Page 229 - Adams and Stashak's Lameness in Horses, 7th Edition

Diagnostic Imaging 195

• Pregnant personnel should have limited role in X‐ray tract. The volume needed for injection is not consistent,
examination and use additional monitoring (waist and when fistulograms are performed, the injection of
VetBooks.ir • Chemical restraint and positioning devices should be material should be injected until back pressure is felt on
an insufficient volume is a common error. Contrast
badge under lead gown).
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used to minimize motion and personnel exposure and
the syringe plunger or external leakage is observed.
limit retake examinations A single radiograph can also be made to determine if an
• Use a radiation‐monitoring system adequate volume of contrast material was injected. After
contrast material has been injected to delineate the
The veterinarian in charge is responsible for the radia­ entire tract, orthogonal radiographic views should be
tion safety practices used by his/her employees. Providing made for complete evaluation. Any contrast material
necessary radiation safety equipment and following that leaks onto the skin surface should be removed
these rules should keep exposure levels below the limits before radiographs are made.
recommended by the National Council on Radiation A fistulogram may demonstrate (1) the extent and
Protection and Measurements. 40
direction of the tract to aid in surgical exploration, (2)
communication with underlying soft tissue structures
CONTRAST EXAMINATIONS (e.g. tendon sheaths or synovial joints) (Figure 3.6a), (3)
osseous involvement (e.g. sequestra or osteomyelitis
A contrast radiographic examination consists of associated with surgical implants), and (4) filling defects
using a radiocontrast agent to better define suspicious (which appear radiolucent because of displacement of
15
lesions detected clinically or radiographically but not contrast material) (Figure 3.6b). Filling defects may be
distinctly seen on survey radiographs. Nonionic, iodi­ caused by fibrous reaction within the tract or by foreign
nated contrast material is most useful for contrast exam­ bodies. Fibrous tissues generally have irregular borders,
inations in lame horses. While previously cost prohibitive, whereas foreign bodies such as wood splinters have
nonionic formulas are now more affordable and readily sharp, straight borders. Small foreign bodies may not
available and are safer than ionic contrast material. be identifiable on a fistulogram because of the overlying
Positive‐contrast agents are commercially available in opacity of the contrast material. In such cases, ultrasound
an injectable form and include iohexol (Omnipaque imaging may provide additional diagnostic information. 13
[Nycomed Inc, 90 Park Avenue, New York, NY]) and
iopamidol (Isovue [Bracco Diagnostics, Princeton, NJ]).
The use of negative‐contrast agents (gas) has been Myelography
reported but has not found widespread, routine accept­ Myelography in the horse is used to substantiate
ance. Procedures most commonly performed are injec­ cervical spinal cord compression suspected from a neu­
3
tion of a draining tract (sinography or fistulography) rologic and/or radiographic examination. It also serves
and myelography. Other contrast examinations such as to identify the location, extent, and type of compressive
arthrography and tendonography have become less lesion present, which is necessary for determining the
popular and in the majority of cases have been replaced prognosis and indication for surgical intervention.
by ultrasound, computed tomography, magnetic reso­ Prior to myelography, survey radiographic examina­
nance imaging, or arthroscopy. tion of the entire cervical spine is necessary. Survey radi­
ographs should include neutral lateral images as well as
Fistulography or Sinography right 45–55° dorsal‐left ventral and left 45–55° dorsal‐
right ventral oblique images. The radiographs should be
Fistulograms provide valuable diagnostic informa­ assessed for malalignment, cervical vertebral malforma­
tion when chronic draining tracts or recent traumatic tion and stenosis, articular facet pathology, fractures
puncture wounds are present. Survey radiographs of and osteochondrosis, disk space or endplate changes,
the area should be made first. If the source or cause of the and any other potentially relevant lesions (Figure 3.7).
draining tract or puncture wound is not clearly identified The accuracy of the subjective evaluation of survey radi­
on the survey radiographs, a fistulogram can be per­ ographs for predicting a compressive lesion has been
formed to obtain additional diagnostic data. 13,15,33,36 reported to be 70% at C3–C4 and only 40% in all other
The technique consists of injecting undiluted water‐ levels of the cervical spine. 37
soluble triiodinated contrast material into the draining There are different methods for quantitatively assessing
tract as aseptically as possible. Water‐soluble contrast cervical spine survey radiographs to try to predict a possible
material is used because it is less viscous and penetrates compressive spinal cord lesion. Moore et al., in 1994,
chronic draining tracts more easily than oil‐based con­ suggested the intravertebral sagittal ratio that represents
trast material. To avoid contrast material draining from the ratio of the minimum sagittal diameter of the verte­
the tract after injection, an inflatable, cuffed (Foley) bral canal to the maximum sagittal diameter of the cor­
catheter or a small polyethylene tube inserted some dis­ responding vertebral body, obtained at the cranial aspect
tance into the tract can be used before injecting the of the vertebra and perpendicular to the vertebral
38
material. Filling is best accomplished if the contrast canal. A sagittal ratio of less than or equal to 50%
material is injected under pressure; thus, some form of from C4 to C6 or less than or equal to 52% at C7 is a
occlusion of the tract opening is necessary. strong predictor (26.1–41.5 likelihood ratio) of verte­
When the distal extremity is being examined, it is bral canal narrowing. The sensitivity and specificity of
important to flex and extend the region slowly. This this method for detecting cervical stenotic myelopathy is
allows a tract that may be closed while the horse is greater than or equal to 89% at each vertebral site from
standing to open up, permitting the contrast to enter the C4 through C7. Mayhew and Green, in 2000, reported
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