Radiation Protection and Dosimetry

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Radiation Protection and Dosimetry

  1. 1. DOSIMETRY AND RADIATION PROTECTION Dr. Vibhuti Kaul
  2. 2. HISTORY • Wilhelm Conrad Roentgen • Accidentally discovered • Working with cathode rays tube • Fluorescence of screens • Observation of bones of hand
  3. 3. • 1st radiograph of hand of Bertha, Roentgen’s wife
  4. 4. Introduction • Radiation protection deals with dose received by populations, and avoidance of effects • Radiological protection, is the science of protecting people and the environment from the harmful effects of ionizing radiation, which includes both particle radiation and high energy electromagnetic radiation. • The doses involved are measurable even if the effects are not
  5. 5. Sources of Radiation
  6. 6. SOURCES • We are exposed to radiation everyday of our lives • Background radiation comes from natural sources • 2 types of sources: 1. External 2. Internal
  7. 7. External Sources • Cosmic & Terrestrial radiation • Cosmic radiation includes: - energetic subatomic particles - photons of extraterrestrial origin - interactions of primary cosmic radiation with atoms & molecules of earth’s atmosphere # Also greater at higher altitudes # Exposure from airline travel
  8. 8. • Terrestrial radiation: Radioactive nuclides in soil
  9. 9. Experiments are impossible, but scientific panels have evaluated the cumulative data, assessed the risks and developed standards • International Commission on Radiological Protection – report ICRP 60 (1991) • National Council on Radiation Protection – report NCRP 91 (1987)
  10. 10. Exposure groups Dose limits are governed by laws and regulation: to cover three distinct exposure groups 1. Occupational 2. Medical – patients who benefit from radiation exposure for diagnosis or treatment 3. Non-intentional – the public who do not benefit from exposure
  11. 11. The ALARA philosophy As low as reasonably achievable
  12. 12. Basic methods of protection against exposure to ionizing radiation Three basic factors • time • distance • shielding
  13. 13. Time Distance Shielding
  14. 14. TIME • The less time that people are exposed to a radiation source, the less the absorbed dose. • It's easy to understand how to minimize the time for external (direct) exposure. • Gamma and x-rays are the primary concern for external exposure.
  15. 15. Time Exposure rate =10mGy/h X Time = Total dose 1 hour = 10 mGy 2 hours = 20 mGy
  16. 16. Distance
  17. 17. DISTANCE • The farther away people are from a radiation source, the less their exposure. • It depends on the energy of the radiation and the size (or activity) of the source. • Distance is a prime concern when dealing with gamma rays. • As a rule, if the distance is doubled, the exposure is reduced by a factor of four.
  18. 18. DISTANCE
  19. 19. Inverse square law 150 mSv/h 0.06 mSv/h d=50cm
  20. 20. SHIELDING • Used when neither time nor distance is effective method to reduce the exposure. • Interposing a material between source of radiation & the point at which the it is desired to reduce the exposure. • The greater the shielding around a radiation source, the smaller the exposure.
  21. 21. SHIELDING • Degree of exposure reduction depends on physical characteristics of material: 1. Atomic number 2. Density 3. Thickness • For fixed X-ray imaging facilities most common materials are lead and concrete
  22. 22. SHIELDING • Standard design practice evaluating shielding is to measure the halving thickness of a material, the thickness that reduces gamma or x-ray radiation by half.
  23. 23. Penetrating power of radiation
  24. 24. Shielding photons
  25. 25. • The aim of radiation protection in dentistry is to obtain the desired clinical information with minimum radiation exposure to patients, dental personnel and the public. • National council on radiation protection (NCPR) and International council on radiation protection (ICPR). MPD (Maximum Permissible Dose) Maximum dose that a person or specified parts thereof shall be allowed to receive in a period.
  26. 26. • MPD differs for nonoccupationally & occupationally exposed persons. • Non occupationally exposed– 0.005 Sv/year • For occupationally exposed persons, the MPD is calculated by using formula, MPD = (Age – 18) x 5 rem • Occupationally exposed: 0.05 Sv/year
  27. 27. Exposure & Dose Concept • Exposure is the property of the x-ray beam. • It is measure of the no. of x-rays making up the beam of radiation and the energy transferred from x-rays to molecules of air under very standardized conditions • Old unit: Roentgen (R) • SI unit: Air kerma (Gy) • 1 Gy = 1 joule/kg
  28. 28. Exposure & Dose Concept What is absorbed dose? • It is a measure of energy absorbed by any type of ionizing radiation per unit mass of any type of matter • Old unit rad (radiation absorbed dose) • SI is Gray where 1 Gray = 1 J/kg • 1 Gy = 100 rads
  29. 29. Exposure & Dose Concept Incident dose • Incident dose = the dose measured on the intended surface of the patient, but without the presence of the patient • The SI unit used to measure the incident dose is the Gray, where 1 Gy = 1 J/kg
  30. 30. Exposure & Dose Concept Surface dose • The surface dose is measured with the body in the path of the beam by including the amount of scattered radiation. • Surface dose = incident dose + scattered radiation from the body The SI unit is the Gray (Gy)
  31. 31. Exposure & Dose Concept Body dose and effective dose • The body dose is the comprehensive concept for the organ or partial-body dose equivalent and the effective dose. • Body dose = sum of all organ or partial-body doses • Effective dose <= patient dose limit • The SI unit = sievert • 1 sievert = 1 Sv = 1 Joule/kilogram = 1 Gray
  32. 32. Methods of exposure & dose reduction • The guiding principle of diagnostic radiology in dentistry is to enhance the diagnostic benefits of dental radiographs and minimize the associated risks to patients and staff** ** from ADA Council on Scientific Affairs
  33. 33. There are four main concerns when dealing with radiation hazards. • First, patients should not be subjected to unnecessary dental radiography. • Second, patients need to be protected from unnecessary exposures. • Third, personnel in dental facilities be protected from unnecessary exposure to radiation in the course of their work. • Finally, the public requires adequate protection.
  34. 34. Methods of exposure & dose reduction Patient selection • Professional judgment • Radiographic selection criteria to guide are clinical or historical findings • Must justify taking any radiographs, not a blanket screening for all patients. • There are disadvantages and risks - must weigh up benefits against risks.
  35. 35. Conduct of examination 1. Choice of equipment 2. Choice of technique 3. Operation of equipment 4. Processing & Interpretation of radiographic image
  36. 36. Choice of equipment 1) Selection of image receptor • Faster films should be used • E speed Ekta speed group
  37. 37. Choice of equipment 2) Intensifying screens • For all extra oral radiographs • Reduce patient exposure by 55% • Two types of crystals calcium tungstate & rare earth
  38. 38. Choice of equipment 3) Focal spot to film distance • 32% decrease in surface exposure with longer distance • X- ray beam is less divergent
  39. 39. Choice of equipment 4) Collimation • Limits size of X-ray beam • Reduces patient exposure • Increased image quality • Scatter radiation is decreased
  40. 40. Choice of equipment 5) PID Film fog is decreased with the Use of rectangular position indicating device & film holders with rectangular collimators
  41. 41. Choice of equipment 6) FILTRATION • Selective filtration of low energy radiation • Decreased patient exposure • With no loss of radiological information • Aluminum
  42. 42. Choice of equipment 7) Leaded aprons & collars • Minimize patient exposure to radiation • ALARA principles • Attenuate 98% scatter radiation to gonads • Thyroid collar – 92% dose reduction to thyroid gland
  43. 43. Choice of Technique 1) BISECTING ANGLE TECHNIQUE An early method for aligning the x-ray beam and film with the teeth and jaws the bisecting angle technique.
  44. 44. Choice of Technique 2)PARALLELING TECHNIQUE In this method placing film parallel with long axis to the tooth. In this technique film is positioned towards the midline of oral cavity away from the teeth.
  45. 45. Choice of Technique Film holders to position the receptor BISECTING ANGLE TECHNIQUE PARALLELING TECHNIQUE
  46. 46. Operating the equipment • Selection of x-ray generating units Exposure settings be established • Kilovoltage – 70-90 kVp Decrease Kilovoltage – image contrast increases, vice versa • Milliampere- Seconds Image density is controlled by quantity of X-ray, this is controlled by mAs 2.2 mAs – E speed film – 90 kVp 4.2 mAs – E speed film – 70 kVp
  47. 47. Processing the film • Proper processing equipment • Darkroom with safelights • Automatic processor with appropriate safelight hood • Saves needless exposure of patient & operating costs
  48. 48. Interpretation of Images • Semi darkened room with light transmitted only through films • Variable intensity light source • Magnifying glasses
  49. 49. Protection of Personnel • Leave room & take position behind suitable barrier or wall • Walls of sufficient density and thickness,10.4-15.4 cm • Gypsum wallboard Lead between layers of wood • Thickness of 1.3-2.4 mm lead used for 60-70 kVp • Barium plaster or barium concrete
  50. 50. Protection of Personnel Position and Distance Rule If no barriers are available, you should stand at least six feet away from the patient at an angle of 90-135 degrees to the direction of the x-ray beam.
  51. 51. Protection of Personnel • Operator should never hold films in place, film holding instruments should be used. • Neither patient nor operator should hold radiographic tube housing
  52. 52. Protection of Personnel Personnel monitoring devices • Film Badges Thermo luminescent dosimeters (TLD)
  53. 53. Protection for General Public • Radiation area should be at corner of building • One extra brick with barium plaster for walls • No person is allowed while exposing • Warning board and light
  54. 54. SOURCES OF RADIATION
  55. 55. 1. Primary beam is defined as radiation originating from the focal spot. 2. Scattered or secondary radiation is the radiation originating from the irradiated tissues of the patient. 3. Leakage or stray radiation is the radiation from the X- ray tube had housing. 4. Scattered radiation is the radiation from filters and cones. 5. Scattered radiation is the radiation coming from the objects other than the patient such as the walls and furniture that the primary beam may strike.
  56. 56. MEANS OF PROTECTION FOR OPERATOR FOR PATIENT FOR ENVIRONMENT
  57. 57. Sources of Radiation 1. Primary X-ray beam 2. Scattered radiation Other Sources of Lesser Importance Include: • Leakage through the head housing • Scattered X-ray from the filters, cones • Scattered radiation coming from the objects other than the patient, such as walls and furniture that the primary beam may strike.
  58. 58. • Secondary Barriers - for surfaces receiving secondary or leakage radiation LEAD standard material - Viewing windows : lead glass - Lead salts or metallic lead added to rubber or plastics : protective gloves or aprons made
  59. 59. PROTECTION AGAINST PRIMARY BEAM Primary beam: It is defined as radiation emitted by the focal spot of the target
  60. 60. i. Effort must be made so that the operator can leave the room or take a suitable position behind a barrier or wall during exposure. ii. Dental Operatory should be designed and constructed to meet the minimum shielding requirements. iii. Position Distance rule- states that the operator should stand at least six feet away from the source of radiation or the operator should be at an angle of 90o to 135o, with respect to the direction of the central ray.
  61. 61. This rule takes advantage of the inverse square law to reduce the intensity and also considers that in this position the patient’s head will absorb the most scattered radiation.
  62. 62. iv. Behind a barrier, made of suitable material, or v. If there is no shield or barrier the operator should use a lead apron vi. The film should never be held by the operator vii. There should be no use of fluorescent mirrors in the oral cavity at the time of exposure viii. Avoid holding the X-ray tube head of the machine.
  63. 63. PROTECTION AGAINST LEAKAGE RADIATION Leakage or Stray radiation is defined as radiation emitted by any other part of the X-ray tube other than the focalspot.
  64. 64. i. Neither the tube housing nor the cone should be hand held during the exposure. ii. The machine should be periodically checked for leakage.
  65. 65. PROTECTION AGAINST SECONDARY AND SCATTERED RADIATION Secondary radiation is defined as the radiation emitted by a substance through which X-rays are passing. Scattered radiation is defined as that radiation that has under gone change in direction during passage through a substance.
  66. 66. i. Use of high speed films ii. Replace the short plastic cone with an open ended lead lined cone iii. Adequate filtration of the primary beam iv. Use of collimator, to reduce the diameter of the beam v. Use of film badge/ TLD badge/ Pocket Dosimeter, for personnel radiation monitoring , to avoid accumulated over exposure.
  67. 67. PATIENT SELECTION • High yield or referral criteria, which is the clinical or historical findings that identify patients for whom a high probability exists that a radiographic examination will provide information affecting their Rx & prognosis.
  68. 68. CONDUCT OF THE EXAMINATION 1. Selection of the Image Receptor: a. Use of high speed films b. Use of screen films
  69. 69. 2. Use of Intensifying Screens 3. Focal Spot Film Distance: As X-rays are less divergent at a longer distance, there is a decrease in the volume of the patient exposed tissue volume. Longer FSFD results in 32% reduction in exposed tissue volume.
  70. 70. 4. Collimation of the beam: collimation helps to control the size and shape of the X-ray beam, allowing only the useful mean to emerge. The beam should be limited to as small as an area possible for a particular radiographic examination. The recommended beam size is not more than 2 ¾” in diameter at the patient’s face, when the source film distance is 18 cm or more. Collimation decreases the risk of radiation and decreases the fog, with a sharper image and better contrast.
  71. 71. Collimation Types of collimators 1. Diaphragm 2. Rectangular 3. Tubular • Using rectangular PID – reduces the area of the patient’s of skin surface exposed by 60% over that of a round • Reduction in beam size makes aiming the beam difficult • Film holders
  72. 72. 5. Filtration: Filtration preferentially absorbs low energy photons which are undesirable as they add to the patient’s skin dose but do not have enough energy to penetrate the tissue and bring about the image formation. Operating range of machine Amount of filtration Below 50 kVp 0.3 to 0.5 mm of Al 50-70 kVp 1.2-1.5 mm of Al Above 70kVp 2.1-4.1 mm of Al
  73. 73. 6. Use of high kVp: Higher kVp is used to keep the incident skin doses acceptable. The equipment should be capable of operating at a kilo voltage of 60 kVp or higher. 7. Use of positioning indicating device: These help to minimize the volume of tissue irradiated in intraoral radiography, it is necessary to increase the target film distance by using longer position indicating devices to direct the X-ray beam.
  74. 74. 8. Film holding devices: These offer protection to the patient, because - their use often reduces frequency of retakes, as the film can be positioned more accurately in the patient’s mouth. - they also provide an external guide to indicate the film position. - The possibility of cone-cuts is also reduced. -Some of the holders also collimate the beam to the size of the film being used. - Exposure to the patient’s fingers is also reduced.
  75. 75. 9. Timers: Most equipment are provided with ‘dead man’ timers. This timer requires a continuous pressure on the button (switch) during the exposure cycle in order to continue the operation of the X-ray machine. If the button is released the exposure is terminated. Dental X-ray machine timers generally automatically reset once the exposure has been terminated. Care should be taken that they are not capable of initiating another exposure until the switch is pressed again. Also, it should not make an exposure if the timer set to zero or off position.
  76. 76. 10. USE OF PROTECTIVE BARRIERS: a. Leaded aprons b. Gonadal shields c. Thyroid shields
  77. 77. DOSIMETRY
  78. 78. TERMS • Determining the quantity of radiation exposure or dose is termed dosimetry. • The term dose is used to describe the amount of energy absorbed per unit mass at a site of interest. • Exposure is a measure of radiation based on its ability to produce ionization in air under standard conditions of temperature and pressure (STP).
  79. 79. UNITS OF MEASUREMENT • EXPOSURE Exposure is a measure of radiation quantity, the capacity of radiation to ionize air. SI unit: air kerma (kinetic energy released in matter) Traditional unit: roentgen
  80. 80. UNITS OF MEASUREMENT • ABSORBED DOSE Absorbed dose (DT)is a measure of the energy absorbed by any type of ionizing radiation per unit mass of any type of matter. SI unit: Gray (Gy) Traditional unit: rad (radiation absorbed dose)
  81. 81. UNITS OF MEASUREMENT • EQUIVALENT DOSE Equivalent dose (HT) is used to compare the biologic effects of different types of radiation to a tissue or organ. Relative biologic effectiveness of different types of radiation is called the radiation- weighting factor (WR). TYPE OF RADIATION WEIGHTING FACTOR Photons 1 5 keV Neutrons/Protons 5 α particles 20
  82. 82. UNITS OF MEASUREMENT • EQUIVALENT DOSE SI unit: Sievert (Sv) Traditional unit: rem (roentgen equivalent man) 1 Sv = 100 rem
  83. 83. UNITS OF MEASUREMENT • EFFECTIVE DOSE (E) Estimate the risk in humans Allows comparison of risk of exposure to one region with another in body. Considers radiosensitivity of different tissues for cancer formation or heritable effect. Comparative radiosensitivities of different tissues are measured by tissue-weighting factor (WT).
  84. 84. TISSUES TISSUE WEIGHTING FACTOR Red bone marrow, breast, colon, lung, and stomach 0.12 Gonads 0.08 Bladder, esophagus, liver & thyroid 0.04 Bone surface, brain, salivary glands & skin 0.01 Others 0.12
  85. 85. UNITS OF MEASUREMENT • RADIOACTIVITY The measurement of radioactivity (A) describes the decay rate of a sample of radioactive material. SI unit: Becquerel (Bq) = 1 disintegration/second Traditional unit: Curie (Ci) => activity of 1 g of radium (3.7 x 1010 disintegrations/second) 1 mCi = 37 megaBq 1 Bq = 2.7 x 10-11 Ci
  86. 86. • Dosimetry tracks exposures and monitors external radiation exposures. • Dosimetry use ensures that we are following the principle of ALARA, keeping exposures as low as reasonably achievable. • Dosimetry only measures external radiation exposure and offers no protection from radiation.
  87. 87. Dosimetry • A Whole Body Dosimeter at all times when working with or around radiation sources. • Where to wear dosimetry? • whole body dosimeter should be worn on the torso, closest to the source of radiation.
  88. 88. Dosimetry • TLD rings are worn on the hand used most often to handle radioactive materials • Most dosimeters (Whole Body and Finger Ring) are issued for three months
  89. 89. • Badges are slim & lightweight • Can be worn on the body or extremities. • Film encased in a specially designed molded plastic holder. • Multi-filter system • radiation will reach the exposed film after penetrating the five different filter areas: open window (OW), aluminum (Al), copper (Cu), lead/tin (Pb/Sn), and plastic (Pl). • A complex algorithm is deployed to analyze the results of these filter areas and report dose. Film badges
  90. 90. The badge has six filters: 1. An open window which allows all incident radiation that can penetrate the film wrapping to interact with the film. 2. A thin plastic film which attenuates beta radiation but passes all other radiations 3. A thick plastic filter which passes all but the lowest energy photon radiation and absorbs all but the highest beta radiation. 4. A dural filter which progressively absorbs photon radiation at energies below 65keV as well as beta radiation. 5. A tin/lead filter of a thickness which allows an energy independent dose response of the film over the photon energy range 75keV to 2Mev. 6. A cadmium lead filter where the capture of neutrons by cadmium produces gamma rays which blacken the film thus enabling assessment of exposure to neutrons.
  91. 91. TLD (Thermoluminiscent Dosimetry) • Some materials absorb energy from ionizing radiation, store it such that later it can be recovered in the form of light when the materials are heated. • Amount of light released is directly proportional to the energy absorbed from the ionizing radiation • Hence to the absorbed dose the material received. • Lithium fluoride (LiF) and calcium fluoride
  92. 92. TLD (Thermoluminiscent Dosimetry) • Higher sensitivity, • Wider range of exposure measurement, • Greater precision, • Extended wear period (3 months) • Can measure gamma/x- ray exposures down to 1 mrem and beta exposures down to 10 mrem.
  93. 93. TLD • Have a precision for approximately 15% for low doses, improves to 3% for high doses.
  94. 94. MECHANISM OF TLDs • When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal and deposits all or part of the icident energy in that material. • Some of the atoms in the material that absorb that energy become ionized producing free electrons and areas lacking one or more electrons, called holes. • Imperfection in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.
  95. 95. MECHANISM OF TLDs • Heating the crystal causes the crystal lattice to vibrate releasing the trapped electrons in the process. • Released electrons return to the original ground state, releasing the captured energy from ionization as light. Hence, the name thermoluminiscent. • Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor.
  96. 96. MECHANISM OF TLDs • Instead of reading the optical density (Blackness) of a film, as is done with the film badges, the amount of light released vs the heating of the individual pieces of thermoluminiscent material is measured. • The glow curve produced by this process is then related to the radiation exposure. • The process can be repeated many times.
  97. 97. ADVANTAGES • Small in size & light in weight • Chemically inert • Almost tissue equivalent • Usable over wide range of radiation qualities & dose values • Accurate & reproducible readings • Automation compatible • No wet chemistry reqd • Reusable • Economical • Read out simple & quick • Apart from initial fading, can store dose over long period of time
  98. 98. DISADVANTAGES • Read out is destructive, giving no permanent record, results cannot be checked or reassessed. • Only limited information provided on type of energy of the radiation • Dose gradients are not detectable.
  99. 99. Finger dosimeters • For those who handle radioisotopes or who perform interventional radiographic procedures • Single use • Can be worn under surgical gloves
  100. 100. IONIZATION CHAMBER • An ionization chamber is used to determine the radiation exposure of a room. • It consists of two oppositely charged plates, separated by a known volume of air. • The plates are connected to a galvanometer to measure the charge. • Before using, standard charge is applied to the plates. • When the X-ray beam exposes the air, there is formation of ion-pairs. The positive & negative ions get attracted to the oppositely charged plates thus resulting in the partial discharges. • Accordingly the radiation exposure depends on the number of ion pairs produced & dropped in the potential.
  101. 101. IONIZATION CHAMBER
  102. 102. THANK YOU

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