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There are a number of techniques that can be used to record the selective passage of x-rays through an animal's body. The major portion of this manual deals with the technique of radiography whereby the x-ray photon that passes through the animal's body stimulates crystals on x-ray film. Following correct processing of this film, a radiograph is available for evaluation. It is obvious that this technique will not permit evaluation of motion of the animal's body or the passage of a bolus of contrast medium .
The first part of this section on special radiographic equipment deals with methods that can be used to study motion. The last part of this section deals with additional types of equipment for the technique of:
Fluoroscopy is a radiographic technique whereby the selective passage of the primary x-ray beam through the patient presents a diagnostic image on a fluoroscopic screen in a darkened room viewed with the naked eye. Fluoroscopy is used primarily to:
The physical description of the fluoroscopic unit is that of a fluoroscopic screen suspended over the examination table with a fixed attachment to an x-ray tube under the table. The screen is coated with cadmium zinc sulfide crystals which emit green light. The eye is most sensitive to light of this wavelength, and perception is better with green light than with blue light which is given off from conventional intensifying screens. The equipment is installed in such a manner that the x-ray tube and fluoroscopic screen are coupled so that the primary x-ray beam is always directed at the fluoroscopic screen and the leaded glass (1.5 mm Pb equivalent) positioned over the screen. Lead shutters are used to keep the field of exposure as small as possible. A lead drape should be positioned between the patient and operator. The drape should be at least 0.25 mm of lead equivalent. A slot in the table for a bucky tray should also be shielded by a lead drape. A spot-film device may be included that permits the instantaneous recording of the fluoroscopic image on a conventional radiograph. The spot-film device provides for the quick positioning of a cassette in the primary beam and switches the generator to a different combination of factors (kVp, mA, and time) for radiography. The cassette can usually be divided and a predetermined number of exposures made on a single film (Fig. 20-1).
Eye accommodation is necessary prior to fluoroscopy since the examination is performed in a darkened room. A stationary grid may be slid into position if the thickness of the animal requires its use. A manually reset cumulative timing device is required that will either indicate elapsed time by an audible signal or by turning off the apparatus.
The major disadvantage of fluoroscopy is the radiation exposure received by the patient and those in the room during the examination. Even when using low mA (3-5) and high kVp (85), the exposure at table top in the primary beam may reach 10 r/minute. Scatter radiation at right angles to the primary beam may reach 10-20 mr/minute at 25 to 75 cm (10 to 30 inches) distance. Therefore, the use of short intermittent exposures and good eye accommodation is important. A suspect lesion should only be identified under fluoroscopy and the spot-film device used to obtain a radiograph that can be evaluated at length following the fluoroscopic examination. This will obviously decrease the time of fluoroscopy.
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A second disadvantage of fluoroscopy is that the shades of darkness on the screen are reversed from those seen when viewing a radiograph on a view box.
Fluoroscopic units have been phased out of human radiology and replaced by image intensifying units. This means that used fluoroscopic units may be available for purchase. They should be used in veterinary radiography only when the hazard from radiation exposure is well understood and the resulting personnel exposure is carefully monitored.
An image intensifier is an electronic unit that takes the primary beam after it has selectively passed through the patient's body and permits it to fall on the input phosphor (layer of fluorescent material) of the image intensifier tube. The photoelectric surface opposite the fluorescent layer gives off electrons in direct proportion to the intensity of the fluorescence. The electrons are accelerated and focused onto the output phosphor (second fluorescent layer) where the energy is converted to visible light. This light may be viewed through a suitable optical system or photographed in several ways.
The uses of an image intensifier are the same as for a basic fluoroscopic unit with intensifying screen. In addition, there is the possibility to record motion of an organ (cinefluorography). The potential in Veterinary Medicine is great but the cost of the equipment is high.
The size of the field viewed varies, i.e., 5"-6", 9", 12" with a capability of dual sizes&endash;9"/5". Different channel attachments are available for handing the image from the output phosphor.
Image intensified fluoroscopy can be observed in two ways. One is through a system of optical lenses and mirrors and is called mirror optics system. The other way is through a television monitoring system. In the mirror optics system the image is viewed directly by looking at the fluorescent layer on the output phosphor through a system of lenses and mirrors that magnify the image. The field of view is small and allows only one or two persons to observe the image at a time. The television monitoring system is more expensive than the mirror optics system. The output phosphor is coupled directly to a television pickup tube called a vidicon, plumbicon or image orthicon. The television pickup tube converts the light image pattern into an electric signal and conveys this signal to the television monitor. An advantage of this system is that the brightness level and contrast can be electronically controlled and it is easy for more than one person to view the image at the same time. There are several modes of operation that can be attached to the image intensifier tube. We have discussed direct optic viewing and the use of a TV pickup tube connected to a TV monitor. In addition it is possible through coupling optics to direct the image to cine camera (cinefluorography) which may be 16 or 35 mm size. Another method to record is a spot film camera which may be a 70 or 105 mm size, while another possibility is to direct the image to a video recorder. This may be a disk or tape mode (Fig. 20-2).
70 mm or 105 mm spot camera. The image from the output phosphor of the image intensifier is transferred to film in a camera and exposures made individually or in rapid sequence. This is useful in procedure where motion is not too fast and sequential studies are helpful. This spot camera is usually incorporated in addition to the optical viewer or TV chain and serves as the method of permanent record. It can be used to replace the spot-film device in many installations.
16 or 35 mm cine camera. The image is transformed from the output phosphor to a rapid speed camera that films at speeds of 71/2, 15, 30, and 60 frames per second. This is most useful in cardiac angiography. Cine cameras are usually installed in addition to a TV chain and serve as the technique for permanent record.
Video recorder. The image from the output phosphor can be transferred to a video recorder for storage and replay. The advantage is immediate replay to confirm a successful injection of contrast media. Some radiologists feel that it has sufficient quality of picture to compete with a cine camera for diagnostic studies. Two modes are currently available, disk and tape.
The use of the image intensifier has overcome two big problems originally associated with fluoroscopic examinations. The image intensifier is used with a much lower mA setting with resulting decrease in radiation exposure to both patient and technician. Since the image can be viewed optically on TV, the examination is performed in a lighted room. This is especially desirable when one considers the problems of immobilizing a large patient in a darkened room.
The only disadvantage of the image intensifier is the high cost of purchase and maintenance. Used units are available but the cost of maintenance and installation should be examined prior to purchase.
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Another technique of obtaining studies that reproduce motion is to make multiple serial exposures on large conventional size x-ray film. The technique produces full size radiographs of excellent quality.
The decision between rapid serial radiography and Cine fluorography is based on the need for film quality as compared with recording motion. The Cine fluorography provides recording of motion but with some loss of image quality. The rapid serial radiography provides high radio graphic quality but sacrifices the study of motion made available with a larger number of exposures/second.
Two basic types of equipment are available, one that moves a loaded cassette and the other that moves only radiographic film in a cut or roll format. Sizes are usually 14" x 14" or 10" x 10". Number of exposures varies from a maximum of 20. The sequence of exposures may be constant for all exposures or there may be the possibility of a delay and more than one filming sequence, i.e., delay 3 seconds, then 2 exps/sec for 3 seconds, then 4 exps/sec for 2 seconds, then I exp/sec for 3 seconds. Units can be arranged in a biplane sequence.
Disadvantages of rapid serial radiographs are the high cost of purchase plus the high cost of film. It is possible to expose 40 films per examination if two projections are recorded. However, there are old Sanchez-Perez changers that are available. These have a maximum speed of 2 exposures per second. Another disadvantage is the high level of radiation exposure during rapid serial radiography. This is a conventional radiographic technique that uses intensifying screens and conventional diagnostic film. Another problem may be the heat level generated during the multiple exposures. The tube heating chart must be examined to insure that the level of anode heat can be tolerated.
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It is possible to manufacture a tunnel device that enables the quick shifting of cassettes so that exposures can be made at a rate of one per second. A "pusher" is used to move the cassettes. This type of changer can be of value in non-selective cardiac angiography (Fig. 20-3).
In conventional radiography, the images of all the levels in the subject are superimposed on one another on the film. Tomography is a general term for a technique that provides a distinct image of any selected plane through the body while the images of structure lie above and below that plane are blurred. Synonyms include body section radiography, laminagraphy, and planography. The procedure is accomplished basically by moving the x-ray tube and cassette in opposite directions during the exposure. It is also possible to accomplish the same thing by moving the body while leaving the tube and film fixed in position.
The simplest method of tomography can be accomplished on a conventional table and tube stand. More specialized units perform only tomography and are very expensive.
Tomography is used in human radiology, especially in diagnoses of disease in the head. Reports in Veterinary Medicine describe its use in diagnosis of disc disease in dogs or study of skeletal disease in dogs. This type of examination in man has almost completely been replaced by computerized serial tomography units (Geary, 1967; Gibbs, 1973).
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Magnification is a technique that could be used to great at advantage in veterinary radiology and at not great additional cost. The technique is based on the theory that an increase in object-film distance results in an increase in the size of the image on the film. When a large focal spot is used, the increased object-film distance results in an increase in the image size but such a blurring of the image that little is gained in diagnostic quality. If, however, an x-ray tube is used with a small focal spot size (< 0.3 mm), the blurring is not sufficient to be objectionable and the magnification results in a more diagnostic film. This technique has value in diagnosis of skeletal lesions and has been used in magnifying vessels following their filling with a contrast agent.
Magnification studies are not commonly used in veterinary radiology because of the increased cost of an x-ray tube with a small focal spot size. This is unfortunate because the technique is a simple one and requires no other additional equipment or training.
Subtraction technique makes important radiographic information easier to see on the radiograph because it subtracts less important information. Minimal pathologic changes which are almost impossible to identify on a conventional radiograph become strikingly clear after use of the subtraction procedure. Small vessels filled with contrast material following injection during angiography may be difficult to visualize because of overlying bony structures. Subtraction removes all bony images leaving only the clinically important small vessels.
Subtraction studies are the sum of the differences between a radiograph made prior to injection and one following injection. The first step in a subtraction procedure involves making a copy (mask) of the preliminary radiograph with the radiographic densities reversed. Light areas on the preliminary radiograph become dark and dark areas become light. Following injection of the contrast agent, a second radiograph is made with the patient in the same position. When the reversal film of the preliminary radiograph is placed over the angiogram, the overlying bony structures are erased. The mask and the angiogram can be viewed together, or an additional combined print made that shows only the injected vessels.
Special subtraction film must be used to produce the mask. A special film printer has been produced to provide good contact between film during production of the mask or the final print. This same printer can be used with a different light source to make duplicate radiographs.
Photofluorography is a combination of radiography and photography in which the image on a fluorescent screen is photographed on a small film. Screen and film are separated and the image is focused on the film by a camera lens. Miniature radiographs (35, 70, or 100 mm) are viewed as a radiograph and a permanent record is possible.
Photofluorography has in the past provided an economical means for mass chest radiography and has been used extensively for tuberculosis surveys.
The equipment consists of a photoradiographic screen with blue fluorescence, or green fluorescence, and is contained within a hood. When sufficient radiation reaches the screen, an exposure is automatically recorded on miniature radiographic film (35, 70, or 100 mm). Mirror optics systems have been developed that permit the recording of the exposure on the radiographic film with smaller exposure to the patient.
The exposure to patient is considerably higher than if a routine diagnostic radiograph is made. There is probably no use for this technique in veterinary radiology today.
