Ionising Radiation - Lecture 13 PDF

Lecture № 13 Ionizing radiation. Classification. Radioactivity. Alpha- and beta- radioactive conversion. Gamma rays. Interaction of charged particles with matter. X-rays. Interaction of photon ionizing radiation with matter. Dosimetric quantities and units. Dosimetry of ionizing r...

Lecture № 13 Ionizing radiation. Classification. Radioactivity. Alpha- and beta- radioactive conversion. Gamma rays. Interaction of charged particles with matter. X-rays. Interaction of photon ionizing radiation with matter. Dosimetric quantities and units. Dosimetry of ionizing radiation.  Classification of radiation Radiation is classified into two main categories, nonionizing and ionizing, depending on its ability to ionize matter. The ionization potential of atoms (i.e. the minimum energy required to ionize an atom) ranges from a few electronvolts for alkali elements to 24.5 eV for helium (noble gas). Ionization is the ejection of one or more electrons from an atom or molecule to produce a fragment with a net positive charge (positive ion). In the electromagnetic spectrum, radiation in the visible or longer wavelength range does not have sufficient quantum energy to ionize an atom, so we classify it as non-ionizing radiation. The threshold for ionization occurs somewhere in the ultraviolet range, with the specific threshold depending upon the type of atom or molecule. It typically takes a photon with energy in the range of a few electron volts to ionize an atom.  VIS, NIR, MIR, FIR Directly ionizing radiation (charged particles): electrons, protons, α particles and heavy ions. Indirectly ionizing radiation (neutral particles): photons (X rays and γ rays), neutrons.  Ionizing radiation Depending on their nature, ionizing radiations are of two types - electromagnetic (photon) and corpuscular [kɔ:'pʌskjulə] radiation from charged particles. The former include X-rays and γ -rays, and the latter - α and β-rays, streams of other charged particles such as electrons, positrons and neutrons with non- zero rest mass and sufficiently large kinetic energy.  IONIZING RADIATION terminology γ rays – ЕМR with a discontinuous energy spectrum arising from atomic nuclei or from the annihilation of a particle with an antiparticle X-ray radiation – a mixture of bremsstrahlung and characteristic radiation Bremsstrahlung – EMF with a continuous energy spectrum, occurring when electrically charged particles slow down Characteristic radiation - EMR with discrete energy spectrum arising from electron transitions to the inner layers of the electron shell of the atom. Photon ionizing radiation - general term for X-ray radiation and/or γ radiation. X-Rays These are electromagnetic radiations emitted as a result of electrons jumping from higher-energy level into lower-energy level in an atom. X-rays emitted as a result of the electronic transitions inside an atom are called characteristic X-rays. On the other hand, slowing down of a fast-moving electron in an electric field also results in the emission of electromagnetic radiation or X-rays called continuous X-rays or bremsstrahlung radiation. X-rays possess energy, frequency, wavelength, and linear momentum. The energy of an X-ray photon is related to its frequency, wavelength, and momentum by the following equations as any other photon of EM radiation: hν h h h E = hν p= = λ= = c λ p mv X-rays are highly energetic radiation and could damage normal tissues if exposed. Therefore, in spite of their frequent and wide use in medicine, shielding of normal tissue is mandatory. In practical applications in medicine, the energy of x-ray is usually given in terms of the generating voltage. The energy ranges of x-rays, in terms of the generating voltage, are given below (Ahmad 1999; Grupen 2010; Stagg et al. 2001): 0.1–20 kV Low-energy or soft x-rays 20–120 kV Diagnostic-range x-rays 120–300 kV Orthovoltage x-rays 300 kV–1 MV Intermediate-energy x-rays 1 MV and above Megavoltage x-rays X-rays are frequently used in both therapeutic and diagnostic medical physics.  RADIOACTIVITY Radioactivity may be defined as spontaneous nuclear transformations in unstable atoms that result in the formation of new elements. These transformations are characterized by one of several different mechanisms, including Alpha particle emission, Beta particle and positron emission, and Orbital electron capture. Each of these transformations may or may not be accompanied by gamma radiation. About 339 natural (natural) nuclides exist on earth, of which about 269 are stable, and the remaining 70 possess the property of radioactivity and are therefore called radionuclides. The isotopes of all chemical elements from the end of the periodic system after 83Bi (A=209) are radioactive, as well as some isotopes of lighter elements, such as 19K with А=40 37 Rb with А=87 , 62Sm with А=147, 64Gd with А=152. The nucleus that undergoes radioactive decay is called the parent nucleus, and the resulting nucleus is called the daughter nucleus. The latter, as a rule, turns out to be excited, and its transition to its ground state is accompanied by the emission of a gamma quantum. Depending on whether radioactive nuclei exist in nature or are obtained through nuclear reactions, one speaks of natural or artificial radioactivity. There is no fundamental difference between the two types of radioactivity and they obey the same laws. Antoine Henri Becquerel discovered the natural radioactivity of uranium ore in 1896 Nobel prize 1903 Antoine Henri Becquerel ) was a French engineer, physicist, Nobel laureate, and the first person to discover evidence of radioactivity. For work in this field he, along with Marie Skłodowska-Curie and Pierre Curie, received the 1903 Nobel Prize in Physics. The SI unit for radioactivity, the becquerel (Bq), is named after him. Research of radioactive ores of Ac and Th Discovery of the elements Ra and Po Nobel prize in physics for1903 Nobel prize in chemistry for1911 Induced radioactivity, also called artificial radioactivity or man-made radioactivity, is the process of using radiation to make a previously stable material radioactive. The husband and wife team of Irène Joliot- Curie and Frédéric Joliot-Curie discovered induced radioactivity in 1934, and they shared the 1935 Nobel Prize in Chemistry for this discovery. Alpha Emission An alpha particle is a highly energetic helium nucleus that is emitted from the nucleus of an unstable atom when the neutron- to-proton ratio is too low. It is a positively charged, massive particle consisting of an assembly of two protons and two neutrons. Since atomic numbers and mass numbers are conserved in alpha transitions, it follows that the result of alpha emission is a daughter whose atomic number is two less than that of the parent and whose atomic mass is four less than that of the parent. In the case of Polonium (210Po), for example, the reaction is: The alpha particle is relatively large and heavy. As a result, alpha rays are not very penetrating and are easily absorbed. A sheet of paper or a 3cm layer of air is sufficient to stop them. Its energy is transferred within a short distance to the surrounding media. In heavy nuclei A >200 , due to the increased size of the nucleus, the Coulomb repulsion between the protons increases and can compete in value with the nuclear forces that are short-range. The mentioned circumstances lead to the fact that the nuclei undergo alpha decay, in which, reducing their size, they increase their stability. 1/ 3 R = R0 A R0 = (1,3 ÷ 1,7).10 m, -15 During the alpha-radioactive transformation, all three parameters of the parent nucleus change: its mass decreases by 4 atomic units, the electric charge - by 2 elementary charges, and the energy - by Eα. A nucleus or an element that emits these radiations is called parent nucleus or parent element, and the newly produced nucleus as a result of the emission of radiation is called daughter nucleus. Natural radioactive nuclei emit alpha particles with energies between 2 to 10 MeV. Alpha particles have a strong ionizing ability. An alpha-particle can ionize 105 ion pairs in air until it comes to a complete stop. The alpha particle cannot penetrate the outer layer of the skin, but is dangerous if inhaled or swallowed. The delicate internal workings of the living cells forming the lining of the lungs or internal organs, most certainly will be changed (mutated) or killed outright by the energetic alpha particle. The number of lung cancer cases among uranium miners from inhaled and ingested alpha sources is much higher than those of the public at large. Radon, the gas produced by the decay of radium-226, also emits alpha particles which possess a hazard to the lungs and airways when inhaled.  Alpha radioactivity Alpha radioactive are mainly the nuclei of heavy chemical elements (226Ra, 232Th, 235U) with A >200 and Z > 82 since the radius of the nucleus is R ∼ A1/3 which is greater than 1 fm. If X is the parent nucleus, Y presents the daughter nucleus and 42He is the alpha particle, the reaction is : A Z X → ZA−−42Y + 42 He Seaborgium-263 is an isotope of seaborgium with a half- life of about 1 second.  Alpha radioactive conversion of 226Ra The most popular example is alpha- the radioactive conversion of radium-226 in radon-222. The energy Еα can have one or more Alpha rays do not values. The two types of alpha particles – have practical α1 and α 2 are emitted with a different meaning for probability, called the radiation yield, 6% and 94%, respectively. medicine.  Beta Emission Beta rays are ionizing radiations that originate in the nucleus of an atom. A β- ray is exactly like an electron with the only difference being the separate origins of the two. As mentioned, a β- ray originates from the nucleus of an atom, while an electron resides in an orbit around a nucleus. Beta rays are subdivided into two types: β- and β+. β- has the same charge and mass as possessed by an electron, given as m0 = 9.109 10-31 kg and e = - 1.6 10-19 C. β+ is like a positron and has the same mass as possessed by an electron but opposite charge. The charge number (Z) and mass number (A) of a β are 1 and 0, respectively. β+ has Z =1 and A = 0. When a β is emitted by a nucleus, the charge number of that atom increases by 1 unit, and the mass number remains unaffected according to the following reactions.  β+ Emission In some instances where the neutron-to-proton ratio is too low and alpha emission is not energetically possible, the nucleus may, under certain conditions attain stability by emitting a positron. The positron does not exist independently in an atom, rather, it is believed that the positron results from a transformation within the nucleus, of a proton to a neutron. In the nucleus, the proton borrows energy from the other nucleons. 1 1 0 0 1 p → 0 n + +1 e + 0ν A Z X→ A Y+ Z −1 0 +1 e + 0 0ν ~ν ) are also Additional particles called neutrino (ν) and anti-neutrino (
u) emitted along with beta rays in order to conserve spin in the above reactions. 4 27 31 30 1 β+ decay: 2 He + 13 Al→ 15 P → 15 P + 0 n 30 15 P → 30 14 Si + 0 +1 e + 0 0ν This type of nuclear decay is only observed with artificial radioactivity : mp < mn. In 1934, Irene and Joliot Curie discovered positron emission in 30 the reaction above where 15 P undergoes β+- decay. A Z X → Z −A1Y + β- - Decay In this case, inside the nucleus, a neutron decays according to the scheme : 0~ 0 0~ 1 0 n →1 1 p + 0 −1 e + 0ν A Z X → Z +A1Y +−1 e+ 0 ν The existence of the neutrino was predicted by Pauli in 1930, but it is an elusive particle because it has zero mass at rest (more precisely, according to experimental data from 2002, its mass at rest is ∼5eV ≈ 10-35 kg) and is without charge but with spin ћ/2. The anti-neutrino (anti- particle of the neutrino – without a charge and with almost a zero mass at rest, but with an opposite spin). For example, the nuclei of the radioactive isotope 14С undergo such decay 14 6 С→147 N + −10 e + ~ ν 0 -1  Orbital electron capture (K-, L- capture) β- decay also refers to the process in which the nucleus captures an electron from the electron shell of the atom, usually from the K-shell (less often from the L- or M-shell) and emits an antineutrino This is a process in which the nucleus captures an electron from the electron shell of the atom and emits a neutrino. And in this case, as in β+- decay, a proton in the nucleus turns into a neutron according to the scheme 1 1 0 0 1 p → 0 n + +1 e+ 0 ν Since electrons are most likely to be captured from the K-shell closest to the nucleus, this process is usually called K-capture, but it can also be L- or M-capture. For example, by K-capture, the nucleus of 74 Be a becomes a nucleus of Li:  Obtaining β- rays Beta particles are electrons and positrons. They are produced in three nuclear reactions: Energy spectrum of beta particles  Beta radiation energy spectrum of 32Р The spectrum of beta radiation is continuous, composed of particles with max energy from 0 to Eβ  max Eβ The energy Е β мах released during the beta-radioactive conversion is distributed randomly between the two emitted particles - an electron and an antineutrino or a positron and a neutrino. Therefore the energy spectrum of beta radiation is continuous. It contains electrons or positrons with energy from Е β мах theoretically to zero (as seen in fig.). The energy Е β мах havethe beta particles, which acquire all the energy of the radioactive transformation, and zero energy — the particles that are born in pairs with neutrinos or antineutrinos, which receive from the parent nucleus all the energy Е β мах The most popular example of beta - radioactive conversion is the radioactive the radionuclide phosphorus 32:  A unique example is the radionuclide 40K, which possesses all three types of beta radioactivity. Much of the radioactivity in the human body ∼3 kBq is due to this isotope  Radionuclides for positron emission tomography (PET) Short-lived β+--radionuclides are used for PET, included in compounds that participate in exchange processes (table). With the help of 11СО-hemoglobin, blood can be visualized, with Н215О- water, and with 13NH3 - the liver, since ammonia participates in the urea cycle. nucl half-life T1/2 Purpose of research ides Glucose metabolism in the brain, tumors Cardiac perfusion Blood flow Brain, tumors Spinal cord Cancer and metabolic disorders Bone formation Thyroid disorders including cancer  Emission from beta-radioactive nuclei What is the origin of electrons and the positrons emitted from beta-radioactive nuclei? They are obtained in these nuclei in spontaneous transformations between neutrons and the protons in them:  Gamma transition A nucleus that has undergone α- or β-decay very often turns out to be excited. On returning to the ground (or lower energy) state, the nucleus emits a γ- quantum. Schematically this is described as: A Z X → A Z X + γ, Excited state  Gamma transition of 222Rn The excited daughter nucleus of radon (222Rn) after the alpha-conversion of the radioactive radium (226Ra) has released the excess energy by emitting one gamma photon with energy: Gamma transition of 60Ni In the beta – radioactive conversion of 60Co the excited daughter nickel nucleus releases the excess energy through sequentially emission of two gamma photons with energies: E γ1 = 1.17MeV and E γ2 = 1.33 Mev Alpha and beta particles are a product of the mother nucleus. Gamma rays are a product of the daughter nucleus in cases where it is in excited state. Release from the excess energy of excitation happens after a very short time (up to 10-6 s) after the alpha- or beta-radioactive conversion of the mother nucleus. That is why it is said 60Co to be a beta-gamma radionuclide.  Statistical law of radioactive decay: The number of nuclei dN that decay in time dt is proportional to the total number of nuclei N in the sample : dN = – λNdt. If at time t = 0, the number of radioactive nuclei is N0, and at moment t their number is N : N t dN N ∫N N = −λ ∫0 dt ⇒ ln N0 = −λt 0 N = N 0 e − λt The half-life Т1/2 of a radioactive substance is the time during which the number of radioactive nuclei decays to half of the initial value N0 present at time t = 0: ½ ½ ½ ½ ½ ½ Фиг. 7.42 ln 2 0,693 The half-life of known radioactive nuclei varies T1 / 2 = = widely from 10-7 s до 1015 а. λ λ  Decay activity Activity of parent nuclei A(t) at time t is given as: − λt A= dN dt = λN λ N = λ N 0 e − λt А = А0 e In 1Ci = 3,7.1010 Bq. It gives the number of decaying nuclei in 1 g of radium in 1 second 1 Bq is a small unit of measurement 40-Potassium in every person is > 1000Bq;  Many radioactive sources are with activity > 100,000Bq  The radioactive sources in radiotherapy are with activity > 100,000,000Bq Derivative units of the Activity dimension  Interaction of charged particles with matter Charged particles that have enough large kinetic energy, pass through the substance by interacting with the electrons from different shells of the atoms, are scattered by the electric field of the nuclei, cause nuclear reactions and other processes. As a result, a change in the parameters of both the particles and the substance occur. The detection and measurement of corpuscular radiation and also their biological action is due to such interactions. A particle that does not interact with the substance of the detector cannot be registered. Quantities characterizing the interaction of charged radiation with matter will be presented.  Linear Energy Transfer (LET) Linear Energy Transfer, or LET, is another concept taken into account when radiation dose is provided to patients or radiation exposure of workers occurs in radiation area. When passing through a medium, ionizing radiation may interact with it during its passage and, as a result, deposit energy along its path (called a track). LET describes the energy deposition ability of a charged radiation and is defined as the amount of energy deposited per unit length of a tissue or a material by radiation. SI unit of LET,, is J/m. However, Joule is a big unit of energy. Similarly, for a dE J eV keV small body section or a material, meter LET = − , [LET] = , , dx m cm μm is also a big unit measuring length. Therefore, keV/μm is used as a where the negative sign shows more loss in common unit for LET. energy with bigger length The linear energy transfer (LET) is also referred to as restricted linear collision stopping power (S) of a material, for charged particles. It represents the energy lost dE by a charged particle due to soft and hard collisions in traversing a distance dx : dE J eV S = − , [S] = , dx m cm Linear ionization of a charged particle NL is the elementary number charges of the same sign dn, created on an elementary section of the path of the particle in a certain substance, divided by the length of this section dx: dn −1 The quantities S and NL depend on NL = , [NL ] = m the charge, mass, Ek of the particle, dx as well as on the type and density of the substance. Particles with high stopping power also have high linearity ionization. Such are the α-particles, the fragments Linear ionization of α- of heavy nuclei and the "hot" particles and β-particles in air ejected from nuclear reactors during accidents. Accelerated electrons and β- rays have much less stopping power and linear ionization than them (for the same energy) due to their smaller charge and higher speed (table). Mean linear distance R is the distance that a charged particle penetrates in a given substance, averaged over a group of particles with the same energy. The actual path of the particles is greater than R, especially for light charged particles such as electrons, which, due to scattering, change their direction of travel many times. The mean linear distance is different from the mean free path l - the average distance between two consecutive acts of interaction of a given type in a given substance. Both quantities are measured in meters. The average linear distance R depends on the charge, mass and speed of the charged particle and mainly - on the density of the substance (Table). Its value for β particles and accelerated electrons in water and biological tissues, expressed in cm, is almost half of the maximum particle energy E, expressed in MeV. Mean linear distance R of α and β particles and accelerated electrons energy Mean linear distance R particle Air Al Biological tissue particles particles accelerated electrons  Energy losses of charged particles When interacting with matter, the particles do not change their structure and therefore they scatter elastically or inelastically. Elastic scattering is when particles exchange Ек with nuclei and change their direction of motion. In inelastic scattering, the Ек of the particles is converted into another type of energy. Particles give up energy for ionization and excitation, collectively called ionization losses. They are the main energy losses during the passage of heavy charged particles (alpha part., protons, parts of nuclei) through the substance. Then the electrons released during ionization (δ electrons) can have significant energy so that they themselves cause ionization. It is known from electrodynamics that an electric charge that moves with acceleration emits EM radiation with energy proportional to the acceleration. These energy losses are transformed in radiation, called bremsstrahlung. In this way, the bremsstrahlung X-ray radiation and stopping of e- at the anode of the X-ray tube is obtained.  Radiation losses of the accelerated electrons The stopping power (S) of a material for е- : S ∼ c Z2Ek c – concentration of atoms Radiation losses for electrons of lower energy are significantly smaller than ionization losses. For example, for energy about 1 MeV they are negligibly small, since ionization losses are about 95% and elastic scattering losses - about 5%. At higher electron energy, radiation losses for a given substance become comparable to ionization losses. This energy is about 10 MeV in lead and about 100 MeV in water. As a result of the interaction processes discussed above the kinetic energy of the charged particle decreases and reaches the values ​of the energy of thermal motion (< 0.1 eV). With such an energy the electron can remain free, to form an ion with a neutral molecule, to recombine with an ion, or to annihilates with a positron.  Annihilation of electron and positron The annihilation of an electron and a positron consists in formation of a coupled system of the two particles with a very short time of lifetime (on the order of 10-7 s or 10-10 s), called positronium. The masses at rest of the electron and the positron are then converted into the energy of annihilation γ-photons. The most likely number of these γ-photons is 2, and according to the laws of conservation of energy and momentum, they have an energy of 0.511 MeV and propagate in opposite directions. 0 0 −1 e + +1e → 2 γ hν ≥ 2me0c2 = 1,02 MeV Annihilation radiation is used in the newest method of radionuclide diagnostics — PET (positron emission tomography).  Positron annihilation with electron where photons of annihilation radiation are produced. The two annihilation radiation photons propagate in opposite directions  Protection from alpha, beta particles and accelerated e- The use of protective screens when working with alpha-radioactive sources is not necessary, because the range of even the highest-energy alpha- particles is less than the thickness of the skin, and with external irradiation they cannot penetrate through it, but care is taken not to exceed the permissible energy norms. Protection when working with beta-radioactive sources has two tasks. One is protection from direct (primary) radiation, which aims to reduce external and exclude the possibility of internal radiation. The first is achieved with the help of protective screens, most often made of plastic up to 1.5 cm thick, since the Еβмах for beta-radionuclides does not exceed 3 MeV. Greater thickness of the screens is necessary for the accelerated e- , whose energy in medicine is up to 25 MeV. In this case, the task is solved simultaneously with X-radiation protection, since the accelerators used in radiation therapy are alternative sources of accelerated electrons and X- rays.  The other task is protection from secondary bremsstrahlung, which occurs in all substances that are irradiated with beta particles and accelerated electrons. Shielding from bremsstrahlung is a more serious problem because it has much greater penetrating power than primary beta particles and electrons. In electron microscopy, secondary bremsstrahlung exposure is reduced by using narrow beams of accelerated electrons and by reducing electron scattering. The thick glass screen is used to protect against bremsstrahlung radiation in color televisions whose kinescope voltage reaches 25-28 kV. Beta-radioactive sources are also dangerous for internal irradiation. Therefore, pure gamma radionuclides are recommended in in vivo radionuclide diagnostics, where the gamma radiation is not accompanied by beta particles.  Х-Rays Discoverer of Х-Rays in 1896 Nobel prize for physics 1901  Х-Rays In German physicist Wilhelm Conrad Röntgen, experimenting with high voltage discharge crux pipes found a new region of the EM spectrum  New radiation plays a huge role in atomic physics and almost immediately finds application in medicine for diagnostics and therapy, and later in industry. The first X-ray photograph, shown at Roentgen's lecture in front of the Medical Society physics at Wurzburg, on December 22 -th1895. The famous X-ray photograph taken by Roentgen on December 22, 1895 and known as “the first X-ray photograph" and Roentgenogram of the hand of Mrs. Roentgen. 1901 - Nobel Prize for outstanding merits in honor of the discovery and exploration of the extraordinary rays, later named after his name. Modern diagnostics (Conventional X-ray and CT – Computed Tomography) demonstrate most vividly the enormous importance of extraordinary rays – X-rays for medicine. The wide range of application and the high informativeness of x-ray diagnostics research makes them first in importance between imaging methods in medicine.  X-rays spectrum Modern x-ray diagnostics demonstrate most vividly the enormous importance of X-rays for medicine. Diagnosis of diseases of the musculoskeletal system, gastrointestinal tract and lungs is unthinkable without X-rays. The very wide scope of application and the high informativeness of X- ray diagnostic studies make them the first in importance among the imaging methods. X-rays are EM waves, which in the scale of EM radiation occupy a place after UV light in the low-energy part of gamma rays. Their wavelength λ is approximately between 10 nm and 3 pm, and for X- rays used in medicine: between 124nm and 6 pm. These values ​of λ correspond to photon energies between 124 eV and 420 keV and between 10 and 200 keV, respectively.. The electromagnetic spectrum in energy, frequency, and wavelength ranges  Roentgenograms X-ray imaging in Non-destructive testing. Basic applications - in medicine, X-ray structural analysis, defectoscopy.  X-RAY PRODUCTION X-ray production occurs whenever electrons of high energy strike a heavy metal target (i.e. metals with high atomic number, like tungsten or copper). When electrons hit this material, some (a fraction) of the electrons will approach the nucleus of the metal atoms where they are deflected because of their opposite charges (electrons are negative and the nucleus is positive, so the electrons are attracted to the nucleus). This deflection causes the energy of the electrons to decrease, and this decrease in energy then results in forming an x-ray. These x-rays are commonly called brehmsstrahlung or "braking radiation".  Schematic diagram of X-ray emission (braking radiation) Electrons hitting the anode region can possibly emit Bremsstrahlung or lead to emission of characteristic X-rays by ionizing the anode atoms  The characteristic X-ray emission which is shown as two sharp peaks in the illustration above occur when vacancies are produced in the or K-shell of the atom and electrons drop down from above to fill the gap. The X- rays produced by transitions from the to levels are called K-alpha X-rays, and those for the transition are called K-beta X- rays. Transitions to the or L-shell are designated as L X-rays ( is L-alpha, is L-beta, etc.). The continuous distribution of X-rays which forms the base for the two sharp peaks at left is called "bremsstrahlung" radiation. The cathode is a filament heated by passage of current, and electrons are released by thermionic emission. These tend to collect around the cathode, forming space charge and inhibiting further emission of electrons. The anode, placed at some distance from cathode, is held at positive potential relative to cathode and attracts these electrons. At low voltage, some of electrons stream out of space charge and reach the anode, while cathode keeps on releasing electrons. Consequently, current is established from cathode to anode. The space charge near the cathode tends to limit the value of this current. As anode voltage is increased, more and more electrons escape the space charge region and tube current increases, while the space charge tends to be depleted. At certain high anode voltage, the space charge disappears altogether, and the tube current reaches maximum value called the “saturation current.” The ultimate value of this saturation tube current proportionately depends on the filament heating and in turn on the filament heating current. The electrons are accelerated as they move from cathode to anode gaining kinetic energy. Just before hitting anode, they have maximum value of kinetic energy 2 mv = eU 2 For example, in case of mammography, the value of U is around 30 keV; for chest X-ray, it is around U ∼ 70 keV; and for skull X-ray, U ∼ 120 keV is needed. On impact with the anode, part of the kinetic energy is converted into heat Q, and the rest is emitted as an X-ray photon with energy: hν = m v 2 / 2 − Q Because Q has different values ​for the random hits, the energy of the emitted photon is different. This explains the continuous spectrum of radiation. The above dependence can also explain the short-wavelength limit λmin of the spectrum, which has no interpretation in the classical theory. From Planck's formula it follows that νmax (λmin) depends on the applied voltage: E max = hν max = eU ⇒ λ min = c / ν max = hc / eU When the applied voltage increases above a certain critical value, against the background of the continuous spectrum, a linear spectrum consisting of sharp intense stripes appears (fig.), which is the result of the interaction of fast electrons with the particles of the metal of the anode. This radiation is called characteristic because it depends on the chemical nature of the anode.  X-ray tube Acceleration of electrones Coolidge thermoelecron tube: a hermetically sealed glass tube with highly rarefied air of 10 mP in which there are two electrodes - a cathode and an anode. XR are produced when high-energy accelerated electrons (2) in a strong electric field.(U = 104 - 105 V) bombard a heavy metal (W, Pt, etc.) target anode. The source of accelerated electrons is the cathode, made of tungsten, which is heated to a very high temperature by electric current катод анод heating circuit high U 62 The variable heating voltage with magnitude Uо 10-15 V is obtained from a step-down transformer. At a constant magnitude of the accelerating voltage U, the electric current I in the X-ray tube is adjusted by changing Uо. All modern x-ray diagnostic tubes have a rotating anode. Exceptions are tubes in low-power mobile devices ("kugels") and in mammographs. The rotating anodes are made of a tungsten alloy with about 10% rhenium (75 Re) - Rhenium is added to improve the strength of the anode subjected to severe mechanical loads. Automated protection prevents X-ray equipment from starting up if the cooling system is turned off or malfunctions. The target cooling problem is milder with X-ray tubes used in radiation therapy because their thermal foci have a much larger area (over 35 mm2). These tubes have stationary anodes that are oil or water cooled. The fraction of energy in the electron beam converted into x-rays is given by f e = 0,001.ZU Z – atomic number of the target in the X ray tube; U – Voltage across the X ray tube.  X-ray structural analysis (XRSA) With the methods of so-called X-ray structural analysis, X-ray diffraction is used to determine the composition and mutual arrangement of atoms and molecules in crystals, in metal alloys, crystals, in biological objects with a periodic structure (for example, the molecules of DNA, hemoglobin and etc.). Wulf’s and Bragg's condition 2d sin α = nλ; n = 1, 2, … , d is the spacing between diffracting planes, α is the incident angle, n is any α integer, and λ is the wavelength of the beam. Фиг. 7.17 An X-ray diffraction pattern obtained by passing X-rays through a crystal of salt. NaCl  Biophysicochemical applications The results of biophysicochemical applications of X-ray structural analysis (XRA) are mainly in two directions. One involves establishing the spatial structure of a number of important biological macromolecules - hormones (for example insulin), antibiotics (penicillin, tetracyclines), vitamins (D2, D3, Bi2), nucleic acids, ionophores (valinomycin, nonactin, gram- cidin A), integral membrane proteins, toxins, phospholipids, etc. The other direction refers to structural studies of enzymes and enzyme- substrate complexes, identification of functional groups of active centers, etc. Such are the studies on respiratory enzymes with substrate O2 (hemoglobin, myoglobin, their changes during oxygenation), on the enzyme lysozyme with a substrate oligosaccharide from the cell walls of bacteria, on ribonuclease, chymotrypsin, carboxypeptidase, etc. XRSA is the most effective modern method for studying the structure not only of crystals but also of biological macromolecules and is widely applied in molecular biology, biochemistry and biophysics.  Total X-ray intensity. X-ray quality. Filtering The total intensity of the X-rays generated by the X-ray tube target is determined by the photons' bremsstrahlung and characteristic radiation throughout their spectrum. It is proportional to the area of t​ he figure enclosed by the intensity curve and the abscissa for λ..The total intensity increases with the square of the accelerating voltage because higher energy bremsstrahlung photons are created at higher U. I = kZU2Ia I of the radiation can also be changed by the anode current without changing its wave composition. In general, the intensity depends on U, Iа and on the atomic number of the element of the anode Z. The intensity – I of the X rays is determined by the electric parameters of the x-ray tube. I = kZU2Ia The quality - Q of the X-rays is determined by the U – accelerating voltage and from the half-value layer HVL The overall intensity of the X-rays used for diagnosis and treatment also depends on their filtering. X-ray devices with voltages up to 75 kV have their own filtration up to 2 mm aluminum. Devices with voltages above this limit necessarily work with additional filters that change the quality of the radiation by increasing its half- attenuation layer. Additional filters inevitably reduce the overall intensity of the X-ray beam. The compromise between the requirement to exclude from the spectrum the lowest energy X-ray radiation (the softest radiation) and the need for a sufficiently large total intensity for practice is achieved by appropriate selection of the additional filters. Aluminum filters with a thickness of 2 to 4 mm are used for X-ray diagnostic devices (with a voltage of more than 75 kV), and for therapeutic devices with a voltage of 180-250 kV, combined filters of copper and aluminum with a thickness of 1-3 mm are used. Effect of additional filtration  INTERACTION OF PHOTON IONIZING RADIATION WITH MATTER Like light, some of the photon ionizing radiation that enters the substance is absorbed, another part is scattered, and a third passes through unchanged. Depending on the energy of the quanta, several interaction processes are possible: photoelectric absorption (internal photoeffect), Compton scattering and formation of electron-positron pairs. Understanding the interaction of X-ray and γ-ray with matter is very important because it is the same interaction that occurs when these radiations come across the human body and tissues. All these mechanisms of interaction result in the transfer of energy to the electrons of matter or tissues.  Photoelectric absorption The photoelectric absorption or effect is the most important interaction of low-energy 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 applications. Photoelectric effect occurs when a bound electron from the atom is ejected after interaction with a photon of energy hν : hν = Ei + Ek hν > Еi Еi is the work function of the material (the ionizing energy of the electron from the atom), and Ек is the kinetic energy of the ejected photoelectron. The effect consists in the complete absorption of the energy of the photon hν (it ceases to exist) and the release of an electron with kinetic energy Ек (called a photoelectron) from the inner layers of the atom. The place of the photoelectron in the atom is occupied by an electron from the outer layers, in which a photon characteristic X-ray radiation is emitted. In comparison to light photoelectric effect, EM photons interact with the valence electrons of the metals, which are separated from the atoms or weakly connected to them. Photon energy is used to separate the electrons from the surface of the metal (separation work) and to transfer kinetic energy to the electron. The energy of X-rays and γ -photons is much greater than that of the light ones and can be fully devoted only to the separation of the electrons tightly bound to the nucleus. Therefore, the probability of their photoelectric absorption is greatest for the innermost electrons of the K-shell of atoms and increases with increasing atomic number Z of the element. In the photoelectric absorption all of the photon's energy is transferred to a single electron in the innermost shell of the atom. This effect is the predominant effect in biological tissues at photon energies up to 60 keV, and in lead at energies above 500 keV. This is one of the reasons lead is used for radiation shielding. An incident photon with Падащ фотон photoelectron with energy N Фотоелектрон с енергия energy с енергия hν > Eйонизац. i Eк hν = Eйонизац. i + Ек  Photoelectric cross section τ or the probability of occurring photo- electric effects The photoelectric cross section τ or the probability of occurring photoelectric effect depends upon the incident photon energy as well as on the charge number Z of the interacting material. As the energy of a photon decreases, the probability of photoelectric absorption or the photoelectric cross-sectional “τ” increases rapidly. The Z and E dependence on the photoelectric cross section τ near 100 keV is approximate: It is proportional to the density ρ and the fourth power of the atomic number Z for simple substances (chemical elements) and decreases strongly with increasing quantum energy: :  Effective atomic number In case of a complex substance (composed of different chemical items) effective atomic number Z τ is applied: it is equal to the atomic number of such a simple substance which has the same linear photoelectric absorption coefficient as the complex substance. For comparison: muscles and water have Zτ = 7,42, and bones have Z τ = 13,8 - almost as much as aluminum (Z = 13). This allows for experiments and calculations in radiation protection to use water instead of soft tissues and aluminum instead of bones. Photoelectric absorption dominates over other types of interactions at quantum energies up to about 60 keV for biological tissues and up to 500 keV - for lead. This is one of the reasons lead to be used for radiation shielding material.  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). Compton scattering gets special importance in therapeutic medical physics because many of the treatment planning by photon beam are carried out in this energy range. A tumor and normal tissues surrounding the tumor, scatter radiation in various directions, making Compton scattering an important phenomenon to concentrate on. Interaction of gamma rays or X-rays with the human body or matter. Compton scattering is dominant over an energy range of 0.5 MeV – 12 MeV, and pair production dominates for photonenergy above 12 MeV. Compton effect occurs when photons interact with free or weakly bound electrons in the γ -ray incident beam. In this incoherent scattering, all atomic electrons act independently of one another. In Compton scattering, a single photon strikes an electron, giving a part of its energy and momentum to the electron. This electron is stationary or almost stationary and is called target electron. As a result, the photon scatters with a reduced energy and longer wavelength. The difference in the energy of photon before and after scattering is taken away by the electron as its kinetic energy. the matter or tissues. Compton’s effect differs from the photoelectric effect in that the photons are not completely absorbed, but only transfer part of their energy to the electrons. Particular cases of the Compton effect are the scattering of X-rays from the electron shells of atoms and the scattering of γ -rays from atomic nuclei.  Compton’s effect P-light substance (graphite, paraffin, aluminum, etc.), whose electrons are weakly λ λ connected to the nuclei of λ′ atoms, i.e. are almost free. ∆λ C = 2λ C sin 2 (θ / 2)   hν pe = m v p= λ С= 0, 241 Å is the so- c ϕ called Compton's constant. θ hν′ Its numerical value is p′ = Фиг. 6.51 c determined for θ = 90°  Compton incoherent scattering    ЗЗЕ: h ν = E k + hν ′ ЗЗИ p = pe + p′ Energy and momentum are conserved.The solution of the system of equations gives the physical meaning of the Compton constant for the electron : λ C = h /(m0 c) = 0,2426 Å The probability of occurring Compton scattering “σ” slowly varies with the charge number Z of the host material of the interacting electron. It is directly proportional to the density ρ of the substance and the concentration of free electrons in it n0 and inversely proportional to the energy of the photons: This is the most dominant interaction mechanism in tissue.  At low energies of the incident photons (hν ≈ 10 keV), they retain their energy, and as a result of the interaction with the atom only change their direction of propagation. Coherent отлитащ Thompson фотон scattering с енергия hν’ incident фотон Падащ photons of photon(томпсаново) ν with energy hразсейване кохерентно с енергия with energy hν = hν’ Комптонов електрон N с енергия Е electron with к energy Ek ≮φ Падащphotons incident с енергия with energy фотон ↺ ↺ ≮θ Разсеян фотон hν = hν’‘+ Ек с енергия hν’ Compton incoherent scattering of photon with energy hν ‘ dominates for (0.5 MeV – 12 MeV)  Production of electron-positron pair In this phenomenon, a γ -ray or X-ray photon with energies hν > 1,022 MeV, passing near the nucleus of an atom is subjected to strong electric field effects from the nucleus and splits into an electron-positron pair. Broadly speaking, a negatively charged electron (e−) and a positively charged positron (e+) are created from a photon interacting with the electromagnetic field while energy and momentum are conserved. Since electron (e −) and positron (e+) possess particle nature and take part in the construction of matter, therefore, pair production is also called materialization of energy. 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). If the energy of the photon is more than the sum of the rest mass energies of both e − and e+, then the remaining energy is their kinetic energies. Double rest mass energies should satisfy : Emin = hν ≥ 2me0c2 = 1,02 MeV. This energy is determined by Einstein's famous formula for the relationship between mass and energy: Е = mc2. If hν > Emin the residual hv - Emin , called suprathreshold energy, is transmitted to as Ек of electron and positron. In order to conserve the momentum, electron and positron must move in the opposite directions after being created. Moreover, pair production dominates overphotoelectric absorption and Compton incoherent scattering when the energy of photon is bigger than 10 MeV. The probability of this effect increases with increasing atomic number due to the fact that pair production is caused by an interaction with the electromagnetic field of the nucleus. The probability “χ” of forming an electron-positron pair increases linearly with the suprathreshold quantum energy hν - Emin and the density of the substance ρ, as well as with the second power of its atomic number Z of the material. (Thus, high-Z materials like lead are more favorable for pair production to take place.) The electron and positron pair do not exist free for long time and recombine through a process called annihilation of matter. In the annihilation process, the electron and positron combine with each other, disappear, and give rise to two γ-ray photons each with an energy of 0.511 MeV. The two photons move in opposite directions to conserve momentum.  Overall Interaction of Photons with Matter When a photon beam passes through a material or a tissue it interacts at the same time with all possible mechanisms described earlier. For example, when a 2 MeV gamma ray photon interacts with a tissue, it dominantly interacts by Compton scattering. During this process, a photon loses part of its energy every time it interacts with a tissue cell. As a result, the energy of the photon continuously decreases and reduces to a range of energies where photoelectric absorption is dominant. At this stage, the photon is absorbed by the tissue by the process of photoelectric absorption. The attenuation of an X-ray gamma ray photon beam is governed by the linear attenuation law given below: − µd I = I 0e where μ represents the linear attenuation coefficient and d gives the thickness of the attenuator.  Overall interaction probability µ The interaction probability “μ” of a photon with matter by all τ is the photoelectric effect interaction probability (IP) three processes is simply the σ is the Compton scattering IP. sum of each individual χ is the pair production IP probability of occurrence μ is the overall interaction probability or the linear attenuation coefficient I = I 0 e −µd  Half-Value Thickness (HVT or HVL) The half-value thickness, or half-value layer, is the thickness of the material that reduces the intensity of the beam to half of its original value. When the attenuator thickness is equivalent to the HVT, I/I0 is equal to ½ ln 2 d1 / 2 = µ This value is used clinically quite often in place of the linear attenuation coefficient. Here are example approximate half-value layers for a variety of materials against a source of gamma rays (Iridium-192): Concrete: 44.5 mm Steel: 12.7 mm Lead: 4.8 mm Tungsten: 3.3 mm Uranium: 2.8 mm Half-Value Layer is the layer thickness of a certain material that reduces the intensity of the ionizing photon radiation to half of its original value  Half-value layer in X-ray diagnostics and in X-ray therapy The half-attenuation layer in X-ray diagnostics is defined in millimeters of aluminum, and in X-ray therapy - in millimeters of beryllium, aluminum and copper.  Law of Diminishing Intensity with the distance It has been found experimentally that with increasing distance r from the photon source 2, 3, 4, etc. times, the I intensity decreases accordingly times  General attenuation law of photon Ionizing radiation The two laws of radiation attenuation act simultaneously and independently from one another. Therefore, in the propagation of photon ionizing radiation their intensity decreases according as follows: e −µx I = I0 2 r  Biological action of X-rays Like radioactive radiation, X- rays have ionizing power. They ionize the atoms and molecules that make up living cells, on which their biological action is based. Absorbed radiation causes physical changes in cells, such as breaking down molecules, stopping enzymes from working, rupturing chromosomes, and other damage. The cells that grow the fastest are the most sensitive to radiation. Therefore, X-rays are used in medicine to attack tumor formations, whose cells multiply much faster than normal cells and are more sensitive to ionizing radiation.  Safety and radiation dosimetry – BASIC PRINCIPLE Radiation exposure to hospital staff and public must be carefully controlled to bebelow the regulatory limits and based on the principle of ALARA (As Low As Reasonably Achievable) (NCPR - National Council on Radiation Protection 1993). This is usually achieved by proper radiation shielding of the CT room. The details of radiation shielding design for CT-scanner rooms and radiation therapy centers can be found in in the National Council on Radiation Protection and Measurement (NCPR) report No. 147 (NCPR 2004). MECHANISMS OF RADIATION DAMAGE Ionizing radiation affects living things on an atomic level, by ionizing molecules inside the microscopic cells. When ionizing radiation comes in contact with a cell, any of the following may happen: (i) It may pass directly through the cell without causing any damage. (ii) It may damage the cell but the cell will repair itself. (iii) It may affect the cell’s ability to reproduce itself correctly, possibly causing a mutation. (iv) It may kill the cell. The death of one cell is of no concern but if too many cells in one organ such as the liver die at once, the organism will die. Radiation which is absorbed in a cell has the potential to impact a variety of critical targets in the cell, the most important of which is the DNA. Evidence indicates that damage to the DNA is what causes cell death, mutation and carcinogenesis. The mechanism by which the damage occurs can happen via one of two scenari: i) Direct Action In the first scenario, radiation may impact the DNA directly, causing ionization of the atoms in the DNA molecule. This can be visualized as a “direct hit” by the radiation on the DNA, and this is a fairly uncommon occurrence due to small size of the target; the diameter of the DNA helix ['hi:liks] is only about 2nm. It is estimated that the radiation must produce ionization within a few nanometers of the DNA molecule in order for this action to occur. (ii) Indirect Action In the second scenario, the radiation interacts with non-critical target atoms or molecules, usually water. This results in the production of free radicals, which are atoms or molecules that have an unpaired electron and thus are very reactive. These free radicals can then attack critical targets such as the DNA (see figure below). Because they are able to diffuse some distance in the cell, the initial ionization event does not have to occur so close to the DNA in order to cause the damage. Thus, damage from indirect action is much more common than damage from direct action, especially for radiation that has a low specific ionization. When the DNA is attacked, either via direct or indirect action, damage is caused to the strands of molecules that make up the double-helix structure. Most of this damage consists of breaks in only one of the two strands and is easily repaired by the cell, using the opposing strand as a template. If however, a double-strand break occurs, the cell has much more difficulty repairing the damage and may make mistakes. This can result in mutations, or change to the DNA code, which can result in consequences such as cancer or cell death. Double- strand breaks occur at a rate of about one double-strand break to 25 single-strand breaks. Thus, most radiation damage to DNA is reparable. DETERMINANTS OF BIOLOGICAL EFFECTS Rate of Absorption The rate at which the radiation is administered or absorbed is most important in the determination of what effects will occur. Since a considerable degree of recovery occurs from the radiation damage, a given dose will produce less effect if divided (thus allowing time for recovery between dose increment) than if it were given in a single exposure. Area Exposed The portion of the body irradiated is an important exposure parameter because the larger the area exposed, other factors being equal, the greater the overall damage to the organism. This is because more cells have been impacted and there is a greater probability of affecting large portions of tissues or organs. An example of this phenomenon is in radiation therapy, in which doses which would be lethal if delivered to the whole body are commonly delivered to very limited areas, e.g. to tumor sites. Variation in Species and Individual Sensitivity There is a wide variation in the sensitivity of various species. Lethal doses for plants and microorganisms, for example, are usually hundreds of times larger than those for mammals. Even among different species of rodents, it is not unusual for one to demonstrate three or four times the sensitivity of another. Within the same species, individuals vary in sensitivity. For this reason the lethal dose for each species is expressed in statistical terms, Variation in Cell Sensitivity Within the same individual, a wide variation in susceptibility to radiation damage exists among different types of cells and tissues. In general, those cells which are rapidly dividing or have a potential for rapid division are more sensitive than those which do not divide. It is possible to rank various kinds of cells in descending order or radiosensitivity. Most sensitive are the white blood cells called lymphocytes, followed by immature red blood cells. Epithelial cells, which line and cover body organs, are of moderate high sensitivity; in terms of injury from large doses of whole-body external radiation, the epithelial cells which line the gastrointestinal tract are often of particular importance. Cells of low sensitivity include muscle and nerve, which are highly differentiated and do not divide.  Interaction of IONIZING RADIATION (IR) with matter radiation matter change in radiation effects the direction of physical propagation chemical  the energy biological The radiation effect depends on the energy, which the radiation transmits to the substance.  DOSIMETRY OF IONIZING RADIATIONS Dosimetry (from the Greek word dose - part, portion) is a section of applied nuclear physics and measurement technique, which examines the quantities characterizing the action of ionizing radiation on matter, as well as the methods and devices for their measurement. Its importance is determined by the fact that a person does not have sensory organs for ionizing radiation, but the consequences of their action on him depend on the degree of irradiation (dose), appear later and can be irreversible. Therefore, accurate measurements in dosimetry are particularly important, but unlike many others, their implementation is more difficult for two reasons - the statistical nature of radioactive decay and the presence of a natural radiation background.  Basic dosimetric quantities and units All effects of ionizing radiation on matter (physical, chemical, biological) are the result of the transmitted energy from the radiation to the substance. The main quantity introduced to evaluate all types of ionizing radiation in their interaction with all substances is called absorbed dose. The name of the section dosimetry comes from it. The absorbed dose (or just dose) D is the ratio of the transmitted energy dE from the radiation per unit mass of the body D = Е/m. The absorbed dose (or just dose) D is the ratio of the transmitted radiation energy per unit mass of the body. dE J D= , [ D] = = Gy dm kg Its SI unit is gray [D] = 1 Gr = 1 J/kg. Old unit is rad (100 rad = 1 Gr).  Absorbed Dose D D can be defined as the average energy absorbed per unit mass of any material. It is a non-stochastic quantity and is applicable to both directly and indirectly ionizing radiations. dE J D= , [ D] = = Gy dm kg Absorbed Dose Old standard unit Gy is a relatively large unit of measure- ment. Skin dose of photons 6 - 8 Gy – skin erythema In case of radiotherapy – for one session 1 – 8 Gy.The skin dose for a dental photograph – 2.5 - 5 mGy  Power of absorbed dose The radiobiological effect depends on:  the absorbed dose  the time for which the dose is received Power of absorbed dose is the ratio of the dose and time for its accumulation. Unit of dD measurement – Gy/s, Gy/h, mGy/h PD = dt  Exposure for photon ionizing radiations (IonRs) The X- or γ radiation interaction with matter leads to the production of ion pairs. The simplest way to measure the quantitative effects of these radiations is to measure the number of ion pairs Х = dQ/dm [С/ kg] It is measured in C/kg in SI. The conventional unit Roentgen is also used (R): [X] = 1 R = 2,58.10-4 C/kg. It is defined as the amount of X-ray radiation that creates 1.61. 1035 ion pairs per kilogram of air.  Exposure power Photonic IonRs (X-rays and gamma rays) Exposure Exposure - a measure of the ionizing ability of photons in the air. Exposure power PX is the ratio of exposure dX and time interval dt, dX A PX = , [ PX ] = dt kg  Exposure-absorbed dose conversion factor There is a simple relationship between the exposure X and the absorbed dose D of photon ionizing radiation: Gy. kg J D = fX , [ f ] = = C C The proportionality factor f is called the exposure-absorbed dose conversion factor and has a different value depending on the energy of the photons and the type of substance. It is proportional to the total linear attenuation coefficient µ and shows how much of the energy of the primary photons is converted into Ek energy of the resulting charged particles and then into ionization losses  EQUIVALENT DOSE The radiobiological effect also depends on the type and energy of ionizing radiation. Equivalent Dose = (Absorbed Dose) x (Radiation Weighting factor) wR - Radiation Weighting factor = 1 for X-rays, gamma-rays, beta-rays, accelerated electrons = 5 - 20 for neutrons of different energy = 5 for protons = 20 for alpha rays  EQUIVALENT DOSE Equivalent Dose = (Absorbed Dose) x (Radiation Weighting factor) unitless quantity D unit gray H unit sievert Old standard measure is ber (or rem – from “rad equivalent man”): 1 Sv = 100 rem = 1 J/kg.  Weighting factor International Committee for Radiological Units (ICRU and International Commission on Radiological Protection (ICRP) introduced a new term called Quality Factor that represents the effectiveness of a particular radiation that interacts with human body. Later its name was changed by a new term Radiation Weighting factor ‘WR’ When exposed to several types of radiation, the equivalent dose is the sum of the products of the absorbed dose and the corresponding radiation weighting coefficient: Power of the equivalent dose Рн is defined analogously to its similar values ​and is measured in the unit sievert per second dH Sv PH = , [PH ] = dt s  Equivalent dose - example The equivalent dose accounts for the difference biological effect of radiation exposure of a different kind and with a different energy. 1. Irradiation with gamma radiation absorbed dose 10 mGy ⇒ equivalent dose 10 mSv 2. Irradiation with alpha radiation absorbed dose 10 mGy ⇒ equivalent dose 200 mSv The biological effect in the second case is 20 times larger.  Tissue Weighting Factor and Effective Dose Another important factor that must not be ignored when radiation is used to deliver dose to human body in diagnoses and treatment applications is the tissues response to radiation. Even if the same equivalent dose is provided to various parts of the body, the response of different tissues is different. Some tissues are more sensitive to radiation while some others are relatively less sensitive. Thus, even under the same equivalentdose, the damage provided to different tissues is different. In order to take into account for the response of various tissues into the interacting radiation a new factor called Tissue Weighting Factor ‘WT’ is introduced by ICRU and a new quantity called Effective Dose ‘E’: E = H.WT H =D.WR E = D.WR.WT Effective dose is always used as a measure of risk. [E] = Sv Tissue weighting factors for various tissues are given in Table.  KERMA Kerma is an acronym for kinetic energy released per unit mass. It is a nonstochastic quantity applicable to indirectly ionizing radiations such as photons and neutrons. It quantifies the average amount of energy transferred from indirectly ionizing radiation to directly ionizing radiation without concern as to what happens after this transfer. In this context, the kerma is defined as the mean energy transferred from the indirectly ionizing radiation to charged particles (electrons) in the medium per unit mass dm: dEtr K= dm The energy of photons is imparted to matter in a two stage process. In the first stage, the photon radiation transfers energy to the secondary charged particles (electrons) through various photon interactions (the photoelectric effect, the Compton effect, pair production, etc.). In the second stage, the charged particle transfers energy to the medium through atomic excitations and ionizations. The unit of kerma is joule per kilogram (J/kg). The name for the unit of kerma is the gray (Gy), where 1 Gy = 1 J/kg. CEMA Cema is the acronym for converted energy per unit mass. It is a nonstochastic quantity applicable to directly ionizing radiations such as electrons and protons. The cema C is the quotient of dEc by dm, where dEc is the energy lost by charged particles, except secondary electrons, in collisions in a mass dm of a material: dE c K= dm The unit of cema is joule per kilogram (J/kg). The name for the unit of cema is the gray (Gy).  Dosimeters Measuring devices of dosimetric quantities are called dosimeters. Dosimeters are the devices that serve for measurement of quantities: absorbed dose - D exposure – X and their power. Radiometers are the devices with Multichannel which we measure the activity – A dosimeter of radioactive sources and radiopharmaceuticals Dose calibrator (activity meter) Individual dosimeters  Basic principles of radiation protection in Ordinance 30 Rationale [ræʃə'na:li] for Irradiation  Irradiation optimization. Principles: the dose for diagnostic tests should be as much as low as is reasonably achievable to provide for necessary diagnostic information to ensure the necessary protection of the radiation sensitive organs. Technical and organizational measures Provision of radiological equipment with which the necessary diagnostic information and/or therapeutic outcome is possible.  Quality Assurance Programs. To ensure geometric and dosimetric accuracy, rigorous quality assurance (QA) program needs to be developed and implemented. The QA testing is usually distributed among daily, monthly, and annual tests. monitoring of the patient’s dose QA of the machine developing X-ray films QA of all types of X-ray systems QA at the computer room tomography  Types of Radiations in medicine Photon Radiation – X rays and γ rays, electrons and protons are the most often used and the most important radiations in medicine for now!  РЕНТГЕНОВА ДИАГНОСТИКА Основа на рентгеновата диагностика е силната зависимост на общия коефициент на отслабване на рентгеновите лъчи от вида на биологичните тъкани. Изследваната част от тялото се пролъчва с рентгеново лъчение с енергия между 20 и 140 keV, което при преминаването си през тъканите намалява своя интензитет (отслабва). Това отслабване е резултат от два вида взаимодействие на рентгеновите лъчи с тъканите – при по-ниски енергии преобладава фотоелектричното поглъщане, а при по-високи енергии Комптоновия ефект. Отслабването на лъчението е по експоненциален закон и освен от μ зависи и от дебелината d на структурите: Тук е разпределението на мощността на енергийния пренос (интензитетът) на лъчението в равнината (х,у), която е перпендикулярна на посоката на разпространение на рентгеновите лъчи z.  РЕНТГЕНОВА ДИАГНОСТИКА Най-големи са стойностите на µ за костите, многократно по-малки за меките тъкани и най-малки – за въздушните кухини. Затова интензитетът на рентгеновите лъчи, преминали през костните структури е най-малък, а зад въздушните кухини – най-голям. Разликата между стойностите на µ за тези тъкани е по-голяма при по-ниски енергии на фотоните, отколкото при по-високи. Това определя по-големият контраст на рентгеновия образ при по-ниски енергии на фотоните, получаван обаче за сметка на по-голяма погълната лъчева енергия в тялото при по-големи стойности на µ. Получаването на образа в класическата, наричана още конвенционална рентгенова диагностика, при която диагностичният образ представлява двумерна проекция на пролъчените тримерни структури.  РЕНТГЕНОВА ДИАГНОСТИКА Растер: правоъгълна матрица от пиксели, т.е. всеки пиксел в изображението има числова стойност, която съдържа информация за цвета в него.  РЕНТГЕНОВА ДИАГНОСТИКА Преминалите през тъканите без да взаимодействат с тях рентгенови лъчи формират невидимо лъчево изображение, което се преобразува във видим образ чрез равнинен детектор, наричан преобразовател на образа. При рентгеновата графия преобразователят е рентгеновият филм и образът представлява разпределение на оптичната плътност (почерняването) на филма: най-светли са костите, по-тъмни – меките тъкани и най-тъмни – белите дробове, т.е. образът е негативен. Поради малката абсорбционна ефективност на филма за рентгеновите лъчи, той се експонира в “сандвич” между два флуоресциращи слоя – т.нар. усилващи фолиа, които преобразуват рентгеновите лъчи в светлина, към която филмите имат много по-голяма чувствителност. Използването на тези филм-фолийни комбинации намалява многократно облъчването на пациента. Със същата цел през последните години рентгеновият филм се замества с дигитален детектор.  РЕНТГЕНОВА ДИАГНОСТИКА При рентгеновата скопия образът се получава върху флуоресциращ екран, като яркостта на екрана е най-голяма зад белите дробове, по-малка зад меките тъкани и най-малка зад костите. За увеличаване на яркоста, в модерните рентгенови скопични уредби образът се усилва чрез електронно-оптичен преобразовател (ЕОП), след което чрез телевизионна система се наблюдава върху монитор. Вместо ЕОП най- новите скопични уредби използват плоски панелни дигитални детектори с директно или индиректно преобразуване.  РЕНТГЕНОВА ДИАГНОСТИКА Втората важна стъпка в развитието на образната диагностика е направена с изобретяването през 1972 г. на рентгеновата компютърна томография (СТ). Това е метод за получаване на двумерен образ от тънък слой тъкани, лежащи между две успоредни близкоразположени равнини в тялото. Докато при конвенционалните рентгенови методи информацията за тримерния обект се представя като двумерен образ, поради което в него има препокриване на някои пролъчвани тъкани, при СТ това се избягва и рентгеновият образ е с по-голям контраст  РЕНТГЕНОВА ДИАГНОСТИКА В основата на СТ са публикуваните през 1917 г. от Радон уравнения за получаване на образ на двумерен или тримерен обект от неговите едномерни проекции, както и разработената от Мак Кормак през 1956 г. математична теория за реконструиране на образа. Първият рентгенов компютърен томограф е създаден от Хаунсфийлд за изследване на главен мозък. През 1979 г. Мак Кормак и Хаунсфийлд получават Нобелова награда по физиология и медицина за създаването на СТ.  РЕНТГЕНОВА ДИАГНОСТИКА Формирането на диагностичния (томографския) образ при СТ става в три фази. Първата фаза е сканирането: тесен колимиран сноп рентгенови лъчи преминава под различни ъгли през тънък слой тъкани между две напречни равнини на тялото, като сигналите от преминалото лъчение се измерват с голям брой срещулежащи детектори – малки сцинтилационни кристали или ксенонови йонизационни камери, механично свързани с тръбата, т.нар. гентри. Сигналите на всички едномерни проекции (линейни интеграли) постъпват в паметта на компютър, който управлява и целия диагностичен процес.  РЕНТГЕНОВА ДИАГНОСТИКА Във втората фаза – компютърното реконструиране, коефициентът на отслабване в определена точка от среза µ (х,у) се представя като сума от линейните интеграли за всички лъчи, преминали през точката. След реконструкцията коефициентът на отслабване на всеки елемент (х,у) от матрицата (пиксел) се представя чрез т.нар. Хаунсфийлдови единици, HU: µ( x, y ) − µ вода HU ( x, y ) = 1000 µ вода където µвода е общият коефициент за отслабване за вода. Скалата на Хаунсфийлдовите единици е от минус 1000 (за въздух) до плюс 3000 (за плътни кости); меките тъкани имат HU от минус 300 до минус 100; по дефиниция HU=0 за водата.  РЕНТГЕНОВА ДИАГНОСТИКА При третият етап на формирането на образа – визуализирането, на всяка HU се приписва определена яркост, като черният цвят съответства на най-малките стойности на HU, а белият на най- големите. Използват се 212 или 216 степени на сивота. Раздели- телната способност е по-малка от тази на конвенционалните рентгенови методи – от 0,4-0,5 mm, и е ограничена от физичните размери на детекторите и големината на цифровата матрица. Най- голямото предимство на СТ е възможността за разграничаване на разлики в рентгеновата плътност на тъканите – от 0.25-0.5%, постигана чрез визуализиране на сигналите само от тънкия слой тъкани, както и чрез допълнително компютърно увеличаване на контраста чрез “прозоречна техника” или други начини. Увеличеният контраст повишава способността на зрението да открива структури с малки размери. РЕНТГЕНОВА ДИАГНОСТИКА Показаните на фиг. 8 и фиг. 9 картини са получени върху фотоплака, която почерня- ва при попадане на електрони върху нея. Много важен факт е, че всеки електрон предизвиква почерняване на само едно зърно от емулсията, т.е. поглъщането на електроните във фотоемулсията става както поглъщането на фотоните от метала: в определена точка се поглъща само цяла порция, цяла частица. Светлите петна на фигурите са местата, където са попаднали най-много електрони, а не са резултат от разпръсването (или размазването) на всеки електрон на части В стъклен балон с дебели стени в силно разреден въздух (налягане 1-10 mР а ) са разположени двата електрода на тръбата. Катодът е източник на електрони, отделяни от металната му повърхност при висока температура (термоелектронна емисия). Той представлява волфрамова спирала, захранвана с т. нар. отоплително напрежение Uf. Анодът на рентгеновата тръба е масивен метален електрод с волфрамова мишена (6). Между катода и анода се прилага постоянно високо напрежение ускоряващо U. Изходът на рентгеновите лъчи, наричан прозорец на рентгеновата тръба, е по-тънка част от стъкления балон или тънка берилиева пластина. Прозорецът е разположен в посока, перпендикулярна на посоката на ускоряване на електроните. Причина за това е характерът на разпространение на генерираните от мишената рентгенови лъчи: техният интензитет е най-голям в посока, приблизително перпендикулярна на посоката на електроните в рентгено- вата тръба. По същата причина челната повърхност на анода е наклонена (скосена), за да се намали поглъщането на рентгеновите лъчи в анода Стъкленият балон на рентгеновата тръба — без прозореца — е обвит в метален кожух, който го предпазва от механични удари и осигурява необходимото за лъчезащитата отслабване на рентгеновите лъчи. Освен рентгеновата тръба, основните части на една рентгенова уредба са: токозахранващ блок, регистриращо устройство и пулт за контрол и управление. Токозахранващият блок съдържа високоволтов генератор на постоянно напрежение, съставен о т повишаващ трансформатор (обикновено 6 интервала 10 - 200 kV) и токоизправител. В него има и понижаващ трансформатор (10 -15 V) за отоплителното напрежение. Рентгеновата тръба е генератор на два вида лъчи — спирачно и характеристично рентгеново лъчение, които се различават по механизма на получаването си и по своя спектър. Спирачното и характеристичното лъчение се създават и излъчват едновременно.  ИЗТОЧНИЦИ НА ЙЛ в радиационната медицина I. Радиоактивни източници (радиоактивност): телегаматерапевтични уредби (ТГТУ)^  вътретъканна и вътрекухинна брахитерапия^^  радиофармацевтици за метаболитна брахитерапия II. Генераторни:  рентгенови уредби  медицински ускорители  циклотрони ^ източникът е ЙЛ извън пациента) ^^Брахитерапията е вид вътретъканна лъчетерапия, при която източникът на йонизираща енергия се поставя в самия орган или в близост до него.  Directly ionizing radiation deposits energy in the medium through direct Coulomb interactions between the directly ionizing charged particle and orbital electrons of atoms in the medium. Indirectly ionizing radiation (photons or neutrons) deposits energy in the medium through a two step process: In the first step a charged particle is released in the medium (photons release electrons or positrons, neutrons release protons or heavier ions); In the second step the released charged particles deposit energy to the medium through direct Coulomb interactions with orbital electrons of the atoms in the medium. Both directly and indirectly ionizing radiations are used in the treatment of disease, mainly but not exclusively for malignant disease. The branch of medicine that uses radiation in the treatment of disease is called radiotherapy, therapeutic radiology or radiation oncology. Diagnostic radiology and nuclear medicine are branches of medicine that use ionizing radiation in the diagnosis of disease.