Unit 8 Medical Physics

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Unit 8 Medical Physics

• Paul Kane
• Radiographer

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Aims for this Session

• Understand the production of and uses for
  thermographic images
• Understand X-Ray production
• Understand how X-Rays are used to produce images
• Understand the dangers of X-Rays
• Evaluate the use of both modalities
• Understand Radiation, its uses and dangers

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The Electromagnetic Spectrum
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Thermography

• Infrared detectors pick up IR radiation
• Amount of radiation varies with temperature
• Computer algorithms used to interpret data and
  produce a usable image

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Why is this useful?

• Certain pathologies will cause temperature
  differentials
• Thermography detects these with high sensitivity
  and accuracy
• Non Invasive
• No ionising radiation used

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What sort of diagnoses?

• Sports Injuries
• Ca Breast screening
• Monitoring of post operative infection

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X-Rays
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X-Rays

• Discovered in 1895 by Roentgen
• X Rays because he didnt know what they were!
• An ionising radiation at a higher level on EM
  spectrum
• Higher frequency or shorter wavelength

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XRAY TUBE
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X-Ray Production
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Rotating Anode Tube
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XRAY TUBE
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X-Rays, the risks and dangers

• Ionising Radiation potentially damaging
• Damage is influenced by
• Amount of body tissue irradiated
• Type of body tissue irradiated
• Dose Received
• Dose Rate
• Risk minimised using ALARA principle

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X-Ray Effects

• Stochastic no threshold for damage
• Non Stochastic a quantifiable threshold
• Effects can take place in somatic cells or be
  passed on (hereditary)

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How are effects measured?

• Sievert is unit of measurement equivalent to a
  deposit of 1 joule of energy per kilogram mass of
  tissue
• Relates dose absorbed in tissue to biological
  damage caused effective dose
• This will depend on the type of radiation
• Typical background radiation results in an
  effective dose of 2.4 mSv/year

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Precautionary Measures

• Legislation
• Ionising Radiation Regulations
• IR(ME)R 2000
• In practice we use
• Radiation Protection
• ALARA principle

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Image Production

• Basic form uses photographic film
• Denser structures attenuate the x-rays
• When film is exposed to x rays it turns black
• Image is contrast between two
• Contrast can be manipulated using exposure
  factors and other aids such as contrast media

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Variations in Contrast
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Computerised Tomography
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CT Explained

• Tomography
• Tomos slice
• Graphia describing
• where digital geometry processing is used to
  generate a three-dimensional image of the
  internals of an object from a large series of
  two-dimensional X-ray images taken around a
  single axis of rotation

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CT in practice

• Data is obtained digitally
• Algorithms allow manipulation of data
• Windowing is process of using a variety of
  Hounsfield Units
• Setting a top and bottom of range allows various
  tissue types to be imaged
• Can get rid of that which does not interest you

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CT versus MR

• Principles of data collection are the same
• MR is NON IONISING
• Better at imaging softer tissue

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Which Modality to Use?

• What are you attempting to image?
• What level of information do you wish to obtain?
• How do you wish to manipulate it?
• What protection measures need to be considered?

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Radiation
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What is Radioactivity?

• Certain elements have isotopes which are unstable
• The unstable atoms emit particles or energy
• The particles or energy are radiation
• The process is unpredictable
• It is measured in Bequerels 1 Bq is one decay
  event per second

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Radiation Types

• Alpha helium nuclei stopped by paper!
• Beta electron, can be stopped by light metal
• Gamma EM photon, requires dense material to
  absorb

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Half Life

• The time taken for half of the atoms of a given
  sample to decay.
• Stays the same for a given isotope regardless of
  the actual quantity
• Expressed as a unit of time
• Can be validated using experimentation and
  computer modelling

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Uses for Radioisotopes

• Nuclear Medicine
• Branch of imaging science which uses unsealed
  radioactive sources
• Gamma sources are those of choice

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How does it work?

• Radioactive isotopes are labelled with
  pharmaceuticals
• Now known as radiopharmaceuticals
• Introduced into the body
• Pharmaceuticals influence tissue type which
  aborbs isotope
• Gamma emission is detected by a gamma camera
• Image is digitally produced

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Gamma Camera
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Why do we use Nuclear Medicine?

• Radiopharmaceuticals do not cause much harm in
  proportion to benefit derived
• Body will excrete material
• Radioactivity is short lived matter of hours
• Can be used to image anatomy and physiology
• Can be integrated with other modalities (PET)

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Production

• Most useful isotopes are not natural
• Must be produced by reactors
• Side product of used nuclear fuel
• Milking a cow

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Production Contd

• Used uranium fuel has a content of molybdenum99
• Easily extracted
• Technetium99 is daughter product
• A few micrograms of molybdenum99 will produce
  enough technetium99 to image approx 10,000
  patients

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Precautions

• Unsealed source
• Main protection for staff is time, distance and
  shielding
• Patients only need worry about the period
  immediately around scan

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Time Distance Shielding
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Detectors

• Scintillation Counters uses materials which
  fluoresce when irradiated
• Geiger Counters uses a gas which becomes a
  conductor if irradiated
• Film Badges uses photographic film

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Film Badge

• Piece of wrapped photographic film
• Film holder - a plastic holder containing various
  metal and plastic filters
• Tin, Cadmium, Lead Indium, plastic of differing
  densities.

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Experimenting with Radiation

• Any experiments must be properly regulated and
  kept safe radiation brings other considerations
• Strict international regulations
• Adequate protection measures must be in place
• QA vital
• Participants monitored

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QUESTIONS?