THE PHYSICS OFNUCLEAR MEDICINE RADIOLOGY.pptx

Radiology

1. PHYSICS OF NUCLEAR MEDICINE

2. NUCLEAR MEDICINE Nuclear medicine is a medical specialty that focuses on the use of radiopharmaceuticals for diagnosis, therapy, and medical research. Nuclear medicine determines the cause of medical problem based on organ or tissue function (physiology). Nuclear medicine procedures may be: 1.diagnostic studies, which are tests of body function. 2.therapeutic procedures in which the radiation is used to treat disease.

3. RADIOPHARMACEUTICALS Combination of radioactive material (detection) and a pharmaceutical (carrier). Radiopharmaceuticals are radioisotopes bound to biological molecules able to target specific organs, tissues or cells within the human body. These radioactive drugs can be used for the diagnosis and, increasingly, for the therapy of diseases. Portrays physiology, biochemistry or pathology in the body. Also called radiotracers because they trace physiological or pathological processes.

4. Ideal Radiopharmaceutical Sterile Pyrogen-free Low cost Low dose Available Near stable Effective half-life longer than exam time . Specific and does not affect bio-distribution . Radiopharmaceuticals are designed to mimic a natural physiologic process. Important characteristics of radiopharmaceuticals are that they be nontoxic, and contain no contaminants. Contaminants of radiopharmaceuticals include chemicals and radionuclides.

5. NUCLEAR STRUCTURE ISO T O P ES - atoms with the same number of P rotons (Z). ISOB A RS - atoms with the same A tomic Mass Numbers ( A ). ISOTO N ES - atoms with the same numbers of N eutrons. ISOM E RS - E xcited nuclear state of atoms (Z, A, and N are the same). Nuclides – Any atomic species characterized by the atomic mass number (A), protons (Z), and number of neutron (N). Nuclides are STABLE nuclei. RADIONUCLIDES - are also nuclides but they have too few or too many neutrons and/or protons. Radionuclides are UNSTABLE nuclei.

6. Radioactivity/ Radioactive decay . The process by which an unstable atomic nucleus spontaneously emits particles and energy and transforms itself to another atom to attain stability. It decays by spitting out : Mass (alpha particle), Charge (Beta particle), Energy (Gamma Rays). Parent Nuclide – the original nuclide that undergoes radioactive decay. Daughter Nuclide – the more stable nuclide which results from radioactive decay.

7. Nuclear Half L ife The time of it takes for 1/2 of a certain radionuclide to turn into another radionuclide. The time required for half of a sample to decay. Physical Half-life - is the time required so that activity of radionuclide is reduced to 50%. Biological Half-life - is the time required for the body to eliminate half of an administered dosage of any substance by regular process of deliberation. Effective Half-life - is the time required for a radioactive elimination in the body to be diminished by 50% as a result of the combined action of the Radioactive Decay and Biological Elimination.

8. Decay Modes Those that involve nuclear transformation: 1. Negative Beta Decay (Beta Minus) 2. Positive Beta Decay (Beta Plus) 3. Electron Capture (K - Capture) 4. Alpha Decay Those that does not involve nuclear transformation: 1. Gamma Decay/Isomeric Transition

9. Decay Mode Mass No. Atomic No. Neutron No. Comments Isomeric transition A Z N Metastable if half-life >10 −9 s Beta minus (β−) A Z + 1 N − 1 Emits negatrons & antineutrinos Beta plus (β+) A Z − 1 N + 1 Emits positrons and neutrinos Electron capture A Z − 1 N + 1 Emits neutrinos (and x-rays ) Alpha decay A − 4 Z − 2 N − 2 Dominant decay mode for Z>82

10. ALPHA DECAY - A radionuclide emits an ALPHA PARTICLE consisting of 2 neutrons and 2 protons. Because they are relatively heavy and charged, they have a short range in matter and a high LET. They travel only a few c m. in air and only about 50 μm in tissue. In alpha decay, the atomic number decreases by two and the mass number decreases by four. An alpha particle is the nucleus of a helium atom. Alpha decay is most common in atoms with a high atomic number (Z>82) and atomic mass number (A > 150). – 226 Ra is a common alpha emitter found in nature. – 226 Ra decays to 222 Rn (radon), which is another alpha emitter. PROPERTIES OF AN ALPHA PARTICLE Discrete energy (4 & 7 MeV) Short range (<0.1mm) Specific ionization (40,000 ion pair/cm)

11. Beta Minus Decay - A radionuclide converts a neutron into a proton. The excess energy is released as a NEGATRON (an energetic electron) also called beta minus particle & ANTI-NEUTRINO. Beta minus decay occurs in nuclei with an excess of neutrons (i.e., too few protons). In beta minus decay, the atomic number increases by one and the mass number stays the same. Most Important example of beta minus emitters are: - 131 I - used in nuclear medicine treatment (therapy). It deposits all its its dose locally right within a few 10mm where they are created so its great for treatment. It is used for diagnosis (imaging) as well. Not just it is a beta minus emitter, it also gives off gamma ray. - 32 P and and 14 C is a pure beta emitter as well. PROPERTIES OF A NEGATRON - Same with electron’s properties - 9.1 x 10 31 kg - Possess negative charge - 511 keV - But different ORIGIN

12. Beta Plus Decay ( P ositron E mission ) - In beta plus (β+) decay, a proton inside the nucleus is converted into a neutron. The excess energy is emitted as a positively charged electron called a positron. Positrons have the same properties as electrons, except that their charge is positive (electrons have (-) charges). Beta plus decay (positron emission) occurs in neutron-defificient nuclei (too many protons). It also results in the emission of a neutrino. A neutrino has no electric charge or rest mass and is similar to an antineutrino. - In beta plus decay, the atomic number decreases by one and the mass number stays the same. Energetic positrons lose their energy by ionization and excitation of atomic electrons. - When the positron loses all of its kinetic energy, it annihilates with an electron. The mass of the positron and electron (511 keV each) are converted into two 511-keV photons that are emitted in opposite directions (i.e., 180 degrees apart).

13. Beta Plus Decay ( P ositron E mission ) - Most Important example of positron emitters is 18 F - it is widely used in PET SCANNING. a pure positron emitter. half-life of 110 minutes. The maximum energy of the 18 F positron is 0.63 MeV, and the average positron energy is one third the maximum (i.e., 0.21 MeV). - Many common positron emitters have very short half-lives. 11 C half-life is 20 minutes, and 15 O half-life is 2 minutes. PROPERTIES OF A POSITRON - The same with electron b ut POSITIVELY charged . - Anti-matter (annihilates an electron) .

14. Electron Capture - In electron capture, a proton inside the nucleus is converted into a neutron by capturing an atomic electron. The electron that is captured most likely originates in the K-shell. A neutrino is emitted during electron capture. Electron capture occurs in nuclei defificient in neutrons (too many protons). - In electron capture, the atomic number decreases by one and the mass number stays the same. If the captured electron is from the K-shell, the resultant K-shell vacancy is filled by an outer shell electron. The excess energy is emitted either as a characteristic x-ray or Auger electron. Electron capture may compete with beta plus decay. - Important electron capture radionuclides used in nuclear medicine are 67 Ga, 111 In, 123 I, and 201 Tl. 57 Co is also an electron capture radionuclide used for quality control of scintillation camera uniformity.

15. Gamma Decay/ Isomeric Transition - A radionuclide in its excited state deexcites by emission of one or more HIGH FREQUENCY PHOTON. Higher energy levels (excited states) are known as isomeric states. Isomeric states are always unstable. Excited states will transform into a lower energy level, emitting a gamma ray or internal conversion electron. - A gamma ray is electromagnetic radiation originating in a nuclear transformation. - The excess energy may be transferred to an orbital electron, which is then emitted from the atom as an internal conversion electron . –After an isomeric transition, both parent and daughter nuclei have the same mass number and atomic number. –Isomeric states that have long lifetimes are called metastable. The metastable state of an atom is denoted by a lower case “ m ” following the mass number (e.g., 99m Tc). 88% of 99m Tc atoms energy undergoes gamma decay and 12% is transferred to an orbital electron, which is then emitted from the atom as an internal conversion electron. Gamma ray energy of 99m Tc is 140 Kev. (140.5 kev) . The half life of 99M Tc is 6hrs Note: Most gamma rays are emitted almost immediately (<10 -12 second) after the primary decay process, whether it be alpha decay, negatron decay, positron decay, or electron capture. When the intermediate excited state last longer than 10 -9 second, the term “metastable” is used.