Overview of Positioning

Rules of Positioning

There are a few rules to keep in mind based on ACVR (see Fig. 15.1):

Limb Terminology

1. 1. Cranial (Cr) and caudal (Cd) refer to the portion of the limb proximal to the carpus and tarsus.
2. 2. The descriptors dorsal (D), palmar (Pa), and plantar (Pl) are used for that portion of the limb distal to and including the carpus and tarsus. Palmar (Pa) is used in reference to the forelimbs, whereas plantar (Pl) refers to the hind limbs.
3. 3. In describing the limbs, the terms dorsal, palmar, plantar, cranial, and caudal should take precedence when used in combination with other terms—for example, dorsoproximal.
4. 4. The term oblique (O) is added to the names of those projections in which the central ray passes obliquely (not parallel) to one of the three major directional axes—mediolateral (ML), dorsopalmar/dorsoplantar (DP), or craniocaudal (CrCd)—through the body part. They are named in the same manner as the standard views. The term oblique is generally used in reference to equine limbs.
   • a. Thus in a dorsomedial-palmarolateral oblique (DM-PaLO) image of the carpus, the beam enters the dorsomedial side of the carpus and exits the palmarolateral aspect of the carpus (Fig. 15.3).

     
     

   • b. Technically, if this view was made by positioning the x-ray tube 60 degrees medially from the dorsal side, the designation would be D60°M-PaLO. Oblique terminology is discussed further in Chapter 24.
5. 5. In those views requiring a combination of directional terms, a hyphen should be inserted to separate the point of entry and point of exit.
   • a. For example, dorsoproximal-palmarodistal (DPr-PaDi) of the front digit of a horse means that the beam comes from the front of the foot and exits at the back of the foot (distal to and including the carpus). The plate is placed at the back of or under the foot, depending on the anatomy.
6. 6. The tangential and skyline views require no special designation because the point of entry to point of exit method describes these views concisely.
   • a. For example, palmaroproximal-palmarodistal (PaPr-PaDi) in reference to an equine forelimb navicular region means that the beam is aiming strictly at the palmaro- (back) aspect of the foot starting at the top (proximo-) and ending toward the sole of the hoof (disto-). Further terminology will be discussed in Chapter 24.

Patient Preparation

The patient should be clean and free of any debris. If the hair coat of the patient is wet or full of debris, confusing artifacts can appear on the radiograph. Collars, harnesses, and leashes of any sort, especially those made of metal, should be removed (Fig. 15.5). Remove bandages, splints, and casts before radiography unless there is a definite medical reason for leaving them in place. Pedal radiography of the horse may require removing the shoe and cleaning the frog of the foot to minimize any artifacts that may obscure an area of interest.

For radiography of the small animal abdomen, the gastrointestinal tract must be free of ingesta and fecal material. A cathartic such as an enema or a laxative may be indicated to remove the obstructive material. A more detailed discussion of patient preparation for abdominal studies is provided in Chapter 23.

Positioning Aids for Human Safety

To assist in the positioning of the patient, devices such as sandbags, foam blocks and wedges, wood blocks, and a radiolucent trough can be used (Fig. 15.6). Tape is essential; gauze, rope, bungee cords, and compression bands are also useful positioning aids. Any reusable aids should be waterproof, washable, and stain resistant, as well as easy to store.

Positioning devices are commercially available. Prepared sandbags are generally prefilled with clean silica sand, permanently sealed, and often made of vinyl or nylon with plastic linings. Other material, such as bean bags, which are filled with polyester beads, can also be used; however, they do not offer the same support as sand does. Sand and bean bags are radiopaque, so should not be in the field of interest.

Commercial foam, available in various shapes and sizes, is generally covered in washable heavy vinyl covers (Fig.15.7A). Triangular and rectangular foam blocks are the most common. Foam tends to produce an air density shadow and, if not properly covered, absorbs and retains liquids that may be radiopaque when dry. The cover on foam blocks may also leave density shadows on the processed radiograph.

U- and V-shaped troughs are essential to maintain a patient in dorsal recumbency. Generally they are clear plastic or vinyl covered. The plastic/acrylic troughs are radiolucent, lightweight, and easy to clean (Fig. 15.7B). U-shaped troughs offer good head support. Those with acrylic rods can be used to maintain the shape of the skull. When using troughs for abdominal or thoracic radiographs, ensure that the entire field of view is either on or off the trough; otherwise, the radiograph may display distortion, artifacts, or asymmetry. Positioning troughs used for areas such as the pelvis should be fully outside the collimated area for the same reasons.

Tape, gauze, and compression bands are extremely effective. These must be functional to be satisfactory. Rope and gauze, using a half-hitch knot or variation (placing the end through the loop and then around the object), can be tied to the table if the patient is heavily sedated or anesthetized, or held by a person who can then back away from both the primary beam and secondary radiation. Velcro on the underside of the table allows soft Velcro straps to stick and is effective for many views (Fig. 15.8).

Adhesive tape can be used to extend and hold limbs, rotate limbs, secure the patellae and femurs for a hip dysplasia view, widen the space between the toes, temporarily secure a head out of the field of view, and so on (Fig. 15.9). A wooden spoon can be used to keep a cat's head out of the field of view so the radiographer is farther away from the primary beam.

Compression bands and hook-and-loop tape (Velcro) can be applied. Clothes pegs/clothes pins for cats, commercially available cat scruffers (Fig. 15.10), or nylon towel clamps applied to the dorsal neck region (a feline behavior principle that the queen uses with her kittens) is a valuable tool for feline patients and is almost as effective as sedation, especially if the positioning is quick and the patient is not in pain. Place the sandbag over the device. Wrapping a cat in a towel (“cat burrito”) can be effective if you need to pull out the limbs for imaging or even for abdominal and thoracic views. Sandbagging the towel keeps cats in position. Wrapping does allow many angry cats to become compliant.

Your own devices can be made at a fraction of the cost of the commercially available positioning devices. Sandbags can easily be sewn and filled with sand. Empty sealable bags that can be filled with sand are also available. If you are making your own out of canvas (jean legs work well) or thick vinyl, make sure to use very narrow stitching so the sand does not leak out. Fabric sand bags are not easily disinfected, so before each use, wrap them in disposable plastic. Most fabric stores sell foam that can be cut into desired shapes with a scalpel blade or electric knife. Covering the bags and wedges is essential for disinfection between patients and for keeping the devices dry to minimize radiographic artifacts. With these devices, and sedation if necessary, minimal manual restraint can be used.

Any nonmanual restraint used must always focus on the safety and comfort of the patient. The devices should be quickly applied and released and should never compromise the patient. Positioning aids should give the patient the illusion that it is being held. Strategically placed sandbags or compression devices over the neck and limbs, judicious use of tape, a dimly lit room, calm deliberate movements, and a gloved hand placed over the head and held until the rotor is depressed may keep patients, especially dogs in lateral recumbency, calm long enough for the restrainer to step back at least 6 feet.

The moment the rotor is depressed, slip your hand out of the glove, leaving it over the animal's head so the patient assumes that you are still there. Keep talking to the patient as you quietly step back. Move forward as soon as the radiograph has been taken. Another person should be depressing the exposure buttons on the console; if you are using the foot pedal, step back as far as possible. Increasing your distance from the beam drastically reduces your exposure.

If there is no alternative but to restrain a patient, it is imperative to look away from the field of view and lean back as far as possible while taking the radiograph. At no point should any part of your body be in the field of view. Protective equipment protects you from secondary radiation, not from the primary beam.

Acrylic tubes, stockinette material, pillowcases, paper bags, and plastic containers can also be strategically used for the positioning of exotic animals. Please see Chapter 25 for more detail on this subject.

Equine radiographers are less likely to practice nonmanual restraint because of perceived physical safety for both the handler and the horse. However, every effort should be made to practice the three essential components of radiation safety (time, distance, and shielding). See Chapter 24 for further information on equine positioning.

Required Views and Positioning Guidelines

The following guidelines assist in producing high-quality images:

• • Two views of each anatomical area taken at right angles to each other are the minimum recommended exposures. You are trying to visualize a three-dimensional body on a two-dimensional image, so details will be missed if two perpendicular views are not taken (Fig. 15.11). There may be exceptions if the patient is debilitated or in trauma, or if positions other than the lateral will cause undue stress to the animal. A horizontal beam radiograph should then be considered so that you still have two perpendicular views (Fig. 15.12).

  
  

  
  

• • Position the area of interest closest to the image receptor to reduce distortion and magnification of the area under examination (Fig. 15.13).

  
  

• • When radiographing a limb, especially in immature or older patients, consider imaging the opposite corresponding limb to allow the pathological structure of one limb to be compared with the normal anatomy of the other.
• • When the tabletop technique is used, an image receptor can be divided to image more than one view, limiting the number of films utilized (Fig. 15.14).
  • • Place a lead sheet over half of the cassette (cassette mask) to prevent exposure while the other side is being radiographed.
  • • The image receptor should still be collimated to the image only so that secondary radiation will not cause unnecessary fogging of the image.
  • • Lead sheets, which can be purchased from most x-ray supply companies, are usually supplied in preselected sizes; or larger sheets can be purchased and cut to the desired size.
  • • The lead should be at least 2 mm thick. If a lead sheet is unavailable, a lead glove can be placed over the area to be shielded.
  • • When using the tabletop technique, place a nonslip pad, such as a rubber shelf liner, under the cassette to keep it from slipping (Fig. 15.15).

    
    

  • • When two separate views will be positioned on a radiograph, both views should be facing the same direction. For limbs, this would mean that the toes are facing the same side of the cassette; for the skull, the nose is in the same direction on each view.

  
  

• • In general, the central ray should be centered directly over the area of interest.
  • • If there is a known lesion, however, it is important to center the beam directly over this area, especially to visualize fracture healing in limbs or spinal lesions.
  • • If the central ray is not directly over the area of interest, distortion and misdiagnosis may occur.
• • The measurement for any anatomical region is generally taken over the thickest part.
  • • This practice ensures that all regions of the area of interest will be penetrated with sufficient exposure factors.
  • • A caliper is used to measure the anatomical area of interest so that proper exposures can be made. This is an inexpensive device that measures part thickness in centimeter increments (Fig. 15.16).

    
    

• • The patient should be measured in the same position used for the radiograph; if the animal is measured while standing, for example, the tissue thickness measurement will be greater than when the animal is recumbent, especially for soft tissue studies (see Fig. 15.16A-B).
  • • If there is a large difference in tissue thickness between the cranial and caudal borders, which is not uncommon in the abdomen and thorax of a deep-chested dog, two separate exposures may need to be taken.
  • • A compromise is best made if there is only a small difference in tissue density.
• • Use an image receptor that is large enough to cover the body area being radiographed. Specific anatomy must be included for each anatomical area.
  • • For example, all radiographs of long bones (humerus and femur) should include the shaft of the bone, as well as the joints both distal (Di) and proximal (Pr) to the bone.
  • • For joint radiography, the central ray must be centered over the joint space, and the beam should include a portion of the long bones distal and proximal to the joint.
  • • Proper centering and inclusion is further expanded in Chapters 18 and 19.
• • Have the thickest part of the area of interest toward the cathode side so that the most penetrating x-rays can assist in proper exposure (Fig. 15.17; see Chapter 2).