le at shoulder level (Berner et al 2012). To further assess intervertebral articulations, oblique projections are obtained with the beam angled 20˚ from ventral to dorsal and centred 15-20cm ventral to dorsal midline (Butler et al. 2000; Girodroux et al. 2009). The X-ray plate should be aligned perpendicular to the beam and as close as possible to the horse. The highly-curved shape of the ribs is causing superimpositions with to the thoracic IVAs. In the lumbar spine there is superimposition of the transverse processes and abdominal structures.
• Ultrasonography of the back should be combined with the radiographic examination and is particularly useful to examine superficial soft-tissue structures and the intervertebral articulations. To assess all structures of interest, both linear and convex transducers are required.
For the supra- and interspinous ligaments (SSL and ISL) longitudinal views are most useful. The images are best acquired with a high-frequency linear transducer using a stand-off pad. The head should be kept in a neutral position as high head carriage may cause relaxation of the SSL and therefore a hypoechogenic appearance. Superficial fibres of the SSL have a horizontal orientation whereas deep fibres of the ISL run in caudoventral direction appearing hypoechogenic in comparison. Care should be taken to position the probe exactly midline at the level of the dorsal spinous processes. The aponeuroses of the epaxial musculature can be imaged off-midline with a parallel fibre pattern similar to the SSL.
In the thoracic (saddle) region the SSL is thin and wide, whereas further caudal in the lumbar region the ligament is thick and narrow with a more echogenic, homogenous appearance. The transverse view of the SSL is less useful due to being more prone to artifacts. In order to achieve the best image quality, the transducer should be tilted cranially and caudally. Published scientific data about the ultrasonographic appearance of the SSL in normal horses and horses with back pain offering information about the potential clinical significance is limited (Henson et al 2007). The clinical significance of heterogenous regions in the SSL should therefore be questioned and further tests, such as diagnostic local anaesthesia, should be applied.
Deep to the ligament the fibrocartilaginous cap, visualised as a hypoechogenic line of approximately 1mm, is visible and runs parallel to the hyperechoic line of the DSP bone surface. The ISS cannot be assessed fully and a statement about potential iminging DSPs should not be made based on ultrasonography alone. The intervertebral articulations can be examined ultrasonographically, both in sagittal and frontal planes, using a convex transducer (Denoix 1999a; Reisinger and Stanek 2005).
The anatomy and ultrasonographic appearance of the joints change slightly from cranial to caudal. The mammillary process is more prominent in the thoracic spine and the joint is positioned further axial and closer to the DSP compared to the lumbar region. Signs of pathology in the intervertebral articulations (see Figure 4) include new bone formation and an enlarged outline of the facet (Denoix and Dyson 2003).
Muscle asymmetries are common in horses with back pain. Measuring the multifidus muscles may add objective information during back investigations and is easily applied. Furthermore, progress made following treatment and training can be measured by re-assessing m. multifidus symmetry (Halsberghe et al 2017).
• Scintigraphy is commonly applied to further image the back in cases where the standard modalities were inconclusive or specific findings require advanced diagnostic imaging. Mild increased radionuclide uptake (IRU) associated with the DSPs is a common finding in horses without clinical signs of back pain (Erichsen et al 2003, 2004). A study of Zimmermann et al (2011) looking primarily at the spinous processes of the thoracolumbar spine showed that the severity of radiographic lesions was associated with scintigraphic abnormalities. DSPs at T14-T17 were most commonly affected. Horses w