The explosion of magnetic resonance imaging (MRI) has revolutionized neuroimaging. However, other modalities still are valuable when looking at the central nervous system. These include ultrasound and computed tomography (CT). Radiographs have limited usefulness due to the superimposition of structures associated with the cranial vault and vertebral column. Though radiographs are still the primary modality of choice when triaging a fracture. The purpose of this talk to show the benefits of advanced modalities such as ultrasound, CT and MRI for evaluating the nervous system.
Ultrasound is a unique modality since it is non-invasive and readily available to veterinarians. The drawback is that the modality is very user dependent and the images can be difficult to interpret. Since ultrasound uses sound waves to generate an image, the other, and most substantial drawback, is the inability to penetrate bone. For patient preparation, the head can be shaved to optimize the contact between the skull and the ultrasound probe. In addition, acoustic coupling gel is used. A high frequency linear transducer is usually used for the evaluation, but a microconvexed curvilinear probe can also be used. Care must be made not to use isopropyl alcohol near the eyes of the patient.
When imaging the skull with ultrasound, an open fontanel is preferred. This lack of fusion of the bone plates of the calvaria allows for a window though the top portion of the head to allow for the identification of the lateral ventricles, the cerebrum and the pituitary gland. Lateral ventricle to brain ratios can be performed to measure the overall size of the ventricles to the head. In most dogs, a ratio of 30% is considered to be severe enlargement; however, with small breed dogs, a ratio greater than 60% is usually needed for clinical signs.Computed tomography is a modality that uses x-rays to generate an image and is therefore much more intuitive to perform and interpret compared to ultrasound and MRI. The difference between CT and conventional radiography is that with CT, the x-ray tube circles the patient creating a 360-degree image of a region. Then the patient is moved and another image is generated. This eliminates the superimposition seen during radiography, but uses the same opacities as seen with radiographs. Because such a large amount of information is obtained, it is now possible to determine the difference between hemorrhage, mineralized disc herniation, water, fat even see sinusitis. These differences in soft tissue are improved when intravenous contrast medium is used. This iodine-based contrast medium enhances structures with a blood supply and helps separate subtle fluid and soft tissue differences that cannot be seen with conventional radiographs. In addition, due to neovascularization present with tumors, generally tumors of the central nervous system, such as meningiomas and metastatic disease, will enhance with contrast medium making them obvious to identify.