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Positioning devices are one of the most important accessory equipment items used in veterinary radiography. Their use is much more important in radiography of animals than in radiography of man because of the obvious difficulties of maintaining position of the animal throughout the examination. The purposes of these devices are:
An example of positioning a body part is the use of a sponge wedge to oblique the skull and mandible for an oblique projection of the mandible. The positioning device must be of such a nature that it can be utilized in a similar manner on successive cases so that the protocol can be easily repeated. These positioning devices must be stored conveniently near the examination table so they can be quickly placed in use and stored again following the examination. The device must be of a material that is easily cleaned since they commonly become soiled by feces or urine or by contrast media. The set-up time must be short if the technician is to be expected to use the device on a routine basis. The device must be small enough and light enough in weight to make usage easy.
Those devices insuring absence of motion of the patient include those used to tie-down the patient or extend a limb. The major requirement is convenience of use. The technique must be easy for the technician to use. While not exactly a positioning device, those methods used to decrease part thickness often assist in patient immobilization and are therefore included in this section. While a major purpose in the use of positioning devices is obtaining a diagnostic radiograph, a more important consideration is the reduction of exposure to the technician. This must be continually considered as new devices are considered for use.
Anesthesia or use of a tranquilizer should be considered as types of positioning devices in both large and small animals. The use of appropriate drugs plus other positioning devices can ensure that the technician is out of the examination room during the examination.
Sandbags are of great value in positioning of small animals. They have adequate weight to ensure stable positioning as well as immobilizing many patients. The sandbags should be covered with a material that is easily cleaned. Several different sizes can be utilized.
These different shaped positioners are available in many sizes and should be easily cleaned or even sterilized, lightweight, and be available in the size and shape needed. These devices obviously have an advantage over sandbags in that they are radiolucent and therefore can be positioned within the primary beam. It may be possible to use the devices in a plastic bag or covered with a plastic wrap in an effort to avoid them being soiled and requiring continual washing (Fig. 19-1).
A type of sponge is available that is filled with thousands of tiny, plastic beads. The pad is molded around the patient and the air exhausted from the pad using a regular wall suction or an aspirator pump . With evacuation of the air, the pad forces the beads together so they cannot move and the pad becomes a solid immovable mass. The pads are radiolucent so that exposures can be made through the pad. It should be appreciated that use of this type of device with unaesthetized patients may be questionable since a tooth or claw can make a hole that destroys the vacuum. After completion of the study, the valve is opened and air admitted which returns the pad to its previous form (Fig. 19-2).
A rope is one of the easiest positioning devices to use and yet it is frequently ignored. It can immobilize the patient or assist in achieving desired positioning of a part. It is easiest if the rope can be fastened to a cleat on the table edge or other convenient method of attachment. Ropes play a slightly different role in large animal radiography where they are often used in positioning and immobilization. Velcro straps have provided an excellent alternative to a rope.
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Gauze is of greatest value in assisting in positioning of the skull and mandible of the dog or cat. By permitting the canine teeth to bite through the gauze, it is relatively easy to open the mouth of the patient and radiograph dental or oral structures. Gauze may also be valuable in positioning a single digit. Gauze can replace the use of rope or velcro when working with smaller patients . Since the gauze is relatively radiolucent it can be super imposed over anatomical structures of interest and not compromise the study.
Use of adhesive tape can be a convenient method for holding an extremity to the table top or used in holding the mouth open. It has the advantage in that it will adhere to the table top or other positioning device but has the disadvantage that it may be a problem in releasing the limb and in removing the tape from the patient. It creates a minimal radiographic shadow but does not badly compromise the quality of the study.
Plastic or foam devices are available for positioning the head of small animal patients and may be of great value in making the open mouth or frontal sinus studies.
A mouth speculum can be used to assist in dental radiography or studies of the mandible. A metal speculum will cast a dense shadow on the film and may be contraindicated while use of a roll of gauze may function as well in positioning the mouth open and not create a radiographic shadow.
A compression band across the patient's body is of great value in assisting in positioning. The band can attach to the table top and be tightened across the patient. Other compression bands wrap around the patient's body and do little for patient immobilization. An inflatable bladder can be placed beneath the compression band to achieve organ displacement. Other compression devices such as a plastic paddle or wooden spoon can be used in flattening the foot of a dog or cat for the dorso-palmar projection. It is often possible to use a small radiolucent paddle in holding a cat's skull in position for dorso-ventral study. Other paddles with a convex surface are of value in achieving localized abdominal compression. This is often required to shift gas or fluid filled loops of small bowel. Large wooden spoons are excellent for this purpose. Commercially available paddles have an inflatable bulb that has value because it compresses and also displaces abdominal organs. Remember that compression devices decrease tissue thickness and therefore have the potential to improve radiographic quality by decreasing secondary radiation. Since they will decrease tissue thickness, the radio graphic technique may need to be decreased.
Tubing. The use of short pieces of plastic tubing may be of value in positioning birds or small mammals. The entire animal can be placed within the tube and positioned as desired. If the tube is of a low density material there is usually no requirement to alter the radiographic technique. This positioning technique may superimpose body parts and thus compromise the study but may be a technique of value in certain circumstances.
There are a group of padded metal holders that use a suction cup device to maintain attachment to the table top. These are stable and can be of value when used with an anesthetized patient. They are difficult to use as the only method of stabilization of the body part and may be more than is required to position the anesthetized patient. The table top must be clean and dry to ensure good stability.
Molded plastic forms are available for restraint positioners for surgery and radiography. The designs correlate to the anatomical shape of the patient and many postural attitudes can be obtained. These are of greatest value in positioning a patient in a ventro-dorsal position.
Wooden blocks are essential in radiography of the horse's foot. They ensure that the foot or digit and cassette are held in the same position on successive studies and tend to eliminate motion during exposure. The type of blocks required depends entirely on the specific x-ray machine and the type of radiographic examination desired. If the x-ray tube cannot be lowered to floor level, it is necessary to elevate the foot to enable true lateral projections to be made. Unfortunately, these blocks are not usually available commercially and it is necessary to use some ingenuity in their design and fabrication (Figs. 19-3,19-4,19-5, and 19-6).
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Scale drawing of cassete! holder and positioning block for the foot for lateral studies of the digit in the horse.
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Positioning devices are available from a number of distributors. Some of these are:
Cassettes with intensifying screens:
Intensifying screen cleaner
Lead rubber blockers (if beam limiting device is not adjustable):
Grid, Stationary: 35 cm x 43 cm (14" x 17") medium focal range&endash;80-120 cm range (36-44 inch) 40 to 50 lines per cm (103 lines per inch) 8:1 or 10:1 ratio
- &endash;identification marker
- &endash;numbers
- &endash;right and left
- &endash;DV, VD
- &endash;erect marker (arrow&endash;Mitchell markers)
Beam limiting device Cassettes with intensifying screens:
Intensifying screen cleaner
Grid, stationary: 25 cm x 30 cm (10" x 12")
Film markers:
Brush and hoof knife
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