Discussion

Radiography is a commonly performed, mainstay diagnostic imaging technique for routine evaluation of the equine tarsus. Radiography of the tarsus may be performed for screening, evaluation prior to sale or as a component of a lameness examination. Although frequently performed, radiographs of the tarsus remain of the most difficult regions to evaluate due to the complex anatomy (Vanderperren et al. 2009a,b). For this reason, a high quality, complete radiographic examination with adequate numbers of projections is central to maximising the diagnostic yield of this modality.
The definition of a complete radiographic examination will vary depending on the institution or purpose of the study but almost always includes multiple views. The views are chosen based on the demographics of the hospital or regional population or a specific clinical presentation. A routine tarsal examination at our institution includes 5 projections:
1. Dorsal - plantar
2. Dorsal 10degrees proximal to plantar distal oblique
3. Dorsal 45degrees lateral 5degrees proximal to plantar medial distal oblique
4. Lateral - medial
5. Plantar 45degrees 5degrees distal to dorsal medial proximal oblique

The importance of high quality radiographs of the tarsus cannot be overstated. Suboptimal radiographic quality will compromise the clinical utility of the examination and result in both false negative and false positive findings.
Routine projections will not identify all osseous pathology in the tarsus (Daniel et al. 2012). Special projections should be considered in order to more fully characterise a specific lesion or to ameliorate the identification of a lesion for which there is a high clinical index of suspicion. Furthermore, additional imaging (nuclear scintigraphy, magnetic resonance imaging, computed tomography) should be considered when radiography fails to identify a lesion befitting the clinical scenario.
Diagnostic ultrasound is of great utility for the evaluation of the regional soft tissues and the proximal metatarsal region (proximal suspensory ligament) (Vanderperren et al. 2009a,b; Raes et al. 2010). Ultrasound is also useful for the determination and characterisation of septic synovial structures such as the bursae, joints or tarsal sheath (Post et al. 2003). Ultrasound provides a thorough and high quality evaluation of the peripherally located soft tissue structures such as the collateral ligaments and the flexor tendons (Vanderperren et al. 2009a).
Nuclear scintigraphy is invaluable for the assessment of tarsal region lameness. Scintigraphy provides functional information about bone remodelling that helps to delineate between active and inactive radiographic findings or, identify active bone remodelling when radiographic findings are absent (Ross 2005). Scintigraphy is critically important for the evaluation of stress (exercise induced) remodelling in racehorses. Nondisplaced, occult fractures of the cuboidal bones and the talus can occur with little or no radiographic abnormalities (Davidson et al. 2005; Daniel et al. 2012). Often these fractures are first detected using scintigraphy. Scintigraphy is limited by superimposition and its inherent low-resolution. Scintigraphy compares favourably with magnetic resonance imaging for the identification of sclerosis, bone marrow lesions characterised by increased T2 signal intensity and fracture (Biggi et al. 2012; Daniel et al. 2012).
Magnetic resonance imaging (MRI) provides detailed information about all components of the tarsal articulation. Recent reports show that MRI can identify clinically relevant abnormalities (Biggi et al. 2012) that correlate well with nuclear scintigraphy (Daniel et al. 2012). The conditions identified on MRI include tarsal bone sclerosis and fracture, osseous hyperintensity, osteoarthritis, osteochondrosis and soft tissue injury (Daniel et al.
2012). Similar to other anatomic regions, MRI shows great promise for the clinical evaluation of the tarsal region.
Computed tomography (CT) provides detailed information about the osseous structures comprising the tarsus. Computed tomography has been reported for the identification of subchondral osseous cyst-like lesions and fractures (Garcia-Lopez and Kirker-Head 2004; Poulin Braim et al. 2010) and can be used to identify unusual sites of osseous trauma or joint disease.
Tarsus imaging is important in lameness diagnosis. Radiography and ultrasonography should be considered complementary and used for complete evaluation. Other diagnostics including nuclear scintigraphy, computed tomography and magnetic resonance imaging have important merits and should be considered when more routine imaging does not fit with the clinical examination findings.