Radiology Equipment and Positioning Techniques

CHAPTER 1 Radiology Equipment and Positioning Techniques

ANATOMIC REFERENCES

Anatomic drawings demonstrating the spatial relationship of the internal organs are provided in Chapter 2. They should be used as general reference material. Individual organs are not always clearly visualized on all radiographs. There are species variations in the size, shape, and location of internal organs. The radiographic appearance of the viscera is also affected by the birds’ reproductive status and digestive tract contents. In the case of this text, the reproductive organs were labeled as “gonads” (versus ovary or testes) if no specific anatomic structure (i.e., the syrinx in male duck) could be identified on the radiograph to indicate the bird’s sex.

Images in this text were anatomically labeled to coincide with illustrations from several textbooks, including Atlas of Avian Anatomy Osteology-Arthrology-Myology (Chamberlain FW, East Lansing, Mich, 1943, Michigan State College Agricultural Experiment Station), Atlas of Radiographic Anatomy and Diagnosis of Cage Birds (Krautwald ME et al., Berlin and Hamburg, 1992, Paul Parey Scientific Publishers), A Color Atlas of Avian Anatomy (McLelland J, Philadelphia, 1991, Saunders), Anatomy of the Domestic Birds (Nickel R et al., 1977, Berlin and Hamburg, Verlag Paul Parey), and Atlas of Avian Radiographic Anatomy (Smith S, Smith B, 1992, Philadelphia, Saunders).

EQUIPMENT FOR RADIOGRAPHIC STUDIES

RADIOGRAPHIC UNITS

It is sometimes erroneously presumed that the relatively small body size of birds allows for the use of low-capacity radiology equipment. However, in order to make high-quality radiographic images of birds, the x-ray generator should be capable of producing at least 300 milliamps (mA), the exposure time capability should be .017 (1/60) second or shorter, the kilovolt peak (kVp) settings should have a range of 40–90 kVp, and kVp settings should be adjustable in 2 kVp increments. High-frequency x-ray generators are recommended because they produce uniform x-ray output. Radiographic exposure factors are more critical in birds than in mammals. Small variations in x-ray output are very noticeable on avian radiographs, especially those made with lower kVp techniques. Small variations in x-ray tube output are more obvious on avian radiographs than on radiographs of dogs and cats; therefore the x-ray generator must be in excellent condition. Older generators are not recommended because they may have mechanical exposure timers that are less accurate than electronic timers and the x-ray output may vary between exposures with identical settings even though the exposure settings are similar.

Over time, use of x-ray tubes causes pitting of the anode or filament damage, resulting in degradation of x-ray output. Radiographs made with damaged tubes may be diagnostic, but the variation in image quality among studies can affect interpretation, especially in sequential examinations monitoring the response to therapy or the progression of disease (e.g., pneumonia). Lung and air sac image opacities are very susceptible to variations in radiographic techniques. If variables in radiographic techniques are not identified, increased pulmonary or air sac opacity caused by underexposure can be misdiagnosed as pulmonary consolidation or air sac membrane thickening. Overexposure, if unrecognized, may result in a misdiagnosis of a positive response to treatment.

Short exposure times (i.e., .017 [1/60] second or shorter) are essential to minimize motion artifact associated with the rapid respiratory rate and generalized muscular tremors that are common in birds. Short exposure times dictate high mA settings to generate sufficient x-ray output (i.e., milliampseconds [mAs]).

Low kVp techniques (40–60 kVp) are preferred for most film screen systems because they produce a long scale of contrast compared with higher kVp techniques. The ability to make small kVp adjustments is essential to optimize image detail. Digital radiology systems usually use higher kVp techniques than film screen systems, but the ability to adjust the kVp in 2 kVp increments remains desirable.

The generation of the x-ray beam is initiated by the production of electrons in the x-ray tube filament. Diagnostic radiology tubes (except for dental units) have two filaments (focal spots). Smaller focal spot tubes generally produce superior image detail compared with larger focal spot tubes. Focal spot size and tube x-ray output are inversely related. In order to obtain enhanced detail, the smaller focal spot should be selected. Selection of the smaller focal spot may require manual override of preprogrammed settings. For larger birds, higher mAs techniques may not be possible with the small focal spot.

Tabletop (nongrid) techniques are standard for avian radiography. Grids are typically employed to minimize the deleterious effects of scatter radiation generated by larger patients that have a body thickness greater than 10 cm. Although some birds’ bodies are greater than 10 cm, grids are typically not necessary because the air within the air sacs does not generate significant scatter radiation.

The x-ray tube stand should allow for adjustments of the focal film distance (FFD). Adjustment of the FFD is used to make small variations in the effective mAs (x-ray exposure reaching the film). The mAs reaching the film is inversely proportional to the square of the distance from the x-ray source (focal spot). To compensate for the lower mA output of the small focal spot, while maintaining the short exposure time, avian radiographic techniques frequently utilize shorter FFDs than typically used for canine and feline radiography. The alteration of the FFD can also be used to make small adjustments to the effective mAs that cannot otherwise be made. Excessive reduction of the FFD distorts the image; therefore FFDs of less than 30 inches (76 cm) are discouraged.

Many types of equipment have been used for making radiographic images of birds. If patient motion is not a factor (i.e., the patient is anesthetized, thus minimizing the respiratory and muscular motion) and the bird is small (less than 20 gm body weight), dental units may be utilized. However, dental radiography units are generally not well-suited for avian radiography because of their relatively large focal spot size, low mA capacity, lack of adjustable collimation, and inability to make short exposure times. For most birds dental radiography film is too small for imaging the entire head or coelom, and the range of contrast for dental film is much lower than for general radiology film. Nevertheless, some detailed images can be made with dental radiology units and film, especially for examinations of the distal extremities. Portable x-ray radiographic units designed for large animal extremity work are not optimal for the radiographic examination of birds because these units are incapable of producing sufficient mA at the required short exposure times without severely reducing the FFD.

Standard diagnostic radiology equipment in good condition designed for small animal (feline and canine) practice should be utilized for making radiographic images of birds. An Innovet Select X-ray System (Summit Industries, Inc., Chicago) was used to produce the radiographic images in this book.

RADIOGRAPHIC FILM-INTENSIFYING SCREENS

Selection of radiographic film and intensifying screens is based on the speed of the system (i.e., mAs required to produce a high-quality diagnostic image). Film-screen speed and detail are inversely related. Faster systems are usually capable of producing less detail than slower systems. Asymetrix Detail Intensifying Screens (3M Animal Care Products, 3M Center, St. Paul, Minn.) and Ultra Detail Plus or SE + radiographic film (3M Animal Care Products, 3M Center, St. Paul, Minn.) were used to produce the radiographic images in this book. This film-screen combination produced a radiographic system speed of 100 to 350. Table 1-1 summarizes radiographic exposure factors used for creating this text’s radiographic images using a tabletop technique. These settings are intended to be guidelines and may require modification depending on the x-ray generator, film-screen combination, radiographic film processing, and patient size. Other film-screen combinations of similar speed and resolution can also be used if they are of sufficient detail and speed.

Table 1-1 Radiographic exposure guidelines for several species of birds.

The same technique is usually used for the laterolateral and ventrodorsal coelomic radiographic studies. For radiographic studies of the distal extremities (foot and distal portion of the wing), the kVps used for coelomic radiographic studies are usually reduced by 2 to 3 kVp to prevent overexposure of the distal extremities.

DIGITAL RADIOLOGY SYSTEMS

Digital radiographic image capture (e.g., direct digital, computed radiography) is slowly replacing film-screen systems in veterinary medicine and will eventually predominate. Nonscreen film and high-detail film-screen systems produce images with superior detail compared with digital systems; however, digital systems are capable of producing a higher image contrast range than is possible with film. This results in improved image quality. Other advantages of digital radiography include images that can be electronically manipulated, do not require film processing, and are immediately viewable. In addition, digital systems result in fewer repeat exposures caused by incorrect exposure factors and film processing errors. Digital systems often use higher kVp and mAs techniques (10%-15% higher kVp and mAs) than film-screen systems. Special algorithms are required for avian patients, but they are increasingly available from the manufacturers that produce the digital radiology systems.

THE RADIOGRAPHIC EXAMINATION

PATIENT PREPARATION

Birds utilized in this text were healthy and were fasted before radiographic examinations. Birds weighing less than 100 grams were fasted for 2 hours, and larger birds were fasted for 3 to 5 hours before the radiographic procedure. Avian patients, especially those that are debilitated, are more easily compromised by food deprivation than are mammals. The decision to withhold food in a clinical situation is therefore complex. Even more important than restricting free access to food is the recommendation not to administer nutrients by gavage for 4 hours before the radiographic study, especially in debilitated birds in which crop and proventricular emptying times may be prolonged. The stress associated with manual restraint or anesthesia required for the radiographic examination increases the potential for regurgitation and airway aspiration of the digestive tract contents.

Digestive tract filling affects the radiographic appearance of internal organs. An example of this is included in the section on the Moluccan cockatoo. Postprandial digestive tract distention can displace the liver cranially. A distended proventriculus often obscures visualization of the spleen. Some of the misdiagnoses associated with failure to recognize the effects of a recent feeding include hepatomegaly and cardiomegaly. Mass lesions, free coelomic fluid, or enlargement of internal organs can also be obscured by ingesta. This is caused by the added opacity of the ingesta, which alters the appearance of the coelomic organs. When two organs of similar opacity are in direct contact, their individual outlines can merge together. The physical characteristics of the ingesta can also variably affect the appearance of the digestive tract. High-fiber (e.g., beans), high-fluid–content (e.g., fruit), and some pelleted diets can produce dramatic digestive tract distention, simulating pathologic conditions. Pelleted diets can sometimes cause the interface between the contrast medium and digestive tract mucosa to be indistinct in digestive tract contrast studies. This can simulate the radiographic appearance of enteritis or excessive intestinal mucus.

Rapid respiratory movements and muscle fasciculations (fine motor movements) are common in birds and can degrade the radiographic detail. The muscle fasciculations can be associated with hypothermia, stress, or a light plane of anesthesia. All of these factors should be addressed before proceeding with the radiographic study.

TIMING THE RADIOGRAPHIC EXPOSURE

The avian respiratory rate is more rapid than that of most mammals, the lungs are nonexpansile, and during both inspiration and expiration air is continuously moving into the pulmonary parenchyma and the air sacs. In general, the effect of the respiratory cycle on the radiographic appearance of the pulmonary parenchyma is less in birds than in mammals because avian lungs are nonexpansile. In some cases, distention of the abdominal air sacs is preferred because the increased air sac distention can improve the visibility of the viscera, especially in birds with large amounts of coelomic fat. Timing the radiographic exposure to coincide with air sac inflation is difficult in nonanesthetized birds as a result of the rapid respiratory rate and inability to directly visualize the respiratory movements through the pelage. If timing the radiographic exposure to coincide with inspiration is not possible, the exposure should coincide with a pause in the respiratory cycle. In the case of intubated anesthetized birds, air sac inflation can be achieved by applying positive pressure to the anesthetic circuit reservoir bag. Positive pressure ventilation of 4 to 6 cm of water is recommended, with 8 to 10 cm of water being the maximum amount for most avian species.

ANESTHESIA

Radiographic studies for which the birds are anesthetized with inhalation gas anesthetics are generally completed in less time and are of higher quality than studies in which the birds are not anesthetized. Anesthetized birds are easily positioned with less physical restraint, and the potential for iatrogenic fractures is minimized. It is also possible to inflate the air sacs with positive pressure ventilation in the intubated bird. Motion artifacts are also reduced with anesthesia. All birds in this text were healthy, and the majority of the studies were performed using inhalation anesthesia or chemical sedation. Birds seen in clinical practice may be severely debilitated, and general anesthesia may be contraindicated; however, it is recommended whenever the anesthetic procedure is deemed safe.

POSITIONING DEVICES

For production of the images for this text, smaller birds (i.e., less than 100 grams body weight) were positioned directly on the radiographic cassette and secured with masking tape, but an acrylic positioning device (Bird Board, 8205 Alba Ct., Citrus Heights, Calif.) was used to facilitate positioning of larger birds. Many avian positioning devices are commercially available. If positioning devices are interposed between the bird and the film or digital sensor, a small increase in kVp (2–4 kVp) may be required to compensate for x-ray beam filtration caused by the device. The need for exposure compensation is especially necessary with low kVp techniques (40–50 kVp) and thicker positioning devices. Positioning devices placed on the x-ray cassette or used with a digital system increase the object film or sensor distance. The increased object film or sensor distance may decrease image detail, but the magnitude of loss is usually minimal. A modified version of the Bird Board is available that allows for direct contact of the bird and the radiographic cassette (Figure 1-1).

Figure 1-1 Figure positioning equipment.

A modified version of the traditional “Bird Board” is commercially available and allows for direct contact of the bird and the radiographic cassette.

PATIENT POSITIONING

Orthogonal projection radiographs (i.e., two projections made at 90 degrees to each other) are indicated for all radiographic studies unless the patient’s condition is compromised and the stress involved with restraint and/or anesthesia is deemed too great to obtain both projections.

The standard avian radiographic study includes laterolateral and ventrodorsal studies of the coelom. For radiographic images of the coelom right lateral and ventrodorsal projections were standard in this text. Pectoral extremity (wing) studies include the mediolateral and caudocranial projections. Given the natural curvature of the skeletal structures of the wing, the laterolateral and ventrodorsal projections of the coelom result in similar-appearing images (mediolateral and lateromedial) of the wing, thus the necessity to make the caudocranial projection of the wing. Manual positioning is required for the caudocranial projection of the wing. The whole body techniques do result in orthogonal projections of the pelvic extremities and therefore do not require additional positioning techniques.

POSITIONING TECHNIQUES FOR LATEROLATERAL AND VENTRODORSAL RADIOGRAPHIC STUDIES OF THE AVIAN HEAD

Radiographic studies of the head include laterolateral and ventrodorsal radiographic projections and, when necessary, oblique views. Small wedges of radiolucent foam may be of assistance for precise positioning of the head. For laterolateral and oblique projections, the patient is placed in a lateral recumbent position. Oblique radiographic projections require rotation of 15 to 30 degrees or less off the straight lateral projection. Oblique projections are described by the point of entrance of the x-ray beam to the point of exit. For ventrodorsal projections, the patient is positioned in dorsal recumbency and tape is applied to the ventral aspect of the rhinotheca so that the maxilla is closer to the cassette. This positioning of the head changes the orientation from a rostrocaudal to a ventrodorsal projection (Figures 1-2 to 1-3).

Figure 1-2

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May 27, 2016 | Posted by admin in ANIMAL RADIOLOGY | Comments Off

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