Medical Exposure

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Medical Exposure

• Dr. Nabil Maalej

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Exposure To Radiation
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Radiation Dose

• Specification of the radiation absorbed dose in
  Gy or rad is inadequate for complete and accurate
  assessment of radiation hazard

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Dose Modifying Factors

• The type of radiation involved (wR)
• The part of the body exposed (wT) the active
  blood forming organs, the gonads, and the lens of
  the eye are specifically sensitive
• The time span over which the radiation dose is
  delivered
• The age of the exposed individual the developing
  fetus is especially sensitive

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Dose Equivalent

• The dose equivalent is the absorbed dose in
  tissue multiplied by a quality factor wR
• HT,R wR DT,R
• The SI units for dose equivalent is the Sievert
  1 J/kg for wR 1
• 1 Sievert 100 rem

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Radiation Weighting Factors
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Effective Dose

• Effective Dose is the summation of tissue
  equivalent doses each multiplied by the tissue
  weighing factor

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Tissue Weighting Factors
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X-Rays

• Diagnostics radiology is necessary to visualize
  human anatomy and detect disease
• Good image quality and low patient doses are
  frequently conflicting requirements
• The medical practitioner should keep the
  radiation exposure As Low As Reasonably
  Achievable (ALARA) to realize the benefit of
  imaging while minimizing dose to the patient.

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Exposure

• Exposure refers to the amount of ionization of
  air caused by x-ray and ?-ray beams
• Exposure E Q/m
• The unit is the Roentgen (R) the ionization
  liberating an amount of charge equal to 2.58 X
  10-4 coulombs/kg of air
• The SI units is C/kg 3876 R
• Exposure can be measured using ionization chambers

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Exposure Variation With X-ray Machine Settings

• Exposure ? (kVp)2 (1/d)2 (mAs)
• kVp is the applied voltage, d is the distance
  from the focal spot, and mAs is the applied
  current multiplied by the time
• Typically, for d 1 m and kVp 100 and a 2mm
  Aluminum filtration of the beam, the Exposure is
  10 mR/mAs

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Example 1

• Estimate the exposure at a distance d 2 m for
  5 mAs and 150 kVp, assume 2 mm Al filtration?

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Solution
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Typical Entrance Skin Exposure
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The f factor (rads/roentgen)
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X ray Dose to a Patient

• If the exposure is measured at a location , the
  absorbed dose in rads that would be delivered to
  a person at that location can be estimated by
  means of the f factor
• For practical purposes the exposure level in
  roentgens is approximately equal to to the
  absorbed dose in rads which is equal to the dose
  equivalent in rems

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X ray Dose to a Patient

• Surface Doses can also be conveniently measured
  using TLDs
• Doses to inaccessible organs can be measured
  using body equivalent phantoms
• Mathematical phantoms such as MIRD-5 and
  mathematical calculations such as Monte Carlo can
  be used to estimate doses

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Reduction of Exposure

• The medical practitioner should keep the
  radiation exposure As Low As Reasonably
  Achievable (ALARA) to realize the benefit of
  imaging while minimizing dose to the patient.
• Use of contrast media such as barium sulfate and
  organic iodine
• Use digital radiography to capture and store
  images in digital form
• Digital Subtraction Angiography

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Nuclear Medicine

• There is a rapid expansion in the diagnostic use
  of radio-pharmaceuticals
• Some common procedures are bone, thyroid, hart,
  lung, liver and kidney scans
• 99mTc is the radionuclide of choice in 75 of
  nuclear medicine procedures

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Maximum Permissible Dose

• The maximum permissible dose (MPD) to the whole
  body for radiation workers should not exceed 50
  mSv per year as recommended by ICRP
  (International Commission of Radiological
  Protection) and NCRP
• The Nuclear Regulatory Commission NRC recommends
  that and individual lifetime exposure should not
  exceed (N-18) X 50 mSv, where N is the age in
  years

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Maximum Permissible Doses (NCRP)
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Typical Doses
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Radiation Safety

• Beside observing the MPD the rule is to keep
  radiation as low as reasonably achievable (ALARA)
• ALARA can be applied to the handling of radiation
  sources, to storage and shielding techniques, and
  to the design and layout of the laboratory

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Exposure Rate Constant

• The exposure rate constants ? (R.cm2/mCi.hr) for
  a radionuclide is the exposure due to ?-rays and
  x-rays in R.hr, at a distance of 1 cm from an
  unshielded 1 mCi source of that radionuclide
• Radiation exposure levels caused by ?-ray and
  x-ray emitters can be estimated from exposure
  rate constants ?

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External Dose Calculation

• The exposure rate E(R/hr) at a distance d(cm)
  from a source of activity A(mCi) and an exposure
  rate constants ?(R.cm2/mCi.hr)
• E A ?/d2
• Because ?-rays and x-rays are emitted
  isotropically exposure levels decrease as the
  square of distance from the source

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Exposure Rate Constants
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Example

• Calculate the exposure rate at 10 cm and 300 cm
  distances from a syringe containing 30 mCi of
  99mTc? The exposure rate constant for 99mTc is
  0.6 (R.cm2/mCi.hr).

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Answer

• At d10 cm,
• E A ?/d2 30(mCi) X 0.6 (R.cm2/mCi.hr)/(10 cm2)
• 0.18 R/hr 180 mR/hr
• And at d10 cm,
• E 30(mCi) X 0.6 (R.cm2/mCi.hr)/(300 cm2)
• 0.0002 R/hr 0.2 mR/hr
• This illustrates the strong effect of distance on
  radiation exposure

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Reduction of Exposure from External Sources

• The basic principle of reducing doses from
  external exposure are described by the Time,
  Distance, Shielding (TDS) rules
• Decrease the time of exposure
• Increase the distance from the source
• Use shielding where necessary

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Effective Shielding

• Use lead boxes for storage, lead lined syringes,
  lead aprons, lead bricks for lining storage
  area, leaded glass for viewing

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Prevention of Internal Exposure

• No eating, drinking or smoking in hot labs
• Wear lab coats and gloves
• No food storage in hot areas
• Absolutely no pipetting
• Wash hands
• Perform work on absorbent pads and in ventilated
  hoods
• Promptly and appropriately dispose of radioactive
  waste
• Appropriately store radioactive materials
• Use personnel dosimeters and laboratory monitors

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Conclusion

• The exposure from natural radiation is on the
  average higher than the exposure from medical
  radiological procedures
• In order to minimize damage from radiation it is
  necessary to observe the maximum allowable limits
  and to keep exposure As Low As Reasonably
  Achievable
• One can usually measure exposure and estimate the
  dose from a radiation source