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Dosimetry
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- 2. • Determining thequantity of radiation exposure or dose. • Exposure – measure of radiation on the basis of its ability to produce ionization in air under standard conditions of temperature and pressure (STP).
- 3. Absorbed dose (DT) •Energy absorbed by any type of ionizing radiation per unit of mass of any type of matter.
- 4. Equivalent dose (HT) •compare the biologic effects of different types of radiation on a tissue or organ. • HT = WR x DT • Particulate radiations have a high LET & are more damaging to tissue than x- rays – this biologic effectiveness of different types of radiation – WR • Photons – 1 • Neutrons & high-energy protons – 5 • α particles - 20
- 5. Effective dose (E) •Estimate the risk in humans. • E = Σ WT x HT • Radiosensitivity of different tissues measured by WT. • Red bone marrow, breast, colon, lung, & stomach - 0.12 • Gonads - 0.08 • Bladder, esophagus, liver, & thyroid - 0.04 • Bone surface, brain, salivary glands, & skin - 0.01
- 6. Radioactivity (A) • Decayrate of a radioactive material.
- 9. • NCRP recommendsDiagnostic Reference Levels of 1.6 mSv skin dose for IOPA & bitewing. • Average weekly exposure for patient/ operator - 0.001 Sv. • Maximum of 13 week exposure is 0.05 Sv. • Dentist and staffs are allowed to receive 20 mSv/yr - 0.2-1% of allowable exposure How much is acceptable ???
- 11. The principle: • Physical& chemical effects produced by the radiation. 1. Ionization 2. Photographic effect 3. Luminescence 4. Scintillation Radiation monitoring devices
- 12. 1. Ionization Principle: • Ionizationin air by radiation.
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- 16. Advantages: • Most accuratemethod. • Gives immediate information. Disadvantages: • No permanent record of exposure. • No indication of the type of energy of the radiation. • Not sensitive to low energy radiation. • Fragile & easily damaged.
- 17. Principle: • The abilityof radiation to blacken the photographic film. Application: • Film badges. 2. Photographic effect
- 18. Principle: • The propertyof certain materials that emit light when stimulated by - chemical/ electrical/ heat. Application: • TLD badges 3. Luminescence
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- 20. Principle: • Certain crystals- sodium iodide & cesium iodide, absorb radiation & convert it to light. 4. Scintillation
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- 22. • Monitor theindividuals who are exposed to radiation during the course of their work. • Must for all occupationally exposed individuals. • Mandatory to wear personnel dosimeter if the annual dose is greater than 1 mSv. • Pocket dosimeter, the film badge or the thermoluminescent dosimeter. Personnel Dosimeter
- 23. • Radiation measurementby time-integrated dose, i.e. the dose summed over a period of time, usually about 3 months. • The dose is consequently stated as an estimate of the effective dose equivalent to the whole body in mSv for the reporting period. • Dosimeters used for personnel monitoring have dose measurement limit of 0.1–0.2 mSv (10–20 mrem).
- 24. Pocket dosimeter 1. Minometer/Condensor type/ indirect reading type • Principle: ‘air wall’ ionization • Working: • Insertion into a charger - voltage potential. • Radiation penetrating the chamber causes the current to leak in proportion to the radiation exposure. • Reinsert into the charge reader - voltage drop is calibrated. • 2 dosimeters are worn at a time. • Lower of the two readings taken as more accurate.
- 25. 2. Direct ReadingDosimeter • Principle: Gold Leaf Electroscope. • Working: • Charging to 200V - quartz fiber is displaced electro statically. • Fiber is viewed through a lens & focused on a scale. • Exposure to radiation discharges the fiber, allowing it to return to the original position. • Can measure radiation up to 50 C/kg (200 mR).
- 27. Film badge monitoring •Has small X-ray films sandwiched between several filters that detects radiation. • Inexpensive, simple mode of functioning. • Detects radiation at or above 0.1 mSv.
- 29. Wearing of thebadge: • Monitored every 2 weeks/ 4 weeks. • If accidental high exposure be suspected - should be immediately processed.
- 30. Advantages: 1. Good formeasuring any type and energy of radiation. e.g. X- rays, gamma radiations. 2. Continuous assessment is possible. 3. Provides a permanent record of dose received. 4. Simple, robust & relatively inexpensive.
- 31. Disadvantages: 1. They areinsensitive to radiation below 0.1 mSv. 2. They cannot be worn longer than 4 weeks duration at a stretch due to fogging. 3. Accuracy is only 10 to 50%. 4. The results are dependent on processing, strength, type of developer used & speed of the film used. 5. No immediate indication of exposure- all information is retrospective.
- 32. Thermoluminescent dosimetry (TLD) Principle •Property of certain materials to emit light when they are stimulated by heat.
- 34. Working: Few electrons become trappedin higher energy levels Light is collected & measured by photomultiplier tube Energy is released in the form of light by heating 250C Amount of light is proportional to the radiation dose (Sv)
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- 36. Wearing of thebadge:
- 37. Advantages: 1. Can measureexposures to individuals as low as 1.3 C/kg (5 mR). 2. Can withstand certain degree of heat, humidity, & pressure. 3. Crystals are reusable. 4. Instantaneous readings are possible if the department has a TLD analyzer. 5. Read out simple and quick. 6. Can store dose over long period of time.
- 38. Disadvantages: 1. Not costeffective. 2. Read out is destructive, giving no permanent record. 3. Results cannot be checked or reassessed.
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Editor's Notes
- #5 deposition of 1 Gy of high-energy protons causes five times as much damage as 1 Gy of x-ray photons
- #6 for instance, the jaws, the it measures the equivalent whole-body dose. allows the risk from exposure to one region of the body to be compared with the risk from exposure to another region
- #8 In diagnostic radiology : 1R = 1 r = 1 rem For diagnostic radiology 1 Sv equals 1 Gy.
- #11 Increased risk of tumor if exposed to > 100 mGy.
- #13 The process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or ions is termed as ionization.
- #14 electrode positioned in the middle of a cylinder that contains gas. Strength of the current is proportional to the radiation intensity.
- #19 Lithium fluoride will emit light when stimulated by heat. on luminescence property is (TLD)
- #20 Lithium fluoride will emit light when stimulated by heat. on luminescence property is (TLD)
- #22 Radiation on scintillating crystals - absorbs it and emit light - photomultiplier tube converts into an electrical pulse & measured
- #23 The values obtained are valid only when properly worn & irradiated only during occupational exposure & are returned on time.
- #25 recharge the Indirect Reading Pocket Dosimeters after every reading.
- #27 can measure radiation up to 50 C/kg (200 mR)
- #28 In India, film badges have been recently replaced by TLD badges.
- #30 1, as many low energy photons may not penetrate the film.
- #32 2. as an outcome of high temperature & light. 4. as many low energy photons may not penetrate the film. In India, film badges have been recently replaced by TLD badges.
- #37 collar level - outside the lead apron - estimate dose to the unprotected regions H & N. trunk level - underneath the lead apron - estimate of radiation to protected organs. If only one is worn - must be at the collar outside the lead apron, because, the neck receives 10–20 times more radiation
- #38 Application: Radiotherapy- for measuring doses received by patients while actually undergoing the treatment exposures.