Summary Task 5 - MRI, PET, fMRI

TASK 5: MRI, PET, FMRI
STRUCTURAL & FUNCTIONAL IMAGING

 Functional imaging (PET, fMRI) – detect changes in metabolism / blood flow in the brain
while participants are engaged in cognitive tasks
 Don’t directly measure neural events BUT metabolic changes correlated with
neural activity
 Neurons require oxygen & glucose to generate energy
 When brain area is active, more oxygen & glucose are provided by increasing
blood flow to that region
 Measures moment to moment variable characteristics of the brain
 Structural imaging (CT, MRI)
 Different types of tissues have different physical properties – these different
properties can be used to construct detailed static maps of the physical structure
of the brain
 Different magnetic properties, because of the amount of water they have in
their tissue
 CT – different types of tissues absorb X-rays differently  able to differentiate
between tissues
 Spatial resolution – accuracy with which one can measure where a cognitive event is
occurring
 Increasing voxel size  lower spatial resolution  more sensitive to BOLD
response
 Decreasing voxel size  higher spatial resolution  less sensitive to BOLD
response
 Optimal voxel size – 3-4mm
 Temporal resolution – accuracy with which one can measure when a cognitive event is




occurring

PET

THE MECHANISMS BEHIND IT

1. Radioactive substance introduced into bloodstream (tracer)
 Radiation omitted from tracer is monitored by PET instrument

,  Tracer administered at least twice (during control & experimental condition)
2. Radioactive isotopes within injected substance rapidly decay  release a positron from their
atomic nuclei
 Most common isotopes (administered while person is engaged in cognitive task)
 Oxygen-15; O (administered in form of water)
 Fluorine-18 (administered in form of glucose sugar)
3. Positron collides with an electron  two photons / gamma rays are created
 Travels 2-3mm before collision
 BUT need to average across participants in PET  spatial resolution a bit worse
(10mm)
4. Two photons move in opposite directions at speed of light
5. PET scanner detects gamma rays & determines where the collision took place


 Results reported as change in regional cerebral blood flow (rCBF)
 Spatial resolution – PET can resolve metabolic activity to regions (voxels) of approx. 5-
10mm3
 Sufficient to identify cortical & subcortical areas of enhanced activity & functional
variation
 Temporal resolution (bad) – 30s, time it takes for radiation to peak to its maximum
 Specialised molecules that might serve as biomarkers of particular neurological
disorders (e.g., PiB)
 PiB – binds to beta-amyloid (biomarker of Alzheimer’s disease)
 If used as PET tracer in might be able to diagnose Alzheimer’s disease

BLOCK DESIGN EXPERIMENT

 Used for PET, because participants must be engaged continually in single
experimental task ≥ 40s
 Recorded neural activity integrated over a block of time – participant is either presented a
stimulus / performs a task
 Recorded activity pattern compared to other blocks that have been recorded while
doing the same task / stimulus, different task / stimulus, nothing at all
 Advantage: able to detect small effects

(DIS-)ADVANTAGES

 Less susceptible to signal distortion around air cavities than fMRI
 Poor temporal resolution compared to e.g., ERPs
 Difficult to interpret data – huge data sets, comparisons of experimental & control
conditions produces many differences
 Difficult to make inferences about each area’s functional contribution  correlation ≠
causation