Medical Physics - Electromagnetic Radiation.pdf

Medical Physics Medical Physics Electromagnetic Radiation Electromagnetic Radiation Contents : Electromagnetic Radiation 3 X-ray 5 Types of Interaction with Matter 11 Reasons for performing x-rays 23 Biological effect of ionising radiation 24 Radiation Therapy Process 31 Ultraviolet rays 32 Electrom...

Medical Physics Medical Physics Electromagnetic Radiation Electromagnetic Radiation Contents : Electromagnetic Radiation 3 X-ray 5 Types of Interaction with Matter 11 Reasons for performing x-rays 23 Biological effect of ionising radiation 24 Radiation Therapy Process 31 Ultraviolet rays 32 Electromagnetic Radiation Electromagnetic Radiation: Field is a physics term for a region that is under the influence of some force that can act on matter within that region. Stationary electric charges produce electric fields, whereas moving electric charges produce both electric and magnetic fields. Regularly, repeating changes in these fields produce what we call electromagnetic radiation. Electromagnetic radiation transports energy from point to point. Electromagnetic Radiation X-ray Electromagnetic Radiation X-ray: X-rays are electromagnetic radiation emitted by charged particles (usually electrons). X-rays and gamma rays have identical properties, only differing in their origin. In X-ray emission tubes, X-ray is emitted by the acceleration of Electrons. Electromagnetic Radiation The practical range of photon energies emitted by radioactive atoms extends from a few thousand eV up to over 7 MeV. On the other hand, linear accelerators are able to produce more energetic photons. When a γ-ray or X-ray passes through a medium, an interaction occurs between the photons and matter resulting in energy transfer to the medium. Electromagnetic Radiation The wavelength range of X-rays is 1 nm to 0.001 nm. The interaction can result in a large energy transfer or even complete absorption of the photon. However, a photon can be scattered rather than absorbed and retain most of the initial energy while only changing direction. An X-ray or γ-ray photon transfers its energy to the atoms and molecules of the body cells they interact with. Electromagnetic Radiation As a result, the exposed cells are affected. Those cells take part in the construction of tissues which are also affected by the interaction. The tissues compose the whole body which is eventually affected by the interaction of such X-rays or γ-rays. A schematic arrangement of these steps is given in Fig. Interaction of gamma rays or X-rays with the human body or matter Electromagnetic Radiation Overall, when an X-ray beam or gamma radiation passes through an object, three possible fates await each photon, as shown in Fig. : 1. It can penetrate and transmit 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. Interaction of gamma rays or X-rays with the human body or matter Types of Interaction with Matter Electromagnetic Radiation Types of Interaction with Matter: There are three major mechanisms by which photons can interact with matter. Those are photoelectric absorption, Compton scattering, pair production. All these mechanisms of interaction result in the transfer of energy to the electrons of matter or tissues. The electrons then transfer this energy to matter. Electromagnetic Radiation Photoelectric Absorption: The photoelectric absorption or effect is the most important interaction of lowenergy photons with matter. This effect is dominant in the 0–0.5 MeV photon energy range. Due to its dominancy in low-energy range, this phenomenon plays a major role in radiation dosimetry, diagnostic imaging, and low-energy therapeutic application. Electromagnetic Radiation It has been determined experimentally that when light shines on a metal surface, the surface emits electrons. This leads to the explanation of photoelectric effect which can be defined as the phenomenon in which a light photon interacts with a material and gives up all its energy to detach and move out an electron from the surface of the material. The photoelectric effect requires the presence of photons with energy equivalent to approximately 1 MeV in elements with large atomic numbers. Electromagnetic Radiation Photoelectric effect occurs when a bound electron from the atom is ejected after interaction with a photon of energy hν. Since the incident photon transfers its energy in two portions, therefore, the following mathematical equation can be developed to express this transfer of energy. Where h is the Planck’s constant, hν is the energy of incident photon, Φ = hν0 is the work function of the material, and T is the kinetic energy of the ejected photoelectron. Electromagnetic Radiation Compton Scattering: Compton’s scattering is the most important interaction in both therapeutic and diagnostic medical physics. For low-Z materials such as air, water, and human tissues, Compton scattering dominantly occurs in approximately 100 keV– 30 MeV photon energy range. In high-Z materials, its dominant energy range is 0.5 MeV– 10 MeV. Electromagnetic Radiation Compton scattering (or the Compton effect) is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound electrons from the outer valence shells of atoms or molecules. Compton effect occurs when photons interact with free or weakly bound electrons in the γ-ray incident beam. Electromagnetic Radiation If a beam of photons is interacting with matter or body tissues, then a number of photons are scattered by the electrons of the atoms that compose the matter or tissues. Electromagnetic Radiation The initial photon has energy hc/λi and collides with a stationary or almost stationary electron, as seen in the figure. The electron has no initial kinetic energy or momentum. The scattered photon has lower energy of hc/λf. Applying the law of conservation of energy and the law of conservation of momentum along the direction of initial photon and perpendicular to that, we obtain the following relationship between the wavelengths of the photon before and after scattering. Electromagnetic Radiation Where m is the mass of the electron. The term Δλ is called wavelength shift and is equal to the difference in the wavelength of photon after and before scattering, and h is the Planck’s constant. Electromagnetic Radiation Pair Production: Pair production is dominant in interactions of higher-energy photons with matter. In this phenomenon, a γ-ray or X-ray photon passing near the nucleus of an atom is subjected to strong field effects from the nucleus and splits into an electron-positron pair. Positron is a positively charged electron and is created as result of the conservation of momentum when a photon passes near the nucleus of an atom. Electromagnetic Radiation The photon should have at least 1.022 MeV or more energy to take part in this process, which is the sum of the rest mass energies of an electron (0.511 MeV) and a positron (0,511 MeV). Moreover, pair production dominates over photoelectric absorption and Compton scattering when the energy of photon is bigger than 10 MeV. Electromagnetic Radiation Reasons for performing x-rays: 1. Identify fractures or infections in bones and teeth. 2. Diagnosis and evaluation of the structure of the oral cavity and jaw. 3. Pick up signs of joint changes that indicate arthritis. 4. Detection of tumors on bones. 5. Bone density measurement to diagnose osteoporosis. 6. Detection of pneumonia, tuberculosis, or lung cancer. 7. Sterilizing food. 8. X-rays are used in crystallography through the diffraction property of light. Biological effect of ionizing radiation Electromagnetic Radiation Biological effect of ionizing radiation: The biological effects of ionising radiation have been known for many years. The first case of human injury was reported in 1895 after the discovery of x-rays by Roentgen, while the genetic effects of radiation were reported at the beginning of 1902. Early evidence of harmful biological effects due to exposure to high radiation dose was obtained for persons working in the radium industry in the 1920s and 1930s. Electromagnetic Radiation Knowledge of these effects increased through the atomic bombings of Hiroshima and Nagasaki in 1945, U.S. nuclear testing in 1952- 1953 and nuclear accidents, such as the Goiania accident and the Chernobyl. The International Commission on Radiological Protection (ICRP) has determined limits for annual radiation dose at 20 mSv for workers and 1 mSv for members of the public to prevent stochastic effects. Electromagnetic Radiation Somatic and genetic effects of ionizing radiation Electromagnetic Radiation The biological effect of ionising radiation is due to the energy transfer to cells and the production of ionisation and excitation in their constituent atoms. The effect can be acute, e.g. following the exposure of a subject to a high radiation dose, greater than 100 mGy, within a short period of time (a few days). Electromagnetic Radiation The interaction of ionising radiation with cells may damage the chromosomes, which are considered the most important parts of the cell as they carry the DNA, which is the genetic material of most living organisms. Potential damage to the chromosomes can cause cell mutations resulting in genetic effects and the development of cancer. The various stages of the physical and biological effects of radiation are shown in Figure. Electromagnetic Radiation Electromagnetic Radiation Radiation Therapy Process: Radiotherapy treatment planning is the process of determining optimal treatment parameters for patient irradiation. Depending on the type, extent, and location of the tumor and the patient condition, the tumor is either surgically removed or treated with radiation and/or chemotherapy agents. Frequently there is a remnant of the tumor, even after surgery, which needs to be irradiated to avoid recurrences. Ultraviolet rays Electromagnetic Radiation Ultraviolet rays: It covers wavelengths ranging from about 4 × 10–7 m (400 nm) down to 6 × 10–10m (0.6 nm). Ultraviolet (UV) radiation is produced by special lamps and very hot bodies. The sun is an important source of ultraviolet light. But fortunately, most of it is absorbed in the ozone layer in the atmosphere at an altitude of about 40 – 50 km. UV light in large quantities has harmful effects on humans. Electromagnetic Radiation Electromagnetic Radiation Exposure to UV radiation induces the production of more melanin, causing tanning of the skin. UV radiation is absorbed by ordinary glass. Hence, one cannot get tanned or sunburn through glass windows. Welders wear special glass goggles or face masks with glass windows to protect their eyes from large amount of UV produced by welding arcs. UV lamps are used to kill germs in water purifiers. Electromagnetic Radiation Sources: Sources of radiation can be grouped by the manner in which the radiation is originated. When the temperature of some material is elevated, many energy transitions occur, and energy is emitted. A main source of UV radiation on the earth comes from the sun. However, when materials are heated to incandescence, some UV radiation may be emitted. Electromagnetic Radiation The spectrum (wavelengths) emitted and the intensity is related to the temperature (absolute degrees Kelvin) of the material. Depending on the source, the radiation emitted can be a broad band (so many wavelengths that it appears as a continuum) or narrow, specific wavelengths (i.e. line spectra from low pressure discharge lamps). The emitted spectrum is an important factor in evaluating the radiation. Electromagnetic Radiation Lasers have been developed which emit UV radiation. Specific lamps are manufactured to produce UV radiation in narrow spectral lines. These are usually low-pressure mercury vapor sources that emit visible and UV wavelengths with 95% of the energy at 253.7 nm. Electromagnetic Radiation Biological Effects: a) The absorption of UV radiation can cause biological effects. The two primary organs of concern are the eye and the skin. b) The UV spectral band of UVA (315-400 nm) is less photobiologically active than the rest of the ultraviolet; UVB (280-315 nm) and UVC (180-280 nm). c) The adverse effects of UV radiation have been shown to be a result of photochemical reactions rather than thermal damage. The maximum sensitivity of the human eye occurs at approximately 270 nm and this wavelength is used as a reference for effectiveness of other UV wavelengths to elicit a biological response. Electromagnetic Radiation d) Acute effects. 1. Erythema (i.e., redness) is a response to excessive exposure by UVB and UVC radiation. The dose required to produce erythema varies with skin pigmentation and the thickness of the skin. There may be a latent period of 4-8 hours between the exposure and the effects. Effects may range from simple skin reddening to serious burns. Darkening of the skin and thickening of the skin offers some protection against future exposures. Erythema production is dependent only on the total radiant exposure dose (i.e., product of irradiance and exposure duration). Electromagnetic Radiation 2. An inflammation of the cornea after excessive exposure to UVB or UVC radiation. This is also known as snow blindness or welder’s flash burn. Although the injury is extremely painful, it is usually temporary because of the recuperative powers of the epithelial layer. The latent period is usually 4-12 hours from the time of exposure and is spectral and dose dependent. There is a sensation of “sand” in the eyes, photophobia, blurred vision, lacrimation and painful uncontrolled excessive blinking. Symptoms may last up to 24 hours with the corneal pain being severe. Recovery takes one to two days. The peak of the action spectrum is 265-275.