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 position
16 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 electron 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 interaction 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 photon’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 scattering 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
2 A Primer on Imaging Anatomy and Physiology photon comes close to a nucleus, its energy may be transformed into two new parti cles: an electron and a positron(excess energy from the photon is transferred as kinetic energy to these two particles)(Fig. 2.1d). For the most part, pair production is rare in medical x-rays given the high level of energy needed The degree to which a given substance allows an x-ray to pass through(versus absorb ing or scattering the x-ray) is referred to as attenuation. Denser materials, particularly comprised of larger atoms, such as the calcium in bone, will absorb more x-rays than soft tissue or fluids. Indeed, photoelectric effects are proportional to the cube of the atomic number of the material. a projectional image is thus formed by capturing those x-ray photons that are successfully transmitted from a source through an object to a detector that is designed to capture the photons Dosage. We briefly touch upon the issue of ionizing radiation and patient exposure Typically, we speak of radiation dosage to describe the amount of radiation absorbed by tissue. The amount of radiation absorbed by tissue is measured in terms of energy absorbed per unit mass; this unit is called a gray(Gy), and is defined as: 1 Gy =1 J/kg a dose equivalent is a weighted measure that accounts for the fact that some types of radiation are more detrimental to tissue than others the unit for this measure is called a sievert (Sv). A sievert is defined as: 1 Sv= 1 J/Kg x radiation weight factor, where he radiation weight factor(RWF) depends on the type of radiation. For example, the RWF for x-rays is 1: for neutron radiation, the RWF is 10; and for a-particles, the RWF is 20. The average dose of radiation that a person receives annually from natural sources is -360 HSv. Regulations state that the maximal allowable maximal amount for most individuals is 1 mSv/year; and for those individuals working closely with radiation, 50 mSv/year. As a point of comparison, a single chest x-ray provides-500 LSV. Ultimately, a key drive of imaging technology is to minimize the total amount of ionizing radiation exposure to the patient while balancing the ability of the modality to provide diagnostic images ig. 2.2 outlines the rudimentary idea behind using x-rays as a means to create medical images. A controlled and focused source of x-rays is allowed to pass through the ana- my of interest; a detector is then responsible for quantifying the amount and pattern of x-ray photons, converting the information into a visual image. Detailed discussions of projectional image formation can be found in[36, 39 X-ray generation. X-rays are generated when electrons of sufficient energy hit certain materials. Generally speaking, a source of electrons is generated by heating a metal cathode (filament) made of tungsten coil; an electrical current is used to induce thermionic emission. These released free photoelectrons are then accelerated toward a rotating target anode, usually made of tungsten, copper, or molybdenum. On hitting this surface, the photoelectrons decelerate, leading to the emission of x-ray radiation nd thermal energy. In particular, the x-rays are created when the accelerated photo- electrons release some of their energy in interacting with an atom. Two processes enerate these x-rays: 1 )bremsstrahlung(German for"breaking radiation"), where the electron collides with a nucleus and its kinetic energy is completely converted into e-ray photons; and 2 )K-shell emission, in which the accelerated electron hits another lower-energy bound electron resulting in the same outcome as the photoelectric effect (a photoelectron and characteristic radiation are generated). X-rays produced by the
2 A Primer on Imaging Anatomy and Physiology 17 photon comes close to a nucleus, its energy may be transformed into two new particles: an electron and a positron (excess energy from the photon is transferred as kinetic energy to these two particles) (Fig. 2.1d). For the most part, pair production is rare in medical x-rays given the high level of energy needed. The degree to which a given substance allows an x-ray to pass through (versus absorbing or scattering the x-ray) is referred to as attenuation. Denser materials, particularly comprised of larger atoms, such as the calcium in bone, will absorb more x-rays than soft tissue or fluids. Indeed, photoelectric effects are proportional to the cube of the atomic number of the material. A projectional image is thus formed by capturing those x-ray photons that are successfully transmitted from a source through an object to a detector that is designed to capture the photons. Dosage. We briefly touch upon the issue of ionizing radiation and patient exposure. Typically, we speak of radiation dosage to describe the amount of radiation absorbed by tissue. The amount of radiation absorbed by tissue is measured in terms of energy absorbed per unit mass; this unit is called a gray (Gy), and is defined as: 1 Gy = 1 J/kg. A dose equivalent is a weighted measure that accounts for the fact that some types of radiation are more detrimental to tissue than others; the unit for this measure is called a sievert (Sv). A sievert is defined as: 1 Sv = 1 J/Kg x radiation weight factor, where the radiation weight factor (RWF) depends on the type of radiation. For example, the RWF for x-rays is 1; for neutron radiation, the RWF is 10; and for α-particles, the RWF is 20. The average dose of radiation that a person receives annually from natural sources is ~360 μSv. Regulations state that the maximal allowable maximal amount for most individuals is 1 mSv/year; and for those individuals working closely with radiation, 50 mSv/year. As a point of comparison, a single chest x-ray provides ~500 μSv. Ultimately, a key drive of imaging technology is to minimize the total amount of ionizing radiation exposure to the patient while balancing the ability of the modality to provide diagnostic images. Imaging Fig. 2.2 outlines the rudimentary idea behind using x-rays as a means to create medical images. A controlled and focused source of x-rays is allowed to pass through the anatomy of interest; a detector is then responsible for quantifying the amount and pattern of x-ray photons, converting the information into a visual image. Detailed discussions of projectional image formation can be found in [36, 39]. X-ray generation. X-rays are generated when electrons of sufficient energy hit certain materials. Generally speaking, a source of electrons is generated by heating a metal cathode (filament) made of tungsten coil; an electrical current is used to induce thermionic emission. These released free photoelectrons are then accelerated toward a rotating target anode, usually made of tungsten, copper, or molybdenum. On hitting this surface, the photoelectrons decelerate, leading to the emission of x-ray radiation and thermal energy. In particular, the x-rays are created when the accelerated photoelectrons release some of their energy in interacting with an atom. Two processes generate these x-rays: 1) bremsstrahlung (German for “breaking radiation”), where the electron collides with a nucleus and its kinetic energy is completely converted into x-ray photons; and 2) K-shell emission, in which the accelerated electron hits another lower-energy bound electron resulting in the same outcome as the photoelectric effect (a photoelectron and characteristic radiation are generated). X-rays produced by the
D. Aberle et al X-ray beam Figure 2.2: An x-ray source is focused into a beam that penetrates the patient, result- ing in attenuated x-rays. A filter then removes scatter generated from photon-electron interaction, and the x-rays are detected by a scintillating material that transforms the signal (e.g, into light or an electrical current). The result is a detectable latent image former phenomenon are the most useful, and are sometimes referred to as white radia tion. Fig. 2. 3a shows the structure and components of an x-ray tube. A voltage is applied to produce a current across the cathode/anode; and as the voltage increases, the current also increases until a maximal point is reached, the saturation current, in which current is limited by the cathode temperature. An x-ray beams"intensity"is thus measured in terms of milliamperes(mA). Note that the number of x-ray photons generated by the tube is dependent on the number of electrons hitting the anode; this quantity is in turn ultimately controlled by the cathode material's saturation current Changing the cathode material will therefore result in a different beam intensity. Addi tionally, the x-rays are of varying energy levels(ie, polychromatic); for medical im- aging, we typically want to use only a portion of this spectrum. For example, there is no reason to expose a patient to non-penetrating x-rays(< 20 keV). The glass encasing the vacuum in which the cathode/anode apparatus exists within an x-ray tube helps to remove some low-energy x-rays. Further filters constructed of thin aluminum can also e placed in the path of the x-ray photons: for instance, a 3 mm layer of aluminum wil attenuate more than 90% of low-energy x-rays. This filtering process to remove the lower-energy x-rays is called beam hardening. Similarly, copper layers are also some times used as filters in order to block high-energy x-rays. The choice of material and the thickness of the filter will determine preferential removal of high-and low-energy x-rays. The x-ray photons generated from this process emanate in all directions; there- fore, the x-ray tube is encased in (lead) shielding, with a small aperture to permit se of the x-rays to escape. A collimators used to further refine the beam, limiting its size and controlling the amount permitted to pass through to the patient Grids. As the x-rays pass through an object, photons generated as a result of scattering effects occur (e.g, Compton effect), thus resulting in signal noise that degrades end image quality(the consequence is sometimes called radiographic fog). To minimize this effect, a(anti-scatter) grid made of high attenuation material is typically placed in front of the detector to block scatter: regularly spaced gaps (or x-ray transmitting material) allow select rays through based on directionality(Fig. 2. 3b). By way of illus tration, the grid may consist of alternating strips of aluminum and lead, the former material transmitting and the latter absorbing the x-rays. The geometry of the grid ultimately affects the degree of scatter that impacts image formation. Image contrast In x-ray images, contrast refers to the difference in visible grayscales seen as a result of differences in attenuation. Given the process of generating a pro- jectional image, there are in general four variables that control the contrast seen in a latent image: 1)thickness, in which two objects of the same composition, but one
18 D. Aberle et al. Figure 2.2: An x-ray source is focused into a beam that penetrates the patient, resulting in attenuated x-rays. A filter then removes scatter generated from photon-electron interaction, and the x-rays are detected by a scintillating material that transforms the signal (e.g., into light or an electrical current). The result is a detectable latent image. former phenomenon are the most useful, and are sometimes referred to as white radiation. Fig. 2.3a shows the structure and components of an x-ray tube. A voltage is applied to produce a current across the cathode/anode; and as the voltage increases, the current also increases until a maximal point is reached, the saturation current, in which current is limited by the cathode temperature. An x-ray beam’s “intensity” is thus measured in terms of milliamperes (mA). Note that the number of x-ray photons generated by the tube is dependent on the number of electrons hitting the anode; this quantity is in turn ultimately controlled by the cathode material’s saturation current. Changing the cathode material will therefore result in a different beam intensity. Additionally, the x-rays are of varying energy levels (i.e., polychromatic); for medical imaging, we typically want to use only a portion of this spectrum. For example, there is no reason to expose a patient to non-penetrating x-rays (< 20 keV). The glass encasing the vacuum in which the cathode/anode apparatus exists within an x-ray tube helps to remove some low-energy x-rays. Further filters constructed of thin aluminum can also be placed in the path of the x-ray photons: for instance, a 3 mm layer of aluminum will attenuate more than 90% of low-energy x-rays. This filtering process to remove the lower-energy x-rays is called beam hardening. Similarly, copper layers are also sometimes used as filters in order to block high-energy x-rays. The choice of material and the thickness of the filter will determine preferential removal of high- and low-energy x-rays. The x-ray photons generated from this process emanate in all directions; therefore, the x-ray tube is encased in (lead) shielding, with a small aperture to permit some of the x-rays to escape. A collimator is used to further refine the beam, limiting its size and controlling the amount permitted to pass through to the patient. Grids. As the x-rays pass through an object, photons generated as a result of scattering effects occur (e.g., Compton effect), thus resulting in signal noise that degrades end image quality (the consequence is sometimes called radiographic fog). To minimize this effect, a (anti-scatter) grid made of high attenuation material is typically placed in front of the detector to block scatter: regularly spaced gaps (or x-ray transmitting material) allow select rays through based on directionality (Fig. 2.3b). By way of illustration, the grid may consist of alternating strips of aluminum and lead, the former material transmitting and the latter absorbing the x-rays. The geometry of the grid ultimately affects the degree of scatter that impacts image formation. Image contrast. In x-ray images, contrast refers to the difference in visible grayscales seen as a result of differences in attenuation. Given the process of generating a projectional image, there are in general four variables that control the contrast seen in a latent image: 1) thickness, in which two objects of the same composition, but one
2 A Primer on Imaging Anatomy and Physiology anode vacuum tube-. collimator ectron beam Figure 2.3:(a) Cutaway illustration of a x-ray vacuum tube and its components A potential difference is created between a cathode/anode, resulting in electrons hitting a metal surface. The result is x-ray photons, which are emitted through a collimator. The entire assembly is encased in a vacuum tube and typically shielded. (b)a grid is used to remove smatter arising from the Compton effect thicker than another, when imaged together the thinner object will produce more con- trast;2)density, where more dense materials(e.g, a solid vS. a liquid) will produce higher x-ray attenuation; 3)material, where the effective atomic number and attenua- on curve dictate interaction with x-ray photons; and 4)x-ray tube voltage, which controls the energy of the photons and hence the degree of penetration(higher voltage increases contrast). The first three of these variables can be explained by examining an x-ray's intensity as it passes through a material. X-ray intensity, I through a material is given by the following equation: I= loe where lo is the incident x-ray intensity, u is the linear attenuation coefficient, and t is the thickness of the material. u reflects the removal of x-ray photons from a beam through the interaction of electrons in the material the higher the electron density, the more likely an interaction between an electron and Conventional image formation. Photographic films coated with materials sensitive to x-rays are still perhaps the most commonly used means of forming images. The pro- cedure of exposing film to photons generates a latent image that can then be processed to create a visible image. The film itself is usually a transparent plastic sheet that is covered with a radiation-sensitive emulsion; silver halide(i.e, a compound formed by silver and a halogen, such as silver bromide)crystals in gelatin is often used for this purpose. In brief, when a silver halide crystal absorbs x-ray photons, imperfections in the crystal (so-called sensitivity specks) will turn into regions of metallic silver. If a sufficient number of silver atoms are present in an area, the crystal is rendered develop- able so that the use of a developing solution will change the entire crystal into silver Hence, those areas that are exposed to more photons will be developed more On film, developed regions are shown as black. Because of the relatively low effective atomic lumber of the film, only 3-5% of the x-rays will actually react with the emulsion(the rest pass directly through). Lower-energy light photons are actually easier for film to capture. Based on this fact, an intensifying screen is used to enhance the interaction between the film and the x-ray photons. One intensifying technique is to use a fluores- cent screen made up of a layer of phosphor that absorbs the x-rays and re-emits visible radiation that is picked up by the silver halide crystals. Current screens can achieve intensification of up to 250x. Thus, combine ed screen-film systems can reduce the exposure time- but at the cost of some loss of detail due to diffusion effects fror fluorescence
2 A Primer on Imaging Anatomy and Physiology 19 thicker than another, when imaged together the thinner object will produce more contrast; 2) density, where more dense materials (e.g., a solid vs. a liquid) will produce higher x-ray attenuation; 3) material, where the effective atomic number and attenuation curve dictate interaction with x-ray photons; and 4) x-ray tube voltage, which controls the energy of the photons and hence the degree of penetration (higher voltage increases contrast). The first three of these variables can be explained by examining an x-ray’s intensity as it passes through a material. X-ray intensity, I, through a material is given by the following equation: I = I0e -μt where I0 is the incident x-ray intensity, μ is the linear attenuation coefficient, and t is the thickness of the material. μ reflects the removal of x-ray photons from a beam through the interaction of electrons in the material: the higher the electron density, the more likely an interaction between an electron and x-ray photon. Conventional image formation. Photographic films coated with materials sensitive to x-rays are still perhaps the most commonly used means of forming images. The procedure of exposing film to photons generates a latent image that can then be processed to create a visible image. The film itself is usually a transparent plastic sheet that is covered with a radiation-sensitive emulsion; silver halide (i.e., a compound formed by silver and a halogen, such as silver bromide) crystals in gelatin is often used for this purpose. In brief, when a silver halide crystal absorbs x-ray photons, imperfections in the crystal (so-called sensitivity specks) will turn into regions of metallic silver. If a sufficient number of silver atoms are present in an area, the crystal is rendered developable so that the use of a developing solution will change the entire crystal into silver. Hence, those areas that are exposed to more photons will be developed more. On film, developed regions are shown as black. Because of the relatively low effective atomic number of the film, only 3-5% of the x-rays will actually react with the emulsion (the rest pass directly through). Lower-energy light photons are actually easier for film to capture. Based on this fact, an intensifying screen is used to enhance the interaction between the film and the x-ray photons. One intensifying technique is to use a fluorescent screen made up of a layer of phosphor that absorbs the x-rays and re-emits visible radiation that is picked up by the silver halide crystals. Current screens can achieve intensification of up to 250x. Thus, combined screen-film systems can reduce the exposure time – but at the cost of some loss of detail due to diffusion effects from fluorescence. Figure 2.3: (a) Cutaway illustration of a x-ray vacuum tube and its components. A potential difference is created between a cathode/anode, resulting in electrons hitting a metal surface. The result is x-ray photons, which are emitted through a collimator. The entire assembly is encased in a vacuum tube and typically shielded. (b) A grid is used to remove smatter arising from the Compton effect
D. Aberle et al Other techniques have also been explored for generation of a latent image. lonography is one means of detection predicated on a chamber filled with a gas(such as xenon)at high pressure(-5-10 atmospheres). A high potential difference is generated across the chamber, resulting in a strong electric field. The chamber also contains electrodes, one of which is covered by a thin foil. When the x-ray photons interact with the gas mole- cules, ion pairs are generated inside the ionization chamber. The ions are attracted to the chamber sides while the free electrons move toward the electrodes The electrons thus form a charge pattern on the foil based on the concentration of x-ray photon xposure:this pattern is the desired latent image. Xeroradiography is another method of x-ray image formation: a plate formed of layers of aluminum, selenium, and aluminum oxide is charged and subsequently exposed to the x-rays. When the x-ray photons aping on the selenium, a positive charge discharges in proportion to the amount of x-ray exposure; this technique exploits the principle of photoconduction. The selenium surface thus forms a latent image. For the most part, ionography and xeroradiography are less common today given the advent of digital detectors(see below) Computed radiography. Unlike film, which must act both as an image receptor and as the image display medium, computed radiography( CR) systems separate the task of photon detection and image display. Photostimulable luminescent phosphor plates (PSL phosphor plates) are used as the primary image receptor material in CR. These imaging plates"are similar in concept to conventional radiographic intensifying screens. The major difference between CR and conventional intensifying screens is in he luminescence process. Conventional screens are designed so that the x-ray photon energy absorbed within the phosphor results in prompt fluorescent emission. PSL imaging plates on the other hand are designed so that a large portion of the absorbed X-ray energy is stored within the PSl phosphor material as trapped excited electrons This stored energy gives rise to a sort of latent image in the PSl plate itself. As such, computed radiography systems are often referred to as storage phosphor systems Once the Psl plate is exposed, the next stage is CR image formation. At a high level, is process is based on the use of a laser to stimulate the d electrons to emit visible light, and a photomultiplier tube(PMT) that captures the light signal and trans forms it into a detectable current that quantifies the degree of x-ray photon exposure The image formation process can be broken up into three major steps 1. Image pre-read. Before the imaging plate is actually scanned, a pre-read of the plate is performed. A survey of points is made using a low power laser to deter mine the minimum, maximum, and mean exposure values on the plate. These values are used to optimize the high voltage and amplifier gain settings on the photomultiplier and signal conditioning circuits. The minimum, maximum, and mean exposure values are also used to determine the appropriate digital transformation tables(see below)for optimal image display. The pre-read stimulates only a small percentage of the total trapped electrons on the plate so that the latent image is relatively unaltered. a pre-read is analogous to exposure readings performed on autofocus/auto-exposure cameras that automatically set shutter speed and aperture size based on survey information about the light intensity in various image zones 2. Image main read, Given the information from the pre-read, the main read samples the imaging plate at several points(-4 million over an 8 x 10"area). Each sampled point details the number of trapped electrons in a particular area of the imaging plate. When a point on an exposed plate is stimulated by the laser beam(spot size between 80-200 um), the trapped electrons in the spot are released and return to a
20 D. Aberle et al. Other techniques have also been explored for generation of a latent image. Ionography is one means of detection predicated on a chamber filled with a gas (such as xenon) at high pressure (~5-10 atmospheres). A high potential difference is generated across the chamber, resulting in a strong electric field. The chamber also contains electrodes, one of which is covered by a thin foil. When the x-ray photons interact with the gas molecules, ion pairs are generated inside the ionization chamber. The ions are attracted to the chamber sides, while the free electrons move toward the electrodes. The electrons thus form a charge pattern on the foil based on the concentration of x-ray photon exposure; this pattern is the desired latent image. Xeroradiography is another method of x-ray image formation: a plate formed of layers of aluminum, selenium, and aluminum oxide is charged and subsequently exposed to the x-rays. When the x-ray photons impinge on the selenium, a positive charge discharges in proportion to the amount of x-ray exposure; this technique exploits the principle of photoconduction. The selenium surface thus forms a latent image. For the most part, ionography and xeroradiography are less common today given the advent of digital detectors (see below). Computed radiography. Unlike film, which must act both as an image receptor and as the image display medium, computed radiography (CR) systems separate the task of photon detection and image display. Photostimulable luminescent phosphor plates (PSL phosphor plates) are used as the primary image receptor material in CR. These “imaging plates” are similar in concept to conventional radiographic intensifying screens. The major difference between CR and conventional intensifying screens is in the luminescence process. Conventional screens are designed so that the x-ray photon energy absorbed within the phosphor results in prompt fluorescent emission. PSL imaging plates on the other hand are designed so that a large portion of the absorbed x-ray energy is stored within the PSL phosphor material as trapped excited electrons. This stored energy gives rise to a sort of latent image in the PSL plate itself. As such, computed radiography systems are often referred to as storage phosphor systems. Once the PSL plate is exposed, the next stage is CR image formation. At a high level, this process is based on the use of a laser to stimulate the trapped electrons to emit visible light, and a photomultiplier tube (PMT) that captures the light signal and transforms it into a detectable current that quantifies the degree of x-ray photon exposure. The image formation process can be broken up into three major steps: 1. Image pre-read. Before the imaging plate is actually scanned, a pre-read of the plate is performed. A survey of points is made using a low power laser to determine the minimum, maximum, and mean exposure values on the plate. These values are used to optimize the high voltage and amplifier gain settings on the photomultiplier and signal conditioning circuits. The minimum, maximum, and mean exposure values are also used to determine the appropriate digital transformation tables (see below) for optimal image display. The pre-read stimulates only a small percentage of the total trapped electrons on the plate so that the latent image is relatively unaltered. A pre-read is analogous to exposure readings performed on autofocus/auto-exposure cameras that automatically set shutter speed and aperture size based on survey information about the light intensity in various image zones. 2. Image main read. Given the information from the pre-read, the main read samples the imaging plate at several points (~4 million over an 8 x 10" area). Each sampled point details the number of trapped electrons in a particular area of the imaging plate. When a point on an exposed plate is stimulated by the laser beam (spot size between 80-200 μm), the trapped electrons in the spot are released and return to a