Basic physics of nuclear medicine

Transcription

1 Basic physics of nuclear medicine

2 Nuclear structure Atomic number (Z): the number of protons in a nucleus; defines the position of an element in the periodic table. Mass number (A) is the number of nucleons in a nucleus

3 Binding Energy The stability of the nucleus is explained by the presence of strong binding force (nuclear force) that outcomes the repulsive forces of protons Nuclear force is equal among all nucleons and exists only in the nucleus having no influence outside the nucleus

4 Nuclear stability curve

5 Nuclear nomenclature Nuclide: an atomic species with a definite number of protons and neutrons Radionuclide: unstable nuclide that decays by emission of particles or by electromagnetic radiation

6 Nuclear nomenclature Isotope: nuclides having same atomic number but different mass number. Example: 11 6C, 12 6C, 13 6C Isotones: nuclides having same number of neutrons but different number of protons, example: Cs, Xe, I

7 Nuclear nomenclature Isobars: nuclides with the same number of nucleons; that is the same mass number but different combination of neutrons and protons. Example: 82 Y, 82 Sr, 82 Rb, 82 Kr. Isomers: nuclides with the same number of protons and neutrons but different energy states ( 99 Tc and 99m Tc); the excited state of a nuclide is called the isomeric state; when the isomeric state is long lived it is called a metastable state and denoted with m

8 Radioactivity There are about 2,450 known isotopes of the elements in the Periodic Table The unstable isotopes lie above or below the Nuclear Stability Curve These unstable isotopes attempt to reach the stability curve by splitting into fragments (fission) or by emitting particles and/or energy (radiation)

9 Radioactivity When people like Henri Becquerel and Marie Curie were working initially on these strange emanations from certain natural materials it was thought that the radiations were somehow related to another phenomenon which also was not well understood at the time - that of radio communication. It seems reasonable on this basis to appreciate that some people considered that the two phenomena were somehow related and hence that the materials which emitted radiation were termed radio-active

10 Radioactive decay Beta minus decay Certain nuclei which have an excess of neutrons may attempt to reach stability by converting a neutron into a proton with the emission of an electron. The electron is called a beta minus particle.

11 Radioactive decay Beta minus decay Certain nuclei which have an excess of neutrons may attempt to reach stability by converting a neutron into a proton with the emission of an electron. The electron is called a beta minus particle.

12 Radioactive decay Beta plus decay When the number of protons in a 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

13 Radioactive decay Beta plus decay When the number of protons in a 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

14 Radioactive decay Electron Capture An inner orbiting electron is attracted into an unstable nucleus where it combines with a proton to form a neutron, the vacant site left in the K-shell is filled by an electron from an outer shell. The filling of the vacancy is associated with the emission of X ray

15 Radioactive decay Electron Capture An inner orbiting electron is attracted into an unstable nucleus where it combines with a proton to form a neutron, the vacant site left in the K-shell is filled by an electron from an outer shell. The filling of the vacancy is associated with the emission of X ray

16 Gamma Rays The energies of γ-rays emitted from a radioactive source are always distinct. For example: 99m Tc (Technetium 99m) emits γ-rays which have an energy of 140 kev. 51 Cr (Chromium-51) emits γ-rays which have an energy of 320 kev. The effects described here are also of relevance to the interaction of X-rays with matter since as we have noted before X- rays and γ-rays are essentially the same entities. 16

17 Radioactive decay Gamma Decay Gamma decay involves the emission of energy from an unstable nucleus in the form of electromagnetic radiation

18 Radioactive decay Gamma Decay X rays and gamma rays are high energy electromagnetic rays and are therefore virtually the same.

19 Radioactive decay Gamma Decay The difference between them is not what they consist of but where they come from.

20 Radioactive decay Gamma Decay In general we can say that if the radiation emerges from a nucleus it is called a gammaray and if it emerges from outside the nucleus it is called an X-ray

21 Radioactive decay There are two common forms of gamma decay (a) Isomeric Transition (b) Internal Conversion

22 Radioactive decay There are two common forms of gamma decay (a) Isomeric Transition A nucleus in an excited state may reach its ground or unexcited state by the emission of a gamma-ray 99m Tc 99 Tc + γ (b) Internal Conversion Here the excess energy of an excited nucleus is given to an atomic electron, e.g. a K-shell electron. The ejected electron is called the conversion electron. This is followed by the emission of characteristic X ray or by emission of an orbital electron (Auger electron)

23 Interaction of Radiation with Matter

24 Interaction of Radiation with Matter Photoelectric effect Gamma-ray collides with an orbital electron of an atom of the material through which it is passing it can transfer all its energy to the electron. Gamma-ray energy is totally absorbed in the process.

25 Interaction of Radiation with Matter Photoelectric effect Occurs primarily at low energy range Its occurrence increases with increasing atomic number of the absorbing crystal

26 Photoelectric Effect When a γ-ray collides with an orbital electron of an atom of the material through which it is passing it can transfer all its energy to the electron and thus cease to exist. On the basis of the Principle of Conservation of Energy we can deduce that the electron will leave the atom with a kinetic energy given by: kinetic energy = energy of the γ-ray - orbital binding energy The resulting electron is called a photoelectron. The following phenomena are of importance: An ion results when the photoelectron leaves the atom. The γ-ray energy is totally absorbed in the process. X-ray emission can occur when the vacancy left by the photoelectron is filled by an electron from an outer shell of the atom (electron capture). 26

27 Interaction of Radiation with Matter Compton Effect Gamma-ray transfers only part of its energy to a valance electron which is essentially free

28 Compton Effect (Scattering) Here a γ-ray transfers only part of its energy to a valance electron which is almost free. The electron leaves the atom and may act like a β-particle The γ-ray deflects off in a different direction to that with which it approached the atom. This deflected or scattered γ-ray can undergo further Compton scatterings within the material. 28

29 Attenuation of Gamma-Rays The photoelectric and the Compton effects give rise to both absorption and scattering of the radiation beam. The overall effect is referred to as attenuation of γ-rays. Remember: γ-rays and X-rays are essentially the same physical entities. 29

30 Specific Gamma Ray Constant (G) It is defined as the exposure rate per unit activity at a certain distance from a source. SI units: C kg -1 s -1 Bq -1 (at 1 m) Traditional units: R h -1 mci -1 (at 1 cm) 30

31 Specific Gamma Ray Constant G 241 Am 201 Tl 57 Co 99m Tc 99 Mo 131 I 111 In 137 Cs (msv h -1 GBq -1 at 1 m) Nuclide γ-ray Constant Co

32 Specific Gamma Ray Constant and Dose Given that an object at distance (d) m away from the source, and that the source activity is (A) Bq, one can compute the dose (D) in Sv/h as follows: D G A d 2 32

33 Specific Gamma Ray Constant and Dose Given that an object at distance (d) m away from the source, and that the source activity is (A) Bq, one can compute the dose (D) in Sv/h as follows: D G A d 2 If you know that Gamma Ray Constant of 99m Tc = 0.017, and its activity = 1.7 x 10-5 curies, calculate its dose in Sv/h at 1 m from the 33 source.

34 Nuclear Medicine Scans In a nuclear medicine scan, a radiopharmaceutical is administered to the patient, and an imaging instrument that detects radiation is used to show biochemical changes in the body. Nuclear medicine imaging, in contrast to imaging techniques that mainly show anatomy (e.g., conventional ultrasound, computed tomography [CT], or magnetic resonance imaging [MRI])*, can provide important quantitative functional information about normal tissues or disease conditions in living subjects. * Exceptionally with the emergence of advanced (functional) MRI methods the pure anatomical role of these traditional imaging techniques is slowly reaching an end. 34

35 Because human senses cannot sense radiation, instruments that detect radiation are essential tools. After a nuclear disaster detecting radiation becomes particularly invaluable, as high levels of radiation can become hazardous to life. Regular monitoring while using radioactive substances is critical to the safety of personnel. 35

36 Detection of radioactivity is necessary to ascertain their presence and Intensity Detection indirect (based on the effects of radioactivity) Darkening of photographic plates Ionization of atoms 36

37 Next Lecturer Ch 7 & 8. 37