MAGNETIC RESONANCE IMAGING CONTRAST ENHANCEMENT OF EXTRA-OCULAR MUSCLES IN DOGS WITH NO CLINICAL EVIDENCE OF ORBITAL DISEASE STEPHEN JOSLYN,SUZANNA RICHARDS,SUSANNE BOROFFKA,MARK MITCHELL,GAWAIN HAMMOND,MARTIN SULLIVAN Enhancement of extra-ocular muscles has been reported in cases of orbital pathology in both veterinary and medical magnetic resonance imaging. We have also observed this finding in the absence of orbital disease. The purpose of this retrospective study was to describe extra-ocular muscle contrast enhancement characteristics in a group of dogs with no known orbital disease. Magnetic resonance images (MRI) from dogs with no clinical evidence of orbital disease and a reportedly normal MRI study were retrieved and reviewed. Contrast enhancement percentages of the medial, lateral, ventral, and dorsal rectus muscles were calculated based on signal-to-noise ratios that were in turn determined from hand-traced regions of interest in precontrast, immediate postcontrast and 10-min postcontrast scans. Comparison measurements were made in the pterygoid muscle. Contrast enhancement of the extra-ocular muscles was observed in all patients (median contrast enhancement percentage 45.0%) and was greater than that of pterygoid muscle (median contrast enhancement percentage 22.7%). Enhancement of the extra-ocular muscles persisted 10 min after contrast administration (median contrast enhancement percentage 43.4%). Findings indicated that MRI contrast enhancement of extra-ocular muscles is likely normal in dogs. C  2013 American College of Veterinary Radiology. Key words: contrast, enhancement, extra-ocular, muscle, MRI. Introduction M AGNETIC RESONANCE IMAGING (MRI) of the orbit and peri-orbital structures is used routinely in the di- agnostic work up of many ophthalmological conditions. 1–5 The superior soft tissue contrast resolution and mul- tiplanar acquisition allow detailed lesion detection and understanding of the complex anatomical structures of the orbit. 4, 6 Contrast administration, using gadolinium chelates, improves the detection of pathological processes in MRI. 7 However, highly vascular normal structures may also show marked enhancement following contrast admin- istration and may mask, or even appear as, pathological processes. 8–10 Contrast enhancement of extra-ocular mus- cles has been reported in canine cases of suspected inflam- matory diseases such as extra-ocular polymyositis, mas- ticatory myositis, or peri-orbital cellulitis. 1, 3, 5 However, at our practice, contrast enhancement of extra-ocular muscles has also been seen in canine cases without orbital disease. From the Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 (Joslyn, Mitchell); Small Animal Hospital, University of Glasgow, Glasgow, UK (Richards); Clinic for Companion Animals, Utrecht University, Utrecht, The Nether- lands (Boroffka); Schoolof Veterinary Medicine, University of Glasgow, Glasgow, UK (Hammond, Sullivan). Address correspondence and reprint requests to Stephen Joslyn, at the above address. E-mail: sjoslyn@illinois.edu Received February 23, 2013; accepted for publication June 17, 2013. doi: 10.1111/vru.12083 Contrast enhancement of extra-ocular muscles has been re- ported to be a normal finding in human MRI but, to the author’s knowledge, this has not been documented in the veterinary literature. 11, 12 The purpose of this retrospective study was to describe the contrast enhancement character- istics of extra-ocular muscles in a group of dogs with no known orbital disease. Materials and Methods Dogs with no clinical signs or history of orbital disease and that had MRI examinations of the head between De- cember 2009 and May 2010 were included in the study. Dogs were excluded if there were any abnormalities in hematol- ogy, serum biochemistry or cerebrospinal fluid analysis, and if the MRI examination revealed any abnormalities or if post contrast MRI sequences were unattainable. All MRI data were acquired using a 1.5 Tesla scan- ner (Siemens Magnetom Essenza, Siemens AG, Erlangen, German) with a human head/neck coil. For all dogs, T2- weighted sagittal, dorsal, and transverse; FLAIR trans- verse; T2*-gradient echo transverse and T1-weighted trans- verse sequences were acquired. The T1-weighted transverse sequences (TR = 455–674 ms, TE = 13) were repeated im- mediately following contrast medium administration and Vet Radiol Ultrasound, Vol. 55, No. 1, 2014, pp 63–67. 63