Experimental and Basic Research Studies Echocardiographic identification of atrial-related structures and vessels in horses validated by computed tomography of casted hearts T. VANDECASTEELE † , P. CORNILLIE † , G. VAN STEENKISTE ‡ , K. VANDEVELDE † , I. GIELEN § , K. VANDERPERREN § and G. VAN LOON ‡ * † Department of Morphology, Ghent University, Merelbeke, Belgium ‡ Department of Large Animal Internal Medicine, Ghent University, Merelbeke, Belgium § Department of Veterinary Medical Imaging and Small Animal Orthopaedics, Ghent University, Merelbeke, Belgium. *Correspondence email: gunther.vanloon@ugent.be. Received: 13.02.18; Accepted: 18.05.18 Summary Background: Echocardiography is the imaging technique of choice for the equine heart. Nevertheless, knowledge about ultrasonographic identification of dorsally located structures and vessels, related to the atria, in horses is scarce. Objectives: To describe the echocardiographic approach and the identification of structures and vessels in relation to the atria in healthy horses. Methods: CT images from two equine hearts, casted with self-expanding foam, were segmented and used to identify atrial-related structures and vessels. These images were compared with standard and nonstandard ultrasound images from ten healthy horses obtained from a left and right parasternal view optimised to visualise the dorsal cardiac area. Results: On new standard ultrasound views, specific atrial anatomical landmarks such as vena cava, pulmonary arteries, intervenous tubercle and oval fossa were identified in all horses. In addition, ultrasound views were defined to visualise the brachiocephalic trunk, pulmonary veins and their ostia. The 3D segmented CT images from casted hearts were used to reconstruct slices that corresponded with the echocardiographic images and allowed correct identification of specific structures. Main limitations: Ultrasound examinations and casts were from different animals. A small number of casts and horses were used; therefore, anatomical variation or individual differences in identifying structures on ultrasound could not be assessed. Conclusions: Important cardiac structures and vessels, even the different pulmonary veins, could be identified on standard and nonstandard ultrasound images in adult horses. This knowledge is important to guide and develop interventional cardiology and might be useful for diagnostic and therapeutic purposes. Keywords: horse; ultrasound; pulmonary veins; cardiac Introduction While fluoroscopy, MRI and CT are widely used in human cardiology [1–3], these techniques have limited value (fluoroscopy) or cannot be applied in adult horses (MRI, CT), and in horses, echocardiography is the principal cardiac imaging tool. Currently, for routine cardiac assessment, the echocardiographic examination is performed in standard, predefined long- and short-axis image planes from the left and right thorax that allow identification and measurements of common structures such as atria, ventricles, myocardial walls, valves and the origin of aorta (Ao) and pulmonary artery (PA) [4]. While these rather ventrally located structures have been described in detail, there is limited information regarding more dorsally located structures, such as dorsal atrial walls, cranial (CrVC) and caudal vena cava (CaVC), brachiocephalic trunk (BT) and pulmonary veins (PV) because they are partially covered by air-filled lungs, and therefore more difficult to visualise [5–7]. Recently, based on a topographical study, the ultrasonographic identification of PV in horses was described [8]. Invasive cardiac procedures are of increasing interest in equine cardiology. Navigation of equipment through vessels and cardiac chambers relies on good ultrasonographic anatomical landmarks. More and more centres offer transvenous electrical cardioversion which requires accurate positioning of catheters in right atrium (RA) and left pulmonary artery (lPA) [9–14]. Pacemaker lead implantation, cardiac biopsy procedures and occluder implantation are performed with ultrasound guidance [15–18]. Equine electrophysiological studies are performed both for research and for diagnostic purposes [19–21]. Recently, the first successful 3D electro- anatomical cardiac mapping has been reported in adult horses [22]. Ultrasound-guided electro-anatomical mapping from within the RA, left atrium (LA) and PV would be of great interest in horses treated for atrial fibrillation to assess PV ectopy [23], and could lead to the development of new treatment strategies. The importance of the PV in atrial fibrillation pathophysiology has been well established in humans [24], but information is currently lacking in horses. Visualisation of the PV would also be of interest to assess PV size and flow. The aim of this study was to identify left and right atrial-related anatomical landmarks and blood vessels on ultrasound. In order to do so, CT images from casted equine hearts were segmented, 3D reconstructed and subsequently used for comparison with ultrasound images to allow correct identification of the different structures. Materials and methods Cardiopulmonary sets of four adult horses (2 Warmblood geldings, one Thoroughbred mare and one Quarter horse stallion; body weight 530– 600 kg; age 2.5–6 years), subjected to euthanasia for noncardiovascular reasons, were removed immediately after death with preservation of the proximal part of CrVC, CaVC and Ao. Via an incision in the left and right auricle, after removal of blood clots, one-component self-expanding polyurethane construction foam a was injected through a tube, placed into the right (RV) and left ventricle (LV). While the foam was injected, the tube was slowly retracted towards the atria. Two hours after foam injection, the cardiopulmonary set was subjectively evaluated to represent a close to normal expansion of all cardiac chambers and vessels (Fig 1). Of the two best casted hearts, distal parts of the lungs were removed, leaving the PV intact, and computed tomographic (CT) scans were made with a four-slice helical CT device at 120 kV and 140 mA (CT scanner, LightSpeed, Qx/I b ). 90 Equine Veterinary Journal 51 (2019) 90–96 © 2018 EVJ Ltd Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/evj.12969