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MRI: Where to Start

Bess P. Brosey, MZS, DVM, Diplomate ABVP & ACVIM

Imaging

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August 2004

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Magnetic resonance imaging (MRI) is a rapidly evolving technique that is becoming the imaging method of choice in small animal medicine because of its ability to produce superior, high-contrast, anatomically detailed images. Traditionally used for diagnosing diseases of the central nervous system, applications have broadened to include diagnosis of many common disorders, among them musculoskeletal injuries and diseases, nasal neoplasia, eye and orbital disease, and vascular malformations. More will probably be added. This article simplifies the science behind MRI and compares it with computed tomography (CT), radiography, and ultrasonography, highlighting the reasons for choosing MRI over other methods and challenging conventional wisdom. Technologic advances in both MRI and CT have made distinguishing the two techniques more subtle, with advantages and disadvantages in some cases related more to cost and availability than to image quality. Because MRI does not use radioactive isotopes, it virtually eliminates risk for local or systemic adverse reactions to contrast media. It documents the extent of disease more completely, which is usually borne out surgically or histologically. Radiography, while helpful, lacks the tissue specificity required for locating the origins of disease. Other advantages of MRI are the range of organ systems that can be imaged and the value of sequential magnetic resonance images in planning biopsy, surgery, and therapy. However, MRI is not considered appropriate or cost-effective in cases where radiography or ultrasonography would suffice. Indications for MRI use and the basic science of MRI are covered in detail and a continuing education test is included at the end of the article.

Indications
Neoplasms-Tumors, cysts, papillomas, carcinomas, abnormal cell infiltrates, and some benign tumor types (with contrast medium, such as gadolinium diethylenetriamine pentaacetic acid)
 

Inflammatory/nonneoplastic conditions-Encephalitis caused by viruses, neurologic manifestations of infections, hypoplasias (confirmed with histopathologic evaluation)
 

Infarction-Nonhemorrhagic, hemorrhagic
 

Seizures-Intracranial lesions (no metabolic disease in patient)

Spinal cord and vertebral column disorders-Spinal pain, paralysis, ataxia, arachnoid cysts, lumbosacral disorders, cervical myelopathy

Nasal tumors-Chronic discharge, bleeding
 

Eye and orbital disease-Tumors, optic nerve disease, third eyelid disease, abscess, foreign body reaction, metastatic disease, intracranial vascular lesions
 

Musculoskeletal conditions-Use when radiography is not sufficient; superior to CT for nonmineralized cartilaginous fragments, changes in cartilage, osteophytosis, intraarticular loose bodies
 

Vascular abnormalities-Both the noninvasive MRI and magnetic resonance angiography closely resemble conventional angiography for malformations, fistulas, aneurysms, thrombi, shunts, cavernous sinus
 

Skin disorders-Bullae

COMMENTARY: MRI is a valuable imaging method that has become more commonly used as a diagnostic tool for a broad range of applications (aside from the usual central nervous system studies). This article is an excellent review of the theory, applications, and advantages/disadvantages of MRI-a great place to start for any clinician considering using this tool in small animal practice.

Magnetic resonance imaging in small animal medicine: clinical applications. Pooya HA, Sequin B, Tucker RL, et al. COMPEND CONTIN EDUC PRACT VET 26:292-301, 2004.

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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