Nuclear Medicine - PET

Question 1

PET isotopes: what is the half life of a C-11?

Answer 1

20 mins

Question 2

What is the half life of O-15?

Answer 2

2 mins

Question 3

What is the half life of F-18?

Answer 3

110 mins

Question 4

What is the half life of Ga-68?

Answer 4

68 mins

Question 5

What is the half life of Cu-64?

Answer 5

12.7 hours

Question 6

How is the signal produced in a PET scan?

Answer 6

Positron released from the nucleus.
Annihilates with an electron
Annihilation creates two gamma-ray (511keV each) travelling in opposite directions
PET scanner detects gammas and uses time difference of event to calculate the position of annihilation

Question 7

How is the cyclotron used to generate PET isotopes and which isotopes is it commonly used to generate?

Answer 7

High energy particles are fired at a target (commonly in negative H ions)
Used for low said isotopes (C-11, F-18 etc)

Question 8

How is a generator used to produce PET isotopes, and which isotopes is it commonly used to generate?

Answer 8

Different isotopes with long half-life decays to produce positron emitter.
Ge-68: t1/2 = 271 days decays to form Ga-68: t1/2 = 68mins.
Only suitable for certain isotopes.

Question 9

What are the properties of a radiotracer?

Answer 9

Traces the biochemical pathway, physiological function or fate of a drug.
May be identical to a natural compound.
High specific activity not required.
Needs to be selective for metabolic pathway or studied process.
Used in Pharmacokinetic studies - metabolism needs to be understood.
Labelling in more than one position - greater understanding.

Question 10

What are the properties of a radiogland?

Answer 10

Targets high-affinity binding sites.
Based on synthetic compounds.
High specific activity required.
Selective for target site.
Little to no metabolic interference.

Question 11

Why is fluorine used as a radiotracer?

Answer 11

Fluorine has a longer half-life than carbon-11 - so is useful for slower biochemical processes.
C-F bonds are stronger than C-H bonds - enzymatic cleavage gives non-toxic fluoride.
Many drugs contain fluorine - increased bioactivity and lipophillicity.
It’s a steric mimic for -OH or H so can be accepted as an enzyme substrate with no reaction.