Radiation in Medicine

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Radiation in Medicine
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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)

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Dosimetry

• The exposure to radiation can be measured in 3
  ways

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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

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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.

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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)

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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.

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

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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
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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
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Firemen at Chernobyl

• 20 000 mSv

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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)

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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.

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Radiation safety

• Intensity decreases with distance

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Radiation safety

• Dose received is proportional to time exposed

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Radiation safety

• Shielding alpha is stopped by paper, beta by a
  sheet of aluminium, gamma by a few cm of lead.

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Monitoring

• Film badge photographic film is a cheap and
  effective way of monitoring absorbed dose

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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.

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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
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Questions
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Radiation therapy
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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

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External radiation to kill cancerous cells

• Gamma fired from different directions to
  intersect at tumour which gets the highest dose

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Internal radiation

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

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Choice of isotope

• Energy High energy will be mnore damaging to
  cancer cells , but also may pass through the
  cancer into neighbouring healthy cells

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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

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Choice of isotope

• Chemical properties Some elements collect in
  certain organs iodine in the thyroid for
  example.

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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.

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Different half lives

• Due to normal radioactive decay (physical
  half-life)

Amount of material
time
Physical half-life (T?)
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Different half lives

• Due to biological processes (excretion
  respiration etc.)

Amount of material
time
Biological half-life (TB)
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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
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Effective half-life TE

• 1/TE 1/T? 1/TB

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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

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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.

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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

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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

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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.

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Imaging using tracers

• The radiation emitted from a radioactive tracer
  can be used to produce an image of an organ.

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PET Scans

• Positron Emission Tomography

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PET

• This uses an isotope of carbon, carbon-11, as a
  tracer

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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.

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PET

• When the carbon-11 decays, it decays by positive
  beta decay (positron emission).
• 11C 1ß 11B

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PET

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

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PET

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

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PET

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

Detector surrounding patient
?
?
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PET

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

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Medical Physics

• Thats it!