Radiography of the equine back: equine limitations
Back pain, whether primary or (more frequently) secondary, is common in horses and so imaging of the equine back is relatively frequently performed in equine practice. Acquiring good quality radiographs of the spinous processes, vertebral bodies and articular facet joints is important and can be challenging with respect to both practicality and equipment factors. This article will focus on equipment factors.
Most portable X-ray generators have enough power to image the dorsal aspect of the thoracic spinous processes in an average sized horse. However, imaging only the top portion of the spinous processes is not adequate for a comprehensive assessment. When considering close or impinging spinous processes (so-called ‘kissing spine’), although the summits of the spinous processes are commonly seen to be the site of impingement, impingement can be present throughout the entire length of the spinous processes. Failure to visualise the entire lengths of the spinous processes can lead to underestimation of the extent of impingement or missing pathology which involves the summits and not the ventral portions of spinous processes.
One major issue with radiographing this region is that the thickness of the soft tissue mass overlying the lower portion of the spinous processes is dramatically thicker than that overlying the dorsal aspect. The result is that with adequately exposed dorsal aspects of the spinous processes the lower portions are often underexposed. If a higher exposure is utilised to adequately expose the lower portions, then the dorsal aspects are overexposed and appear ‘burnt out’. The use of aluminium filter wedges can compensate for this disparity in tissue thickness, reducing exposure to the dorsal aspects whilst allowing greater exposure of the lower portions, thereby allowing the entire lengths to be adequately imaged in one radiograph. Portable generators used with a digital radiography (DR) system will often be able to produce diagnostic quality images of the spinous processes. However, some systems or use in large, well-muscled horses may result in poor image quality of the spinous processes due to the limitation of the output range of the generator.
Imaging the vertebral bodies and articular facet joints is important in the assessment of horses with clinical signs of back pain (Girodroux et al. 2009). Portable generators are designed to be lightweight, however the downside of their design is that they cannot provide the high outputs that the larger generators can. This makes them ideal for imaging the distal limb, where a lightweight machine is preferable, and the exposure factors required for a high quality image are low.
With most portable generators it is a struggle to acquire good quality images of the vertebral bodies. Underexposure is the main reason for failed radiography of vertebral bodies. Some low powered generators simply cannot produce the mA required. Exposure time needs to be longer to give the required mAs value and will increase the risk of motion artefact. Reducing the distance between the generator and the plate will act in the same way as increasing mAs and may contribute to improved image quality. The simplest solution is to use a higher power generator with a generator stand, instantly improving image quality.
As technology has advanced, it is now possible to acquire high quality images using much lower exposure settings, and as such acquisition of high quality radiographs of the spine using portable generators is becoming possible. Wired DR systems often cope better with spinal radiography than the automated, automatic exposure detection (AED) systems. This is because the synchronisation box of the wired DR system controls when the radiation is produced. As the trigger is pressed, it ensures that all the equipment is ready and communicating before allowing the generator to expose. This process ensures that the plate will record the exposure correctly. In AED systems, the radiation is controlled only by the operator firing the generator (there is no synchronisation box) completely relying on the AED panel correctly sensing and recording the exposure. Because of X-ray photon attenuation, when imaging the vertebral bodies the AED panel may not detect the exposure and no image will be produced.
X-ray photon scatter can be a real issue when radiographing the spine and there are several measures that can be taken to minimise its negative effects. Placement of a lead sheet behind the cassette and on top of the horse’s back (when not imaging the summits of the spinous processes!) will help to reduce some of this scatter (also increasing staff safety). Use of a grid with a ratio of 10:1 or even 12:1 will improve image quality but will also mean that even higher exposure factors are required, which can be problematic if equipment limitations come into play.
Girodroux M., Dyson S., Murray R. (2009) Osteoarthritis of the thoracolumbar synovial intervertebral articulations: clinical and radiographic features in 77 horses with poor performance and back pain. Equine Vet J 41:130-138.
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