Brain and Spinal MRI
When performing Magnetic Resonance Imaging (MRI) on a patient, a magnetic field is applied to the body to align particles within the tissues. The magnetic field is interrupted and the coils monitor the particles return to their nature alignment. This modality is ideal for evaluating tissues with various amounts of hydrogen, particularly water content. MRI is commonly used to examine the brain, spinal cord, spinal canal, muscles and soft tissues of the abdomen.
CT myelography is often done following standard myelography. CT provides exquisite spatial resolution. The acquisition of slices through a specific abnormal area previously identified on standard myelography may more clearly define the exact location and side of the lesion along the vertebral column.
This is particularly important for further defining the extent and sidedness of disc extrusion prior to surgery, although is often employed for any disease entity causing a focal abnormality on a standard myelogram.
X-ray energy is used in a similar manner to radiography in CT scanning. In CT imaging, the x-ray tube rotates quickly around the patient, within the gantry. The detectors rotate in a synchronized manner, on the opposite side of the gantry, absorbing the x-rays that pass through the patient. Based on the absorbed or detected energy, the computer builds a 3D image of the patient, as a sum of many small volumes (voxels) of data. The whole patient may be thought of as a loaf of bread. Each CT image is a slice of bread that is viewed on the cut surface. The loaf may be sliced in any direction. This modality is ideal for examination of complex body parts (i.e., skull, pelvis, vertebrae) where superimposed, irregularly shaped structures make radiographs challenging to interpret.
CT is ideal for examination of the thorax (chest) and abdomen (belly) for small masses that may not be apparent by routine radiographs or ultrasound exam. CT is helpful for evaluation of the skull and brain in cases of acute head trauma when skull fractures and recent bleeding are a concern. Assessment of the temporomandibular joints, temporal bullae, nasal passages, sinuses, pharynx, larynx, orbit and periorbital structures, maxilla and mandible are common applications of this modality. Identifying the extent of a soft tissue or bone mass for surgery or radiation therapy planning is also done by CT.
Spinal Radiography and Fluoroscopy
Radiography of the vertebral column
Following neurological examination, if the presenting problem is identified to be located in the spine, survey radiographs of the vertebral column (spine) can provide very important information. The vertebral column is composed of individual vertebra and each is slightly different in shape due to change in function at different levels of the column. There are seven cervical (neck) vertebra, thirteen thoracic vertebrae (chest), seven lumbar vertebrae (abdomen), three sacral vertebra- fused (pelvis), and multiple coccygeal vertebra (tail). Between each vertebra is an intervertebral disc which functions to allow restricted movement of the spine.
Radiographs are generally inexpensive, readily accessible, and are rapidly acquired. Radiography of the vertebral column is particularly useful when a disease or abnormality is of the vertebrae itself, or results in displacement of the vertebrae. Some examples of diseases include: malformations of the vertebrae, vertebral fractures or luxations, tumors (cancer) affecting the bone causing destruction or proliferation of bone, or bone infection causing loss of bone. Some diseases may cause mild changes to the bone or width of the intervertebral disc space such as intervertebral disc extrusion or protrusion. Care should be taken not to over interpret survey spinal radiographs in intervertebral disc disease. The utility of radiographs should not be underestimated especially where there is concern for column instability such as fracture or luxation.
Each vertebra has a complex three-dimensional shape, and yet imaging with radiography provides only two dimensional images of these complex structures. Consistency and care regarding the positioning of the patient and radiographic technique (x-ray beam energy) is essential because otherwise subtle abnormalities may be overlooked or normal structures may be misinterpreted as being abnormal. It is important to collect images in at least two planes (orthogonal): lateral (lying on the side) and dorsoventral (lying on the back), unless column instability is likely (trauma). Oblique images between these two orthogonal planes may also be used. To adequately position an animal, supportive devices such as foam pads may be required. Adequate sedation and pain control, or preferably, anesthesia is needed for good radiographs of the column. Manual restraint should not be performed as it is important that the technician not remain in the room when the radiograph is taken. Continued exposure to high levels of ionizing radiation (x-rays) is harmful, and for this reason, there is NO reason a technician should be exposed to the x-ray beam or scatter radiation.
Limitations of survey column radiography are that only the bony elements are adequately imaged; the spinal cord, nerve roots and meninges (covering spinal elements) are not able to be visualized. For this reason, additional advanced diagnostic techniques are used such as myelography, computed tomography and magnetic resonance imaging.