X-Ray
X-Ray Equipment
Production of X-rays • Image Detection: Screen Film Combinations • Image Detection: X-Ray Image Intensifiers with Televisions • Image Detection: Digital Systems
X-Ray Equipment
Conventional x-ray radiography produces images of anatomy that are shadowgrams based on x-ray absorption. The x-rays are produced in a region that is nearly a point source and then are directed on the anatomy to be imaged. The x-rays emerging from the anatomy are detected to form a two-dimensional image, where each point in the image has a brightness related to the intensity of the x-rays at that point. Image production relies on the fact that significant numbers of x-rays penetrate through the anatomy and that different parts of the anatomy absorb different amounts of x-rays. In cases where the anatomy of interest does not absorb x-rays differently from surrounding regions, contrast may be increased by introducing strong x-ray absorbers. For example, barium is often used to image the gastrointestinal tract.
X-rays are electromagnetic waves (like light) having an energy in the general range of approximately 1 to several hundred kiloelectronvolts (keV). In medical x-ray imaging, the x-ray energy typically lies between 5 and 150 keV, with the energy adjusted to the anatomic thickness and the type of study being performed.
X-rays striking an object may either pass through unaffected or may undergo an interaction. These interactions usually involve either the photoelectric effect (where the x-ray is absorbed) or scattering (where the x-ray is deflected to the side with a loss of some energy). X-rays that have been scattered may undergo deflection through a small angle and still reach the image detector; in this case they reduce image contrast and thus degrade the image. This degradation can be reduced by the use of an air gap between the anatomy and the image receptor or by use of an antiscatter grid.
The equipment of conventional x-ray radiography mostly deals with the creation of a desirable beam of x-rays and with the detection of a high-quality image of the transmitted x-rays. These are discussed in the following sections.
Production of X-Rays
X-Ray Tube
The standard device for production of x-rays is the rotating anode x-ray tube, as illustrated in Fig. 61.1. The x-rays are produced from electrons that have been accelerated in vacuum from the cathode to the anode. The electrons are emitted from a filament mounted within a groove in the cathode. Emission occurs when the filament is heated by passing a current through it. When the filament is hot enough, some electrons obtain a thermal energy sufficient to overcome the energy binding the electron to the metal of the filament. Once the electrons have "boiled off" from the filament, they are accelerated by a voltage difference applied from the cathode to the anode. This voltage is supplied by a generator (see below).
After the electrons have been accelerated to the anode, they will be stopped in a short distance. Most of the electrons' energy is converted into heating of the anode, but a small percentage is converted to x-rays by two main methods. One method of x-ray production relies on the fact that deceleration of a charged particle results in emission of electromagnetic radiation, called bremmstrahlung radiation. These x-rays will have a wide, continuous distribution of energies, with the maximum being the total energy the electron had when reaching the anode. The number of x-rays is relatively small at higher energies and increases for lower energies.
A second method of x-ray production occurs when an accelerated electron strikes an atom in the anode and removes an inner electron from this atom. The vacant electron orbital will be filled by a neighboring electron, and an x-ray may be emitted whose energy matches the energy change of the electron. The result is production of large numbers of x-rays at a few discrete energies. Since the energy of these characteristic x-rays depends on the material on the surface of the anode, materials are chosen partially to produce x-rays with desired energies. For example, molybdenum is frequently used in anodes of mammography x-ray tubes because of its 20-keV characteristic x-rays.
Low-energy x-rays are undesirable because they increase dose to the patient but do not contribute to the final image because they are almost totally absorbed. Therefore, the number of low-energy x-rays is usually reduced by use of a layer of absorber that preferentially absorbs them. The extent to which low-energy x-rays have been removed can be quantified by the half-value layer of the x-ray beam.
It is ideal to create x-rays from a point source because any increase in source size will result in blurring of the final image. Quantitatively, the effects of the blurring are described by the focal spot's contribution to the system modulation transfer function (MTF). The blurring has its main effect on edges and small objects, which correspond to the higher frequencies. The effect of this blurring depends on the geometry of the imaging and is worse for larger distances between the object and the image receptor (which corresponds to larger geometric magnifications).
To avoid this blurring, the electrons must be focused to strike a small spot of the anode. The focusing is achieved by electric fields determined by the exact shape of the cathode. However, there is a limit to the size of this focal spot because the anode material will melt if too much power is deposited into too small an area. This limit is improved by use of a rotating anode, where the anode target material is rotated about a central axis and new (cooler) anode material is constantly being rotated into place at the focal spot. To further increase the power limit, the anode is made with an angle surface. This allows the heat to be deposited in a relatively large spot while the apparent spot size at the detector will be smaller by a factor of the sine of the anode angle. Unfortunately, this angle cannot be made too small because it limits the area that can be covered with x-rays. In practice, tubes are usually supplied with two (or more) focal spots of differing sizes, allowing choice of a smaller (sharper, lower-power) spot or a larger (more blurry, higher-power) spot.
The x-ray tube also limits the total number of x-rays that can be used in an exposure because the anode will melt if too much total energy is deposited in it. This limit can be increased by using a more massive anode.
Generator
The voltages and currents in an x-ray tube are supplied by an x-ray generator. This controls the cathode-anode voltage, which partially defines the number of x-rays made because the number of x-rays produced increases with voltage. The voltage is also chosen to produce x-rays with desired energies: Higher voltages makes x-rays that generally are more penetrating but give a lower contrast image. The generator also determines the number of x-rays created by controlling the amount of current flowing from the cathode to anode and by controlling the length of time this current flows. This points out the two major parameters that describe an x-ray exposure: the peak kilovolts (peak kilovolts from the anode to the cathode during the exposure) and the milliampere-seconds (the product of the current in milliamperes and the exposure time in seconds).
The peak kilovolts and milliampere-seconds for an exposure may be set manually by an operator based on estimates of the anatomy. Some generators use manual entry of kilovolts and milliamperes but determine the exposure time automatically. This involves sampling the radiation either before or after the image sensor and is referred to as phototiming.
The anode-cathode voltage (often 15 to 150 kV) can be produced by a transformer that converts 120 or 220 V ac to higher voltages. This output is then rectified and filtered. Use of three-phase transformers gives voltages that are nearly constant versus those from single-phase transformers, thus avoiding low kilovoltages that produce undesired low-energy x-rays. In a variation of this method, the transformer output can be controlled at a constant voltage by electron tubes. This gives practically constant voltages and, further, allows the voltage to be turned on and off so quickly that millisecond exposure times can be achieved. In a third approach, an ac input can be rectified and filtered to produce a nearly dc voltage, which is then sent to a solid-state inverter that can turn on and off thousands of times a second. This higher-frequency ac voltage can be converted more easily to a high voltage by a transformer. Equipment operating on this principle is referred to as midfrequency or high-frequency generators.
Image Detection: Screen Film Combinations
Special properties are needed for image detection in radiographic applications, where a few high-quality images are made in a study. Because decisions are not immediately made from the images, it is not necessary to display them instantly (although it may be desirable).
The most commonly used method of detecting such a radiographic x-ray image uses light-sensitive negative film as a medium. Because high-quality film has a poor response to x-rays, it must be used together with x-ray-sensitive screens. Such screens are usually made with CaWo2 or phosphors using rare earth elements such as doped Gd2O2S or LaOBr. The film is enclosed in a light-tight cassette in contact with an x-ray screen or in between two x-ray screens. When a x-ray image strikes the cassette, the x-rays are absorbed by the screens with high efficiency, and their energy is converted to visible light. The light then exposes a negative image on the film, which is in close contact with the screen.
Several properties have been found to be important in describing the relative performance of different films. One critical property is the contrast, which describes the amount of additional darkening caused by an additional amount of light when working near the center of a film's exposure range. Another property, the latitude of a film, describes the film's ability to create a usable image with a wide range in input light levels. Generally, latitude and contrast are competing properties, and a film with a large latitude will have a low contrast. Additionally, the modulation transfer function (MTF) of a film is an important property. MTF is most degraded at higher frequencies; this high-frequency MTF is also described by the film's resolution, its ability to image small objects.
X-ray screens also have several key performance parameters. It is essential that screens detect and use a large percentage of the x-rays striking them, which is measured as the screen's quantum detection efficiency. Currently used screens may detect 30% of x-rays for images at higher peak kilovolts and as much 60% for lower peak kilovolt images. Such efficiencies lead to the use of two screens (one on each side of the film) for improved x-ray utilization. As with films, a good high-frequency MTF is needed to give good visibility of small structures and edges. Some MTF degradation is associated with blurring that occurs when light spreads as it travels through the screen and to the film. This leads to a compromise on thickness; screens must be thick enough for good quantum detection efficiency but thin enough to avoid excess blurring.
For a film/screen system, a certain amount of radiation will be required to produce a usable amount of film darkening. The ability of the film/screen system to make an image with a small amount of radiation is referred to as its speed. The speed depends on a number of parameters: the quantum detection efficiency of the screen, the efficiency with which the screen converts x-ray energy to light, the match between the color emitted by the screen and the colors to which the film is sensitive, and the amount of film darkening
for a given amount of light. The number of x-rays used in producing a radiographic image will be chosen to give a viewable amount of exposure to the film. Therefore, patient dose will be reduced by the use of a high-speed screen/film system. However, high-speed film/screen combinations gives a "noisier" image because of the smaller number of x-rays detected in its creation.
Image Detection: X-Ray Image Intensifiers with Televisions
Although screen-film systems are excellent for radiography, they are not usable for fluoroscopy, where lower x-ray levels are produced continuously and many images must be presented almost immediately. Fluoroscopic images are not used for diagnosis but rather as an aid in performing tasks such as placement of catheters in blood vessels during angiography. For fluoroscopy, x-ray image intensifiers are used in conjunction with television cameras. An x-ray image intensifier detects the x-ray image and converts it to a small, bright image of visible light. This visible image is then transferred by lenses to a television camera for final display on a monitor.
The basic structure of an x-ray image intensifier is shown in Fig. 61.2. The components are held in a vacuum by an enclosure made of glass and/or metal. The x-rays enter through a low-absorption window and then strike an input phosphor usually made of doped CsI. As in the x-ray screens described above, the x-rays are converted to light in the CsI. On top of the CsI layer is a photoemitter, which absorbs the light and emits a number of low-energy electrons that initially spread in various directions. The photoelectrons are accelerated and steered by a set of grids that have voltages applied to them.
The electrons strike an output phosphor structure that converts their energy to the final output image made of light. This light then travels through an output window to a lens system. The grid voltages serve to add energy to the electrons so that the output image is brighter. Grid voltages and shapes are also chosen so that the x-ray image is converted to a light image with minimal distortion. Further, the grids must be designed to take photoelectrons that are spreading from a point on the photoemitter and focus them back together at a point on the output phosphor.
X-ray image intensifiers can be described by a set of performance parameters not unlike those of screen/film combinations. It is important that x-rays be detected and used with a high efficiency; current image intensifiers have quantum detection efficiencies of 60% to 70% for 59-keV x-rays. As with film/screens, a good high-frequency MTF is needed to image small objects and sharp edges without blurring. However, low-frequency MTF also must be controlled carefully in image intensifiers, since it can be degraded by internal scattering of x-rays, photoelectrons, and light over relatively large distances. The amount of intensification depends on brightness and size of the output image for a given x-ray input. This is described either by the gain (specified relative to a standard x-ray screen) or by conversion efficiency [a light output per radiation input measured in (cd/m2)/(mR/min)]. Note that producing a smaller output image is as important as making a light image with more photons because the small image can be handled more efficiently by the lenses that follow. Especially when imaging the full input area, image intensifiers introduce a pincushion distortion into the output image. Thus a square object placed off-center will produce an image that is stretched in the direction away from the center.
Although an image intensifier output could be viewed directly with a lens system, there is more flexibility when the image intensifier is viewed with a television camera and the output is displayed on a monitor. Televisions are currently used with pickup tubes and with CCD sensors.
When a television tube is used, the image is focused on a charged photoconducting material at the tube's input. A number of materials are used, including SbS3, PbO, and SeTeAs. The light image discharges regions of the photoconductor, converting the image to a charge distribution on the back of the photo-conducting layer. Next, the charge distribution is read by scanning a small beam of electrons across the surface, which recharges the photoconductor. The recharging current is proportional to the light intensity at the point being scanned; this current is amplified and then used to produce an image on a monitor. The tube target is generally scanned in an interlaced mode in order to be consistent with broadcast television and allow use of standard equipment.
In fluoroscopy, it is desirable to use the same detected dose for all studies so that the image noise is approximately constant. This is usually achieved by monitoring the image brightness in a central part of the image intensifier's output, since brightness generally increases with dose. The brightness may be monitored by a photomultiplier tube that samples it directly or by analyzing signal levels in the television. However, maintaining a constant detected dose would lead to high patient doses in the case of very absorptive anatomy. To avoid problems here, systems are generally required by federal regulations to have a limit on the maximum patient dose. In those cases where the dose limit prevents the image intensifier from receiving the usual dose, the output image becomes darker. To compensate for this, television systems are often operated with automatic gain control that gives an image on the monitor of a constant brightness no matter what the brightness from the image intensifier.
Image Detection: Digital Systems
In both radiography and fluoroscopy, there are advantages to the use of digital images. This allows image processing for better displayed images, use of lower doses in some cases, and opens the possibility for digital storage with a PACS system or remote image viewing via teleradiology. Additionally, some digital systems provide better image quality because of fewer processing steps, lack of distortion, or improved uniformity.
A common method of digitizing medical x-ray images uses the voltage output from an image-intensifier/TV system. This voltage can be digitized by an analog-to-digital converter at rates fast enough to be used with fluoroscopy as well as radiography.
Another technology for obtaining digital radiographs involves use of photostimulable phosphors. Here the x-rays strike an enclosed sheet of phosphor that stores the x-ray energy. This phorphor can then be taken to a read-out unit, where the phosphor surface is scanned by a small light beam of proper wavelength. As a point on the surface is read, the stored energy is emitted as visible light, which is then detected, amplified, and digitized. Such systems have the advantage that they can be used with existing systems designed for screen-film detection because the phosphor sheet package is the same size as that for screen films.
A new method for digital detection involves use of active-matrix thin-film-transistor technology, in which an array of small sensors is grown in hydrogenated amorphous silicon. Each sensor element includes an electrode for storing charge that is proportional to its x-ray signal. Each electrode is coupled to a transistor that either isolates it during acquisition or couples it to digitization circuitry during readout. There are two common methods for introducing the charge signal on each electrode. In one method, a layer of x-ray absorber (typically selenium) is deposited on the array of sensors; when this layer is biased and x-rays are absorbed there, their energy is converted to electron-hole pairs and the resulting charge is collected on the electrode. In the second method, each electrode is part of the photodiode that makes electron-hole pairs when exposed to light; the light is produced from x-rays by a layer of scintillator (such as CsI) that is deposited on the array.
Use of a digital system provides several advantages in fluoroscopy. The digital image can be processed in real time with edge enhancement, smoothing, or application of a median filter. Also, frame-to-frame averaging can be used to decrease image noise, at the expense of blurring the image of moving objects.
Further, digital fluoroscopy with TV system allows the TV tube to be scanned in formats that are optimized for read-out; the image can still be shown in a different format that is optimized for display. Another advantage is that the displayed image is not allowed to go blank when x-ray exposure is ended, but a repeated display of the last image is shown. This last-image-hold significantly reduces doses in those cases where the radiologist needs to see an image for evaluation, but does not necessarily need a continuously updated image.
The processing of some digital systems also allows the use of pulsed fluoroscopy, where the x-rays are produced in a short, intense burst instead of continuously. In this method the pulses of x-rays are made either by biasing the x-ray tube filament or by quickly turning on and off the anode-cathode voltage. This has the advantage of making sharper images of objects that are moving. Often one x-ray pulse is produced for every display frame, but there is also the ability to obtain dose reduction by leaving the x-rays off for some frames. With such a reduced exposure rate, doses can be reduced by a factor of two or four by only making x-rays every second or fourth frame. For those frames with no x-ray pulse, the system repeats a display of the last frame with x-rays.
Defining Terms
Antiscatter grid:
A thin structure made of alternating strips of lead and material transmissive to x-rays. Strips are oriented so that most scattered x-rays go through lead sections and are preferentially absorbed, while unscattered x-rays go through transmissive sections.
Focal spot:
The small area on the anode of an x-ray tube from where x-rays are emitted. It is the place
where the accelerated electron beam is focused. Half-value layer (HVL): The thickness of a material (often aluminum) needed to absorb half the x-ray in a beam.
keV: A unit of energy useful with x-rays. It is equal to the energy supplied to an electron when accelerated through 1 kilovolt.
Modulation transfer function (MTF):
The ratio of the contrast in the output image of a system to the contrast in the object, specified for sine waves of various frequencies. Describes blurring (loss of contrast) in an imaging system for different-sized objects.
Quantum detection efficiency:
The percentage of incident x-rays effectively used to create an image.
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