D. Aberle et al ranslates to photons with an energy level of 0. 12-125 keV. Above a certain energy level (-12 ke V), x-rays are able to penetrate different materials to a varying degree: it is this phenomenon that is taken advantage of in projectional x-ray imaging. Recall from basic physics that when a photon hits an atom, there is a chance of interaction between the photon and any electrons. There are essentially three different ways that an x-ray can interact with matter within the diagnostic energy range Photoelectric effect. The well-known photoelectric effect involves the interaction of a photon with a low-energy electron. If the photon has sufficient energy, then the electron is separated from the atom, with any excess energy from the photon being transformed into the electrons kinetic energy(Fig. 2.1a). The emitted elec- tron is referred to as a photoelectron. Given the absence of an electron in the lower energy levels, an electron from a higher energy level moves down to take its place; but in order to do so, it must release its extra energy, which is seen in the form of a photon(characteristic radiation). Thus, the photoelectric effect generates three products: a photoelectron; a photon(characteristic radiation); and an ion(the positively charged atom, hence the phrase ionizing radiation). This type of inter action typically occurs with the absorption of low-energy x-rays 2. Compton effect. Rather than being absorbed, when a high-energy photon collides with an electron, both particles may instead be deflected. a portion of the pho- ton's energy is transferred to the electron in this process, and the photon emerges with a longer wavelength; this effect is known as Compton scattering(Fig. 2.1b) This phenomenon is thus seen largely with higher-energy x-rays. Compton scat- tering is the major source of background noise in x-ray images. Furthermore, Compton scattering is a cause of tissue damage 3. Coherent scattering. Lastly, an x-ray can undergo a change in direction but no change in wavelength(energy)(Fig. 2.1c). Thompson and Rayleigh scatter are examples of this occurrence. Usually 5% of the radiation undergoes this effect. A fourth type of interaction is possible, known as pair production. Pair production involves high energy x-rays and elements of high atomic weight. When a high-energy x-ray scattered x- x-ray deflected x- -ray photon photoelectron photon ray photon photon ray photon photon Figure 2.1: Interaction of x-rays with matter, envisioning an atom and its electrons in terms of a nucleus and orbitals. (a) The photoelectric effect results in the complete transfer of the energy from an x-ray photon to an electron, which leaves the atom as a photoelectron. Another electron then moves from a higher to lower orbit and in the orocess emits characteristic radiation. (b) The Compton effect results in scattering of the x-ray photon with a portion of the photon's momentum transferred as kinetic energy to the electron. (c) Coherent scattering involves the deflection of the x-ray photon a new direction. (d) Pair production occurs when the x-ray photon interacts with the nucleus, its energy being transformed into two new particles, an electron and position16 D. Aberle et al. translates to photons with an energy level of 0.12-125 keV. Above a certain energy level (~12 keV), x-rays are able to penetrate different materials to a varying degree: it is this phenomenon that is taken advantage of in projectional x-ray imaging. Recall from basic physics that when a photon hits an atom, there is a chance of interaction between the photon and any electrons. There are essentially three different ways that an x-ray can interact with matter within the diagnostic energy range: 1. Photoelectric effect. The well-known photoelectric effect involves the interaction of a photon with a low-energy electron. If the photon has sufficient energy, then the electron is separated from the atom, with any excess energy from the photon being transformed into the electron’s kinetic energy (Fig. 2.1a). The emitted elec￾tron is referred to as a photoelectron. Given the absence of an electron in the lower energy levels, an electron from a higher energy level moves down to take its place; but in order to do so, it must release its extra energy, which is seen in the form of a photon (characteristic radiation). Thus, the photoelectric effect generates three products: a photoelectron; a photon (characteristic radiation); and an ion (the positively charged atom, hence the phrase ionizing radiation). This type of inter￾action typically occurs with the absorption of low-energy x-rays. 2. Compton effect. Rather than being absorbed, when a high-energy photon collides with an electron, both particles may instead be deflected. A portion of the pho￾ton’s energy is transferred to the electron in this process, and the photon emerges with a longer wavelength; this effect is known as Compton scattering (Fig. 2.1b). This phenomenon is thus seen largely with higher-energy x-rays. Compton scat￾tering is the major source of background noise in x-ray images. Furthermore, Compton scattering is a cause of tissue damage. 3. Coherent scattering. Lastly, an x-ray can undergo a change in direction but no change in wavelength (energy) (Fig. 2.1c). Thompson and Rayleigh scatter are examples of this occurrence. Usually < 5% of the radiation undergoes this effect. A fourth type of interaction is possible, known as pair production. Pair production involves high energy x-rays and elements of high atomic weight. When a high-energy Figure 2.1: Interaction of x-rays with matter, envisioning an atom and its electrons in terms of a nucleus and orbitals. (a) The photoelectric effect results in the complete transfer of the energy from an x-ray photon to an electron, which leaves the atom as a photoelectron. Another electron then moves from a higher to lower orbit and in the process emits characteristic radiation. (b) The Compton effect results in scattering of the x-ray photon with a portion of the photon’s momentum transferred as kinetic energy to the electron. (c) Coherent scattering involves the deflection of the x-ray photon in a new direction. (d) Pair production occurs when the x-ray photon interacts with the nucleus, its energy being transformed into two new particles, an electron and position