Computed tomography (CT) scanning has many uses in equine medicine and surgery. It can provide a great amount of information
useful in the diagnosis and treatment of equine leg and foot lameness, including the evaluation of soft tissue injuries and
fractures. It is also beneficial for evaluating equine head lesions such as skull fractures, sinus and dental problems, bone
cysts and bone infections.
CT is an anatomical imaging modality using X-rays and X-ray attenuation to produce a cross-sectional image. It allows practitioners
to gain a better understanding of the structure of the imaged body tissue. It is a digital modality, so images can be computer-manipulated
to enhance clarity, contrast and brightness and to zoom, rotate and measure. Additionally, specialized computer software allows
images to be reformatted into different imaging planes.
While conventional radiography produces summed images of an object, tomographic scanners rotate the object around, dividing
and organizing it into spatially consecutive image slices. Compared with common radiography, the elimination of superimposition
and improved resolution are the major advantages of CT. Images are produced as the portion of the equine anatomy on the CT
table moves through the circular tunnel of the CT gantry (Photo 1). An X-ray tube within the CT housing emits X-rays as it
encircles the patient within the gantry tunnel. A detector array, opposite the X-ray tube, measures the X-rays that pass through
the tissues, and computer-generated cross-sectional images are constructed from the data.
Photo 1: A horse positioned on a specialty CT table undergoing a CT scan. All horses are anesthetized for the procedure to
prevent movement and injury.
Helical scanning now allows the patient to advance through the center of the gantry as it continues to rotate. This means
that there is no waiting between image slices while the table is moved. The number of slices obtained per revolution depends
on the number of CT detector rows, but it can range from 1 to more than 126. Each slice can vary in thickness from less than
1 mm to 15 mm. In general, it takes about one second per revolution, making some CT scanners ultrafast.
Originally designed for human medicine, computed tomography angiography (CTA) takes CT technology one step further. For this
technique, contrast medium is used to visualize blood flow through vessels. CT then creates cross-sectional images, which
are assembled by computer into three-dimensional pictures of the area being studied. Using CTA, vasculature of the equine
distal limb can be assessed after traumatic injuries, thereby allowing equine surgeons to assess injury to blood vessels as
well as to the bone.
This technique for CTA can be also be used to provide contrast enhancement of the soft tissues of the equine limb. This enhanced
form of CT helps to clearly identify and evaluate tendon and ligament lesions. Contrast agents are used with both magnetic
resonance imaging (MRI) and CT to define areas of increased perfusion and altered permeability, which allows for better margin
definition of neoplastic and inflammatory lesions.