PET and MRI

Question 1

PET

Answer 1

Measurement of radioactivity over time using sequential scans following i.v. injection of radiolabelled tracer

Question 2

What does PET allow?

Answer 2

1. Observation of radioisotope uptake and decay as function of time
2. Quantitative measures of brain metabolism and perfusion
3. Detection of binding densities of radioactive ligands

Question 3

How do you choose a radiotracer?

Answer 3

• Long enough half-life to gather measurements
• Ability to bind to drug of interest (biochemistry)
  Most common: F-18 and C-11

Question 4

Molecular PET

Answer 4

Tracer may not be specific for a particular receptor so distributes uniformly

Binding a small amount of drug allows the tracer to bind specifically to target receptor

Shows drug crosses the BBB and is pharmacologically active

Does not provide relationship between dose and occupancy

Question 5

PET radiodisplacement

Answer 5

Small amount of drug bound to tracer -> at baseline the response shows areas with higher conc. depending on where the drug binds

True dose of drug with no tracer given -> hot drug is displaced by normal drug

More drug given, greater receptor occupancy

Can be used in both animals and human -> increases translatability

Question 6

Amyloid PET Biomarkers

Answer 6

High sensitivity but low specificity
-> very sensitive to amyloid but does not distinguish between amyloidoses

• Amount of amyloid does not linearly correlate with AD progression

Question 7

Volumetric MRI

Answer 7

Measures structures and volumes to create 3D images of the brain

Question 8

MRI vs PET

Answer 8

MRI: very safe, can be done as many times as needed, BUT loud and claustrophobic, must check for metallic implants

PET: more sensitive BUT requires radioactivity, can only be performed a certain number of times

Question 9

MRI Brain Atrophy

Answer 9

MRI can measure the amount of brain tissue lost by looking at differences in the brain over time in the same individual

But the drug trial would need to last the same amount of time as the imaging in order to detect an effect

Very hard to detect differences between scans performed close together

Question 10

Functional MRI

Answer 10

Measures function rather than volume
Measures brain at various axes

Question 11

BOLD fMRI

Answer 11

Detects the increase in cerebral blood flow during performance of a task

Increased blood flow = increased Hb

Ratio between diamagnetic Hb and paramagnetic deoxy-Hb

Question 12

Translatability

Answer 12

Imaging techniques can be used in humans and animals, increasing the translatability