1. Understand the principles of radiation and X-ray production.
2. Describe the different forms of photon and charged particle interactions.
3. Define the concepts of exposure, absorbed dose, equivalent dose, and effective dose.
4. Describe the main aspects of image quality.
5. Understand the basic principles of digital imaging.
6. Explain the structure and functions of picture archiving and communication system (PACS).
radio frequencies is used as the transmission and reception signal for MRI.
remove an electron. For example, the minimum energy necessary to remove an electron (ie, the ionization potential) from water is 12.6 eV.
Table 1.1 FUNDAMENTAL PROPERTIES OF PARTICULATE RADIATION
It is the most commonly utilized particulate radiation in diagnostic imaging. An electron has a rest mass of 9.109 × 10-31 kg and a rest energy of 511 keV. Electrons carry a negative charge and exist in atomic orbits. Electrons may also be emitted by the nuclei of some radioactive atoms. In such a case, they are referred to as beta-minus particles (β–), negatrons or “beta particles.”
Positrons are positively charged electrons. It is the antiparticle or the antimatter particle of electrons.
During radioactive decay, positrons may be emitted from some nuclei.
Protons are found in the nuclei of all atoms. A proton has a single positive charge and is the nucleus of a hydrogen-1 atom.
Neutrons are uncharged nuclear particles that have a mass slightly greater than a proton. Neutrons are released by nuclear fission and are used for radionuclide production.
An alpha particle (α2+) consists of two protons and two neutrons; therefore, it carries a net charge of +2 and is identical to the nucleus of a helium atom (4He2+). Certain naturally occurring radioactive materials emit alpha particles, such as uranium, thorium, and radium. In such emissions, the α2+ particle will eventually acquire two electrons from the surrounding medium and become a neutral helium atom (4He).
The radiation emitted when an outer-shell electron fills a vacancy in an inner shell of an atom is characteristic of each atom, because the electron binding energies depend on atomic number (Z). Radiation emissions from electron transitions exceeding 100 eV are called characteristic X-rays.
Characteristic X-rays are not always the product of an electron cascade. Auger electron emission is a competing process that predominates in low Z elements. In Auger electron emission, the energy released is transferred to an orbital electron, typically in the same shell as the cascading electron. The ejected Auger electron possesses kinetic energy equal to the difference between the transition energy and the binding energy of the ejected electron.
have energy in excess of the ground state, they are said to be in an excited state. Atoms in the excited state have lifetimes with a vast range. Excited states can last from 10-16 s to more than 100 years. Excited states that exist longer than 10-12 s are referred to as metastable or isometric states. These excited nuclei can be denoted by the letter m after the mass number of the atom (ie, Tc-99m).
Unstable nuclei have a surplus of internal energy compared with a stable arrangement of neutrons and protons. The transformation from unstable nuclei to stable nuclei is achieved through the conversion of a neutron to a proton or vice versa, and these events are accompanied by the emission of energy. Nuclides that transform (ie, decay) to more stable nuclei are said to be radioactive and the transformation process is called radioactive decay. The radionuclide at the beginning of a particular decay sequence is called the parent, and the nuclide produced by the decay is called the daughter. Several decays may occur before a stable configuration is achieved. Therefore, the daughter nuclide may be either stable or radioactive.
FIG. 1.8 • Plot of the number of neutrons (N) versus the number of protons (Z) for all stable nuclides (points in pink). The line corresponding to equal number of protons and neutron (N = Z) is also shown. When the proton number is low, the ratio N/Z is close to 1 for stable nuclides; when the proton number is high, that ratio approaches approximately 1.5. Radionuclides lie on both sides of the stability curve, with those above for neutron-rich radionuclides and those below for proton-rich ones. Reprinted with permission from Chandra R, Rahmim A. Nuclear Medicine Physics: The Basics. 8th ed. Philadelphia, PA: Wolters Kluwer; 2017.
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