Unit 8 - Medical Physics

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Unit 8 - Medical Physics

• Nikki Kelso

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Aims of this Session

• Production of and uses of thermographic images
• Introduce the production of dangers of using x-
  rays
• Stochastic Non Stochastic effects
• Somatic Hereditary effects
• Uses of Radioisotopes Nuclear Medicine
• Production uses of Medical Ultrasound
• Magnetic Resonance Imaging (MRI)

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Thermography

• Infra-red detectors pick up IR radiation
• Amount of radiation increases with
  temperature therefore thermography allows
  you to visualise variations in temperature
  
• computer algorithms used to interpret data and
  produce a usable image

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Why is this Useful?

• Certain pathologies cause temperature
  differentials
• Thermography detects these with high sensitivity
  accuracy
• Non invasive
• NO Ionising Radiation used

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Types of Diagnosis

• Sports injuries
• Breast cancer screening
• Monitoring of post operative infection

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What we do in Radiology Departments

• Plain film radiography
• Contrast studies
• Computerised Tomography
• Radioisotope imaging
• Ultrasound
• Magnetic resonance imaging
• Bone density measurement
• Positron emission tomography

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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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X-ray Production
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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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Precautionary Measures

• Legislation
• Ionising Radiation Regulations 1999
• IR(ME)R 2000
• In Practice we use
• radiation protection
• ALARA principle

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Staff Protection

• Not place themselves in the primary beam
• Use of the inverse square law
• Use of lead glass panels
• Use of lead rubber coats/thyroid shields/lead
  glasses
• Limit of time spent in fluoroscopy especially
  during pregnancy
• QA of the equipment
• Dose monitoring

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Patient Protection

• Correct exposure factors
• QA done daily on equipment
• Collimation of the primary beam
• Correct focus/film distance
• Use of appropriate lead rubber protection where
  appropriate ie gonads/eyes/thyroid
• Appropriate examination
• Well trained staff

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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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Stochastic Effects

• Probability of the effect of radiation which can
  be either radiation induced cancers or genetic
  effects.
• No safe dose limit
• Statistically generated
• Lower doses of radiation

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Non Stochastic Effects

• Also called deterministic effects
• There is usually a threshold below which the
  effect will not occur
• Examples are erythema (skin reddening) or
  epilation (hair loss)
• Doses are large eg following radiotherapy or as a
  result of a radiation accident (Chernobyl)

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Damage caused by radiation

• SOMATIC caused to the individual
• GENETIC passed onto future generations

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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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Examples of Doses

• Were all exposed to background radiation
• Chest few days
• Skull few weeks
• Spine/Abdo Few months or a year
• CT Chest few years
• Additional risk of cancer per exam
• 1 in 1,000 to 1 in 1,000,000
• Risk of cancer 1 in 3

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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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Using contrast media
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Factors affecting contrast

• Transmission x-ray photons that pass through
  the patient unchanged.
• Absorbtion x-ray photons that transfer their
  energy to the patient.
• Absorbtion is proportional to the degree of
  attenuation thickness, density atomic number
• Scatter radiation that changes direction or is
  modified by decrease in energy as it passes
  through a body
• Attenuation process that x-rays lose power as
  it travels through matter

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Plain film radiography
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Mobile Radiography

• Mobile unit can be moved to patients bedside, AE
  dept or theatre
• Can be mains or battery powered
• Can produce images as good as purpose built
  units.

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Digital imaging

• Images stored on computer
• No films
• Image manipulation
• Multiple viewing
• Storage
• Volume
• Physical principles remain the same

• But because its Windows based

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C arm for angiography
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Ultrasound

• Ultrasound uses sound waves to produce images
• Becoming highly skilled
• Increasingly specialised
• Images are very dependent on the
  ultrasonographers skill

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Ultrasound images
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Ultrasound images
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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 what you are not interested in

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Magnetic Resonance Imaging

• The latest imaging tool
• Images are similar in appearance to CT but
  produced without radiation
• Technology utilises radio waves and a huge magnet
  to produce images
• The magnet must be kept cool to allow
  superconductivity. It has to be cooled with
  liquid helium to -270 degrees.

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MR scanner
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MR Precautions

• Not everybody can have an MRI scan
• Metal implants eg cardiac pacemakers, aneurysm
  clips
• Tattoos
• Metallic foreign bodies
• Pregnant women
• claustrophobics

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MRI Images
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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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Radioisotope Imaging

• What is an isotope?
• Nuclei of atoms consist of protons and neutrons.
• The number of protons is called the atomic number
• The number of protons and neutrons is called the
  mass number
• All the atoms of one element with the same atomic
  number but different mass number are called
  isotopes

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Radioisotopes

• Isotopes behave chemically the same
• some of the radioisotopes will be radioactive ie
  emit radiation
• By attaching these radioactive isotopes to
  certain pharmaceuticals we can use the emitted
  radiation to produce images
• Most commonly used isotope is Technetium99m
  because it decays by gamma emission

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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 Becquerels 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 modeling

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Uses for Isotopes

• Nuclear Medicine
• Branch of imaging science which uses unsealed
  radioactive sources
• Gamma sources are isotope 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
  absorbs isotope
• Gamma emission is detected by a gamma camera
• Image is digitally produced

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Gamma Camera

• Detects individual Gamma photons
• Builds up an image over a period of several
  minutes
• Useful to show biological (metabolic) processes
  eg infections/secondary boney cancer deposits

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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
• Used uranium fuel has a content of molybdenum99
• Easily extracted
• Technetium99 is a daughter product
• A few micrograms of molybdenum99 will produce
  enough technetium 99 to image 10,000 patients

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Radioisotope/NM Images
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Positron Emission Tomographylatest radiology
tool to image patients
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Positron Emission TomographyPET
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QUESTIONS?