24.3.1Medical physics and astronomy (A Level)
Radioactive Tracers
5 min read 10 parts Lesson 11 of 22 1 quiz
Lesson summary
24.3.1 Radioactive Tracers — AS PHYSICS 9702 notes: Define a radioactive tracer, explain why fluorine-18 and technetium-99m suit medical use and how tracers are used in PET.Radioactive tracer = a substance containing radioactive nuclei that is introduced to the body and absorbed …
- Attachment
- Function not structure
- Half-lives
- Short half-life
- Penetrating radiation
- Chemically suitable
Section 1 of 10Radioactive tracer
Radioactive tracer
Radioactive tracer
Radioactive tracer a substance containing radioactive nuclei that is introduced to the body and absorbed by tissue to study the structure and function of organs.
Attachment
- The isotope is attached to a molecule that the body handles in a known way.
Detection
- Radiation passes out of the body, so detectors locate where the tracer collects.
Function not structure
- A tracer shows where a chemical is being used, whereas an X-ray shows structure.
Types
- In PET the tracer is a positron emitter, and in other nuclear medicine the tracer is a gamma emitter such as technetium-99m.
Section 2 of 10Suitable isotopes
Binding
- Technetium-99m and fluorine-18 bind to organic molecules such as glucose or water that are readily available in the body.
Radiation
- Both give radiation that leaves the body and decay into stable isotopes.
Half-lives
- Technetium-99m has a half-life of 6 hours and fluorine-18 an even shorter half-life of 110 minutes.
- The patient is exposed to radiation for a shorter time.
Section 3 of 10Properties of a good tracer
Short half-life
- Minutes or hours limits the dose, but it must be long enough to make, deliver and scan.
Penetrating radiation
- The final gamma rays can be detected outside the body.
Chemically suitable
- It attaches to a biologically active molecule and is excreted soon after.
Low activity
- Only the minimum needed for a clear image is used.
Section 4 of 10Fluorine-18 and FDG
FDG
- Fluorodeoxyglucose is a glucose-like molecule in which one hydroxyl group is replaced by fluorine-18.
Uptake
- Cells with a high metabolic rate take up FDG like glucose.
- It cannot be fully broken down, so it stays trapped in the cell.
Hot spots
- Active tissues such as the brain, heart muscle and fast-growing tumours accumulate more FDG and appear bright.
Procedure
- The tracer is injected or swallowed and the patient waits, typically about an hour, before the scan.
Uses
- Diagnosis of cancer, areas of reduced blood flow in the heart, and brain disorders including Alzheimer's disease and dementia.
Monitoring
- It lets doctors follow the progress of a disease and the effect of treatment.
Section 5 of 10Beta-plus decay of the tracer
Decay
- Fluorine-18 is neutron-poor and decays by β+ emission as a proton changes into a neutron.
- 189F → 188O + 0+1β + νe.
Other positron emitters
- Carbon-11 , nitrogen-13 (10 minutes) and oxygen-15 (2 minutes).
- (about 20 minutes)
Chemistry
- These are elements found in organic molecules, so they can be built into biological compounds without changing the chemical behaviour.
Production
- Most are made in a cyclotron, often on site, because their half-lives are short.
Section 6 of 10Decay of the tracer activity
Activity law
- A = A0e−λt with λ = 0.693/t½.
Preparation
- Activity falls between production and injection, so the prepared dose must be higher than needed.
Distance
- Tracers with the shortest half-lives can only be used where the cyclotron is next to the scanner.
Section 7 of 10Dose and half-lives
Dose
- It depends on the activity injected and on how long the isotope stays in the body.
Physical half-life
- The time for radioactive decay to halve the activity.
Biological half-life
- The time for the body to remove half the tracer, for example through the kidneys.
Effective half-life
- It is shorter than either, with 1/teff = 1/tphys + 1/tbio.
Patient advice
- Drink plenty of fluid to speed excretion and avoid close contact with infants and pregnant women for several hours.
Section 8 of 10More from the specification
More from the specification
Positron emission and annihilation
- In positron emission tomography (PET), an emitted positron slows down and collides with an atomic electron,
- causing them to annihilate and convert their combined rest mass into pure electromagnetic energy.
Energy released in pair annihilation
- By Einstein's mass-energy equivalence equation, the total rest energy released (2 x 0.511 MeV ~ 1.022 MeV) produces two identical gamma-ray photons travelling in opposite directions.
- (E = 2 * me * c2
Photon momentum conservation
- Conservation of linear momentum requires the two emitted gamma-ray photons to travel in exactly opposite directions, each photon carrying energy E = h f = 511 keV and momentum p = E / c.
- (180 degrees apart)
Scintillation crystal detection
- Annihilation gamma-ray photons strike a ring of scintillation crystal detectors, generating flashes of light converted into electrical signals to localize the tracer source.
- (e.g. bismuth germanate)
Gamma-ray photon
- A high-energy photon from a nuclear process or annihilation, 0.511 MeV for annihilation of an electron and positron.
Nuclear equation
- Equation in which nucleon number and charge are conserved.
Electron
- Negatively charged particle in an atom.
Annihilation
- The process in which a particle and its antiparticle meet and their mass is converted into the energy of gamma photons.
Photon
- A quantum of electromagnetic energy, E = hf
Momentum
- P = mv, a vector.
Frequency
- Revolutions per second, f = 1/T.What the symbols mean: T = period, the time for one complete wave (s) · f = frequency (Hz)
E
E hf = mec2
What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
E = hf
What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Radioactive tracer | A substance containing radioactive nuclei introduced into the body and absorbed by tissue to study the structure and function of organs. |
| Fluorodeoxyglucose (FDG) | A glucose-like molecule containing fluorine-18, used as a PET tracer. |
| Positron emission tomography (PET) | A nuclear medicine technique that images tissues by measuring the metabolic activity of cells. |
| Effective half-life | The half-life of a tracer in the body, combining physical decay and biological removal. |
| Cyclotron | A particle accelerator used to make short-lived positron-emitting isotopes. |
Section 10 of 10Exam technique
1
Thinking the tracer needs a long half-life to last — a short half-life reduces the patient's dose.
2
Saying PET tracers emit gamma rays directly — they emit positrons, which then annihilate to produce gamma rays.
3
Describing PET as showing the structure of an organ — it shows function through metabolic activity.
4
Forgetting that the prepared dose must be higher than needed — activity falls before injection.
5
Believing the radioactive atom changes the chemistry of the molecule greatly — it behaves like the normal atom.