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  1. 1. Basic Sciences of Nuclear Medicine
  2. 2. Magdy M. Khalil (Ed.)Basic Sciences of NuclearMedicine
  3. 3. Magdy M. KhalilImperial College LondonHammersmith CampusBiological Imaging CentreDu Cane RoadW12 0NN LondonUnited Kingdommagdy khalil@hotmail.comISBN 978 3 540 85961 1 e ISBN 978 3 540 85962 8DOI 10.1007/978 3 540 85962 8Springer Verlag Heidelberg Dordrecht London New YorkLibrary of Congress Control Number: 2010937976# Springer Verlag Berlin Heidelberg 2011This work is subject to copyright. All rights are reserved, whether the whole or part of the material isconcerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting,reproduction on microfi lm or in any other way, and storage in data banks. Duplication of this publicationor parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965,in its current version, and permission for use must always be obtained from Springer. Violations are liableto prosecution under the German Copyright Law.The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply,even in the absence of a specific statement, that such names are exempt from the relevant protective lawsand regulations and therefore free for general use.Product liability: The publishers cannot guarantee the accuracy of any information about dosage andapplication contained in this book. In every individual case the user must check such information byconsulting the relevant literature.Cover design: eStudio calamar, Figures/BerlinPrinted on acid free paperSpringer is part of Springer ScienceþBusiness Media (
  4. 4. I dedicate this book to my parents
  5. 5. (76)“. . . and over every lord of knowledge, there is ONE more knowing” Yousef (12:76)
  6. 6. AcknowledgmentThanks to my God without him nothing can come into existence. I would like tothank my parents who taught me the patience to achieve what I planned to do. Specialthanks also go to my wife, little daughter and son who were a driving force for thisproject. I am grateful to my colleagues in Department of Nuclear Medicine at KuwaitUniversity for their support and encouragement, namely, Prof. Elgazzar, Prof. GaberZiada, Dr. Mohamed Sakr, Dr. A.M. Omar, Dr. Jehan Elshammary, Mrs. HebaEssam, Mr. Junaid and Mr. Ayman Taha. Many thanks also to Prof. Melvyn Meyerfor his comments and suggestions, Drs. Willy Gsell, Jordi Lopez Tremoleda andMarzena Wylezinska-Arridge, MRC/CSC, Imperial College London, Hammersmithcampus, UK. Last but not least would like to thank Sayed and Moustafa Khalil fortheir kindness and indispensible brotherhood. ix
  7. 7. ContentsPart I Physics and Chemistry of Nuclear Medicine 1 Basic Physics and Radiation Safety in Nuclear Medicine . . . . . . . . . . . . . 3 G.S. Pant 2 Radiopharmacy: Basics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Tamer B. Saleh 3 Technetium-99m Radiopharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . 41 Tamer B. Saleh 4 Radiopharmaceutical Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Tamer B. Saleh 5 PET Chemistry: An Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Tobias L. Roß and Simon M. Ametamey 6 PET Chemistry: Radiopharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . 103 Tobias L. Roß and Simon M. AmetameyPart II Dosimetry and Radiation Biology 7 Radiation Dosimetry: Definitions and Basic Quantities. . . . . . . . . . . . . 121 Michael G. Stabin 8 Radiation Dosimetry: Formulations, Models, and Measurements . . . . 129 Michael G. Stabin 9 Radiobiology: Concepts and Basic Principles . . . . . . . . . . . . . . . . . . . . 145 Michael G. StabinPart III SPECT and PET Imaging Instrumentation10 Elements of Gamma Camera and SPECT Systems . . . . . . . . . . . . . . . . 155 Magdy M. Khalil11 Positron Emission Tomography (PET): Basic Principles . . . . . . . . . . . 179 Magdy M. Khalil xi
  8. 8. xii Contents Part IV Image Analysis, Reconstruction and Quantitation in Nuclear Medicine 12 Fundamentals of Image Processing in Nuclear Medicine . . . . . . . . . . . 217 C. David Cooke, Tracy L. Faber, and James R. Galt 13 Emission Tomography and Image Reconstruction . . . . . . . . . . . . . . . . 259 Magdy M. Khalil 14 Quantitative SPECT Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Michael Ljungberg 15 Quantitative Cardiac SPECT Imaging. . . . . . . . . . . . . . . . . . . . . . . . . . 311 Magdy M. Khalil 16 Tracer Kinetic Modeling: Basics and Concepts . . . . . . . . . . . . . . . . . . . 333 Kjell Erlandsson 17 Tracer Kinetic Modeling: Methodology and Applications . . . . . . . . . . 353 M’hamed Bentourkia Part V Pre-Clinical Imaging 18 Preclinical Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379 Ali Douraghy and Arion F. Chatziioannou Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
  9. 9. Part IPhysics and Chemistry of Nuclear Medicine
  10. 10. Basic Physics and Radiation Safety in Nuclear Medicine 1 G. S. PantContents 1.1.2 Modern Atomic Theory1.1 Basic Atomic and Nuclear Physics . . . . . . . . . . . . . . . . . . . 3 1.1.1 Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Modern Atomic Theory . . . . . . . . . . . . . . . . . . . . . . . . 3 Wave-Particle Duality 1.1.3 Radioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.4 Interaction of Radiation with Matter . . . . . . . . . . 11 According to classical physics the particle cannot be1.2 Radiation Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 a wave, and the wave cannot be a particle. However, 1.2.1 Types of Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.2.2 Control of Contamination . . . . . . . . . . . . . . . . . . . . . 17 Einstein, while explaining the photoelectric effect 1.2.3 Radioiodine Therapy: Safety (PEE), postulated that electromagnetic radiation has Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 a dual wave-particle nature. He used the term photon 1.2.4 Management of Radioactive Waste . . . . . . . . . . . 20 to refer to the particle of electromagnetic radiation. HeReferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 proposed a simple equation to relate the energy of the photon E to the frequency n and wavelength l of electromagnetic wave. c E ¼ hn ¼ h (1.1)1.1 Basic Atomic and Nuclear Physics l1.1.1 Atom In this equation, h is Planck’s constant (6:634Â 34 10 J.s) and c is the velocity of light in a vacuum. De Broglie generalized the idea and postulated thatAll matter is comprised of atoms. An atom is the all subatomic particles have a wave-particle nature. Insmallest unit of a chemical element possessing the some phenomena, the particle behaves as a particle, andproperties of that element. Atoms rarely exist alone; in some phenomena it behaves as a wave; it neveroften, they combine with other atoms to form a mole- behaves as both at the same time. This is called thecule, the smallest component of a chemical com- wave-particle duality of nature. He suggested the fol-pound. lowing equation to relate the momentum of the particle p and wavelength l: h l¼ (1.2) p Only when the particles have extremely small mass (subatomic particles) is the associated wave appre-G.S. Pant ciable. An electron microscope demonstrates theConsultant Medical Physicist, Nuclear Medicine Section,KFSH, Dammam, KSA wave-particle duality. In the macroscopic scale, thee mail: De Broglie theory is not applicable.M.M. Khalil (ed.), Basic Sciences of Nuclear Medicine, DOI: 10.1007/978 3 540 85962 8 1, 3# Springer Verlag Berlin Heidelberg 2011
  11. 11. 4 G.S. Pant1.1.2.2 Electron Configuration Pauli in 1925 added a complementary rule for arrangement of electrons around the nucleus. The pos-Electrons around a nucleus can be described with tulation is now called Pauli’s exclusion principle andwave functions [1]. Wave functions determine the states that no two electrons can have all quantumlocation, energy, and momentum of the particle. numbers the same or exist in identical quantum states.The square of a wave function gives the probability The filling of electrons in orbitals obeys thedistribution of the particle. At a given time, an elec- so-called Aufbau principle. The Aufbau principletron can be anywhere around the nucleus and have assumes that electrons are added to an atom startingdifferent probabilities at different locations. The with the lowest-energy orbital until all of the electronsspace around the nucleus in which the probability is are placed in an appropriate orbital. The sequence ofhighest is called an orbital. In quantum mechanics, energy states and electron filling in orbitals of a multi-the orbital is a mathematical concept that suggests electron atom can be represented as follows:the average location of an electron around thenucleus. If the energy of the electron changes, this 1s À 2s À 2p À 3s À 3p À 4s À 3d À 4p À 5s À 4daverage also changes. For the single electron of a À 5p À 6s À 4f À 5d À 6p À 7s À 5f À 6d À 7phydrogen atom, an infinite number of wave func-tions, and therefore an infinite number of orbitals,may exist. An orbital can be completely described using the Electron Binding Energiescorresponding wave function, but the process istedious and difficult. In simple terms, an orbital can The bound electrons need some external energy tobe described by four quantum numbers. make them free from the nucleus. It can be assumed The principal quantum number n characterizes the that electrons around a nucleus have negative potential energy and shell size in an atom. It is an integer and energy. The absolute value of the potential energy is can have a value from 1 to 1, but practically n is called the binding energy, the minimum energy always less than 8. The maximum number of elec- required to knock out an electron from the atom. trons in orbital n is 2n2. The shells of electrons are labeled alphabetically as Kðn ¼ 1Þ; Lðn ¼ 2Þ; Mðn ¼ 3Þ; and so on based on the principal quan- Atomic Emissions tum number. The orbital quantum number l relates to the angu- For stability, electrons are required to be in the mini- lar momentum of the electron; l can take integer mum possible energy level or in the innermost orbi- values from 0 to n À 1. In a stable atom, its value tals. However, there is no restriction for an electron to does not go beyond 3. The orbital quantum num- transfer into outer orbitals if it gains sufficient energy. ber characterizes the configuration of the electron If an electron absorbs external energy that is more orbital. In the hydrogen atom, the value of l does than or equal to its binding energy, a pair of ions, not appreciably affect the total energy, but in the electron and the atom with a positive charge, is atoms with more than one electron, the energy created. This process is termed ionization. If the exter- depends on both n and l. The subshells or orbitals nal energy is more than the binding energy of the of electrons are labeled as sðl ¼0Þ, pðl = 1Þ, electron, the excess energy is divided between the dðl = 2Þand fðl = 3Þ. two in such a way that conservation of momentum is The azimuthal or magnetic quantum number ml preserved. relates to the direction of the angular momentum If an electron absorbs energy and is elevated to the of the electron and takes on integer values from À l outer orbitals, the original orbital does not remain to þ l. vacant. Soon, the vacancy will be filled by electrons The spin quantum number ms relates to the electron from the outer layers. This is a random process, and angular momentum and can have only two values: the occupier may be any electron from the outer orbi- À½ or +½. tals. However, the closer electron has a greater chance
  12. 12. 1 Basic Physics and Radiation Safety in Nuclear Medicine 5to occupy the vacancy. In each individual process of remain within the nucleus due to a strong attractivefilling, a quantum of energy equal to the difference force between nucleons that dominates the repulsivebetween the binding energies E2 À E1 of the two force and makes the atom stable. The force is effectiveinvolved orbitals is released, usually in the form of a in a short range, and neutrons have an essential rolesingle photon. The frequency n and wavelength l of in creating such a force. Without neutrons, protonsthe emitted photon (radiation) are given as follows: cannot stay close to each other. In 1935, Yukawa proposed that the short-range c E2 À E1 ¼ DE ¼ hn ¼ h (1.3) strong force is due to exchange of particles that he l called mesons. The strong nuclear force is one of the When an atom has excess energy, it is in an unsta- four fundamental forces in nature created betweenble or excited state. The excess energy is usually nucleons by the exchange of mesons. This exchangereleased in the form of electromagnetic radiation can be compared to two people constantly hitting a(characteristic radiation), and the atom acquires its tennis ball back and forth. As long as this mesonnatural stable state. The frequency spectrum of the exchange is happening, the strong force holds theradiation emitted from an excited atom can be used nucleons together. Neutrons also participate in theas the fingerprint of the atom. meson exchange and are even a bigger source of the strong force. Neutrons have no charge, so they approach other nuclei without adding an extra repul- Nuclear Structure sive force; meanwhile, they increase the average dis- tance between protons and help to reduce the repulsionThere are several notations to summarize the nuclear between them within a nucleus.composition of an atom. The most common is A XN , Zwhere X represents the chemical symbol of the ele-ment. The chemical symbol and atomic number carrythe same information, and the neutron number can be Nuclear Binding Energy and Mass Defectcalculated by the difference of A and Z. Hence, for thesake of simplicity the brief notation is A X, which is It has been proved that the mass of a nucleus ismore comprehensible. For example, for 137Cs, where always less than the sum of the individual masses of137 is the mass number (A þ Z), the Cs represents the the constituent protons and neutrons (mass defect).55th element (Z = 55) in the periodic table. The neu- The strong nuclear force is the result of the masstron number can easily be calculated (A À Z = 82). defect phenomenon. Using Einstein’s mass energyTable 1.1 shows the mass, charge, and energy of the relationship, the nuclear binding energy can be givenproton, neutron, and electron. as follows: Eb ¼ Dm:c21.1.2.6 Nuclear Forces where Dm is the mass defect, and c is the speed of light in a vacuum.Protons in a nucleus are close to each other The average binding energy per nucleon is a mea-ð% 10 15 mÞ. This closeness results in an enormously sure of nuclear stability. The higher the average bind-strong repulsive force between protons. They still ing energy is, the more stable the nucleus is.Table 1.1 Mass and charge of a proton, neutron, and electron Particle Symbol Chargea Massb Mass (kg) Energy (MeV) Relative mass Proton p þ1 1.007276 1.6726 Â 10À27 938.272 1,836 À27 Neutron n 0 1.008665 1.6749 Â 10 939.573 1,839 À À31 Electron e 1 0.000548 9.1093 Â 10 0.511 1a Unit charge 1.6 Â 10À19 coulombsb Mass expressed in universal mass unit (mass of 1/12 of 12C atom)Data from Particles and Nuclei (1999)
  13. 13. 6 G.S. Pant1.1.3 Radioactivity the right of the line (area II) are neutron deficient, and those above the line (area III) are too heavy (excess of both neutrons and protons) to be stable.For all practical purposes, the nucleus can be An unstable nucleus sooner or later (nanosecondsregarded as a combination of two fundamental parti- to thousands of years) changes to a more stablecles: neutrons and protons. These particles are proton-neutron combination by emitting particlestogether termed nucleons. The stability of a nucleus such as alpha, beta, and gamma. The phenomenon ofdepends on at least two different forces: the repulsive spontaneous emission of such particles from thecoulomb force between any two or more protons and nucleus is called radioactivity, and the nuclides arethe strong attractive force between any two nucleons called radionuclides. The change from the unstable(nuclear forces). The nuclear forces are strong but nuclide (parent) to the more stable nuclide (daughter)effective over short distances, whereas the weaker is called radioactive decay or disintegration. Duringcoulomb forces are effective over longer distances. disintegration, there is emission of nuclear particlesThe stability of a nucleus depends on the arrange- and release of energy. The process is spontaneous, andment of its nucleons, particularly the ratio of the it is not possible to predict which radioactive atom willnumber of neutrons to the number of protons. An disintegrate first.adequate number of neutrons is essential for stability.Among the many possible combinations of protonsand neutrons, only around 260 nuclides are stable; Modes of Decaythe rest are unstable. It seems that there are favored neutron-to-proton The radionuclide, which decays to attain stability, isratios among the stable nuclides. Figure 1.1 shows the called the parent nuclide, and the stable form sofunction of number of neutron (N) against the number obtained is called the daughter. There are situationsof protons (Z) for all available nuclides. The stable when the daughter is also unstable. The unstablenuclides gather around an imaginary line, which is nuclide may undergo transformation by any of thecalled the line of stability. For light elements following modes.(A 50), this line corresponds to N ¼ Z, but withincreasing atomic number the neutron-to-proton ratioincreases up to 1.5 (N ¼ 1.5Z). The line of stability Nuclides with Excess Neutronsends at A ¼ 209 (Bi), and all nuclides above that andthose that are not close to this line are unstable. Beta EmissionNuclides that lie on the left of the line of stability Nuclides with an excess number of neutrons acquire a(area I) have an excess of neutrons, those lying on stable form by converting a neutron to a proton. In this process, an electron (negatron or beta minus) and an antineutrino are emitted. The nuclear equation is given 100 III as follows: n ! p þ e þ v þ EnergyNeutron number (N) 80 I 60 where n, p, e, and v represent the neutron, the proton, the negatron (beta minus), and the antineutrino, 40 II respectively. The proton stays in the nucleus, but the electron and the antineutrino are emitted and carry the 20 released energy as their kinetic energy. In this mode of decay, the atomic number of the daughter nuclide is one more than that of the parent with no change in 0 20 40 60 80 100 mass number. The mass of the neutron is more than the Atomic number (Z) sum of masses of the proton, electron, and the antineu-Fig. 1.1 The line of stability and different regions around it. trino (the daughter is lighter than the parent). This(Reproduced from [3]) defect in mass is converted into energy and randomly
  14. 14. 1 Basic Physics and Radiation Safety in Nuclear Medicine 7shared between the beta particle and the antineutrino. is called the annihilation reaction, and the photons soHence, the beta particle may have energy between created are called annihilation to a certain maximum level (continuous spec- 2. Electron capturestrum). The antineutrino has no mass and charge and A nucleus with excess protons has an alternativehas no clinical application. way to acquire a stable configuration by attracting Radionuclides in which the daughter acquires a one of its own electrons (usually the k electron) tostable state by emitting beta particles only are called the nucleus. The electron combines with the proton,pure beta emitters, such as 3H, 14C, 32P, and 35S. Those producing a neutron and a neutrino in the process.that cannot attain a stable state after beta emission andare still in the excited states of the daughter emit p þ e ! nþvgamma photons, either in a single transition or throughcascades emitting more than one photon before attain- The electron capture creates a vacancy in the innering a stable state. 131I, 132Xe, and 60Co emit beta electron shell, which is filled by an electron fromparticles followed by gamma emissions. the outer orbit, and characteristic radiation is emit- ted in the process. These photons may knock out orbital electrons. These electrons are called Auger electrons and are extremely useful for therapeuticNuclides that lack Neutrons applications (targeted therapy) due to their short range in the medium.There are two alternatives for the nucleus to come to a Electron capture is likely to occur in heavy ele-stable state: ments (those with electrons closer to the nucleus),1. Positron emission and subsequent emission of anni- whereas positron emission is likely in lighter ele- hilation photons ments. Radionuclides such as 67Ga, 111In, 123I, and 125 In this mode of decay, a proton transforms to a I decay partially or fully by electron capture. neutron, a positron, and a neutrino. p ! n þ eþv Nuclides with Excess Protons and Neutrons The neutron stays in the nucleus, but a positron and There are two ways for nuclides with excess protons a neutrino are ejected, carrying the emitted energy and neutrons (region III) to become more stable: as their kinetic energy. In this mode of decay, the atomic number of the daughter becomes one less 1. Alpha decay than that of the parent with no change in mass There are some heavy nuclides that get rid of the number. The mass of the proton is less than the extra mass by emitting an alpha particle (two neu- masses of the neutron, the positron, and the neu- trons and two protons). The atomic number of the trino. The energy for creation of this mass daughter in such decay is reduced by two and mass (E 1.022 MeV) is supplied by the whole nucleus. number is reduced by four. The alpha particle The excess energy is randomly shared by the posi- emission may follow with gamma emission to tron and the neutrino. The energy spectrum of the enable the daughter nucleus to come to its ground positron is just like that of the beta particle (from or stable state. Naturally occurring radionuclides zero to a certain maximum). The neutrino has no such as 238U and 232Th are alpha emitters. mass and charge and is of no clinical relevance. 2. Fission Some of the positron-emitting radionuclides are It is the spontaneous fragmentation of very heavy 11 C, 13N, 15O, and 18F. nuclei into two lighter nuclei, usually with the Just a few nanoseconds after its production, a positron emission of two or three neutrons. A large amount combines with an electron. Their masses are converted of energy (hundreds of million electron volts) is into energy in the form of two equal-energy photons also released in this process. Fission nuclides them- (0.511 MeV each), which leave the site of their crea- selves have no clinical application, but some of tion in exactly opposite directions. This phenomenon their fragments are useful. The fissile nuclides can
  15. 15. 8 G.S. Pant be used for the production of carrier free radio- in knocking out an orbital electron from its own atom. isotopes with high specific activity. This process is called internal conversion, and the emitted electron is called a conversion electron. The probability of K conversion electron is more than L orGamma Radiation and Internal Conversion M conversion electrons, and the phenomenon is more common in heavy atoms. The internal conversion isWhen all the energy associated with the decay process is followed by emission of characteristic x-rays or Augernot carried away by the emitted particles, the daughter electrons as the outer shell electrons move to fill thenuclei do not acquire their ground state. Such nuclei can inner shell in either an excited state or a metastable (isomeric) It should be noted that there is no differencestate. In both situations, the excess energy is often between an x-ray and a gamma ray of equal energyreleased in the form of one or more gamma photons. except that the gamma ray originates from the nucleusThe average lifetime of excited states is short, and energy and has a discrete spectrum of energy, whereas x-rayis released within a fraction of a nanosecond. The aver- production is an atomic phenomenon and usually has aage lifetime of metastable states is much longer, and continuous spectrum.emission may vary from a few milliseconds to few daysor even longer. During this period, the nucleus behavesas a pure gamma-emitting radionuclide. Some of the Laws of Radioactivitymetastable states have great clinical application. Thetransition of a nucleus from a metastable state to a There is no information available by which one canstable state is called an isomeric transition. The decay predict the time of disintegration of an atom; theof 99mTc is the best example of isomeric transition. The interest really should not be in an individual atomdecay scheme of 99Mo-99mTc is shown in Fig. 1.2. because even an extremely small mass of any element There are situations when the excited nuclei, consists of millions of identical atoms. Radioactiveinstead of emitting a gamma photon, utilize the energy decay has been found to be a spontaneous process 67h 99 Mo μ 42 1.11 0.3% 0.922 17% 1.0% 0.513 82% 0.181 6h 0.142 0.140 2.12 x 106yFig. 1.2 Decay scheme of 99 T99 Mo. (Reproduced from [3]) 43 C
  16. 16. 1 Basic Physics and Radiation Safety in Nuclear Medicine 9independent of any environmental factor. In other half-life and decay constant are related by the follow-words, nothing can influence the process of radioac- ing equation:tive disintegration. Radioactive decay is a randomprocess and can be described in terms of probabilities 0:693 0:693 T1=2 ¼ or l ¼ (1.6)and average constants. l T1=2 In a sample containing many identical radioactiveatoms, during a short period of time ð@tÞ the number ofdecayed atoms ð@N Þ is proportional to the total num-ber of atoms ðNÞ present at that time. Mathematically, Average Lifeit can be expressed as follows: À @N / N@t The actual lifetimes of individual atoms in a sample are different; some are short, and some are long. The À @N ¼ lN@t (1.4) average lifetime characteristic of atoms is related toor the half-life by @N ¼ ÀlN Tav ¼ 1:44  T1=2 (1.7) @t In this equation, the constant l (known as the decay The average life is a useful parameter for calculat-constant) has a characteristic value for each radionu- ing the cumulated activity in the source organ in inter-clide. The decay constant is the fraction of atoms nal dosimetry.undergoing decay per unit time in a large number ofatoms. Its unit is the inverse of time. For simplicity, the decay constant can be defined asthe probability of disintegration of a nucleus per unit Radioactive Equilibriumtime. Thus, l ¼ 0.01 per second means that the proba-bility of disintegration of each atom is 1% per second. In many cases, the daughter element is also radioactiveIt is important to note that this probability does not and immediately starts disintegrating after its forma-change with time. tion. Although the daughter obeys the general rule of The exact number of parent atoms in a sample at radioactive decay, its activity does not follow theany time can be calculated by integrating Eq. 1.4, exponential law of decay while mixed with the parent.which takes the following form: This is because the daughter is produced (mono- exponentially) by disintegration of its parent while it N ¼ No à expðÀltÞ (1.5) disintegrates (monoexponentially) as a radioactive element. So, the activity of such elements changeswhere No is the initial number of atoms in the sample, biexponentially: First the activity increases, thenand N is the number present at time t. reaches a maximum, and then starts decreasing. The The term @N shows the number of disintegrations rate at which the activity changes in such a mixture of @tper unit time and is known as activity. The SI unit of radionuclides depends on the decay constant of bothactivity is the becquerel (Bq; 1 decay per second). The the parent and the daughter.conventional unit of activity is the curie (Ci), which is If we start with a pure sample of a parent with aequal to 3.7  1010 disintegrations per second (dps). half-life of T1 and a decay constant l1 and it containsThis number 3.7  1010 corresponds to the disintegra- ðN1 Þ0 atoms initially, the decay of this parent can betions from 1 g 226Ra. expressed by N1 ¼ ðN1 Þ0 e l1 t (1.8)Half-Life The rate of decay of the parent is the rate of forma-The time after which 50% of the atoms in a sam- tion of the daughter. Let the daughter decay at the rateple undergo disintegration is called the half-life. The l2 N2 , where l2 is the decay constant of the daughter
  17. 17. 10 G.S. Pantand N2 is the number of atoms of the daughter. The net Decay of 113Snrate of formation of the daughter can be given by 100 @N2 ¼ l1 N 1 À l2 N 2 (1.9) 50 @t Radioactivity The solution of this equation in terms of activity Growth of 113mIncan be given as follows: T1 T 1 0:693ð T T2 Þt 10 A2 ¼ A1 ð1 À e 1 T2 Þ (1.10) T1 À T2 5where A1 and A2 are the activity of the parent and 0 2 4 6 9 10 14daughter, respectively; T1 and T2 are their respective No. of daughter half-livesphysical half-lives; and t is the elapsed time. Thisequation is for a simple parent-daughter mixture. In Fig. 1.3 Secular equilibriumgeneral, three different situations arise from Eq. 1.10.(a) Secular equilibrium 100 Decay of 99Mo When the half-life of the parent (T1 ) is too long in comparison to that of the daughter (T2 ), Eq. 1.10 may be expressed as 50 Radioactivity Growth of 99mTc 0:693t A2 ¼ A1 ð1 À e T2 Þ (1.11) After one half-life of the daughter (t ¼ T2), A2 will 10 become nearly A1 =2; after two half-lives the daughter may grow up to three fourths of the parent, and after four half-lives (of the daughter) 20 0 10 30 40 50 60 this increases to about 94% of the parent activity. Time (h) Thus, activity of the daughter gradually increases, and after a few half-lives the activity of the parent Fig. 1.4 Transient equilibrium and daughter become almost equal (Fig. 1.3); they are said to be in secular equilibrium. The growth of the daughter for multiples of(b) Transient equilibrium T2 ðT2 ; 2T2 ; 3T2 ; 4T2 ; etc:Þ will be nearly 50%, The half-life of the parent is a few times ($ 10 times 75%, 87.5%, and 94%, respectively of the activity or more) longer than that of the daughter, but the of the parent. It is therefore advisable to elute the difference is not as great as in secular equilibrium. activity from the technetium generator after every In this case, the activity of the daughter increases 24 h (Mo-99 with 67-h half-life and Tc-99m with and eventually slightly exceeds the activity of 6-h half-life). the parent to reach a maximum and then decays with the half-life of the parent, as can be seen in Fig. 1.4. For a large value of t, Eq. 1.10 can be (c) No Equilibrium written as If the half-life of the daughter is longer than the half- T1 life of the parent, then there would be no equilibrium A2 ¼ A1 for t ) T2 (1.12) T1 À T2 between them.
  18. 18. 1 Basic Physics and Radiation Safety in Nuclear Medicine 111.1.4 Interaction of Radiation electron energy by this mode is termed radiative with Matter loss. The energy lost per unit path length along the track is known as the linear energy transfer (LET) and is generally expressed in kilo-electron-volts perIonizing radiation transfers its energy in full or part to micrometer.the medium through which it passes by way of inter-actions. The significant types of interactions are exci-tation and ionization of atoms or molecules of thematter by charged particles and electromagnetic radi- Range of a Charged Particleation (x-rays or gamma rays). After traveling through a distance in the medium, the charged particle loses all its kinetic energy and comes to rest as it has ample chance to interact with electrons1.1.4.1 Interaction of Charged Particles or the positively charged nucleus of the atoms of the with Matter medium. The average distance traveled in a given direction by a charged particle is known as its rangeThe charged particle loses some of its energy by the in that medium and is influenced by the followingfollowing interactions: factors:1. Ejection of electrons from the target atoms (ioniza- 1. Energy. The higher the energy of the particle is, the tion) larger is the range.2. Excitation of electrons from a lower to a higher 2. Mass. The higher the mass of the charged particle energy state is, the smaller is the range.3. Molecular vibrations along the path (elastic colli- 3. Charge. The range is inversely proportional to the sion) and conversion of energy into heat square of the charge.4. Emission of electromagnetic radiation 4. Density of the medium. The denser the medium is, In the energy range of 10 KeV to 10 MeV, ioniza- the shorter is the range of the charged particle.tion predominates over excitation. The probability ofabsorption of charged particles is so high that even athin material can stop them completely. Interaction of Electromagnetic Radiation The nature of the interaction of all charged particles with Matterin the energy range mentioned is similar. Light parti-cles such as electrons deflect at larger angles than When a beam of x-rays or gamma rays passes throughheavier particles, and there is a wide variation in an absorbing medium, some of the photons aretheir tortuous path. The path of a heavier particle is completely absorbed, some are scattered, and the restmore or less a straight line. When electrons are pass through the medium almost unchanged in energydeflected at large angles, they transfer more energy and direction (transmission). The transferred energyto the target atom and eject electrons from it. These results in excitation and ionization of atoms or mole-electrons, while passing through the medium, produce cules of the medium and produces heat. The attenua-secondary electrons along their track (delta rays). The tion of the beam through a given medium ischarged particles undergo a large number of interac- summarized as follows:tions before they come to rest. In each interaction, theylose a small amount of energy, and the losses are The thicker the absorbing material is, the greater iscalled collision losses. the attenuation. Energetic electrons can approach the nucleus, The greater the atomic number of the material is,where they are decelerated and produce bremsstrah- the greater is the attenuation.lung radiation (x-rays). The chance of such an interac- As the photon energy increases, the attenuationtion increases with an increase in electron energy and produced by a given thickness of materialthe atomic number of the target material. Loss of decreases.
  19. 19. 12 G.S. Pant1.1.4.4 Linear Attenuation Coefficient The negative sign indicates that as dx increases, the number of photons in the beam decreases. Equa-The linear attenuation coefficient m is defined as the tion 1.13 can be rearranged as follows:fractional reduction in the beam per unit thickness asdetermined by a thin layer of the absorbing material. dN m¼ (1.14) N:dx Fractional reduction in a thin layer m¼ The formal definition of attenuation coefficient is Thickness of the layers (cm) derived from the integration of Eq. 1.14, which gives The unit of the m is cm 1. the following relationship: N ¼ No: e mx (1.15) Exponential Attenuation Equation 1.15 can also be expressed in terms of beam intensity:The exponential law can explain the attenuation ofradiation beam intensity. The mathematical derivation I ¼ Io: e mx (1.16)is given next. Let No be the initial number of photons in the beam where I and Io are the intensities of the beam asand N be the number recorded by the detector placed recorded by the detector with and without absorbingbehind the absorber (Fig. 1.5). material, respectively. The attenuation coefficient may The number dN, which gets attenuated, will be vary for a given material due to nonuniform thickness.proportional to the thickness dx of the absorber This is particularly so if the absorbing material isand to the number of photons N present in the malleable. It is therefore better to express the massbeam. The number dN will depend on the number absorption coefficient, which is independent of thick-of atoms present in the beam and the thickness of the ness of the absorbing material. The mass absorptionabsorber. coefficient is obtained by dividing the linear attenua- Mathematically, tion coefficient by the density of the material. The unit of the mass attenuation coefficient is square centi- dN / N: dx meters per gram. The electronic and atomic attenua- (1.13) or dN ¼ Àm:N:dx tion coefficients are also defined accordingly. The electronic attenuation coefficient is the fractionalwhere m is a constant called the linear attenuation reduction in x-ray or gamma ray intensity producedcoefficient for the radiation used. by a layer of thickness 1 electron/cm2, whereas the X X-rays Attenuated No N P PrimaryFig. 1.5 Attenuation of aradiation beam by an absorber.The transmitted beam ismeasured by detector P.(Reproduced from [4])
  20. 20. 1 Basic Physics and Radiation Safety in Nuclear Medicine 13atomic attenuation coefficient is the fractional reduc- in medical applications of radiation. However, it hastion by a layer of thickness 1 atom/cm2. Thus, the application in x-ray crystallography.atomic attenuation coefficient will be Z times the elec-tronic one. Inelastic (Compton) Scattering1.1.4.6 Half-Value Layer Compton elucidated the mechanism of inelastic (Comp- ton) scattering. In this process, the photon interacts withFrom Eq. 1.16, it can be seen that, for a certain thick- loosely bound (free) electrons. Part of the energy of theness (x ¼ d1/2) of the absorbing material, the intensity photon is used in ejecting the electron, and the rest isbecomes half of its original value, that is, I ¼ Io/2. scattered in different directions (Fig. 1.6).Substituting these values, Eq. 1.16 can be rearranged In a so-called head-on collision, the photon turnsas follows: back along its original track (scattered through 180 ), and maximum energy is transferred to the recoil elec- d1=2 ðHVLÞ ¼ 0:693=m (1.17) tron. The change in wavelength dl of the photon is given by The half-value layer or thickness (HVL or HVT)can be defined as the thickness of an absorbing mate- dl ¼ 0:024ð1 À cos ’ÞÅ (1.18)rial that reduces the beam intensity to half of its origi-nal value. Depending on the energy of radiation, where ’ is the angle of scattering of the gammavarious materials are used for the measurement of ˚ photon, and A is the angstrom unit for wavelength.HVL, such as aluminum, copper, lead, brick, and The energy of the scattered photon is expressed asconcrete. The HVL for a broad beam is more than follows:that for a narrow beam. E1 ¼ E0 =½1 þ E0 =me c2 f1 À cos ’gŠ (1.19) Mechanism of Attenuation where E0 is the energy of the incident photon and E1 isThere are many modes of interaction between a photon that of the scattered photon, me is the mass of theand matter, but only the types discussed next are of electron, and c is the velocity of light in a vacuum.importance to us. Compton scattering involves interaction between photons and electrons. The probability therefore depends on the number of electrons present and inde-Photon Scattering pendent of the atomic number. With the exception of hydrogen, all elements contain nearly the samePhoton scattering may or may not result in transfer of number of electrons per gram (practically the sameenergy during the interaction of the photon with an electron density). Compton scattering, therefore, isatom of the medium. γ-ray Electron (e−)Elastic ScatteringIn elastic scattering or unmodified scattering, thephotons are scattered in different directions without L M γ-ray Kany loss of energy. The process thus attenuates thebeam without absorption. In this process, the photoninteracts with a tightly bound electron in an atom. Theelectron later releases the photon in any direction Fig. 1.6 Process of Compton scattering. The incoming photonwithout absorbing energy from it. The contribution ejects the electron from outer orbit and is scattered with reducedof this mode of interaction is relatively insignificant energy in a different direction. (Reproduced from [4])
  21. 21. 14 G.S. Pantindependent of atomic number. This is the choice of x-ray photons. Such photons are characteristic of theinteraction required in radiation oncology, for which atom from which they are emitted. The K, L, M, and sothe delivered dose is homogeneous in spite of tissue on shells of a given atom have fixed energy, so theinhomogeneity within the body. difference in their energies is also fixed. The radiation The total probability s for the Compton process is emitted therefore is termed the characteristic x-rays.given by Three types of possibilities exist during PEE: 1. Radiative transitions s ¼ ss þ sa As explained, during the electron transition from the outer orbit to the inner orbit, a photon is emittedwhere ss and sa are the probabilities for scattering and with energy equal to the difference of the bindingabsorption, respectively. energies of the orbits involved. The vacancy moves to a higher shell; consequently, a characteristic photon of lower energy follows. The probability1.1.4.8 Photoelectric Effect of emission of a photon is expressed as the fluores- cent yield:In the PEE process, the photon disappears when itinteracts with the bound electron. The photon energy Fluorescent yieldhas to be higher than the binding energy of the electron Number of x À ray photons emittedfor this type of interaction to take place. ¼ Number of orbital vacancies created hv ¼ BE þ kinetic energy Mostly, it is the K shell that is responsible for fluorescent yield.where hv is the energy of the photon and BE is thebinding energy of the electron in the shell (Fig. 1.7). If K shell fluorescent yield ( ok)the photon energy is slightly higher than the binding Number of K x À ray photons emittedenergy (BE), then the chance of PEE is high. For ¼example, a photon of energy 100 keV has a high Number of K shell vacanciesprobability of undergoing PEE when it interacts with The yield increases with an increase in atomica Pb atom, for which the K shell binding energy is number.88 keV. The rest of the (100 to 88) 12-keV energy will 2. Auger electronsbe carried away by the ejected electron as its kinetic The characteristic x-ray photon, instead of beingenergy. The ejection of the electron creates a hole in emitted, can eject another orbital electron from thethe inner shell, which is filled by an electron from any atom. These electrons are called Auger electronsof the outer shells. Since the electrons in the outer (Fig. 1.8). The energy of the Auger electron isshells possess higher energy than those in the inner equal to the difference of the x-ray photon energyshells, the difference in their energy is released as and the binding energy of the shell involved in the process. The process competes with radiative tran- sition. The Auger yield is expressed as the ratio of ) n (e electrons emitted due to vacancies in subshell i and γ-ray Ele ctro the total number of atoms with a vacancy in sub- shell i. 3. Coster Kronig electrons The process for Coster Kronig electrons is exactly like the Auger transition except that the electronFig. 1.7 Process of photoelectric absorption. The incoming filling the vacancy comes from the subshell of thephoton disappears (is absorbed), and the orbital electron is same principal shell in which the vacancy lies. Theknocked out. An electron from the outer shell falls (dottedline) into the inner shell to fill the vacancy. (Reproduced kinetic energy of the emitted electrons can be cal-from [4]) culated exactly as for Auger electrons. The energy
  22. 22. 1 Basic Physics and Radiation Safety in Nuclear Medicine 15Fig. 1.8 Mechanism of Characreristic Auger electronAuger electron emission. x-ray e−(Reproduced from [4]) hv L L L K K K of Coster Kronig electrons is so small that they are - (e ) tron quickly absorbed in the medium. Elec g-ray 511 keV Pos it1.1.4.9 Pair Production ion (e +) e- e+ 511 keVWhen a photon with energy in excess of 1.022 MeVpasses close to the nucleus of an atom, it may disap-pear, and in its place two antiparticles (negatron and Fig. 1.9 Schematic representation of pair production. (Repro duced from [4])positron) may be produced as shown in Fig. 1.9. In thisprocess, energy converts into mass in accordance withEinstein’s mass energy equivalence (E ¼ mc2). After While such enthusiasm is appreciable, adequate safe-traversing some distance through the medium, the guards against radiation exposure are also necessary topositron loses its energy, combines with an electron, minimize the radiation risk to occupational workers,and annihilates. During combination, both the antipar- patients, and the public.ticles disappear (annihilation) and two 0.511-MeVphotons are emitted in the opposite direction. 1.2.1 Types of Exposure1.1.4.10 Photonuclear Reaction The following three categories of people are likely to be involved in radiation exposure in medical applica-When photon energy is too high, either a neutron or a tions of ionizing radiation:proton may be knocked out (more likely the neutron)from the nucleus. For the majority of atoms, the 1. Occupational staffthreshold energy for this effect is more than 10 MeV, 2. Patientsand the probability increases with increasing energy 3. Publicuntil a maximum is reached; above this maximum, the Protection is aimed at achieving a dose as low asprobability falls rapidly. reasonably achievable (ALARA) to these categories of people. Spending a minimum of time near the radiation sources, keeping a distance from them, and using1.2 Radiation Safety shielding devices are the cardinal parameters for radia- tion safety. The fourth parameter with unsealed sources in nuclear medicine is to avoid or minimize the chanceThe applications of radiopharmaceuticals for medical of contamination.diagnosis and therapy have rapidly increased due to For safe use of radionuclides in nuclear medicine,the favorable physical characteristics of artificially the following basic requirements should be met:produced radionuclides, progress in instrumentation,and computer technology. Efforts are under way to 1. The nuclear medicine facility should be welldevelop newer radiopharmaceuticals in nuclear medi- planned with a sufficient number of rooms forcine for both diagnostic and therapeutic procedures. intended operations (including the storage and
  23. 23. 16 G.S. Pant disposal of radioactive waste) as approved by the shall be handled carefully to minimize exposure to competent authority. staff and the public.2. All the equipment required for safe handling should be available in each room for proposed operations.3. The staff should be adequate and well trained in Protection of Patients handling radioactive material.4. Radiation monitoring instruments (survey meters, Every practice involving ionizing radiation should be contamination monitors, pocket dosimeters, digital justified in terms of net positive benefit. It is particu- monitors etc.) and decontamination facility should larly important for children, for whom long-term risks be readily available. of exposure to ionizing radiation are larger. Once clinically justified, each examination involving ioniz- ing radiation should be conducted such that the radia- Protection of Staff tion dose to the patient is the lowest necessary to achieve the clinical aim (optimization). ReferenceNuclear medicine procedures demand preparation of and achievable doses for various radionuclide investi-radiopharmaceuticals, their internal movement within gations have been proposed for this purpose by variousthe facility, and finally administration to the patient. organizations. The concept of reference doses isAt each step, there is a possibility of radiation expo- recognized as a useful and practical tool for promotingsure if safety guidelines are ignored. The diagnostic the optimization of patient protection. While reducingprocedures normally do not cause any alarming expo- the radiation dose to the patient, image quality shouldsure to the staff and public. However, patients admi- not be compromised, which may otherwise lead tonistered radioactive substances for therapeutic repeat investigation. Routine quality control (QC)purposes become a source of radiation to the staff tests of imaging systems and radiopharmaceuticalsand their attendants and public. Therapeutic radionu- have to be done before clinical studies. Another con-clides are usually beta emitters that do not pose much sideration in reducing the patient dose is to avoidof a problem from a safety standpoint, and patients misadministration and to provide proper radiationtreated with them can even be treated as outpatients. counseling to patients and their family or othersHowever, patients treated with radioiodine (I-131) involved.need hospitalization if treated beyond a certain doseas per national regulatory requirements due topenetrating gamma radiation. These patients stay in a What Is Misadministration?specifically designed isolation room or ward until thebody burden decreases to an acceptable level for Error in any part of the procedure starting from patientrelease from the hospital. selection to the interpretation of final results may lead to a repeat study. This in turn leads to an increased radiation dose not only to the patient but also to theWork Practice staff. A major contributor to increased radiation dose to the patient is misadministration. MisadministrationGood work practice is an essential component of radi- has several components, such as administration ofation safety. This includes observation of all radiation the wrong radiopharmaceutical or the wrong dose orprotection rules as applicable to nuclear medicine, use giving the dose to a wrong patient or through a wrongof appropriate safety devices, remote handling of route, which ultimately lead to undesirable exposure totools/accessories, and maintaining good housekeeping a patient.habits in the laboratory. The following points should be checked to avoid In addition to the external irradiation, there is a misadministration:chance of radioactive contamination while handling Identity of the patient: The radiopharmaceuticalunsealed sources in nuclear medicine procedures. The should be administered to the patient for whom it isradioactive waste generated during preparation, dis- prepared. Name, age, and medical record numberpensing, and administration of radiopharmaceuticals should be checked before dose administration.
  24. 24. 1 Basic Physics and Radiation Safety in Nuclear Medicine 17 Radionuclide and its physical or chemical form: 6. The storage of radioactive waste shall be done at aThe physical and chemical form of the radiophar- location within the hospital premises with adequatemaceutical should be reconfirmed before admini- shielding to eliminate the public hazard from it.stration. Radiopharmaceuticals should go throughroutine QC procedures to check for any inadequatepreparation. Dose, quantity of radioactivity, QC: The radioac- 1.2.2 Control of Contaminationtivity should be measured in a dose calibrator beforeadministration. The accuracy and precision of the Radioactive contamination can be minimized by care-radionuclide dose calibrator need to be maintained at fully designing the laboratory, using proper handlingall times for accuracy in dose estimation. Similarly, tools, and following correct operating proceduresimaging equipment should be maintained at its opti- together with strict management and disposal of radio-mum level of performance. active waste. In the event of contamination, proce- Route: The physician should confirm the route of dures indicated should be followed to contain theadministration (oral, intravenous) of the radiopharma- contamination.ceutical. Pregnancy and breast feeding: Female patientshould notify if she is pregnant or breast feeding. Management of a Radioactive SpillThis can happen with proper patient education. Proper counseling is also helpful in reducing expo- 1. Perform a radiation and contamination survey tosure to patients and their family members. determine the degree and extent of contamination. 2. Isolate the contaminated area to avoid spread of contamination. No person should be allowed to enter the area. Protection of the Public or Environment 3. Use gloves, shoe covers, lab coat, and other appropriate clothing.To ensure that unnecessary exposure to the members 4. Rapidly define the limits of the contaminated areaof the public is avoided, the following guidelines shall and immediately confine the spill by covering thebe followed: area with absorbent materials with plastic back-1. No member of the public shall be allowed to enter ing. the controlled (hot laboratory and the injection 5. First remove the “hot spots” and then scrub the room/area, imaging rooms) and supervised (con- area with absorbent materials, working toward the soles) areas. center of the contaminated area. Special decon-2. Appropriate warning signs and symbols shall be tamination chemicals (Radiacwash) shall be used posted on doors to restrict access. in the case of a severe spill.3. Relatives or friends of the patients receiving thera- 6. All personnel should be surveyed to determine peutic doses of radioactive iodine shall not be contamination, including their shoes and clothing. allowed to visit the patient without the permission If the radioactive material appears to have of the radiation safety officer (RSO). The visitors become airborne, the nostrils and mouth of possi- shall not be young children or pregnant women. ble contaminated persons should be swabbed, and4. A nursing mother who has been administered radio- the samples shall be evaluated by the RSO. pharmaceuticals shall be given instructions to be 7. Shut off ventilation if airborne activity is likely to followed at home after her release from the hospital. be present (rare situation). The breast-feeding may have to be suspended. 8. A heavily contaminated individual may take a5. An instruction sheet shall be given at the time of shower in the designated decontamination facility release from the hospital to patients administered as directed by the RSO. Disposable footwear and therapeutic doses of radioiodine; the instructions gloves should be worn in transit. should be followed at home for a specified period 9. If significant concentrations of radioiodine have as suggested by the RSO. been involved, subsequent thyroid uptake
  25. 25. 18 G.S. Pant measurements should be made on potentially shower room to avoid the spread of contami- exposed individuals after 24 h. nation.10. Monitor the decontaminated area and all person- Place all the towels and other contaminated nel leaving the area after the cleanup. Particular clothing in a plastic bag for later monitoring attention should be paid to checking the hands and of radiation level for storage and decay. the soles of shoes. Specific hot spots on the skin can be localized11. All mops, rags, brushes, and absorbent materials with a survey meter or appropriate contamina- shall be placed in the designated waste container tion monitor. and should be surveyed by the RSO. Proper radio- Clean the specific areas with mild soap and active disposal should be observed. warm water. Avoid using detergents or vigor-12. The RSO should provide the final radiation survey ous scrubbing for they might damage the skin. report with necessary recommendations or advice The use of a soft brush is adequate. to avoid such an incident in the future. For stubborn contamination, covering a con- taminated area with plastic film or disposable cotton or latex gloves over a skin cream helps remove the contamination through sweating. Personnel DecontaminationContaminated eyes Internal Contamination If eye contamination is found, the eye should be flushed profusely with isotonic saline or Simple expedients such as oral and nasopharyngeal water by covering other parts with a towel to irrigation, gastric lavage, or an emetic and use of prevent the spread of contamination. An oph- purgatives may greatly reduce the uptake of a con- thalmologist shall be consulted if there are taminant into the circulation. signs of eye irritation. Blocking agents or isotopic dilution techniques can appreciably decrease the uptake of the radionuclidesContaminated hair into relatively stable metabolic pools such as bone. If hair is contaminated, try up to three wash- These should be administered without delay. ings with liquid soap and rinse with water. When a contaminated person requires treatment (for Prevent water from running onto the face and wounds) by a physician, the emergency room (ER) shoulders by shielding the area with towels. should be informed. The following points must be Perform a radiation survey. remembered:Contaminated skin Medical emergencies are the priority and must be Remove any contaminated clothing before attended first. Radiation injuries are rarely life determining the level of skin contamination. threatening to the victim and the attending physi- Levels below 0.1 mR/h (1 mSv/h) are consid- cian/staff. ered minimal hazards. Clean the wound with mild detergent and flush with If there is gross skin contamination, it shall be isotonic saline or water. If necessary, a topical given attention first. Wipe with a cotton swab anesthetic, such as 4% lidocaine, can be used to moistened with water and liquid soap using allow more vigorous cleansing. After a reasonable long forceps. Place all swabs in a plastic con- effort, there is no need to attempt to remove all tainer for radiation level measurement and contamination since it will probably be incorporated storage before disposal. into the scab. If a large skin area is contaminated, the person Whenever radionuclides have entered the skin via a should have a 10-min shower. Dry the body needle or sharps, induce the wound to bleed by with a towel in the shower room and monitor “milking” it as a cleansing action in addition to the radiation level over the whole body. Do not the use of running water. allow any water to drip on the floor outside the Perform radiation monitoring at the surface.
  26. 26. 1 Basic Physics and Radiation Safety in Nuclear Medicine 191.2.3 Radioiodine Therapy: Safety form, the vials containing I-131 should be opened only Considerations inside a fume hood using remote-handling bottle open- ers. If these vials are opened outside the fume hood, there is every possibility that the worker involved mayRadioiodine has been effectively used for more than inhale a fraction of the vaporous activity. All opera-five decades to ablate remnant thyroid tissue following tions using I-131 should be carried out wearing facethyroidectomy and for treating distant metastases. masks, gloves, and shoe covers and using remote-Looking at the radiation hazard to staff and the public, handling tools. Radioactive iodine uptake measure-national regulatory bodies have established guidelines ment for the thyroid of staff involved should be donefor the hospitalization and subsequent release of weekly to check for any internal contamination.patients administered radioiodine from the hospital.The limit of body burden at which these patients arereleased from the hospital varies from country tocountry. Groups who may be critically exposed Specific Instructions to the Patientamong the public are fellow travelers during the jour-ney home after release from the hospital and children It is the combined responsibility of the physician andand pregnant women among other family members at the medical physicists or technologists to administerhome. the desired dose to the patient. The patient is nor- The administered dose of radioiodine is concen- mally advised to come with an empty stomach ortrated avidly by thyroidal tissue (thyroid remnant, after a light breakfast and not to eat or drink anythingdifferentiated thyroid cancer). It rapidly gets excreted for 1 2 h after therapeutic administration. After thisvia the kidney and urinary bladder and to a lesser time, they are advised to have as much fluids asextent through perspiration, saliva, exhalation, and possible for fast excretion of radioiodine from thethe gut. The faster biological excretion of the activity kidneys. They are also advised to void the urinaryin a thyroid cancer patient actually poses less radiation bladder frequently and to flush the toilet twice afterhazard to the environment than actually expected. each voiding. This practice not only reduces the radi-Counseling of patients and family members from a ation dose to the kidneys, bladder, and entire body ofradiation safety viewpoint is necessary before thera- the patients but also helps in their fast release frompeutic administration. the hospital. In female patients of reproductive age, two impor- tant aspects need to be considered: Radiation Monitoring 1. Possibility of pregnancy: Radionuclide therapy is strictly prohibited during pregnancy.Routine monitoring of all work surfaces, overcoats, 2. Pregnancy after radionuclide therapy should beexposed body parts, and so on is essential before avoided for at least 4 months (4 6 months) or asleaving the premises. Both Gieger-Muller (G.M.)-type advised by the treating physician.survey meters and ionization chamber-type surveymeters are required for monitoring. All personsinvolved in a radioiodine procedure should be covered Discharge of the Patient from theby personnel radiation monitoring badges, and their Hospitalneck counts should also be measured periodically. It isadvisable to carry out periodic air monitoring in these The regulatory authority of each country decides theareas to ensure that no airborne activity is present. maximum limit of activity of I-131 at which the patient may be discharged from the hospital. This can be roughly estimated by measuring the exposure1.2.3.2 Use of a Fume Hood rate from the patient at a 1-m distance with a cali- brated survey meter, which should read approxi-Radioiodine in capsule form poses much less radiation mately 50 mSv/h (5 mR/h) for a body burden ofsafety problems than in liquid form. When in liquid 30 mCi or less.