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Radiation physics in Dental Radiology...
- 1. BY: DR. D.SAHITHI PG I YEAR DEPT. OF ORAL MEDICINE AND RADIOLOGY
- 2. Contents Introduction Radiation Nature of radiation X-rays History of X-rays Properties of X-rays Production of X-rays Factors controlling X-ray beam Interactions of the X-rays with matter Conclusion References
- 3. Introduction Physics: The branch of science concerned with the properties of matter and energy and the relationships between them.
- 4. Atomic Structure Quantum mechanical model - proposed by Niels Bohr in 1913.
- 5. Principle quantum number(n): Maximum number of electrons in a given shell = 2n2 K – 2 L – 8 M - 18 N – 32 O – 50 …. Atomic number(Z): The number of protons in the nucleus of an atom. Atomic mass(A): sum of the protons and neutrons in an atom.
- 6. Ionization energyElectron binding energy The amount of energy required to remove an electron from a given shell. K Shell electrons – 70 KeV L Shell electrons – 12 KeV M Shell electrons – 3 KeV
- 7. Radiation: is defined as the emission and propagation of energy through space or matter in the form of waves or particles Radiation physics: The study of ionizing radiation and its effects on matter.
- 8. Ionization: Ionization is the process of converting an atom into ions. If an electrically neutral atom loses an electron, it becomes a positive ion and the free electron is a negative ion. This process of forming an ion pair is termed ionization Excitation : Displacement of an electron from an inner shell to an outer shell.
- 9. Ionization is more concentrated at the end of particle path and is termed Bragg’s effect Linear energy transfer The rate of loss of energy from a particle as it moves along its track through matter.
- 10. Nature of radiation Particulate radiation • Consists of atomic nuclei or subatomic particles moving at high velocity. • Alpha particles, Beta particles and Cathode rays are the examples
- 11. Electromagnetic radiation is the movement of energy through space as a combination of electric and magnetic fields. Gamma rays, x rays, ultraviolet rays, visible light, infrared radiation(heat), microwaves, and radio waves.
- 12. Electromagnetic radiations are arranged according to their energies termed as electromagnetic spectrum Depending on their energy levels, electromagnetic radiations can be classified as Ionizing Non-ionizing.
- 13. Theories of EMR Wave theory Such waves consist of electric and magnetic fields oriented in planes at right angles to one another that oscillate perpendicular to the direction of motion
- 14. Waves of all kinds exhibit the properties of wavelength (ʎ) and frequency (v) travelling at a velocity of light in vaccum and are related as follows : ʎ x v = c = 3 X 108 meters/sec ʎ is in meters and V is in cycles per second (hertz). C is the velocity of light
- 15. Quantum Particle theory Quantum theory considers electromagnetic radiation as small bundles of energy called “photons”. Each photon travels at the speed of light and contains a specific amount of energy. The unit of photon energy is the electron volt (eV). The relationship between wavelength and photon energy is as follows: E = 1.24/ ʎ
- 16. X-rays X-rays are defined as weightless packages of pure energy (photons ) that are without electrical charge and that travel in waves along a straight line with a specific frequency and speed.
- 17. History of X-rays X-rays were discovered by Professor Wilhelm Conrad Roentgen, November 8-1895 and are also known as Roentgen rays. First dental radiograph – Dr. Friedrich Otto Walkhoff, 1896. First X-ray tube – William David Coolidge, 1913. Father of radiation protection - William Herbert Rolliins
- 18. Physical properties of X-rays X-rays belong to a family of electromagnetic radiations having a wavelength between 10 Å and 0.01 Å. As they travel through space, they can produce an electrical field at right angles to their path of propagation and a magnetic field at right angles to the electric field.
- 19. They cannot be reflected, refracted or deflected by a magnetic or electric field as they do not possess any charge. They show the properties of interference, diffraction and polarization , similar to that of visible light. X-rays are pure energy with no mass and they transfer energy from place to place in the form of quanta (photons)
- 20. X-rays can penetrate various objects and the degree of penetration depends upon the quality of the X-ray beam, and also on the intensity and wavelength of the x-ray beam. An X-ray beam may be attenuated, absorbed or scattered. Due to their energy X-rays can release photoelectrons from the metals, when allowed to fall on them.
- 21. Inverse square law For a given beam the intensity is inversely proportional to the square of the distance from the source. I1/I2= D2 2/D1 2
- 22. Chemical properties of X-rays X-rays induce color changes of several substances or their solutions X-rays bring about chemical changes in solutions which are otherwise completely stable. X-rays can cause destruction of the fermenting power of enzymes, which are vital substances for the metabolism of cells of all living materials.
- 23. Biological properties of X-rays Somatic effect: This ranges from a simple sunburn to severe dermatitis, to changes in the blood supply and/or malignancy. Genetic effect: This effect is due to radiation induced mutation of genes and chromosomes.
- 25. 25 Evacuated glass envelope Cathode with filament Anode with focal spot Schematic Representation of an X-ray Tube STEP-DOWN TRANSFORMER STEP-UP TRANSFORMER Electron cloud Radiation is emitted Window
- 26. Line focus principle The target is inclined at an angle of 20 degrees to the central ray of electrons reducing the actual focal spot size of 1×3mm to the effective focal spot of size 1× 1mm. This is called line focus principle and the 20 degree angle is called as “the angle of truncation”.
- 27. Heel effect The intensity of the x-ray beam that leaves the x-ray tube is not uniform throughout all portions of the beam. This variation is termed the heel effect and is accentuated as the angle of the target is reduced.
- 28. Factors controlling X-ray beam Exposure time When the exposure time is doubled, the number of photons generated at all energies in the x-ray emission spectrum is doubled, but the range of photon energies is unchanged. Tube current As the mA setting is increased, more power is applied to the filament, which heats up and releases more electrons that collide with the target to produce radiation.
- 29. The quantity of radiation produced by an x-ray tube is directly proportional to the tube current (mA) and the time the tube is operated. Tube voltage : Increasing the kVp causes an increase in the (1) the number of photons generated, (2) their mean energy, and (3) their maximal energy.
- 30. Filtration The aluminum preferentially removes many of the lower energy photons which contribute to patient exposure but do not have enough energy to reach the film, with lesser effect on the higher energy photons that are able to penetrate to the film, thereby reducing patient dose.
- 31. Half value layer The HVL is the thickness of an absorber, such as aluminium, required to reduce by one half the number of x-ray photons passing through it.
- 32. Collimation Collimating the beam to reduce the exposure area and thus the number of scattered photons reaching the film can minimize the detrimental effect of scattered radiation on the images. A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam and the volume of irradiated tissue within the patient
- 33. Round collimator :is a thick plate of radio paque material (usually lead) with a circular opening centered over the port in the x ray tube head through which the x-ray beam emerges and these are built in open-ended aiming cylinders.
- 34. • Rectangular collimator:- Restricts the size of the x ray beam to an area slightly larger than size 2 intraoral film and significantly reduces patient exposure. And also increases image quality by decreasing scattered radiation
- 35. X-ray spectra Continuous spectrum General Bremsstrahlung Braking radiation
- 37. Interactions of electrons at the target X-ray-producing collisions Heat-producing collisions
- 40. Combined spectra The final total spectrum of the useful X-ray beam will be the addition of the continuous and characteristic spectra.
- 41. Interactions of X- rays with matter When X-rays strike matter, such as a patient's tissues, the photons have four possible fates: Transmitted unchanged. Absorbed with total loss of energy Scattered with some absorption and loss of energy. Completely scattered with no loss of energy.
- 42. Absorption and Photoelectric effect
- 43. K-edge absorption When the energy of the incident photon is raised to the binding energy of the K-shell electrons of the absorber, the probability of photoelectric absorption increases sharply and the number of transmitted photons is greatly decreased. This is called K-edge absorption.
- 44. Coherent scattering (classical, elastic, or Thompson scattering)
- 46. Pair production Photodisintegration
- 47. Attenuation of the X-ray beam Beam quantity: Number of photons in an X-ray beam. Beam quality: Mean energy of the photons in an X-ray beam Beam intensity: The product of the number and energy of the photons describes the intensity of the beam.
- 48. Attenuation is the reduction in the intensity of an X-ray beam as it traverses matter, by either the absorption or deflection of photons from the beam. The attenuation of a beam depends on both the composition of the absorber and the energy of the incident beam.
- 49. Fate of secondary electrons The secondary electrons give up their energy in the absorber by either of two processes: Collisional interaction with other electrons, resulting in ionization or excitation of the affected atom, and Radiative interactions produce bremsstrahlung radiation, resulting in the emission of low-energy x-ray photons.
- 50. Conclusion
- 51. References Oral radiology Principles and Interpretation, White and Pharoah – Fifth edition Textbook of Dental and maxillofacial Radiology, Freny. R. Karjodkar – 2 edition Essentials of Dental Radiography and Radiology, Eric Whaites – Fourth edition
- 52. Textbook of Radiology – Christensen J. Anthony Seibert, “X-Ray Imaging Physics for Nuclear Medicine Technologists. Part 1: Basic Principles of X-Ray Production” J Nucl Med Technol 2004; 32:139–147
Editor's Notes
- Matter def may be divided into elements def and compounds. Atoms, the fundamental units of elements Fundamental particles of an atom – Electrons, Protons, and Neutrons
- Electrons are maintained in their orbits by the electrostatic force, or attraction, between the positive nucleus and the negative electrons is which known as the binding energy or binding force of an electron. Centrifugal force pulls the electrons away from the nucleus. The balance between electrostatic force and centrifugal force keeps the electrons in orbit around the nucleus
- When the number of orbiting electrons in an atom is equal to the number of protons in its nucleus, the atom is electrically neutral. If an electrically neutral atom loses an electron, it becomes a positive ion and the free electron is a negative ion. Electrons in the K shell of a given element have the greatest binding energy because they are closest to the nucleus. The binding energy of the electrons in each successive shell decreases. For an electron to move from a specific orbit to another orbit farther from the nucleus, energy must be supplied in an amount equal to the difference in binding energies between the two orbits. In contrast, in moving an electron from an outer orbit to one closer to the nucleus, energy is lost and given up in the form of electromagnetic radiation.the binding force is inversly prportional to square the distance btn nucleus and electron.
- If an electrically neutral atom loses an electron, it becomes a positive ion and the free electron a negative ion thus forming an ion pair. when an atom gains the electron it is called negative ion. An ionized atom is not electrically neutral and carries a charge equal to the difference between the number of protons and electrons.
- Non particulate radiation need not penetrate by damaging the tissues
- Gamma rays are photons that originate in the nuclei of radioactive atoms. They typically have greater energy than x rays. X rays, in contrast, are produced extranuclearly from the interaction of electrons with nuclei in x-ray machines. If sufficient energy is associated with the radiation to remove orbital electrons from atoms in the irradiated matter, the radiation is ionizing.
- Ionizing radiation : if there is sufficient energy associated with the radiation such that it can remove orbital electrons from the atoms in the irradiated matter E.g . Cosmic rays, gamma rays & x-rays Non- ionizing radiation :these radiations are not able to remove electron from the atom E.g.visible light, ultraviolet, infrared,microwaves & radiowaves.
- Frequency and wavelength are inversely related; Low-frequency electromagnetic radiations have a long wavelength and less energy. High-frequency electromagnetic radiations have a short wavelength and more energy.
- characterizes electromagnetic radiations as waves and focuses on the properties of velocity, wavelength, and frequency PARTICLE CONCEPT characterizes electromagnetic radiations as discrete bundles of energy called photons or quanta.
- E is energy in kiloelectron volts (keV) , ʎ, is wavelength in nanometers
- X-radiation is a high-energy, ionizing electromagnetic radiation having properties of both waves and particles. X-rays can be defined as weightless bundles of energy (photons) without an electrical charge that travel in waves with a specific frequency at the speed of light.
- . They travel through space in a wave motion. In free space, they travel in a straight line. They travel with a speed that of visible light.
- They are invisible to the eye and cannot be seen, heard or smelt. They cannot be focused by a lens. They do not require a medium for propagation.
- The intensity of an x-ray beam at a given point (number of photons per cross-sectional area per unit exposure time) depends on the distance of the measuring device from the focal spot. where l is intensity and D is distance.
- The high-speed electrons bombarding the target are involved in two main types of collision with the tungsten atoms: Production of xrays depends on three properties of tungsten
- All the x-rays which are produced in the x-ray tube are not same because of having difference in energy and wavelength. Energy and wavelength of x-rays vary based on how the electrons interact with the tungsten atoms in the anode.
- 99%to heat..less than 1% as x rays
- The intensity of the beam is not uniform across the exposure field, because the distance from the anode to a point on the film away from the perpendicular is greater than the centre of the exposure field Heating affect X-rays have a potential to ionize the matter through which they pass. When X-rays fall upon certain materials, visible light is emitted called fluorescence. Phosphorescent materials Flourescent materials Thermoluminescent materials
- For instance.. a machine operating at lOrnA for 1 second (lOmAs) produces the same quantity of radiation when operated at 20 rnA for 0.5 second (lOmAs)
- Although an x-ray beam consists of a spectrum of x-ray photons of different energies, only photons with sufficient energy to penetrate through anatomic structures and reach the image receptor are useful for diagnostic radiology. Those that are of low energy contribute to patient exposure (and but do not have enough energy to reach the film. Consequently, to reduce patient dose, the less-penetrating photons should be removed. In determinations of the amount of filtration required for a particular x-ray machine, kVp and inherent filtration of the tube and its housing must be considered.
- As the average energy of an x-ray beam increases, so does its HVL.
- Dental x-ray beams are usually collimated to a circle 23/4 inches(7 cm) in diameter. A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam and the volume of irradiated tissue within the patient. Round and rectangular collimators are most frequently used in dentistry.
- Electrons traveling from the filament to the target convert some of their kinetic energy into x-ray photons by the formation of bremsstrahlung and characteristic radiation. Bremsstrahlung interactions, the primary source of xray photons from an x-ray tube,
- The incoming electron penetrates the outer electron shells and passes close to the nucleus of the tungsten atom. The incoming electron is dramatically slowed down and deflected by the nucleus with a large loss of energy which is emitted in the form of X-rays • The incoming electron collides with an inner-shell tungsten electron displacing it to an outer shell (excitation) or displacing it from the atom (ionization), with a large loss of energy and subsequent emission of X-rays
- The incoming electron is deflected by the cloud of outer-shell tungsten electrons, with a small loss of energy, in the form of heat • The incoming electron collides with an outer shell tungsten electron displacing it to an even more peripheral shell (excitation) or displacing it from the atom (ionization), again with a small loss of energy in the form of heat Heat needs to be removed quickly and efficiently to prevent damage to the target. This is achieved by setting the tungsten target in the copper block, utilizing the high thermal capacity and good conduction properties of copper.
- When x-ray photons arrive at the patient with energy, one of the following events may occur: X-rays can pass through the patient without any interaction. X-ray photons can be completely absorbed by the patient. X-ray photons can be scattered
- Other interactions: Photodisintegration and Pair production.
- Rare earth elements are sometimes used as filters because their K edges greatly increase the absorption of high-energy photons.
- In both photoelectric absorption and Compton scattering, electrons are ejected from their orbits in the absorbing material after interaction with x-ray photons.