1. Radiation in Medicine

2. A = λN = λN0e(-λt) = A0e(-λt)
Activity
Activity is the number of disintegrations per second. The effect it has largely depends on how ionising the radiation is.
A = λN = λN0e(-λt) = A0e(-λt)

3. Dosimetry
The exposure to radiation can be measured in 3 ways:

4. Exposure (X)
This measures the amount of ionising radiation you would be exposed to in a particular environment.
Exposure (X) = Q/m
Q = Total charge of + ions produced
m = Mass of air in the room
Units = C.kg-1

5. Exposure (X) Exposure X = Q/m
It is only used for X-rays and gamma rays as alpha and beta have a small range in air and anyone standing a few metres away in the same room will not be exposed to them.

6. Absorbed dose (D)
This is the energy absorbed per unit mass of actual tissue
Absorbed dose (D) = E/m
E = Total energy absorbed
m = mass of tissue
Unit = J.kg-1 (called a gray, Gy)

7. Dose equivalent (H) H = QD
This is an attempt to measure the actual damage that occurs in tissues
Dose equivalent = Quality factor x absorbed dose
H = QD
Units = J.kg-1, but this time called a sievert (Sv)
Alpha particles will have a higher quality factor than gamma rays etc.

8. Quality factor Particle/wave Quality factor X-ray 1 Gamma ray
Beta particle (electron)
Alpha particle 20

9. Sievert
To distinguish between absorbed dose and dose equivalent the Sievert is used. Since Q = 1 for X-rays, the absorbed dose and dose equivalent of X-rays is the same. For other radiations, the dose equivalent gives the amount of X-radiation that would give the same harm.

10. Dose equivalents Source Dose equivalent /mSv Background dose in 1 year3
Ankle X-ray
0.02
CT scan of the head
2.0
Barium meal
8.0

11. Dose effects Dose/mSv Effect 1000 Nausea. vomiting 2000
Loss of body hair
4000
Bleeding in the mouth
10 000
Death after 14 days
50 000
Death within 48 hours

12. Firemen at Chernobyl
mSv

13. Biological effects of radiation
Ionisation could cause damage directly to DNA or RNA
Metabolic pathways could be interfered with (the complex chemical reactions that take place in the body)

14. Biological effects of radiation
Damage to a cell could cause it to divide at a rate faster than cells die. The malignant cells could continue to grow until they interfere with the normal workings of the organ/tissue. Death is the result. This is called cancer.

15. Radiation safety
Intensity decreases with distance

16. Radiation safety
Dose received is proportional to time exposed

17. Radiation safety
Shielding – alpha is stopped by paper, beta by a sheet of aluminium, gamma by a few cm of lead.

18. Monitoring
Film badge – photographic film is a cheap and effective way of monitoring absorbed dose

19. Monitoring
Ionising particle causes chemical change in one of the grains that cover the film. When processed the grain turns black.
Different areas of the badge can have different filters in front of the film
The badge is processed about once a month.

20. Balanced risk Risk Benefit Ankle X-ray (0.02 nSv)
Ankle gets repaired correctly
Radiation emitted from a smoke detector
Detector might detect a fire and save lives
CT scan of an unborn baby to find out if it is a boy or girl (2 mSv)
Know whether to paint nursery blue or pink

21. Questions

22. Radiation therapy

23. External radiation to kill cancerous cells
Gamma rays
It will also affect healthy cells
Cancerous cells do not function correctly so are unable to repair themselves as well as normal cells

24. External radiation to kill cancerous cells
Gamma fired from different directions to intersect at tumour which gets the highest dose

25. Internal radiation
Placing a solid radiactive source next to the tumour (brachytherapy) or injecting/ingesting a fluid containing a radiactive isotope.

26. Choice of isotope
Energy – High energy will be mnore damaging to cancer cells , but also may pass through the cancer into neighbouring healthy cells

27. Choice of isotope
Type – Alpha is the most damaging but not very penetrating so needs to be placed close to cancer cells.
Beta and gamma sources need longer exposure time and have more effect on surrounding tissue

28. Choice of isotope
Chemical properties – Some elements collect in certain organs – iodine in the thyroid for example.

29. Choice of isotope
Half-life – If solid and can be retrieved – does not matter, although a long half-life means it can be re-used
If ingested, needs to be shorter so it doesn’t stay in the body too long.

30. Different half lives
Due to normal radioactive decay (physical half-life)
Amount of material
time
Physical half-life (Tρ)

31. Different half lives
Due to biological processes (excretion respiration etc.)
Amount of material
time
Biological half-life (TB)

32. Effective half-life TE
The effective half-life (TE) is the time it takes for the actual number of nuclei in the body to reduce. It obviously depends both on the physical half life (Tρ) and the biological half-life (TB)
TE is a combination of both processes

33. Effective half-life TE
1/TE = 1/Tρ + 1/TB

34. Example
Iodine is a gamma emitter with a half-life of 8 days. It accumulates in the thyroid gland. The biological half-life is 80 days.
Effective half-life?
1/TE = 1/8 + 1/80
TE = 7.3 days

35. Radioactive tracers
Small amount of a radioactive isotope is injected into the blood. Since activity is proportional the amount present (A = -λN) it is possible to find how much isotope is present in any part of the body simply by measuring the activity.

36. Measuring blood volume
Small amount of isotope is put into a known amount of blood and its activity measured.
Blood re-injected into body and allowed to “mix”
New sample taken and “diluted” activity measured
Original activity/Diluted activity =
total volume/sample volume

37. Thyroid activity
Thyroid uses iodine to make hormones so iodine collects in the thyroid.
By using a tracer the flow of iodine in and out of the thyroid can be monitored

38. Calcium build-up in the heart
Build-up of calcium is an indication of a damaged heart muscle.
Radioactive technetium takes the place of calcium in the heart muscle giving an indication of the amount of calcium build-up.

39. Imaging using tracers
The radiation emitted from a radioactive tracer can be used to produce an image of an organ.

40. Positron Emission Tomography
PET Scans
Positron Emission Tomography

41. PET
This uses an isotope of carbon, carbon-11, as a tracer

42. PET
A small amount of carbon monoxide (CO) containing some C-11 is inhaled and is taken up by red blood cells and circulated around the body.

43. PET
When the carbon-11 decays, it decays by positive beta decay (positron emission).
11C +1β + 11B

44. PET
When the positron meets an electron they annihilate each other to produce 2 gamma rays
+1β + -1β 2γ

45. PET
The two gamma rays emitted travel in opposite directions
γ γ

46. PET
The two gamma rays can be detected and the position of their origin calculated with great accuracy
Detector surrounding patient γ γ

47. PET
This is used especially to image a functioning brain (when given specific tasks to do).

48. Medical Physics
That’s it!