There is nothing more frustrating for an owner than lameness which has no apparent cause. In cases of persistent lameness where close examination, periods of rest and ultimately standard X-rays or ultrasound have not established the root cause, your vet may elect to send the horse for a CT scan, (Equine Computed Tomography). Although CT scanners are used regularly to examine human casualties, this technology is not widely available for horses and as yet there are not many equine hospitals in the UK which offer this facility.
A CT scan produces images showing both the bony and soft tissue structures within the horse's leg. The scanner is effectively a rotating X-ray machine contained within a large circular tunnel or tube. X-rays are passed through the horse's body and picked up by a detector on the opposite side of the tunnel. The X-rays are absorbed at different levels depending upon the density of the structures through which they pass. This creates a profile of X-ray beams of varying strengths. A computer then processes this information and produces a series of cross-sectional images of extremely high quality enabling the vet to more easily make an accurate diagnosis.
The scanner produces images of four to 16 slices. The computer can reconstruct these slices into three dimensional images. In the case of leg injuries, a 'contrast study' may also be performed. Injured areas have a distinctive blood supply. A contrast agent, a high-tech dye, is injected into an artery close to the area of interest before the scan. The images produced will enable the vet to establish whether the injury is active or inactive; acute or chronic.
CT scanning is much quicker than its cousin, MRI, and allows larger areas of the horse to be scanned. The main drawback, as well as the expense involved, is that the horse is exposed to a high degree of ionising radiation which can pose a risk if CT scanning is used to excess.
If a lower limb is to be scanned, a short general anaesthetic (20 minutes or so) will be administered before a CT scan can be performed. This is because the horse will need to lie on his side for the duration of the scan. The horse would therefore have to be admitted the day prior to his scan and be starved in preparation for his anaesthesia.
When scanning is complete, the horse will be brought round from his anaesthetic. It is likely that another night as an in-patient will be required, depending on how well he recovers. He will then be ready to go home, hopefully with a definitive diagnosis and a course of treatment which will see him back to full fitness.
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