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Medical Radiation Physics

Medical Radiation Physics. Dr/ Aida Radwan. Topics. (1) Radiation Quantities and units (2 ) Teletherapy Isotope Units (3) High Energy Radiation Machines (4) Interactions of photon & Electron Beams with matter (5) Dose Calculations From Beta & Gamma

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Medical Radiation Physics

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  1. Medical Radiation Physics Dr/ Aida Radwan

  2. Topics (1) Radiation Quantities and units (2) Teletherapy Isotope Units (3) High Energy Radiation Machines (4) Interactions of photon & Electron Beams with matter (5) Dose Calculations From Beta & Gamma Radioactive Isotopes From Human Body (6) Dosimetry of Photons & Electron Beams & Instruments of Absorbed Doses (7) Effect of Radiation on Living Cells (8) Radiation Protection (9) Internationally Allowed Dosimetry

  3. (1) Radiation Quantities and units

  4. Radiation The propagation of energy from a radiative source to another medium is termed radiation. This transmission of energy can take the form of particle radiation or electromagnetic radiation (i.e., electromagnetic waves).

  5. Common features of electromagnetic radiation: It propagates in a straight line. It travels at the speed of light (nearly 300,000 km/s). It transfers energy to the medium through which it passes.

  6. The amount of energy transferred correlates positively with the frequency and negatively with the wavelength of the radiation. The energy of the radiation decreases as it passes through a material, due to absorptionand scattering, and this decrease in energy is negatively correlated with the square of the distance traveled through the material.

  7. Electromagnetic radiation can also be subdivided into ionizing and nonionizing radiations. Nonionizing radiations have wavelengths of 10−7 m. Nonionizing radiations have energies of <12 electron volts (eV); 12 eV is considered to be the lowest energy that an ionizing radiation can possess.

  8. Types of nonionizing electromagnetic radiation: Radio waves Microwaves Infrared light Visible light Ultraviolet light

  9. Non-ionizing radiation (cannot ionize matter). Ionizing radiation (can ionize matter either directly or indirectly): Directly ionizing radiation (charged particles): electrons, protons, and heavy ions. Indirectly ionizing radiation : photons (X rays and γ- rays), neutrons.

  10. Ionizing Radiation Ionizing (high-energy) radiation has the ability to remove electrons from atoms; i.e., to ionize the atoms. Ionizing radiation can be electromagnetic or particulate radiation

  11. Ionizing Electromagnetic Radiation The electromagnetic spectrum comprises all types of electromagnetic radiation, ranging from radio waves (low energy, long wavelength, low frequency) to ionizing radiations (high energy, short wavelength, high frequency).

  12. Comparing Interactions of Photons and Other Charged Particles Photons • Not charged • Zero rest mass • V = c (speed of light) • No Coulomb force • Random and rare interactions • Infinite range (in theory) Electrons • Charged • Finite mass • V < c • Coulomb force • Continuous interactions • Finite range

  13. Isotopes, Isotones, and Isobars An isotope of an element is a particular variation of the nuclear composition of the atoms of that element. The number of protons (Z: atomic number) is unchanged, but the number of neutrons (N ) varies.

  14. Isotones : are atoms of different elements that contain identical numbers of neutrons but varying numbers of protons. Isobars :are atoms of different elements with identical numbers of nucleons.

  15. RADIOACTIVITY Radioactivity may be defined as spontaneous nuclear transformations in unstable atoms that result in the formation of new elements.

  16. Nuclear Stability To overcome the effect of the increase in the electrostatic repulsion when the number of proton increase the number of neutrons must increase more rapidly to contribute sufficient energy to bind the nucleus together It has been found that the most stable isotope for each element has a specific number of neutrons in its nucleus.

  17. Plotting a graph of the number of protons against the number of neutrons for these stable isotopes generates what is called the Nuclear Stability Curve: Note that the number of protons equals the number of neutrons for small nuclei. But notice also that the number of neutrons increases more rapidly than the number of protons as the size of the nucleus gets bigger so as to maintain the stability of the nucleus. In other words more neutrons need to be there to contribute to the binding energy used to counteract the electrostatic repulsion between the protons.

  18. Methods of Radioactive Decay Spontaneous fission which occurs in some heavy nuclei which split into 2 or 3 fragment plus some neutrons. Alpha Decay In this process two proton and two neutron leave the nucleus together in an assembly known as an alpha particle. EX.

  19. Beta Decay (a) Electron Emission Nuclei with too many neutron may reach stability by converting a neutron into a proton with the emission of an electron neutron proton + beta particle

  20. Positron Emission When the number of protons in the nucleus is too large for the nucleus to be stable it may attempt to reach stability by converting a proton into a neutron with the emission of a positively-charged electron. An electron with positive charge is called positron ( beta-plus particle) Ex.

  21. Electron Capture In this third form of beta decay an inner orbiting electron is attracted into an unstable nucleus where it combines with a proton to form a neutron this process is also known as K-capture since the electron is often attracted from the k-shell of the atom Ex.

  22. Gamma radiation (δ) • Gamma is a very high energy photon emitted from an unstable nucleus that is often emitting a beta particle at the same time. • Gamma radiation causes ionization in atoms when it passes through matter, primarily due to interactions with electrons. • It can be very penetrating and only a substantial thickness of dense materials such steel or lead can provide good shielding.

  23. The Radioactive Decay Law Let us say that in the sample of radioactive material there are N nuclei which have not decayed at a certain time, t. The number of radioactive nuclei dN which will decay during the time interval from t to t+dt must beproportional to N and to dt. The minus sign indicating that N is decreasing.

  24. Turning the proportionality in this equation into an equality we can write: - dN= λ N .dt where the constant of proportionality, λ, is called the Decay Constant. Dividing across by N we can rewrite this equation as:

  25. So this equation describes the situation for any brief time interval, dt. To find out what happens for all periods of time we simply add up what happens in each brief time interval. In other words we integrate the above equation. Expressing this more formally we can say that for the period of time from t = 0 to any later time t, the number of radioactive nuclei will decrease from N0 to Nt, so that:

  26. This final expression is known as the Radioactive Decay Law. It tells us that the number of radioactive nuclei will decrease in an exponential fashion with time with the rate of decrease being controlled by the Decay Constant.

  27. The graph plots the number of radioactive nuclei at any time, Nt against time, t. We can see that the number of radioactive nuclei decreases from N0that is the number at t = 0 in a rapid fashion initially and then more slowly in the classic exponential manner.

  28. The influence of the Decay Constant can be seen in the following figure: Notice that when the Decay Constant has a low value the curve decreases relatively slowly and when the Decay Constant is large the curve decreases very quickly.

  29. The Decay Constant is characteristic of individual radionuclides. Some like uranium-238 have a small value and the material therefore decays quite slowly over a long period of time. Other nuclei such as technetium-99m have a relatively large Decay Constant and they decay far more quickly.

  30. Half-Life the length of time it takes for the radioactivity of a radioisotope to decrease bya factor of two. Note that the half-life does not express how long a material will remain radioactive but simply the length of time for its radioactivity to halve

  31. Examples of the half lives of some radioisotopes are given in the following table. Notice that some of these have a relatively short half life. These tend to be the ones used for medical diagnostic purposes because they do not remain radioactive for very long following administration to a patient and hence result in a relatively low radiation dose.

  32. Relationship between the Decay Constant and the Half Life There is a relationship between the Decay Constant and the Half Life. For example when the Decay Constant is small the Half Life should be long and correspondingly when the Decay Constant is large the Half Life should be short.

  33. When We can therefore re-write the Radioactive Decay Law by substituting for Ntand t as follows:

  34. Units of Radioactivity The SI is called the Becquerelwith the symbol Bq. The Becquerel :is defined as the quantity of radioactive substance that gives rise to a decay rate of 1 decay per second. the kilo-becquerel(kBq) and mega-becquerel (MBq) are more frequently used.

  35. The traditional unit of radioactivity the curie, • with the symbol Ci. • The curie is defined as :the amount of radioactive substance which gives rise to a decay rate of 3.7 x 1010decays per second. • For medical diagnostic work the millicurie (mCi) • and the microcurie(μCi) are therefore more • frequently used.

  36. If we have asource of radiation ,a radiation beam and some material which absorbs the radiation. So the quantities which can be measured are associated with the source, the radiation beam and the absorber.

  37. The Radiation Source • When the radiation source is a radioactive one the quantity that is typically measured is the radioactivity of the source. the units used to express radioactivity are the Becquerel(SI unit) and the curie(traditional unit).

  38. The Radiation Beam The characteristic of a radiation beam that is typically measured is called the Radiation Exposure. This quantity expresses how much ionization the beam causes in the air through which it travels.

  39. A straight-forward way of measuring such ionization is to determine the amount of electric charge which is produced. the SI unit of electric charge is the coulomb. The SI unit of radiation exposure is the coulomb/kilogram-and is given the symbolCkg-1. It is defined as the quantity of X-or gamma-rays such that the associated electrons emitted per kilogram of air at standard temperature and pressure (STP) produce ions carrying 1 coulomb of electric charge.

  40. The traditional unit of radiation exposure is the, roentgen is given the symbol R The roentgen is defined :as the quantity of X- or gamma-rays such that the associated electrons emitted per kilogram of air at STP produce ions carrying 2.58 x 10-4coulombs of electric charge. Often it is not simply the exposure that is of interest but the exposure rate, that is the exposure per unit time. The units which tend to be used in this case are the C kg-1s-1and the R hr-1.

  41. The Absorber • Energy is deposited in the absorber when radiation interacts with it. • The quantity that is measured is called the Absorbed Dose The SI unit of absorbed dose is called the grayand is given the symbolGy. • The gray is defined as the absorption of 1 joule of radiation energy per kilogram of material.

  42. So when 1 joule of radiation energy is absorbed by a kilogram of the absorber material we say that the absorbed dose is 1 Gy. The traditional unit of absorbed dose is called the rad: It is defined as the absorption of 10-2joules of radiation energy per kilogram of material. As you can figure out 1 Gy is equal to 100 rad.

  43. Average Life sum of the lifetimes of the individual atoms divided by the total number of atoms originally present.

  44. Ex. Given that the transformation rate constant(decay constant) for 226Ra is 4.38 × 10−4/yr, calculate the half-life for radium.

  45. Ex. Calculate the decay constant for cobalt-60 (T1/2 = 5.26 years) in units of month-1. 2. What will be the activity of a 5,000-Ci 60Co source after 4 years?

  46. Ex. When will 5 mCi of 131I (T1/2 = 8.05 days) and 2 mCi of 32P (T1/2 = 14.3 days) have equal activities? Suppose the activities of the two nuclides are equal after t days. For 131 I For 32P

  47. H.W

  48. (2)Tele therapy Isotope Units

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