The Uses of Advanced Imaging Modalities for Orthopedic and Spinal Cord Diseases
Amy Kapatkin, DVM, Dip. ACVS, Assistant
Professor of Surgery
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are two imaging modalities that are becoming very available to the veterinarians for orthopedic and spinal cord conditions. Many specialty practices, university clinics have these modalities available to them on site and specific imaging clinics are starting to be built. Cost of buying and maintaining the equipment as well as sufficient room and expertise to manage these modalities are probably the only reason why they are not more widely used. With imaging clinics starting to form, hopefully these will be more available for any practitioner to use.
The basic way CT functions is by accumulating multiple X-ray images of a slice of tissue, and then computes them into a single image that represents information from several slices of that particular tissue. The major advantages of CT over conventional radiography are that while radiographs can identify 5 opacities, the CT can decipher up to 2000 densities (Hounsfield units). CT's also improve spatial resolution and gives a perception of depth to the image. Other helpful properties of the CT are its ability to eliminate superimposition of structures, allows viewing in different "windows" to look more for soft tissue versus bony changes and the 3-D CT's can reconstruct the image and make it appear as the bone in space with 360 degree observation. Radiographic contrast can be used intravenously to enhance CT lesions as well as myelography, or arthrography.
There are some limitations of the CT; they include ability to distinguish between soft tissues densities as well as distortion when metal objects are in the field of the scan. For example, a case that had previous hip surgery with a 316L steel plate in the ilium, it will be difficult to get a good scan of the lumbar-sacral spine area. There can be other distortions of the CT scan and therefore, trained radiologists are needed in interpreting the studies in some instances.
The talk will show uses for the CT for clinical cases; OCD lesions, elbow dysplasia, bone tumor margins and pathology and how these lesions were difficult to determine on standard radiography alone. Uses of the CT in the lumbar-sacral spine conditions as well as other spinal lesions will be shown.
MRI uses magnetic properties of tissues and not X-rays. The patient's free protons in tissues are processed by alignment, release and realignment and then a radiofrequency signal is collected and converted to a tissue image. Depending on the sequence of the processing, different tissues will appear as different densities and contrasts on the images generated. In general, the T-1 weighted images emphasize anatomical characteristics of tissues while T-2 weighted images emphasize fluid characteristics of tissues. For example: high intensity on T1 = fat, high protein, gadolineum contrast, melanin, Mg, Ca and low intensity on T1 = water, CSF, calcified tissues. High intensity on T2= water & CSF and low intensity on T2 = fat, high protein, calcified tissue, gadolineum contrast. Other sequences are used to help delineate the pathology in certain diseases and once again, a trained radiologist is often needed to be at the scan. They can determine if these sequences are indicated for a suspected condition while they are looking at the usual sequences. The magnetic field strength is in Telsa; most magnets used in veterinary medicine range between 1-1.5 T.
Contrast used in MRI is gadolinium given intravenously. It is used with T-1 weighted images. Arthrography and myelography can also be used with MRI. MRI's are useful for ligament and tendon injuries, meniscal injuries, bone pathology, and brain and spinal cord lesions. This talk will show examples of the use of MRI in some of these conditions.