Periodic Table of the Elements

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Chapter 2 and Chapter 6
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X-Ray absorption in biologic tissue

• When a human tissue is exposed to electromagnetic
  energy this energy may interact with the atoms of
  the tissue or may not. When there is no
  interaction the x-ray photons passes trough the
  tissue without any effect. When there is
  interaction part of the energy is absorbed by the
  tissue, the amount of energy absorbed per unit
  mass is called absorbed dose. The absorbed dose
  should very low to avoid damages to the tissue.
  The absorption allows us to obtain radiographs
  diagnostic quality.

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Energy of photons in a diagnostic x-ray beam

• The energy of the x-ray beam is directly related
  to the kVp, the higher it is the grater the
  penetrating power of the x-ray beam. In
  diagnostic radiology this energy is expressed in
  thousands of volts and it is the intensity of the
  electrical voltage applied across the tube. 100
  kVp means 100,000 eV bombarding the target. The
  beam contains photons with energy of 100 kEv,
  with an average energy of 33 kEv.

4
X-Ray beam production and energy.

• The x-ray beam is produced when a beam of
  electrons interacts wit the target of the x-ray
  tube, the target is made out of tungsten it has a
  very high melting point and very high atomic
  number. It is located inside the x-ray tube under
  vacuum. When the electrons interact with the
  target x-ray photons are created, and they are
  directed towards the patient, when this bean is
  created it many levels of energy, and the lower
  levels of energy are filtered from the beam by
  and aluminum filter that removes the
  diagnostically useless photons hardening the
  x-ray beam.

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Questions

• What is the average energy on a 100 kEv x-ray
  beam?
• 33 kEv
• Is absorption necessary to obtain good quality
  radiographs?
• Yes
• Why is tungsten used in the manufacture of the
  target?
• High melting point and atomic number

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Attenuation

• When an x-ray beam passes through an object, it
  goes through a process called attenuation
• The term attenuation is rather broad with
  respect to x-rays, it may be used to refer to any
  process decreasing the intensity of the primary
  photon beam that was directed toward a
  destination
• Attenuation is simply the reduction in the number
  of primary photons in the x-ray beam through
  absorption and scatter as the beam passes through
  an object in its path

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Attenuation Transmission

• Direct Transmission- X-ray photons that traverse
  through the body without interacting and reach
  the image receptor
• Indirect Transmission-Primary photons which
  undergo Compton/and Coherent interactions and as
  a result may be reflected and scattered may still
  transverse the object and hit the image receptor

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Photons

• Primary Photon all x-ray photons before
  reaching the object that have been produced by
  the source
• Exit Photon Any x-ray photon that passes
  through the object and reaches the radiographic
  image receptor below it
• Attenuated Photons Any photons which enter the
  object but never reach the receptor due to
  scatter or absorption

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Probability of Photon Interaction

• Because the probability of photon interaction
  with matter is random, it is impossible to
  predict with certainty what will happen to a
  single photon when it enters matter
• When dealing with large numbers of photons it is
  possible to produce an average estimate

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Factors That Influence Photon Interaction

• depends on energy of photon
• depends on type of matter 

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Questions

• What is Attenuation?
• Attenuation is simply the reduction in the number
  of primary photons in the x-ray beam through
  absorption and scatter as the beam passes through
  an object in its path
• What are the two types of transmissions?
• Direct and Indirect
• What are the three types of photons?
• Primary, Exit and Attenuated
• What 2 properties affect photon interactions
• depends on energy of photon and depends on type
  of matter 

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Photon Interaction With Matter
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PHOTON INTERACTIONS

•   Photons are individual units of energy. As an
  x-ray beam or gamma radiation passes through an
  object, three possible interactions occur with
  each photon
• 1. It can penetrate the section of matter without
  interacting.
• 2. It can interact with the matter and be
  completely absorbed by depositing its energy.
• 3. It can interact and be scattered or deflected
  from its original direction and deposit part of
  its energy.

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Photons Entering the Human Body Will Either
Penetrate, Be Absorbed, or Produce Scattered
radiation

•    There are two kinds of interactions through
  which photons deposit their energy both are with
  electrons. In one type of interaction the photon
  loses all its energy in the other, it loses a
  portion of its energy, and the remaining energy
  is scattered.

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Coherent Scattering

• There are actually two types of interactions that
  produce scattered radiation Coherent and Compton
  Scattering.
• Coherent, Thompson, Rayleigh, classical, and
  elastic, is a pure scattering interaction which
  deposits no energy in the material. It is
  generally not significant in most diagnostic
  procedures.
• Less than 30 keV
• Interaction of low energy photon with outer shell
  electron. With a slight change in direction

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Compton Scattering

• A portion of the incident radiation "bounces off'
  or is scattered by the material. The most
  significant object producing scattered radiation
  in an x-ray procedure is the patient's body. The
  portion of the patient's body that is within the
  primary x-ray beam becomes the actual source of
  scattered radiation.
• A Compton interaction is one in which only a
  portion of the energy is absorbed and a photon is
  produced with reduced energy. This photon leaves
  the site of the interaction in a direction
  different from that of the original photon,
• This has two undesirable consequences. The
  scattered radiation that continues in the forward
  . direction and reaches the image receptor
  decreases the quality (contrast) of the image
  the radiation that is scattered from the patient
  is the predominant source of radiation exposure
  to the personnel conducting the examination

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Photoelectric absorption

• The incident photon must have enough energy to
  knock out an inner orbital electron of an atom.
  It is then completely absorbed by the electron,
  hence the name photo-electron.
• The now free electron may interact with other
  atoms causing excitation or ionization until all
  its kinetic energy has been spent. (usually
  within a few micrometers)
• In human body this energy transfer contributes to
  increased patient dose and contributes to
  biological damage

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Pair Production

•    Pair production is a photon-matter interaction
  that is not encountered in diagnostic procedures
  because it can occur only with photons with
  energies in excess of 1.02 MeV.
• The photon interacts with the nucleus in such a
  manner that its energy is converted into matter.
  The interaction produces a pair of particles, an
  electron and a positively charged positron.

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Photodisintegration

• Photodisintegration occurs when a high-energy
  photon is absorbed by an atomic nucleus. The
  nucleus splits to form lighter elements,
  releasing a neutron, proton or alpha particle in
  the process.

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Questions

• Photodisintegration occurs when a high-energy
  photon is absorbed by an atomic nucleus ?
• True
• What are the three photon interactions with
  matter?
• 1. It can penetrate the section of matter without
  interacting.2. It can interact with the matter
  and be completely absorbed by depositing its
  energy.3. It can interact and be scattered or
  deflected from its original direction and deposit
  part of its energy.
• Pair production can only occur at ____MeV
• 1.02

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Radiation Effects on Organ Systems
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Radiation Dose-Response Relationship

• Dose-Response Curves
• It is demonstrated graphically through a curve
  that maps the observed effects of radiation
  exposure in relation to the dose of radiation
  received.
• The graph demonstrates the relationship between
  the dose received (horizontal axis) and the
  biologic effects observed (vertical axis).
• The curve is either linear (straight line) or
  nonlinear (curved to some degree) and depicts
  either a threshold dose or a nonthreshold dose.

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Threshold and Nonthreshold Relationships

• Threshold
• a point at which a response or reaction to an
  increasing stimulation first occurs.
• below a certain radiation level or dose, no
  biologic effects are observed
• Nonthreshold
• any radiation dose will produce a biologic
  effect
• no radiation dose is believed to be absolutely
  safe

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Linear Nonthreshold Curve Linear Quadratic
Curve
Sigmoid
Threshold Curve
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Risk Models Used to Predict Cancer Risk and
Genetic Damage in Human Populations

• Majority of stochastic somatic effects (e.g.
  cancer) and genetic effects as low-dose levels
  from low-LET radiations
• Those employed in diagnostic radiology, appear to
  follow a linear-quadratic nonthreshold curve.
• Currently the committee recommends the use of
  linear nonthreshold curve of radiation
  dose-response for most types of cancer.
• The linear nonthreshold curve implies that the
  biologic response to ionizing radiation is
  directly proportional to the dose.

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Questions

• ______ is demonstrated graphically through a
  curve that maps the observable effects of
  radiation exposure in relation of radiation
  received.                   
• Answer Dose-Response Curve
• The graph demonstrates the relationship between
  the _____ (horizontal axis) and the _____
  observed (vertical axis)                  
• Answer Dose Received Biologic Effects
• _____ is a point at which a response or reaction
  to an increasing stimulation first occurs.
                  
• AnswerThreshold
• _____ is any radiation dose that will produce a
  biologic effect.                
• Answer Nonthreshold
• True or False.     Any radiation dose is
  absolutely "safe"                 
• Answer False

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Radiation Effects on Organ Systems
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Risk Models used to Predict Cancer

• The linear-quadratic, non-threshold curve
  estimates the risk associated with low-level
  radiation.
• The BEIR Committee believes it is a more accurate
  reflection of stochastic, somatic and genetic
  effects at low-dose levels from low-LET
  radiations.
• Leukemia, breast cancer, and heritable damage are
  presumed to follow this curve.

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Risk Models used to Predict Cancer

• The health effects of low-level radiation could
  follow any of these curves. The cost of radiation
  protection depends greatly on which curve is most
  accurate.
• That last idea is a real possibility, argues
  Mossman, who says bone and skin cancers show
  "convincing evidence" for a threshold, and "There
  is good evidence of linearity for breast and
  thyroid cancer." For leukemia, he sees "excellent
  evidence" that low doses have less effect than
  the linear model predicts.

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Rationale for Risk Model Selection

• In establishing radiation protection standards,
  the regulatory agencies have chosen to be
  conservative.
• That means, they use a model that might
  overestimate risk but is not expected to
  underestimate risk.
• The use of the linear dose-response model for
  radiation protection standards may exaggerate the
  seriousness of radiation effects at lower-dose
  levels from low-LET radiation.

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Risk Model used to Predict High-Dose Cellular
Response

• Nonstochastic effects of significant radiation
  exposure may be demonstrated graphically through
  the use of the linear, threshold curve of
  radiation response.
• Here, a biologic response dose not occur below a
  specific dose level.

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Risk Model used to Predict High-Dose Cellular
Response

• The sigmoid or S-shaped (nonlinear), threshold
  curve of radiation dose-response relationship
  indicates the existence of a threshold for
  high-dose cellular response.
• A threshold is a minimal dose of ionizing
  radiation below which observable effects will not
  occur.

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Risk Estimates

• Risk estimates are given in terms of absolute
  risk or relative risk.
• The absolute risk model predicts that a specific
  number of excess cancers will occur as a result
  of radiation exposure.
• The relative risk model predicts that the number
  of excess cancers rises as the natural incidence
  of cancer increases with advancing age in
  population.

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Questions

• Leukemia, breast cancer, and heritable damage are
  presumed to follow the --------------------
  curve.
• Answer linear-quadratic, non-threshold
• Nonstochastic effects can be represented by the
  use of the ----------------- curve of radiation
  response.
• Answer linear, threshold curve
• The risk model used for high-dose cellular
  response is the ------------------- curve.
• Answer sigmoid or S-shaped (nonlinear),
  threshold curve

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Questions

• True/ False
• The use of the linear dose-response model for
  radiation protection standards maybe conservative
  with the seriousness of radiation effects at
  lower-dose levels from low-LET radiation.
• Answer False (may exaggerate the seriousness)
• True/ False
• The absolute risk model predicts that a specific
  number of excess cancers will occur as a result
  of the natural incidence of cancer increases with
  advancing age in population.
• Answer False (absolute risk model predicts
  excess cancers will occur as a result of
  radiation exposure.)

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The Basics of Genetic Effects

• Genetic effects of ionizing radiation are
  biological effects on generations yet unborn
• Characteristics such as eye, hair or skin color
  that make each person unique are called genetic
  traits
• Changes in the genes of DNA can arise naturally,
  or as a result of exposure to radiation or to
  chemical and physical agents
• There are two types of mutations germ line and
  somatic

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Cause of Genetic Mutations

• Mutations that occur at random as natural
  phenomenon are called spontaneous mutations
• Spontaneous mutations in human genetic material
  cause a wide variety of disorders and diseases
• Agents that cause mutation are called mutagens

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Radiation interaction in the Body

• Radiation reacts directly with the DNA
  macromolecules in the body
• A mutation of this type could cause genetic
  disease in subsequent generations

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How Radiation Can Harm a Cell

• When a radioactive particle or wave hits a cell
  in the body, one of four things can happen
• It may pass through the cell without doing
  damage.
• It may damage the cell, but the cell may be able
  to repair the damage before it produces new
  cells.
• It may damage the cell in such a way that the
  damage is passed on when new cells are formed.
• It may kill the cell.

40
Radiation induced Genetic Effects in Humans

• Point mutations mat be either recessive or
  dominant
• The only evidence comes from experimentations on
  lab animals
• Effects in humans due to radiation is still under
  speculation, but proper radiation protection must
  be always used

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Doubling Dose Concept

• Animal studies of radiation induced genetic
  effects have led to the doubling dose concept
• Doubling dose measures the effectiveness of
  ionizing radiation in causing mutations
• The doubling dose for humans is estimated at
  1.56Sv (156 rem)

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Questions

• Genetic effects of radiation affect who?
• Generations yet unborn
• Mutagens are what?
• Anything that could cause a mutation
• Radiation interacts with what part of the body?
• The DNA in cells
• How can radiation harm a cell?

43
Questions

• Evidence on human radiation comes from what?
• Experiments on lab animals
• The Doubling Dose in humans is what?
• The doubling dose for humans is estimated at
  1.56Sv (156 rem)