EM RADIATION &PHYSICS OF XRAYS DR ZOHEIR HUSSEIN
ATOMIC THEORY Matter is comprised of very small particles known as atoms. Atoms are can be further subdivided into 3 basic subatomic particles: - protons (p+) - neutrons (nº) - electrons (e¯)
Historical Overview The Greeks theorized that matter has four basic components: air, water, earth and fire. Named the smallest division of these components the atom. Theory accepted until early 1800s, when English schoolteacher John Dalton (1766-1844) published his work on atomic theory: Elements differentiated from one another based on the characteristic of mass Elements composed of atoms which behaved in an identical fashion during a chemical reaction.
1800s: Russian scientist, Dmitri Mandeleev (1834- 1907), developed the first periodic table of the elements: elements are arranged in order of ascending atomic mass 1911: English physicist Ernest Rutherford (1871-1937) developed a model for the atom which contained a central, small, dense nucleus, which possessed a positive charge and was surrounded by a negative charge. 1913: Niels Bohr (1885-1962), a Denish physicist, expanded on Rutherford’s work and proposed a model for the atom which is considered the most representative of the structure of matter. Bohr’s atom is likened to a miniatures solar system where electrons orbit around a central nucleus just as the planets revolve around the sun.
Basic Atomic Particles The atom - small, dense centre the nucleus, which is surrounded by –vely charged electrons that orbit it at various levels. Nucleus – protons (+ve) and neutrons (neutral) Protons + neutrons = Atomic mass Electron has a relatively insignificant mass, 1/1,828 that of a proton ( 9.109 x 10-31 kg) Proton – 1.673 x 10-27 kg Neutron – 1.675 x 10-27 kg
When the number of positively charged protons equals the number of negatively charged electrons, the atom is neutral or stable. Because of its electrical nature, the atom is dynamic and ever moving and in a constant, vibrating motion because of the strong positive nuclear force field which is surrounded by the negatively charged spinning and orbiting electron.
Atomic Number Each elements has its own specific number of nuclear protons. This is the key characteristic which distinguishes one element from another. The number of nuclear protons in an atom is known as the atomic number or z number. The simplest element, hydrogen, possesses only one proton and therefore has atomic number of 1. Helium comes next on periodic table and has two protons, giving it an atomic number of 2. Lead has an atomic number of 82, indicating that within the nucleus of an atom of lead there are 82 protons.
In a neutral atom the number of protons is equal to the number of electrons. -In a stable, neutral atom of hydrogen there is one proton and one electron. -In a stable atom of lead there are 82 protons and 82 electrons. If an atom gains or loses neutrons, the result is an atom called an isotope. -Isotopes are atoms which have the same number of protons in the nucleus but differ in the number of neutrons. -Deuterium is an isotope of hydrogen, contains the same number of protons as hydrogen but also contains one neutron. If an atom gains or loses an electron, it is called an ion and the atom is said to be ionized.
Ionization is the process of adding or removing an electron from an atom. ie when an electron is removed from an atom, the atom becomes a positive ion; that is the atom possesses an extra positive charge. When an electron is added to an atom, the atom becomes negative ion, ie it possesses an extra negative charge
EM Radiation In terms of modern quantum theory: EM wave is the flow of photons (also called light quanta) through space. Photons are packets of energy that aways move with the universal speed of light (300,000 Km/sec)
Electromagnetic radiation (EM) spans a continuum of wide ranges of magnitudes of energy. This continuum is termed the electromagnetic spectrum. The electromagnetic spectrum details all of the various forms of EM radiation. One of the common properties of all forms of EM radiation is velocity.
Electromagnetic spectrum High energy – Gamma - X- ray - Ultraviolet - VISIBLE LIGHT - Infrared - Microwave - Radar Low energy - Radio
Radiation along the EM spectrum will vary according to the associated frequency and wavelength. Low frequency-long wavelengths are at the bottom of the spectrum with radio waves and microwaves. Visible light is in the centre of the spectrum and at the top of the spectrum are gamma and x-rays having high frequency and short wavelengths.
IONIZING RADIATION IR arises from both natural and manmade sources grouped as Particulate and EM (Photon) radiation Particulate- high energy electrons, protons and neutrons. Produce ionization by direct atomic collisions Photon radiation- X-rays Gamma rays Produce ionization by other types of interactions (photoelectric absorption, compton scattering)
S O U R C E S O F R A D IA T IO N NATURAL A R T IF IC IA L EXTERNAL IN T E R N A L IN D U S T R IA L M E D IC A L C O S M IC T E R R E S T IA L IN G E S T E D IN H A L E D NUCLEAR W EAPONS D IA G N O S T IC T H E R A P E U T ICO UTER SPACE E N V IR O M E N T A L K&C RADON REACTO RS
Properties of Ionization radiations They have a very short wavelength and so can penetrate materials They can cause certain substances to fluoresce They form an image on a radiographic film They produce biological effects which may be useful as in radiation therapy or harmful as to cause disease
RADIO-SENSITIVITY (RS) OF VARIOUS HUMAN ORGANSHigh RS Medium RS Low RSBone Marrow Skin MuscleSpleen Mesoderm BonesLymphatic Organs (liver, Nervous systemNodes heart, lungs)GonadsEye LensLymphocytes
Radio-sensitivity RS - probability of a cell, tissue or organ of suffering an effect per unit of dose. Bergonie and Tribondeau (1906): “RS LAWS”: RS will be greater if the cell:• Is highly mitotic.• Is undifferentiated.• Has a high carcinogenic future.
Biologic effects of ionizing radiations Photons of ionizing radiation absorbed by atoms of water molecules of the cell- 1˚ ionization. The ejected electron has sufficient energy to cause ionization of other atoms in the material- 2˚ ionization. The free formed combine with others to form a new chemicals in the cell- chemical change. This chemical change causes abnormal behavior of the cell- Biological change. The effects of these changes are stochastic and non-stochastic effects.
Stochastic effect Is defined as an effect in which the probability of occurrence increases with increasing absorbed dose, but severity of effect does not depend on the magnitude of the absorbed dose. It is an all-or-none phenomenon, and is assumed to have no dose threshold so higher the dose higher the risk.
3 important consequences of stochastic effect are: Radiation induced carcinogenesis of which leukemia is the commonest neoplasia. Congenital malformations from effect during organogenesis. Mutations due to altered biological code on chromosomes in germ cells (sperm & ova)
Non stochastic effect Is defined as a somatic effect that increases in severity with increasing absorbed dose. So there is a threshold below which the effect will not occur or will be stochastic. These are usually degenerative effects severe enough to be clinically significant. They usually occur in industrial disasters.
1Sv- Temporary depression of blood count but the victim is likely to recover or may develop stochastic effect. 5Sv- Death can occur within weeks due to bone marrow failure, unless radical medical intervention is effected. 10Sv- Death occurs within days due to damage of the GIT lining leading to infections, septicemia or hemorrhage. 20Sv- Death within hours due to severe damages to CNS from the cellular debris and not the radiation directly.
METHODS OF PROTECTION Who should be protected: Parameters available to Patients reduce radiation exposure Staff Time General public Reduce time of exposure Protected against: Distance Primary / useful radiations By inverse square law Stray radiations I.e. Barriers Scatter Filters Leakage Lead shields Concrete walls
PROTECTION TECHNIQUES Use Posters e.g. Radiation symbol Inverse square law (D α 1/S2) Date of LNMP Lead sheets/iron sheets Quality assurance Renewable license
PATIENTS Use of Aluminum filters close to the tube Tube shielding Reduce field size with collimation Use fast film/screen combination Reduce number of repeats Use highest possible kVp and low mAs Use pulsed fluoroscopy rather than continuous Use lowest possible time of exposure Use adequate protective gears
STAFF Only those required in the room should be present All should stand behind the barrier during exposure Must wear adequate protective gear Must be as far as possible from the primary beam Use immobilising devices for restless patients
GENERAL PUBLIC The radiology unit must be reinforced with concrete walls and lead shielded doors. The direction of primary beam should not be directed to the clinics or waiting areas. Those who are not supposed to be in Radiography suit should be kept away. Should use appropriate protective gear when required.
Conclusion There are no ionizing radiations which are completely safeIt is of utmost importance to avoid unnecessary ionizing radiations and the radiation exposure should be kept as low as reasonably achievable (ALARA)