Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

662 Small Animal Radiolo g y and Ultrasono graphy
material under its ventral part, and the pelvis should be supported by placing a small
amount of radiolucent material between the stifles, causing the femurs to be parallel to the
table top.
The ventrodorsal views require the animal to be positioned so that the spine is in a
straight line and the sternum is directly over the center of the vertebral column. This is
most easily accomplished with sandbags or a Plexiglas cradle supporting the patient on
each side of its body.
On occasion, it may be difficult to assess a change on the ventrodorsal view because of
superimposed intestinal gas shadows. This may be overcome partially by using a prolonged
exposure time and allowing the patient to breathe or mechanically ventilating the patient
during the exposure. This will blur the image of the gas-containing structures and will
allow the spine to be seen more clearly.
CONTRAST STUDIES
M Y E L O G R A P H Y
A myelogram is performed to opacify the subarachnoid space and thereby delineate the
spinal cord. Iohexol, a nonionic contrast material, currently is the most frequently used
agent, although both iohexol and iopamidol appear to be equally effective. 2,3 Iohexol is used
routinely at a concentration of 240 mg/ml and iopamidol is used at a concentration of 200
mg/ml, although concentrations varying from 180 to 300 mg/ml have been used. The contrast
medium may be introduced by injection into the subarachnoid space at either the cisterna
magna or in the caudal lumbar spine. In most cases, the site of puncture closest to the
suspected lesion is preferred. In cats, lumbar puncture was superior to cisternal puncture for
evaluation of the thoracolumbar spine, and cisternal puncture was better for evaluation of
the cervical spinal cord. 4 The volume of contrast required will vary with the area of interest
and site of introduction. If a cervical puncture is used for evaluation of the cervical spinal
cord, a dosage of 0.3 ml/kg of body weight is recommended. If examination of the lumbar
spinal cord also is desired, the contrast dosage should be 0.5 ml/kg of body weight. If a lumbar
puncture is used, the dosage is 0.35 ml/kg of body weight for the thoracolumbar region
and 0.5 ml/kg for the cervical region. If the patient is grossly underweight or overweight for
the general body size, the dosage should be based upon the ideal body weight. It is possible
to influence the distribution of the contrast, which has a specific gravity slightly higher than
normal cerebrospinal fluid (CSF), by positioning the patient so that gravity causes the contrast
medium to move to the area of interest. This can be accomplished by tilting the patient
so that the area to which you wish the contrast to flow is dependent; however, in some cases
this may require suspending the patient vertically for at least 5 to 10 minutes. In some
instances obstruction to contrast flow cannot be overcome by gravity.
Contrast injection into the subarachnoid space produces alterations in the CSF. These
alterations include increased numbers of neutrophils, red blood cells, and total protein,
pleocytosis, and a decrease in percentage of mononuclear cells. These changes usually disappear
within 24 hours but can persist in some dogs for up to 72 hours. 5
Because hazards and potential complications can occur, myelography must be performed
with great care. Injections via a cisternal puncture put the patient at risk for cervical
spinal cord injury. In animals with occipital dysplasia, the cerebellum may be displaced
caudally and may be damaged as a result of the puncture. Injection of contrast medium in
the caudal lumbar subarachnoid space requires puncture of the spinal cord. 6 If performed
at the L5-6 space or at spaces more caudal to L6, there usually is no neurologic sequela.
When necessary, the L4-5 space has been used without problem, but there is increasing risk
of iatrogenic spinal cord trauma as the selected site progresses cranially from L5-6.
During myelography, some contrast medium may be inadvertently injected into the
subdural space, extradural space, or central canal. 7-9 In cervical punctures the most likely
difficulty is injection into the subdural space. 8-10 Injection at this site results in the radiographic
appearance on the lateral view of contrast localization dorsally in the vertebral
canal, deep to the dura mater but superficial to the subarachnoid space. The ventral margin
of the contrast typically has a wavy or undulating edge. 8 The dorsal margin is usually
smooth. The ventrodorsal view is usually within normal limits.