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Radiation in Medicine

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Title: Radiation in Medicine


1
Radiation in Medicine
2
Activity
  • Activity is the number of disintegrations per
    second. The effect it has largely depends on how
    ionising the radiation is.
  • A ?N ?N0e(-?t) A0e(-?t)

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)
  • 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 1
Beta particle (electron) 1
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 year 3
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
  • 20 000 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 doesnt
    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 (T?)
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 (T?) and the biological half-life (TB)

TE is a combination of both processes
33
Effective half-life TE
  • 1/TE 1/T? 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 -?N) 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
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 1ß 11B

44
PET
  • When the positron meets an electron they
    annihilate each other to produce 2 gamma rays
  • 1ß -1ß 2?

45
PET
  • The two gamma rays emitted travel in opposite
    directions
  • ? ?

46
PET
  • The two gamma rays can be detected and the
    position of their origin calculated with great
    accuracy

Detector surrounding patient
?
?
47
PET
  • This is used especially to image a functioning
    brain (when given specific tasks to do).

48
Medical Physics
  • Thats it!
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