These procedures would not be possible without helical CT scanning capabilities. The CT gantry is where image acquisition
occurs. The gantry is a circular structure where X-ray generator is placed across from a row of X-ray detectors. During the
helical scanning procedure, the CT gantry continues to spin and acquires images without resetting itself due to "slip-ring"
technology, whereby an electrical connection is maintained without electrical cords. The older machines stopped after a 360-degree
rotation and then would have to back-rotate and repeat its movement. With helical scanning, the machine continues through
360 degrees and continues spinning in the same direction. It is useful in two ways. You can advance the table that the patient
is resting on as the gantry continues to spin creating a helical image path around the patient. The other thing that you can
do is to continue to acquire images within a prescribed amount of time indefinitely. They've taken an image every other second
giving a contrast bolus, stopping the infusion of contrast material and watching how it washes out of the tissue. The idea
is that in the normal non-injured horse with normal tissue, the contrast material will come in and wash out with the normal
vascular flow. As the blood leaves the area, the material will leave the area. In injured tissue, the contrast material just
sits there. In a time density curve, you actually can plot Hounsfield units vs. time on a graph and compare your clinical
case to graphs from normal horses. The horse that has inflammation or contrast retention in the interstitial space will not
wash out in a normal fashion.
Cases of CTA Use
Cases where this technique has shown major benefits usually have a relatively lengthy history of lameness with a consistent
blocking pattern of alleviating the lameness with a palmar digital (PD) nerve block. Specifically, they have identified contrast-enhancing
lesions of the deep digital flexor tendon (DDFT) and CL of the distal interphalangeal (DIP) joints.
"In my opinion, the use of CTA is really critical for people who are dealing with these foot-sore horses," Puchalski says.
There are a few different causes of heel pain in horses. It depends on whose literature you read, but there are definitely
some horses that have pain arising from the navicular bone itself, and another subset of horses that have pain arising from
soft-tissue injury, true tendonitis or desmitis. It is the DDFT that can exhibit true tendonitis or tendon tearing. Those
injuries, as they're below the coronary band and the bulb of the heels, cannot be diagnosed on ultrasound, which would be
the typical modality for diagnosing tendon injuries anywhere else. The hoof capsule is in the way, and the probe has to be
perpendicular to the tendon fibers in order to get a true image. As the tendon wraps down and around the navicular bone, it
changes direction or curves. So it is very difficult to get perpendicular to it while penetrating the hoof capsule. Its anatomy
presents a significant problem to properly assess it with ultrasound. The other place that injuries can occur is the collateral
ligaments of the DIP, which could tear. Desmitis or actual tearing can occur. That can be seen on ultrasound for a limited
length from its origin, but as soon as it goes below the coronary band—which it still has significant length of—it can't be
seen on ultrasound, either. Those are two specific areas that are difficult to diagnose by any other modality.
Advice for the Use of CTA for the Future
MRI is very sensitive to these types of foot-tissue injures, but only gives you a morphologic assessment of the tissues, CTA
provides a functional test, a quantitative measure and more detailed enhancement. MRI is very useful morphologically for these
types of injuries, though you can't do the dynamic studies such as with CT angiography.
CTA is a more-efficient process; scans are done in about 30 minutes, while most facilities doing MR are taking between one
and two hours. The actual CT scanning procedure takes about a minute. The balance of the procedure is involved in positioning
within the CT scanner and the catheterization. The actual CT scan, acquiring the image takes less than a minute. It's a real
key feature of CT vs. MRI, which takes about two hours to acquire an image.
"I think that CT would be a very good investment for a private practice for many reasons," Puchalski recommends.