The medial aspect of the carpus is clipped and aseptically prepared for the catheter insertion. A high-resolution linear ultrasound
probe is used to identify the median artery, and ultrasonographic guidance is used to place a regular over the needle catheter
into the artery. Once the catheter is placed, it is connected via pressure tubing to a remotely controlled pressure injector.
Prior to any contrast administration two scout views—amounting to lateral and DP radiographs—are made. The scout views are
used as guidance to determine the location for the CT scanner. Cross sectional images of 5-millimeter thickness are then obtained
from the mid portion of the first phalanx to the distal extent of P3. This initial study is used to evaluate the bone column
and to compare the cross-sectional post-contrast images. The next study is a repeat of the first run of cross sectional images
except that the pressure injector is connected to the median artery catheter, and diluted iodinated contrast material is injected
for the duration of the second scan. The contrast material will extravasate and accumulate in regions of altered permeability
and perfusion secondary to inflammation. A third study is performed, and this study documents the wash-in and wash-out of
contrast material in a given area, chosen after an abnormality is identified. Using custom-imaging software, a time-density
graph can be developed and compared to the time-density graph of a normal horse for any given area. The graph allows for quantitative
comparison of the amount of contrast in the tissues not in the vasculature.
With CTA, an iodinated contrast material shows up as "white" upon X-ray. It attenuates the X-ray beam more than the surrounding
soft tissues. The contrast material is infused directly into the arteries. It gives you a localized specific delivery of the
contrast agent. If the vasculature is normal, the contrast agent should stay within the vascular space. If there is inflammation,
neoplasia or re-vascularization, then an increased number of blood vessels, will cause an increased amount of contrast material
to go into the area. The beauty of CT is that it can be measured quantitatively. The amount of X-ray attenuation increases
linearly with the amount of contrast material that is present. If it's whiter, then it means there is more contrast material.
With each insult or injury-damaged tissue, it is going to be a range from glaringly obvious to a subtle lesion. There is a
set of values for how much normal tissue should enhance compared to injured tissue. Normal tissue should not show very much
enhancement if at all, while injured tissue would enhance significantly. With CT angiography, there is a quantitative measure
called Hounsfield units.
"What we look for is the number of Hounsfield units that the tissue increases," Puchalski explains.
For example, the DDFT, should not increase more than 10 Hounsfield units. With contrast, with injuries, it enhances more than
that, from 20-40 units. The nice thing about it is that you can actually measure it, which is very helpful when trying to
identify more subtle lesions, i.e. areas of inflammation, tissue edema, and neovascularization, increased blood-vessel recruitment.
With injury, not only is there measurably increased vascularization but also an increase in the amount the blood vessels leak,
or the vascular permeability. The contrast agent leaks out of the blood vessels into the interstitial space and just sits
there. Instead of just washing out with the normal blood flow, it is retained, i.e. leaked into the interstitial space, shown as enhancement.