Principles of Neurological Imaging of Exotic Animal Species

doi:10.1016/j.cvex.2007.05.001

Veterinary Clinics of North America: Exotic Animal Practice

Volume 10, Issue 3, September 2007, Pages 893-907

https://doi.org/10.1016/j.cvex.2007.05.001 Get rights and content

Because the central nervous system (CNS) is encased almost entirely in bone, the means by which the clinician can evaluate it are limited. Additionally, the small size of many exotic companion animals further complicates diagnostic evaluation of the brain and spinal cord. Knowledge of the advantages and limitations of different imaging modalities, along with the neuroanatomical localization and assessment of likely causes of disease, will permit the clinician to choose the most appropriate imaging method for the patient. This article discusses the basic imaging principles of radiology, myelography, CT, and MRI of the nervous system of companion exotic animals to aid exotic animal clinicians in selecting imaging modalities and interpreting the results.

Section snippets

Localizing the lesion for imaging purposes

Before proceeding with any type of imaging, the clinician must be sure he or she is evaluating the appropriate area of the CNS. A thorough neurological examination is necessary to permit localization of a lesion to one (or more) of the main divisions of the nervous system: brain, spinal cord, or neuromuscular system. A patient presenting with tetraparesis could have brain disease, cervical spinal cord disease, or even generalized neuromuscular disease. The diagnostic plan for each of these

Radiography

Radiographic evaluation of the skull and vertebral column is the most economic and readily available imaging modality to the exotic animal clinician, and several publications describe restraint, positioning, and normal radiographic anatomy [1], [2], [3], [4]. Unfortunately, radiographic evaluation of the CNS only will provide information about skeletal abnormalities, and cannot definitely provide the clinician with information about the neural structures. If the patient presents for paraplegia,

Myelography

Myelography is performed after completion of a survey radiographic study of the vertebral column, and it is indicated to evaluate the neural structures. Nonionic iodinated contrast is injected into the subarachnoid space to delineate the spinal cord and identify any compression or distortion of the spinal cord (Fig. 2). If possible, cerebrospinal fluid (CSF) should be collected for analysis, before injection of the radiographic contrast material, to rule out infectious or inflammatory disease.

Computed Tomography (CT)

Routine access to CT is becoming increasingly common, and many specialty practices have a CT scanner on-site. Normal CT anatomy of the skull of some exotic animal species has been published [4]. CT uses a radiograph tube and an opposing detector array that are passed around the patient in a 360-degree arc at different intervals. The information obtained at each of these intervals is processed by the computer to form an image of the section (or slice) of tissue at that site. Each slice is a

Magnetic Resonance Imaging (MRI)

Magnetic resonance technology is very different than radiography and CT. Instead of attenuation of radiographs, MRI uses the molecular composition (particularly the hydrogen atoms, also referred to as protons or spins) of tissue to obtain an image by the principles of nuclear magnetic resonance. The patient is placed within a powerful static magnetic field that aligns the protons in the tissue parallel to the field. Magnet strength is measured in units of Tesla (T), where one T is about 20,000

Increased magnet strength

Increased field strength will increase SNR. High-field magnets (>4 T) produce exquisitely detailed images on small patients, but are less available to clinicians than 0.5 to 1.5 T magnets. Compare the grainy image quality of Fig. 9, obtained with a 0.5 T magnet, with the image quality of Fig. 8, obtained with a 1.5 T magnet.

Coil type

The closer the receiver coils can get to the tissue imaged, the better the SNR. Different types of coils will permit this. Commonly used coils for MRI in veterinary medicine

Summary

Imaging the CNS of companion exotic veterinary patients frequently is required to accurately diagnose and subsequently treat their diseases. Based on the patient's neuroanatomical localization and the knowledge of the perks and pitfalls of the different imaging modalities, the clinician can choose the most appropriate modality to evaluate the brain or spinal cord. Because the availability and use of advanced imaging are increasing in veterinary medicine, it is important for exotic animal

Acknowledgments

The author thanks Richard Larson and Jason Peters, for acquisition of images, John Doval for assistance with figures, and Eric Johnson, DVM, Diplomate ACVR, for technical consultation.

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As a result, these diagnostics are rarely performed in private practice. Another form of contrast radiography, myelography, is used in addition to survey films to more fully evaluate the spinal cord.36 An example of successful myelography in a clinically unremarkable guinea pig has been published.31

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Principles of Neurological Imaging of Exotic Animal Species

Section snippets

Localizing the lesion for imaging purposes

Radiography

Myelography

Computed Tomography (CT)

Magnetic Resonance Imaging (MRI)

Increased magnet strength

Coil type

Summary

Acknowledgments

Seminars in Avian and Exotic Pet Medicine

Vet Clin North Am Exot Anim Pract

Vet Clin North Am Exot Anim Pract

Seminars in Avian and Exotic Pet Medicine

Neuroimage

Atlas of diagnostic radiology of exotic pets

Psittacine skull radiography

Veterinary Radiology

Radiology of rodents, rabbits, and ferrets

Osteomyelitis associted with Salmonella enterica ss. arizonae in a colony of Ridgenose rattlesnakes (Crotalus willardi)

J Zoo Wildl Med

Proliferative osteoarthritis and osteoarthrosis in 15 snakes

J Zoo Wildl Med

Neurology

Birds—their structure and function

Musculoskeletal and neurologic diseases

A myelographic technique for avian species

Vet Radiol Ultrasound

Spinal osteomyelitis in a green iguana,Iguana iguana: cerebrospinal fluid and myelogram diagnosis

Risk factors associated with development of seizures after use of iohexol for myelography in dogs: 182 cases (1998)

J Am Vet Med Assoc

Complications associated with the use of iohexol for myelography of the cervical vertebral column in dogs: 66 cases (1988–1990)

J Am Vet Med Assoc

A prospective clinical trial comparing metrizamide and iohexol for equine myelography

Vet Radiol Ultrasound