"We began using CT angiography to evaluate the vasculature of the distal limb after traumatic injuries and realized that contrast
enhanced CT could be an alternative to magnetic resonance," Puchalski states.
With contrast enhancement, inflammatory lesions tend to show up very well.
Lameness arising from the heel, previously termed navicular syndrome, is one of the most commonly diagnosed lameness conditions
in performance and pleasure horses. The diagnosis is usually non-specific, and treatment programs often are prolonged. Historically,
this pain-causing lameness was thought to arise from the navicular bone itself, but it has been shown that a number of different
soft-tissue injuries—including deep digital flexor tendonitis, collateral desmitis, navicular suspensory ligament injuries
and digital cushion injuries—also possibly are occurring. Current diagnostic techniques, including radiography, ultrasound
and nuclear scintigraphy, frequently fail to identify the injured structure. Contrast enhanced CT allows for evaluation of
both the soft tissue and bony structures in a cross-sectional manner, eliminating the problems of superimposition of bones
and the hoof capsules.
Initially, CTA in the horse was performed to assess the distal limb vasculature. The technique for ultrasound-guided catheterization
of the median artery was developed for this purpose. Conversely, the contrast agent in the enhanced CT is delivered directly
to the distal limb via the arterial supply. Contrast agents are used in cross-sectional imaging of both MRI and CT to define
areas of increased perfusion and altered permeability. This generally allows better margin definition in both neoplastic and
In the equine in general, contrast-enhanced studies are cost and time prohibitive because the dose to an adult horse is between
1 liter and 2 liters. This technique of median-artery catheterization has allowed for selective contrast-enhanced CT studies
of the soft tissues of the distal limb without having to administer large quantities of contrast material in a non-specific
fashion, such as the jugular vein.
Interestingly, the purpose of the CTA procedure for the median artery catheterization was developed for orthopedic breakdown
injuries. After evaluating the images for the vascularity, the hypothesis was raised that horses with soft-tissue injuries
to the foot would have altered vascular supply and possibly contrast enhancing lesions in the soft tissues. The technique
for guiding the catheter into the median artery with ultrasound was refined further. It is a bit tricky approach, but is doable.
You can get it with ultrasound guidance. There is a research group in the College of Veterinary Medicine at the University
of California-Davis that does perfusion and permeability studies in conjunction with the biomedical engineering department.
They have been developing software to do time density curves and the mathematical computation for measurement of contrast
permeability and perfusion.
"We were discussing that and just thought that it would interesting to see what a horse's foot might look like," Puchalski
says. "Our initial hypothesis was that soft-tissue structures of the foot will have a repeatable pattern of contrast enhancement
and that the anatomy of the foot would be readily recognized on CT. Secondarily, we hypothesized that contrast-enhanced CT
will identify specific soft-tissue injuries within the foot by the accumulation of contrast material in inflamed tissues,"
The work to describe the patterns of enhancement and the anatomy of the foot of the horse has been done and they are currently
looking at clinical cases.
CT is recognized in diagnostic imaging as the modality of choice for lesions of bone, and an MRI generally is accepted as
less optimal for bone lesions. Currently, the protocol for imaging the equine distal limb includes evaluation of the bone
structures and the soft-tissue structure, as well as their contrast-enhancing patterns. This procedure is proving to be useful
for identifying both lesions of bone and soft tissue.
The CTA procedure
Each horse is anesthetized and positioned in lateral recumbency within the gantry of the CT scanner. The dependent forelimb
is the limb that generally is scanned, although they recently have scanned both forelimbs of several clinical cases.