRI? What is it’s purpose?

Answer 7

• Yes, but it is usually only done for a specific area of the brain, e.g. the visual centre in the occipital lobe.
• This allows pictures to be taken rapidly but with a degree of anatomical accuracy, producing a mixed result.

Question 8

What are some issues with using MRI as an imaging method?

Answer 8

• Machine is very loud
• Patient has to be in there for a long time
• Claustrophobic, deeply unpleasant for the patient
• Inappropriate for some patients (fat, psychotic, metal implants, claustrophobic, tattoos at the shoulder)
• Get magnetic distortion at the very front of the brain and also by the ears, producing inaccurate results and possibly missing lesions in these areas.

Question 9

What is EEG and how does it work?

Answer 9

• Electroencephalography
• 64/128 leads placed on the outside of the scalp measure changes in electrical activity in the brain and are able to triangulate them to a rough location.
• Very little anatomical accuracy but non-invasive.
• Less accurate than fMRI as only measuring effect of activity on outside of the skull, not activity itself.
• However can provide more accurate time-based data as is constantly recording changes rather than taking pictures at a set rate.

Question 10

What is MEG and how does it work?

Answer 10

• Magneto-encephalography.
• EEG measures changes in electrical activity, MEG measures the tiny changes in magnetic fields generated by brain activity.
• Of little clinical use outside of epilepsy and migraine research.

Question 11

What are some examples of functional neuro-imaging methods?

Answer 11

• fMRI (most common)
• EEG
• MEG
• PET scans

Question 12

What are PET scans and how do they work?

Answer 12

• Positron Emission Tomography
• Uses a compound like Tagged Glucose as a contrast.
• Scan patient before and after injection, areas that have taken up more glucose appear brighter, this suggests greater metabolic activity, greater activation.
• Provides a highly accurate picture of what is happening in the brain in terms of activity.

Question 13

What are some issues with using PET scans?

Answer 13

• Require injection therefore moderately invasive.
• Radioactive compound, which limits use (no kids, no pregnant women, can’t be done in research, can’t be done more than once)
• Very expensive.

Question 14

What is fNIRS and how is it used?

Answer 14

• Functional near-infrared spectroscopy.
• Non-invasive imaging method that relies on how near-IR light responds to different levels of oxygen in the blood to judge perfusion in one area of the brain and therefore activity in that part of the brain.
• Machine measures different levels of reflection and refraction of the light beams.
• Mostly done in babies as non-invasive and works best if you have little hair and a thin skull.

Question 15

What is the BOLD effect? And how is it used in neuro-imaging?

Answer 15

• Blood Oxygenation Level Dependent.
• Essentially oxyHb doesn’t disturb local magnetic fields as much as deoxyHb does.
• This is what an fMRI picks up.
• Increases in regional oxygen delivery causes an increase in oxyHb and a drop in deoxyHb, magnetic fields become less disturbed, area appears brighter on an fMRI.
• E.g: Person hears a noise, blood flow increases to the auditory centres of the brain, these areas appear brighter.
• fNIRS also uses the BOLD effect.

Question 16

What are the important steps in designing an fMRI experiment?

Answer 16

• Hypothesis generation (should be specific).

- Manipulate the exercise/behaviour/stimulus involved to produce a DETECTABLE functional change.

Question 17

What are the different ways you can manipulate an fMRI experiment to try and produce a change that is detectable?

Answer 17

• Change stimulus TYPE or PROPERTY (e.g. audio or visual, vary intensity of stimulus)
• Change stimulus TIMING (block or event-related methodology).
• Change PARTICIPANT INSTRUCTIONS (e.g. implicit or explicit, distractions)

Question 18

Describe Block Design in fMRI studies?

Answer 18

• Multiple repetitions of each experimental condition within a block or ‘epoch’.
• Test involves alternating between one or more condition blocks and control/baseline blocks.
• Blocks are typically 20-30 seconds long.
• Whole test usually lasts between 5 and 10 minutes.
• Most common form of fMRI study.

Question 19

What are the pros and cons of Block Design fMRI studies?

Answer 19

PROS:

• Adequate for many types of experiments.
• Allows for some flexibility.
• Simple to design, carry out and analyse.
• Most powerful design when it comes to statistical analysis.

CONS:

• Can be predictable or boring, leading to rapid habitation from the patient, anticipation and reduced response.
• May be difficult for patients to maintain a specific cognitive state for 30 seconds.
• Cannot extract the response to one stimulus specifically, only to a block.
• Assumes the patient isn’t still thinking about the stimulus in the baseline period.

Question 20

Describe Event-Related Design in fMRI studies?

Answer 20

• Each stimulus becomes an individual epoch so we can associate brain processes with more discrete and therefore specific events.
• Closer to a behavioural study.
• Algorithms exist to help randomise the delivery of these stimuli.

Question 21

What are the Pros and Cons of Event-Related Design in fMRI studies?

Answer 21

PROS:

• Able to detect signals to individual trial events.
• More flexible in their design than Block studies.
• Allows for post-hoc sorting of stimuli (e.g. correct vs incorrect responses, aware vs unaware, remembered vs forgotten , quick vs slow response etc)

CONS:

• Requires a greater understanding of fMRI because the design and analysis is so much more complicated.
• Less statistical power than block design except if we have a lot of well separated single events.

Question 22

Is there a mid ground between Block and Event Design.

Answer 22

Yes, Mixed design.

Pros: Sustained activity (and therefore focus) throughout task AND allows us to model brain response evoked by each trial.
Can also dissociated transient and sustained events.

Cons: Really difficult, expensive, complicated to design.

Question 23

In what circumstances would a different form of study be better than an fMRI?

Answer 23

• If you don’t need them to be doing a task (Resting State MRI).
• If you want a more direct measure of cerebral blood flow (ASL)
• When measuring really fast brain processes (EEG or MEG).
• If you need the results available in real time (real time fMRI).