(Radiographics. 2001;21:227-236.)
© RSNA, 2001
IMAGING & THERAPEUTIC TECHNOLOGY |
Fluoroscopy: Recording of Fluoroscopic Images and Automatic Exposure Control1
Richard A. Geise, PhD
1 From the Department of Radiology, University of Minnesota, Box 292, 420 Delaware St SE, Minneapolis, MN 55455. From the AAPM/RSNA Physics Tutorial at the 1999 RSNA scientific assembly. Received August 14, 2000; revision requested August 18 and received August 25; accepted August 29. Address correspondence to the author (e-mail: geise001@tc.umn.edu).
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Abstract
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Some means of recording images is a necessary part of most fluoroscopic systems. Several methods are available for recording images during fluoroscopy. Screen-film recording methods such as use of spot film devices and automatic film changers provide high-spatial-resolution images. Recording images by using the image intensifier (fluorography) provides film or digital images at relatively lower doses but with poorer spatial resolution. Digitally recorded images have better contrast resolution than analog images but lower spatial resolution and represent a compromise between dose and image quality. Motion picture (cine fluorographic) recording requires extremely high dose rates compared with those of lower-resolution videotape recording of motion. Recording systems in fluoroscopy require automatic exposure control for optimum image quality. The same feedback system used to control fluorographic exposures can be used to control exposure rates during fluoroscopy as well. Automatic brightness control maintains intensifier exposure rates on the basis of subject thickness by adjusting various technique factors. The type of control mechanism depends on the imaging task and the complexity (age and cost) of the equipment. The operator can choose between better image quality (higher contrast) or lower radiation dose.
Index Terms: Fluoroscopy • Physics • Radiography
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Introduction
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There are a number of technologies available for recording images during fluoroscopic procedures. These technologies include spot film devices, automatic film changers, photofluorography, digital fluorography, cine fluorography, and videotape recording. Each of these technologies has its own characteristics and is preferred for particular imaging tasks. It is valuable to be familiar with the general design of these imaging methods and to be able to characterize their image quality in terms of spatial resolution, contrast, and noise. Methods used for automatic exposure control for both still images and fluoroscopic viewing are also important to understand to make optimum use of fluoroscopic and fluorographic systems.
This article describes the technologies used for recording images during fluoroscopy, identifies the components that affect image quality, and discusses their effects on image quality and patient radiation dose. General methods of image recording during fluoroscopy are direct film recording, indirect recording, and recording motion. Automatic exposure control systems used in both fluoroscopy and fluorography are also reviewed. The components of these systems are identified, and their effects on image quality and radiation dose are discussed.
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Direct Film Recording
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Spot Film Devices
Fluoroscopic systems designed for gastrointestinal imaging are generally equipped with a spot film device. The spot film device allows exposure of a conventional screen-film cassette in conjunction with fluoroscopic viewing. This rather familiar system, located in front of the image intensifier, accepts the screen-film cassette and "parks" it out of the way during fluoroscopy (Fig 1). Cassettes may be loaded from the front or rear depending on the design of the system. A two-dimensional positioner centers the portion of the film to be used in the x-ray field, and a formatting mask prevents x-ray exposure of the portion of the film that is not used. The x-ray field size is also reduced automatically by the collimators at the time of exposure to minimize scattered radiation and patient radiation dose. The fluoroscopist can override this automatic collimation to further reduce the x-ray field.
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Figure 1. Standard spot film imaging configuration typical of gastrointestinal fluoroscopy equipment. The screen-film cassette is parked out of the x-ray field until the spot film trigger is pressed, causing both the cassette and the formatting mask to move into position.
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Spot film imaging uses essentially the same technology as conventional screen-film radiography. The details of screen-film imaging have been discussed in a previous tutorial series in this journal (1,2). Herein, we will review only the differences and limitations of spot film imaging compared with general radiography. One major limitation is the range of film sizes available for spot film imaging. Although some older fluoroscopy equipment is limited to a single size, usually 24 x 24 cm, current equipment allows a range of film sizes to be used, from 20 x 25 cm to 24 x 35 cm. Spot film devices usually allow more than one image to be obtained on a single film. Formats typically include one, two, three, four, or six images on a film.
Although some institutions may still be using screen-film systems with lower speeds, screen-film combinations with a speed of around 400 are generally used for spot film imaging. This speed usually yields a system with a rather moderate level of contrast and noise and a spatial limiting resolution of around 7 line pairs per millimeter (lp/mm).
There is slightly more magnification of patients' features with spot film imaging than with general radiography. For a typical gastrointestinal system with an under-table x-ray tube, a source-to-film distance of 76 cm, and a source-to-skin distance of 38 cm, the part of the body closest to the table can be magnified up to two times in the image. Because the patient cannot be moved relative to the x-ray source in such systems, magnification is primarily dependent on the position of the spot film device relative to the patient. Moving the spot film device closer to the patient reduces the amount of magnification and decreases the patient radiation dose.
A number of factors affect patient doses in spot film imaging. The source-to-skin distance is shorter in spot film imaging than in general radiography. Although the automatic exposure control system fixes the exposure to the screen, the shorter source-to-skin distance increases the inverse square reduction in radiation intensity as it passes through the patient. This increase tends to make the skin entrance exposure higher. The field size in spot film imaging is generally smaller than that used in general radiography. This smaller field size reduces scatter and therefore tends to reduce dose. For the same reason, grids used in fluoroscopy generally have a lower grid ratio and therefore a smaller Bucky factor, which also leads to lower dose. These effects tend to offset each other to a large extent.
One of the major shortcomings of conventional spot film devices is the delay involved in moving the cassette into position for exposure. In gastrointestinal imaging, this delay can be overcome by using photofluorography. In vascular imaging, more rapid film movement is achieved with automatic film changers.
Automatic Film Changers
The automatic film changers used in vascular imaging are also screen-film systems. They can be found in several varieties (3). Some are large, floor-mounted boxes, but systems more commonly used today mount on the image intensifier (Fig 2). The system consists of a supply magazine for holding unexposed film, a receiving magazine, a pair of radiographic screens, and a mechanism for transferring the film. When an exposure is required, the screens are mechanically separated, the film is pulled into place between them, and they are closed. After the film is exposed, the screens separate again. The film is moved to the receiver, and another film is pulled into place for the next exposure. The number of films and filming rates must be preprogrammed for proper operation.
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Figure 2. Typical rapid film changer. Approximately 20 films are held in the supply magazine. During exposure, a film is advanced between the high-speed screens, exposed, and removed to the receiving magazine.
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Because the major requirement is capturing rapid motion of a contrast agent, 800-speed screen-film combinations are typically used in film changers. This speed leads to a dose half that of the more general-purpose 400-speed systems used for gastrointestinal imaging. Resolution is also reduced to about 5 lp/mm. Unlike with spot film devices, the requirement for rapid motion limits the automatic changer to one film size, usually 35 x 35 cm. The typical film changer holds up to 30 films in the receiving magazine. At the maximum rate of four films per second, a 7.5-second run is achieved without changing magazines.
A problem common to many film changer systems is that the film changer is mounted perpendicular to the image intensifier (Fig 2). Thus, there is a long delay between fluoroscopic viewing and recording of the images. In some systems, the film changer is mounted in front of the intensifier, limiting the size of the intensifier input field to something smaller than the film size when the changer is in position (4). Other problems associated with film changers are motion blurring, missed exposures, jamming, inadequate density and contrast, and film fogging (3).
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Indirect Recording
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Photofluorography
More rapid filming than that provided by film changers can be achieved by using a photospot (photofluorographic) camera. Photospot cameras can expose as many as 200 films before reloading at rates as high as 12 per second (3). The camera is mounted on the optical coupling system behind the image intensifier (Fig 3). Because direct viewing of the fluoroscopic image is almost always reserved for the television (TV) camera, the photo-spot camera is usually side mounted. The image of the output phosphor is collimated by a lens and reflected by a partially silvered mirror. An iris or diaphragm, placed in front of the camera lens, is used to adjust the amount of light reaching the camera for an optimal trade-off of image noise with patient dose. The camera may have additional mirrors to direct the image onto the film plane (Fig 3). Typically, 105-mm-wide film is used in roll film cameras. Some cameras hold 100-mm-square cut film instead.
When a photospot camera is used, the image is minified in comparison with a screen-film image. The amount of minification depends on the intensifier input diameter and the size of the image on the film. For example, if the image of a 23-cm-diameter intensifier input is exactly framed by a 100-mm-square film, the minification is 0.43 (ie, 100 mm/230 mm).
Because the light output of the image intensifier is much brighter than that of a fluorescent screen, a high-resolution, fine-grain film can be used in a photospot camera. The film therefore contributes little to noise or blurring in the image. The limiting resolution of photospot films is typically more than 200 lp/mm (5). When the minification is taken into account, the equivalent resolution at the input face of the image intensifier is equal to 200 times the ratio of the film size to the intensifier field size. Thus, for the 23-cm-diameter intensifier input of the previous example, the equivalent resolution would be 200 x 100 mm/230 mm or 87 lp/mm. This limitation is negligible compared with the typical 4–5 lp/mm resolution of the intensifier.
Therefore, we probably will not see the same detail with a photospot camera as is available with a 400-speed screen-film system. At a normal viewing distance, the eye blurs out this detail, but it also blurs out some noise in the image. As a result, lower doses can be used for photospot images than for spot film images. In theory, the noise contrast is inversely proportional to the square root of the radiation exposure. For example, if each dimension of a resolvable picture element is increased by 7/5 (ie, 5 lp/mm for a photospot image and 7 lp/mm for a screen-film image), the area of the element is doubled ([7/5]2) and dose could be reduced proportionately. In practice, the dose reduction achieved by using photospot films is typically 25%–50%. This dose reduction also means exposure times can be shorter, thereby reducing motion blurring.
There are several advantages to use of photospot films. The film is cheaper and needs less storage space than radiographic film. There is less delay between fluoroscopy and filming. Higher frame rates and longer runs are possible. It is possible to view the images on the TV monitor as they are being produced. Doses can be reduced. The disadvantages are poorer resolution, more handling by the technologist, and viewing a less than full-size image.
Digital Fluorography
Many radiology facilities are replacing conventional image recording systems with digital technology. It is likely that a radiologist practicing 20 years from now will not use any of the technology discussed earlier in this article. Digital charge-coupled device (CCD) TV cameras are rapidly replacing conventional TV cameras in fluoroscopic systems. An analog, high-resolution (1,023-line) TV camera has a vertical resolution of about 358 line pairs. A high-resolution CCD camera with a 1,024 x 1,024 matrix will provide equivalent vertical resolution. However, the digital camera will have the same vertical and horizontal resolution, whereas the horizontal resolution of the analog camera is defined by its electronic band-pass. For a 15-cm-diameter intensifier input, the limiting resolution of the CCD camera would be 358 lp/150 mm or 2.4 lp/mm. This result is about half the resolution of a photospot film. This resolution loss is made up for by the ability to digitally increase display contrast, reduce noise, and enhance the edges of digital images.
There are several other advantages to digital photospot images. Mechanical devices are not needed for film transport. Film processing is not required. Images can be viewed immediately. The linear response of the digital system makes it very forgiving of under- or overexposure.
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Recording Motion
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Cine Fluorography
Cine fluorography is the standard for imaging the movement of a contrast agent through vessels. The mechanism of a cine camera (Fig 4) differs somewhat from that of a roll film photospot camera. The major differences are that the film is smaller (35 mm wide) and longer (400 ft [120 m] on a roll) and that the camera is capable of operating at up to 90 images (ie, frames) per second. Several frame rates are usually available, typically 15, 30, 60, and 90 frames per second (3). The higher frame rates are usually used in cardiography, particularly for pediatric imaging. Exposure times on the order of 5 msec are needed to stop heart motion. At 90 frames per second, there is enough time for a 5-msec x-ray exposure and another 5 msec to advance the film. The x-ray source has to be turned on and off quickly and synchronized with the rotation of the camera shutter. Grid-controlled x-ray tubes are often used for this purpose.
The usable image on a 35-mm film is 18 x 24 mm. If the circular image of the intensifier is fully visible on the film (a condition called exact framing), the diameter of the image is only 18 mm. If a 23-cm-diameter intensifier input is used, the minification is 18/230 or 0.08 (roughly a reduction of 13 times). As with the photospot camera, we can calculate the effective resolution of the film at the face of the image intensifier. If the film resolves 200 lp/mm, the effective resolution at the intensifier input would be 200 x 18/230 or 16 lp/mm. This result is significantly better than the 5 lp/mm offered by the best intensifiers. Thus, the film has little effect on image quality.
Not too many years ago, film resolution was much poorer (about 60 lp/mm). The film blurring was significant; thus, there was much discussion of whether to increase the magnification optically to improve resolution. Such "overframing" leads to exposure of regions that are not imaged (Fig 5). Mean-diameter framing was considered an acceptable norm (6). Because of improved film resolution, loss of resolution due to minification is no longer a problem. Exact framing can be used to reduce patient dose except in pediatric cardiology (7). However, some manufacturers still overframe the image by as much as 15%.
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Figure 5. Framing of the circular intensifier image on the rectangular film format used in cine fluorography. In exact framing (A), the intensifier image exactly fits within the smallest dimension of the film. In mean-diameter framing (B), the diameter of the intensifier image is the mean of the rectangular film dimensions. Total overframing (C) fills the 18 x 24-mm rectangle with a portion of the intensifier field. Overframing increases image size on the film and causes radiation exposure of areas not visualized on the film. For modern, high-resolution films in the 35-mm format, exact framing provides adequate resolution without unnecessary patient exposure.
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When compared with the resolution of cine fluorography, the resolution of the fluoroscopic image on TV is quite poor. For a 23-cm-diameter intensifier input field, conventional TV resolution is typically around 0.9 lp/mm compared with 5 lp/mm for cine fluorography (8). Another way to look at this difference is that the smallest resolvable object or pixel in a cine image has a dimension 1/5 of that in a TV image (0.9 lp/mm ÷ 5 lp/mm = 0.18) or an area 1/25 of that in a TV image. Although this resolution allows visualization of small vessels, the number of photons in a cine pixel is less than 1/25 of that in a TV pixel. Thus, it is necessary to increase the exposure proportionately to minimize the quantum noise. The radiation exposure required at the input of the image intensifier is about 30 µR (7.7 x 10-9 C/kg) per second for TV viewing. For cine fluorography, it is about 20 µR (5.2 x 10-9 C/kg) per frame (6). At 30 frames per second, this is a dose increase of 20 times! In addition to the effect on patient dose, this high exposure rate requires x-ray generators with high power ratings and tubes with high heat capacities, making these systems very expensive.
Videotape Recording
Most of us are familiar with videotape recording of TV, even if we can't figure out how to program the recorder to capture our favorite show. Videotape recorders used in fluoroscopy are only slightly more sophisticated than those used for home video. They use Super-VHS (or S-video) technology, which is now also available on better home TV systems. Super-VHS recorders sold for medical imaging have an electronic band-pass matched to, or better than, that of the TV so that there is essentially no loss of resolution due to the recorder for conventional 525-line TV. (For a discussion of the effect of band-pass on horizontal TV resolution, see the previous article in this series [8].)
As a replacement for cine fluorography, 1,023-line TV (or 1,024 x 1,024 digital CCD TV) cameras are used. These provide a resolution of almost 2.5 lp/mm when used with a 15-cm-diameter intensifier input field. This resolution is not as sharp as that of cine fluorography but does not require nearly as much radiation to achieve the same quantum noise level. Currently available video recorders have a band-pass of only about 10 MHz and are the weak link in the imaging chain when it comes to horizontal resolution.
High-resolution TV recording is a compromise offering about half the resolution of cine fluorography at about four times the dose of conventional TV. One advantage of using a CCD camera for this purpose is that the images are recorded digitally, rather than on tape. The digital recording process does not degrade the image quality of high-resolution TV images, as tape recording does. The resolution is the same as that mentioned earlier for digital still images, but because the eye integrates the information over several TV frames, noise is reduced. Therefore, the dose can be several times lower for digital recording of motion than for digital photospot imaging with the same resolution.
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Automatic Exposure Control
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Automatic control of the radiation exposure to the image detector is needed for both fluoroscopy and image recording in most cases. The exception to this requirement is the automatic film changer, for which exposure techniques are usually preprogrammed for the body part being examined.
The feedback signal used for automatic exposure control of spot films is obtained from an ionization chamber located between the grid and the film cassette (Fig 6). For cameras that view the image intensifier, the signal can come from an optical detector, typically a photodiode, placed in the light field of the intensifier output phosphor behind the collimating lens. The collimating lens is placed at a distance equal to its focal length from the intensifier output, thus creating a beam of parallel light. A photodetector sensing any portion of this beam receives light from the entire surface of the output phosphor. Therefore, the sensor can be placed near the edge of the light field in a portion of the light field that will be cut off by the camera irises. The same detector can be used for fluoroscopy and fluorography. A lens may be used to refocus the image of the output phosphor onto the photodetector in such a way that it senses light from only about the central 60% of the image, ignoring the relatively less observed areas around the edge. Another method of exposure control that can be used with TV cameras is to feed the TV video output signal back to the x-ray generator.
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Figure 6. Feedback system for automatic exposure control of fluoroscopy and fluorography. The signal from an ion chamber is used for spot film exposure control. The signal from the photodetector is used for generator control in both fluorography and fluoroscopy. The TV video signal is also used to maintain image brightness with or without changes in generator output.
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Whether the signal comes from an ion chamber or photodetector, it is used in the same way. For fluorography or radiography, charge from the sensor is accumulated on a capacitor until a predetermined reference voltage is reached, at which time the exposure is terminated. For a spot film system, an acceptable exposure, and hence the reference voltage, is based on the optical density desired for the film and therefore on the speed of the screen-film system. For a photospot or cine camera, an acceptable exposure is based on the desired level of quantum noise in the image and an iris is used to adjust the light intensity reaching the camera to achieve the desired film density. This type of exposure control system can be used to limit exposure times at a fixed tube kilovoltage and current (milliamperes) or to limit milliampere-seconds at a fixed kilovolt peak in "falling load" systems, in which the milliamperage varies during the exposure.
The same sensor used for control of fluorographic exposures is used to regulate the x-ray output during fluoroscopy. In this case, the signal from the sensor is not accumulated but rather compared continually against a reference signal that corresponds to the desired intensifier input exposure rate, which in turn corresponds to an acceptable level of quantum noise in the TV image. Such systems are usually referred to as automatic brightness control or automatic brightness stabilization systems (9). The x-ray exposure rate can be adjusted by varying generator kilovolt peak, milliamperage, or x-ray pulse width. X-ray absorbing filters can also be selected as part of the exposure control system. The Table shows some possible methods used for automatically controlling brightness during fluoroscopy.
The last two combinations listed in the Table are generally found on fluoroscopic systems designed for vascular applications. The most common configuration for gastrointestinal fluoroscopy uses automatic control of both kilovolt peak and milliamperage. Because it is possible to achieve the same brightness with many combinations of kilovolt peak and milliamperage, modern automatic brightness stabilization systems are programmed to follow a particular curve describing the combinations of kilovolt peak and milliamperage to be used (Fig 7). The shape of this curve has a dramatic effect on both patient dose and image quality. For example, the curve may describe combinations of kilovolt peak and milliamperage that tend to cause higher kilovolt peaks to be used when possible to increase radiation output to penetrate thicker body parts (Fig 7 [curve A]). The milliamperage is increased significantly only after the maximum kilovolt peak has been reached. This curve ensures that the most penetrating radiation is used, hence reducing patient dose. Or, the curve may use higher milliamperage and lower kilovolt peak values to achieve the same brightness level (Fig 7 [curve B]). By keeping the kilovolt peak low, subject contrast is increased at the expense of patient dose. Finally, the curve may offer a compromise between the preceding two situations (Fig 7 [curve C]). Selection of a low, medium, or high dose rate by pressing a button on the intensifier control panel selects a program, similar to those in Figure 7, for the automatic brightness stabilization system to use. In some systems, these same buttons also select filters to be inserted in or removed from the x-ray beam, a process that has an effect similar to changing the kilovolt peak. Good fluoroscopic practice requires the fluoroscopist to start with the low-dose mode and increase the dose only if adequate contrast cannot be achieved.
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Figure 7. Power curves for automatic brightness control system. With curve A, kilovolt peak (kVp) increases first in response to demand for more power, providing maximum penetration of x rays and low radiation dose. With curve B, milliamperage (mA) and kilovolt peak increase simultaneously, providing more contrast but higher radiation dose. Curve C provides a compromise. Curve D is the maximum power loading possible for the system.
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Another curve in this configuration represents the maximum power at which the generator and x-ray tube can be operated (Fig 7 [curve D]). The maximum x-ray output is reached at the point where this power curve reaches the maximum kilovolt peak. At this point, if more tissue is placed in the x-ray beam, the number of x-ray photons reaching the intensifier will go down and the image will appear noisier. However, the brightness of the image will be maintained by another system in the TV camera itself. The camera will simply amplify its signal electronically to maintain the signal intensity required for adequate brightness on the TV monitor. This process is called automatic gain control. The automatic gain control system does not alter the radiation dose rate in the way that automatic brightness control does but only maintains video brightness.
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Conclusions
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A variety of equipment exists for recording fluoroscopic images, including spot film devices, automatic film changers, photospot cameras, CCD cameras, cine fluorographic cameras, and videotape recorders. For still imaging, the best resolution can be obtained with screen-film cassettes by using a spot film device. More rapid acquisition of images can be achieved with rapid film changers at the expense of image sharpness. Photospot cameras allow even more rapid multiple-exposure sequences at lower radiation doses than spot film devices but at the cost of reduced image size. Digital CCD cameras offer a compromise between radiation dose and image quality, with the added advantages of digital image manipulation and storage. For recording of motion, cine fluorography provides the highest-resolution images at very high dose rates. Videotape recording of high-resolution TV images offers a compromise of reduced resolution compared with that of cine fluorography but with lower radiation dose.
Recording systems in fluoroscopy require automatic exposure control for optimum image quality. The same feedback system used to control fluorographic exposures can be used to control exposure rates during fluoroscopy as well. Automatic brightness control maintains intensifier exposure rates on the basis of subject thickness by adjusting various technique factors. The type of control mechanism depends on the imaging task and the complexity (age and cost) of the equipment. The operator can choose between better image quality (higher contrast) or lower radiation dose.
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Footnotes
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Abbreviations: CCD = charge-coupled device,
TV = television
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References
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Abstract
Introduction
