EQUINE VETERINARY EDUCATION / AE / JANUARY 2017


15


Case Report


Nonselective computed tomography angiography for detecting arterial blood flow to the distal limb following trauma in two small equids


W. T. Walker*, N. G. Ducharme, J. Tran and P. V. Scrivani †


†


Department of Clinical Sciences and Department of Biomedical Science, Cornell University, Ithaca, New York, USA.


*Corresponding author email: wtw36@cornell.edu Keywords: horse; angiography; computed tomography; contrast; injury; vasculature


Summary Two equids weighing <250 kg were examined several days after suffering severe mid-diaphyseal dorsal metatarsal lacerations. Distal limb vascular disruption was suspected in both cases. Nonselective computed tomography angiography with contrast medium injected peripherally via the jugular veins was used to evaluate the vascular supply to the distal limb. The use of this imaging technique demonstrated either intact or disrupted distal limb vasculature. The imaging results were verified with a positive long-term outcome in the case with a diagnosis of an intact vasculature and with histological findings of avascular necrosis in the case with a diagnosis of disrupted vasculature.


Introduction


In horses, trauma to the distal limb is especially precarious because of limited blood supply to the distal extremity. In the forelimb, the medial palmar artery provides the majority of blood supply to the distal limb with smaller contributions from the lateral palmar artery and palmar metacarpal arteries (Ghoshal 1975). Similarly, arterial supply to the distal extremity of the hindlimb is primarily provided by the dorsal third metatarsal artery and the anastomosing medial plantar and lateral and medial plantar metatarsal arteries. Distal limb vascular disruption prevents distal limb healing and can lead to avascular necrosis and sloughing of the hoof capsule (White 1983). Therefore, it is vital to be able to assess accurately the extent of arterial blood flow to the distal limb following trauma to determine whether avascular necrosis is eminent. Several techniques have been used to determine the


vascular integrity of the distal limb. These include both radiographic (Ackerman et al. 1975; Rosenstein et al. 2000) and computed tomographic (CT) arteriography (Collins et al. 2004), Doppler ultrasonography (Adair III et al. 2000; Aguirre et al. 2013), and vascular phase nuclear scintigraphy (Galey et al. 1990; Trout et al. 1990; Bell et al. 1995; Ritmeester et al. 1998). Nonselective angiography involves the direct imaging of arteries after administration of high volume contrast material into a peripheral vein. Nonselective CT angiography (CTA) allows improved ease of access and vascular safety when compared to arteriography (Miller-Thomas et al. 2005) and a more comprehensive evaluation of the distal limb vasculature and surrounding anatomy when compared to other conventional techniques. Nonselective CTA is currently considered the gold standard imaging technique for vascular assessment after severe extremity injuries in human patients at trauma centres (Inaba et al. 2006; Peng et al. 2008). The


following report describes the use of nonselective CTA in 2 small equids for the determination of distal limb vascular integrity.


Imaging


Nonselective CTA was performed by acquiring images before and after i.v. administration of contrast material using a 16-slice scanner (Aquilion LB)1. After sedation (xylazine 1 mg/kg bwt i.v.), anaesthesia was induced with a combination of diazepam (0.1 mg/kg bwt) and ketamine (2.2 mg/kg bwt) i.v. and maintained with isoflurane in a semi-closed positive pressure ventilation system. Helical acquisition was performed with the patients in dorsal recumbency and the limbs extended caudally. The acquisition parameters varied slightly between the cases: 0.5 s tube rotation time, 120 kVp, automated mA using high quality SUREExpose (50–75 mAs), 1.6/1.4 or 0.9/0.8mm focal spot, 1 or 2mm slice thickness, 512 × 512 matrix, 40 cm field of view, 11 helical pitch, and 0.688 pitch factor. Nonionic iodinated contrast material (Omnipaque 350 iohexol injection)2 was injected by hand or using an injector (MEDRAD MCT Multi-level CT Injector)3. Scanning was initiated manually immediately after injection of contrast material was complete and the room evacuated. Typical delay was approximately 30 s, allowing for venous return of contrast medium through the heart and subsequent arterial distribution to the extremities. Image acquisition was repeated after contrast medium administration to assess temporal differences in opacification. In Case 1, contrast material (190 ml at 7.5 ml/s) was injected into a jugular vein using a pressure injector. In Case 2, contrast material (460 ml) was injected into both jugular veins by hand within 30 s. Multiple reconstructions were made in the 2 cases: the most helpful scans were multiplanar reconstructions in 2 or 4mm transverse, sagittal, and dorsal planes using a standard soft-tissue display, and surface rendered 3-dimensional scans using a bone display. All images were stored and viewed on a dedicated DICOM workstation (PACS workstation)4 and reviewed by a board-certified veterinary radiologist (P.V.S.).


History, clinical presentation and treatment


Case 1 A 3-year-old 115 kg miniature mule mare was referred for evaluation of a severe traumatic soft tissue injury to the mid-diaphyseal region of the right metatarsus 8 days after initial injury. The mule’s right hindlimb became stuck between a door and doorframe and the traumatic injury ensued during


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