Lameness is a frequent medical problem in horses. Aside from localizing techniques such as palpation and subsequent diagnostic blocking, radiographic investigation of the limbs, neck and back is often the first choice in diagnostic testing. Digital radiography is widespread and arguably considered the standard of practice. The advantages of digital systems include the ability to manipulate images, apply post processing, centrally store and quickly obtain and review images immediately after acquisition. Despite these advances in technology, the basics of radiographic exposure and patient positioning have not changed. When producing radiographic images using digital technologies, the veterinarian or technician must still correctly position and choose a technique appropriate for the body part being imaged. Faulty imaging due to poor positioning or inappropriate exposure can lead to under or over diagnosis of pathologic changes. The goal of diagnostic imaging is to reach an accurate diagnosis to target treatment specifically. Well-positioned radiographic images acquired with the appropriate technique are a key component to reaching the correct diagnosis.
Principles of X-rays
When a radiographic image is obtained, about 90% of the x-ray photons are absorbed by the tissue and 10% of the photons pass through the patient and reach the detector. Many of the absorbed photons generate scattered radiation (Compton scatter). These scattered photons travel in all directions creating noise and degrading image quality. The effect of scattered radiation can be minimized by collimating the x-ray beam to reduce the number scattered photons.
The farther away you are from the patient the less intense the photons are that strike plate. This is especially important for obtaining radiographic images in large body parts with portable generators. The typical film focal distance (distance from the plate to the generator) is 60 cm. When imaging larger body parts such as the caudocranial stifle, neck or back it is important to maintain a 60-cm distance (or closer). We can also stand closer to the image detector and patient to improve image quality when imaging larger body parts when limited by our generators.
By standing closer when obtaining radiographs of larger body parts, the increased quantity and intensity of photons reaching the plate will help to reduce the commonly obtained gray, grainy images (also known as quantum mottle).
Patient and X-Ray Positioning
The ideal conditions for obtaining most radiographs of equine joints is with the horse standing square, except for non-weight-bearing views. Unfortunately, not all of our equine patients are willing to stand square and the equine practitioner must accommodate the patient to make quality radiographs. Radiographic examination of the stifle, especially the caudocranial image, is a good example of this principle. For example, the optimal angle for the caudocranial image is 10 degrees from caudal proximal to cranial distal with the horse standing square (the tuber calcaneus is in line with the tuber ischii). However, if the limb is slightly behind the vertical (camped out), or under the horse (camped under), the angle must be adjusted to accommodate the stance of the horse. If the horse is standing with the limb behind it, the angle (caudoproximal to craniodistal) will increase (be steeper) and vice versa for the horse that is stood under itself. When evaluating the caudocranial stifle image for adequate positioning, the tibial plateau should superimpose itself from cranial to caudal and the tibial tuberosity should be distal (10–15 mm) to the tibial plateau. If the tibial tuberosity is proximal to the tibial plateau, the angle is typically too steep. The x-ray generator should be centered about 8–10 cm proximal to the indentation created by the distal aspect of the thigh musculature as it transitions to the proximal crus area. Joint narrowing in the stifle may be masked or falsely created by inadequate positioning. The most common areas of pathologic change in the stifle are associated with the medial femoral condyle and the lateral trochlear ridge,2,3 the caudo 45° lateral–craniomedial oblique highlights these areas well. A well-positioned Cd45L–CrM oblique should project the medial femoral condyle caudal to the tibial eminence and show the joint clearly. The superimposition of the medial femoral condyle with the tibial eminence can mask subtle concave defects. Just as with the caudocranial projection of the stifle, the angle of the x-ray generator should be 5–10 degrees proximal to distal in a square standing horse. The most common mistakes are to is be at too steep of an angle or being too lateral. Limb positioning also affects acquisition of the lateral radiographic projection. If the horse is standing base wide the x-ray generator will have to be angled distally; if base narrow, the generator angle will be slightly proximal. Judging placement of the x-ray generator in a cranial to caudal fashion is best done by lining up with the heel bulbs and tarsus. The most common mistake is being too far cranial.
Radiographic examination of the neck has seen a dramatic increase in frequency in recent years. Unlike the distal extremities, the radiographer cannot see the x-ray detector on the opposite side of the neck. This creates positioning problems and often results in images with one vertebral body or facet joint that is not centered on the x-ray detector. Furthermore, the articular facets on the lateral radiographs may not be perfectly superimposed, which can lead to interpretation errors. However, if a moment is taken to palpate the transverse processes and apply white tape to these sites, this can serve as a guide to detector placement and x-ray generator focus. Centering just proximal to the transverse process will render well-positioned radiographs, when the horse’s poll is in line with the withers. Symmetrical anatomy of the neck can make lesions difficult to lateralize. Oblique radiographs obtained in a left/right 45–55° ventral to right/left dorsal fashion can help localize lesions.5 The x-ray generator is typically centered at the jugular furrow and the x-ray detector is placed with the transverse process centered at the bottom ⅓ of the x-ray detector. Radiographic images are named from where the x-ray generator is located and subsequently where the x-rays enter to where the x-rays exit the neck, and where the x-ray detector is located. In the example of a L55V–RD oblique the left articular facets will be projected dorsally and the right transverse processes will be projected ventrally. Properly labeled, opposite oblique radiographs should be obtained to accurately localize the lesion. Well positioned oblique radiographs should project one side of the articular facets dorsal and show the intervertebral foramen well. The other articular facet joint will be superimposed over the vertebral body highlighting the joint width. Oblique radiographs obtained with portable units centered at the articular facets at C6–7 are challenging due the shoulder superimposition. This may be overcome by offsetting the forelimbs, with the leg near the x-ray generator pulled caudally.
Radiographic projections are used to highlight specific areas and document pathologic change. The fetlock is an area with a multitude of pathologic change in which appropriately positioned radiographs make the pathologic change easy to identify. For example, palmar/plantar process osteochondral fragmentation is a common abnormality in the fetlock and depending on the location may be a source of lameness. The oblique views (dorso 20° proximo 45° lateral–palmaromedial oblique and dorso 20° proximo 45° medial–palmarolateral oblique) should show these lesions best. However, if the proximal sesamoid bones superimpose this area, the fragmentation could be easily missed. Ideal oblique and DP radiographs of the fetlock project the proximal sesamoid bones proximal to the joint margin.
Patient preparation and positioning, adequate technique and knowing how to correct malpositioned radiographs are skills in achieving diagnostic radiographic images. Taking a moment to assess unintentional obliquity, patient conformation and stance can reduce retakes and reduce radiation exposure.
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