Basic radiation protection & radiobiology

Transcription

1 Basic radiation protection & radiobiology By Dr. Mohsen Dashti Patient care & management 202 Wednesday, October 13, 2010

2 Ionizing radiation. Discussion issues Protecting the patient. Protecting the radiographer. Radiation monitoring.

3 Ionizing radiation What are the sources of ionizing radiation? 1. Natural radiation. - What is natural radiation? -- Sources of radiation that occur spontaneously in nature and can be affected by human activity. - Examples: -- Cosmic radiation.. The sun and other planets. -- Radioactive substances on earth. Uranium and radium. - Natural radiation sources are given less attention to their hazardous potential.

4 Ionizing radiation What are the sources of ionizing radiation? 2. Manmade radiation. - What is manmade radiation? -- Sources of radiation that are developed by humans and used in different fields of technology. - Examples: -- Nuclear industry. Weapons & nuclear power stations. -- Radionuclide. Radioactive elements & radiopharmaceuticals. -- Medical radiation. Medical imaging & dental exposure.

5 Ionizing radiation Manmade radiation. - It is known as x-rays, which is a form of electromagnetic radiation that travels at the speed of light depositing energy randomly. How can we produce x-rays? 1. Source of electrons. 2. Force to move electrons rapidly. 3. Element to stop this movement rapidly.

6 Manmade radiation. Ionizing radiation - What happens to x-rays when they are produced? 1. Absorbed. 2. Scatter. 3. Pass through undistributed.

7 Manmade radiation. Ionizing radiation - How do x-rays interact with matter? 1. Classic coherent scattering. -- Interaction with matter in which a low-energy photon (below 10 kev) is absorbed and released with its same energy, frequency and wavelength but with change of direction. 2. Photoelectric interaction. -- Interaction with matter in which proton strikes an inner shell electron, causing its ejection from orbit with complete absorption of the photon s energy.

8 Ionizing radiation Manmade radiation. - How do x-rays interact with matter? 3. Compton scattering. -- Interaction with matter in which a higher-energy photon strikes a loosely bound outer electron, removing it from its shell, and the remaining energy is released as scatter photon. 4. Pair production. -- Interaction between matter and photon possessing a minimum of 1.02 MeV of energy, producing two oppositely charged particles. 5. Photodisintegration. -- Interaction directly with the nucleus of photon possessing a minimum of 10 MeV, causing excitement followed by emission of nuclear fragment.

9 Ionizing Radiation Standards for regulation of exposure: - What guidelines available to limit radiation dose? 1. No-threshold. -- No dose exists below which the risk of damage does not exist. 2. Risk versus benefit. -- The benefit to the patient performing radiographic procedure far outweigh the risk of possible biologic damage.

10 Radiation risk. Ionizing radiation

11 ALARA Ionizing radiation - To keep radiation dose as low as reasonably achievable. -- The annual whole-body dose-equivalent limit for the occupational worker is 50mSv (5 rem). -- The whole-body dose-equivalent limit for the general population is one tenth the occupational worker s annual limit or 5 msv (0.5 rem). - Sv: unit in the SI system to measure the dose-equivalent or biologic effectiveness of differing radiation; 1 Sv is equal to 100 rems.

12 Ionizing radiation

13 Protecting the patient ALARA concept can be practiced with the patient by utilizing 3 methods: 1. Time: - Time minimization is the most important element to protect the patient from radiation dose. How? -- Applying the rules of radiographic techniques. -- Using the exposure chart to determine the correct amount of radiation to produce an image. -- Minimizing repeat rates to reduce the patient s time in the path of the x-ray beam.

14 2. Distance: Protecting the patient - Distance maximization is another element to reduce patient radiation dose. Why? -- This serve to lessens the skin or entrance dose to the patient. -- Increasing the distance should be kept to a reasonable range so radiation dose will not be affected. How? -- For you to answer??? 3. Shielding: - Use of shield to protect sensitive or unexposed region of the patient s body is another method to protect the patient from radiation dose.

15 3. Shielding: Protecting the patient - The rule indicates that patients should be shielded whenever they are 4-5 cm from the primary x-ray beam. -- Shields are made of lead, which absorbs x-rays through the process of photoelectric effect, thereby minimizing patient exposure. Types of shield: 1. Flat contact shield: made of a combination of vinyl and lead. Placed directly over the gonads of the patient. 2. Shaped shield: cup shaped and made specifically for male patients.

16 Protecting the patient 3. Shadow shield: mounted on the side of the collimator of the x-ray tube and can be manipulated to extend into the path of the beam.

17 Protecting the radiographer The same methods are used to protect the radiographer from extra radiation dose. - The radiographer should spend the least amount of time possible in a room when a source of radiation is active. - Fluoroscopy requires the radiographer to spend longer time in an active radiation room, therefore extra protection should be considered. - Distance is the best measure to protect the radiographer from radiation dose. - Inverse square law should be applied to reduce the impact of radiation dose.

18 Protecting the radiographer Inverse square law: The intensity of radiation varies inversely with the square of the distance. What does it mean? -- For you to answer??? - Submit your answer next week

19 Protecting the patient - Lead shield and aprons must be used by the radiographer whenever radiation is active. - Aprons and lead shields must in in good conditions and crack free to avoid passing radiation into the radiographer. - The minimum permissible amount of lead equivalency for aprons used where the peak kilovoltage is 100 should be 0.25 mm.

20 Radiation monitoring - Discuss the four main radiation monitoring methods used in x- rays: film badges, thermoluminescent dosimeters, pocket dosimeters, and field survey instruments.

21 See you next week