International Veterinary Epilepsy Task Force recommendations for a veterinary epilepsy-specific MRI protocol

Rusbridge et al. BMC Veterinary Research (2015) 11:194 DOI 10.1186/s12917-015-0466-x CORRESPONDENCE Open Access International Veterinary Epilepsy Task Force recommendations for a veterinary epilepsy-specific MRI protocol Clare Rusbridge1,2*, Sam Long3, Jelena Jovanovik1, Marjorie Milne3, Mette Berendt4, Sofie F. M. Bhatti5, Luisa De Risio6, Robyn G. Farqhuar7, Andrea Fischer8, Kaspar Matiasek9, Karen Muñana10, Edward E. Patterson11, Akos Pakozdy12, Jacques Penderis13, Simon Platt14, Michael Podell15, Heidrun Potschka16, Veronika M. Stein17, Andrea Tipold17 and Holger A. Volk18 Abstract Epilepsy is one of the most common chronic neurological diseases in veterinary practice. Magnetic resonance imaging (MRI) is regarded as an important diagnostic test to reach the diagnosis of idiopathic epilepsy. However, given that the diagnosis requires the exclusion of other differentials for seizures, the parameters for MRI examination should allow the detection of subtle lesions which may not be obvious with existing techniques. In addition, there are several differentials for idiopathic epilepsy in humans, for example some focal cortical dysplasias, which may only apparent with special sequences, imaging planes and/or particular techniques used in performing the MRI scan. As a result, there is a need to standardize MRI examination in veterinary patients with techniques that reliably diagnose subtle lesions, identify post-seizure changes, and which will allow for future identification of underlying causes of seizures not yet apparent in the veterinary literature. There is a need for a standardized veterinary epilepsy-specific MRI protocol which will facilitate more detailed examination of areas susceptible to generating and perpetuating seizures, is cost efficient, simple to perform and can be adapted for both low and high field scanners. Standardisation of imaging will improve clinical communication and uniformity of case definition between research studies. A 6–7 sequence epilepsy-specific MRI protocol for veterinary patients is proposed and further advanced MR and functional imaging is reviewed. Keywords: Canine, Feline, Seizure, Imaging, Hippocampus Background Canine epilepsy has an estimated prevalence of 0.62– 0.75 % in primary veterinary practice [1, 2] and as such is one of the most common chronic neurological diseases. Magnetic resonance imaging (MRI) is regarded as an essential diagnostic test however the specificity is limited because the diagnosis of idiopathic epilepsy is one of exclusion and the reliability of diagnosis is limited by available technology and expertise in interpretation. The International League against Epilepsy (ILAE) defines idiopathic epilepsy as an epilepsy of predominately genetic * Correspondence: 1 Fitzpatrick Referrals, Halfway Lane, Eashing, Godalming GU7 2QQ, Surrey, UK 2 School of Veterinary Medicine, Faculty of Health & Medical Sciences, University of Surrey, Guildford GU2 7TE, Surrey, UK Full list of author information is available at the end of the article or presumed genetic origin and in which there is no gross neuroanatomic or neuropathologic abnormality [3]. Therefore by default, MRI examination of an animal with idiopathic epilepsy should be “normal” (in human epilepsy termed MRI–negative). However the ability to detect lesions depends on many factors that affect the quality of the MRI examination (Table 1). Some of these factors can be controlled, such as optimal slice thickness and sequence. Other factors are less easy to influence. For example, the ideal epilepsy protocol in humans (Table 2) would include a gradient echo or similar technique for detecting haemorrhage or calcification. However this sequence is sensitive to susceptibility artefacts arising from the skull bones for example the mastoid area of the temporal bone, which are a more significant problem in veterinary patients that have a greater bone:brain ratio © 2015 Rusbridge et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http:// creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Type Example Notes Protocol Slice thickness Thinner slices give more chance of lesion detection. A routine scan with 5 mm thick slices and 0.5 mm interslice gaps with T1W and T2W transverse image acquisitions and gadolinium contrast enhancement may be adequate to evaluate gross cerebral abnormalities such as large tumours or malformations but may not detect subtle epileptic lesions. Slice thickness of 3 mm or less in at least 2 orientations is recommended for examination of the epileptic brain and larger slice size risks missing lesions less than 5 mm [38]. However MRI machines of 1 T or less cannot provide thin slices with sufficient SNR within reasonable time. For this reason machines under 1.5 T are considered insufficient for the imaging of human epilepsy patients unless there is no alternative [38]. Sequence Failure or inability to select the appropriate sequences to detect lesions. For example in humans, high resolution, volumetric and 3D MRI acquisition is recommended to obtained detailed information on hippocampal anatomy, cortical gyral patterns, improve grey and white matter contrast and to enable co-registration with other modalities or sequential MRI examinations [13, 38]. This requires a good quality machine (1.5 T or more) and careful orientation of slice plane relative to patient position. FLAIR sequence is regarded as the most useful image for detecting epileptic lesions in humans [38] however many low field machines produce FLAIR with low resolution. Magnetic field strength Low field versus high field Imaging with higher magnetic field-strength provides improved signal-to-noise ratio and spatial resolution which allows shorter imaging times for a given resolution and/or higher resolution for a given imaging time. Higher signal-to-noise ratio allows better resolution with smaller voxel size and thinner slice thickness [7]. Coil Type of coil used (for example Knee vs Head coil) Coils with minimum distance between receiving coil and brain surface and minimal diameter increase SNR and therefore image quality. Some coils (for example brain coils) may limit the field of view that can be imaged before significant signal drop-off occurs. The lack of availability of dog-specific coils and variation in dog head size makes coil selection challenging in some cases. Available channels An 8 channel brain coil is usual in veterinary MRI but a 32 channel brain coil will provide much better SNR and contrast resolution. Inexperience / lack of training A fully trained radiography technician understands the physics of (...truncated)