MRI vc CT.

Though both CT and MRI provide cross sectional imaging there are definite differences in the information each can provide.  In general CT tends to be better at imaging osseous structures, though it provides much higher soft tissue contrast resolution than radiography.  MRI provides exceptional soft tissue contrast resolution that is especially valuable in the evaluation of brain and spinal cord, and for soft tissue musculoskeletal problems.  Because of the sophisticated instrumentation needed for MR imaging, it is more costly than CT.  The following list will provide referring veterinarians with information on the indications for MRI or CT imaging for different clinical problems:

• Seizures or central CNS signs: MRI is preferred for brain imaging. Contrast is used to enhance recognition of neoplastic and inflammatory lesions. Much information is obtained regarding prognosis and options for surgery or radiation therapy. In most cases, neoplasia can be differentiated from inflammatory diseases and vascular compromise. Sometimes, CT guided biopsy or recheck scans are necessary for a diagnosis.
• Head trauma: CT is preferred if imaging is performed in the first 24 hours after injury, because acute hemorrhage is visible. After this time the soft tissue injuries are less defined and MRI is preferred.
• Nasal disease: Both MRI and CT are very good at imaging the nasal cavity for neoplasia and rhinitis. Both can identify the extent of disease and evaluate for cribiform compromise. Because of its cost effectiveness and ability to fully evaluate the bony structures, CT is preferred.
• Soft tissue neoplasia: Cross sectional imaging is invaluable in determining the extent of disease for surgical planning, especially in cases of soft tissue sarcoma. MRI is the modality of choice, but contrast enhanced CT is almost as effective.
• Ocular and orbital masses: MRI is indicated to fully visualize the extent of disease and to rule out intra cranial involvement. Ultrasound is also very useful in the initial evaluation of these cases, to differentiate neoplasia from cellulitis, and to obtain biopsies.
• Spinal cord disease: MRI is preferred and is very sensitive in identifying a lesion and differentiating neoplasia, disc herniation, and embolic disease. In cases of lumbosacral degeneration it can determine if surgery is indicated. If the lesion cannot be fully localized to a specific segment of the spinal column, myelography and electrodiagnostics are often indicated as the first procedures.
• Unilateral lower motor diseases: MRI is very sensitive at identifying lateralized disc herniation and nerve root tumors.
• Elbow dysplasia: CT is very useful for identifying coronoid process fractures.
• Partial cruciate rupture: MRI is indicated if confirmation of a damaged cruciate is requested previous to surgery. Meniscal tears are also visible with MRI.
• Shoulder lameness: At this time, the combination of radiography and ultrasound evaluation is indicated to evaluate for bicipital tenosynovitis, supraspinatous injury, or OCD.
• Chronic otitis: CT is effective at identifying otitis media/interna vs. neoplasia, and to determine the need for surgery.
• Thorax: CT is more sensitive than radiography for identifying pulmonary metastasis. It is also useful in the surgical or radiation treatment planning of body wall and mediastinal masses. MRI is limited by the inherent motion in the chest without sophisticated imaging techniques.
• Abdomen: At this time, ultrasonography tends to provide as much or more information in animals.