The start of January means many things to different people. For many of us, it is still dark and cold, and it seems like there is at least a hundred days until the next pay cheque. The more optimistic will look on it as a time for personal growth and rejuvenation. For skiers, it can be an exciting time, getting untold amounts of clobber down from the loft and preparing for that exciting time in the crisp, clear air of the Alps. But for equine diagnostic imagers, it is the chance to combine skiing with professional development!
For the last 15 years (excluding during the COVID-19 pandemic), Hallmarq, the global equine advanced imaging company located in Guildford, has hosted a conference, the International Users Meeting, in Chamonix, at the foot of Mont Blanc (Figure 1).
This meeting has grown from a small group of 20 or so people at the start, to over 100 today. With a break in the middle of the day on Friday, the meeting is ideal for the recreationally focused imager.
There was a consistent theme at the meeting this year: the comparison of cone beam computed tomography (CT), fan beam CT and magnetic resonance imaging (MRI).
This is highly relevant to clinical practice, where, with limited resources, it is increasingly important that veterinary surgeons recommend the advanced imaging modality most likely to yield the diagnosis. The days of MRI being the only other option for the investigation of foot or fetlock lameness have gone!
Cone beam, fan beam or MRI?
Fan beam CT
Fan beam CT is the conventional form of CT with which many people are familiar. In essence, this modality places the region of interest within a “tube”. The X-ray source and opposing detector then circle around the region while the patient slowly advances through the tube.
[Fan beam CT] has been used on horses intraoperatively or under general anaesthesia for many years. It has also been adapted for CT of the horse’s head under sedation
This modality has been used on horses intraoperatively or under general anaesthesia for many years. It has also been adapted for CT of the horse’s head under sedation. In this situation, the horse is often on an air-supported table – a hovercraft – which is slowly advanced through the bore of the CT scanner.
The demand from human medicine for large-bore CT scanners has benefitted equine practice. In particular, one of these CT scanners – the Aquilion, made by Canon – is marketed for veterinary usage by Qualibra. The scanner has a 90cm bore and produces a 32 “slice” dataset. This unit is perceived as one of the highest-quality CT scanners currently on the market. Due to the large bore, it is possible to position the limbs of standing horses within the CT and scan them without general anaesthesia.
Cone beam CT
Cone beam CT is a more recent innovation. Rather than using a narrow receptor, which depends on the subject moving through the fan, a cone beam is analogous to approximately one conventional radiograph per degree, obtained in one pass around the subject.
There are three cone beam units commercially available at the moment: the Pegaso from Epica Medical Innovations, available through IMV in the UK; the “robot arms” unit from Orimtech; and the Hallmarq standing leg CT (Figure 2). The Hallmarq unit involves a 120KV generator and a receptor mounted on concentric circles, which pass around the horse’s limbs as it stands on a platform.
Magnetic resonance imaging
MRI technology is well established in equine practice. The majority of horse MRIs conducted in the world are performed in the Hallmarq standing MRI. This uses a low field or 0.27 Tesla magnet, while many human medical machines use a 3.0 Tesla magnet. Nevertheless, over the years it has been established that diagnostic images of the horse’s foot, fetlock and proximal suspensory regions can be obtained with care and patience using MRI. Images are lower resolution than the high-field scans, but still diagnostic in many situations.
So which is best?
Multiple papers published in the last year have compared the results obtained in Qualibra CT, Hallmarq standing limb CT and Hallmarq MRI.
The evidence
Annamaria Nagy from the University of Budapest presented a longitudinal study of Thoroughbreds (Nagy et al., 2023) from yearlings to three years of age. At the yearling stage, the horses were already showing cortical thickening typical of skeletal remodelling. Fissures in the articular surface, both in the parasagittal groove of the third metacarpus and the sagittal groove of the proximal phalanx – the typical locations for condylar and split pastern fractures – were detected in nearly half of the 40 yearlings. Despite these abnormalities, none of these horses went on to develop clinically significant fractures during their training career up to three years of age.
Despite these abnormalities, none of these horses went on to develop clinically significant fractures
Szu-Ting Lin has published several papers comparing these imaging modalities. The post-mortem study of racehorses euthanised for multiple reasons highlighted the presence of these fissures in the flock joint surface (Lin et al., 2023a). Fissures were common – present in nearly 40 percent of locations examined microscopically. The same authors also looked at osteochondral erosion of the dorsoproximal aspect of the proximal phalanx (Lin et al., 2023b), the common location of chip fracture in the fetlock. Again, these erosions were very common, with nearly two-thirds of fetlocks having clearly visible erosion post-mortem.
In these studies (Lin et al., 2023a, 2023b), the authors cleverly looked at two separate selections of MRI sequences, a “high resolution” set and then a “fast” set, typical of the sequences used to examine live horses in clinical practice.
Conclusion
The comparison of these imaging modalities across studies was surprisingly consistent. In general, fan beam CT was the most sensitive – often over 80 percent – while MRI was the most specific – often over 70 percent. Cone beam CT performed very well; it was marginally less sensitive than fan beam CT but still reasonably specific. The “fast” MRI sequences also performed well. Though less sensitive than other modalities, typically about 50 percent, the modality was highly specific – often nearly 80 percent.
However, the authors of these studies did note that these were all post-mortem specimens, and therefore there was no movement artefact. Cone beam CT is exquisitely sensitive to motion, significantly more so than fan beam. This will reduce the sensitivity and specificity of the modality in clinical (live) cases.
Synovial masses
Let’s head back to the foot, where MRI began. During the 2024 International Users Meeting, Elisabetta Giorio presented their experience with “synovial masses” (Giorio et al., 2023).
These extruded masses of collagen from the dorsal surface of the deep digital flexor tendon in the proximal navicular bursa were consistently detectable on MRI (Figure 3). Indeed, when the results were compared to surgical findings, there were no false-positive cases. Sadly, the authors also found that the results of surgery to remove these masses was not very successful. They quoted a success rate of 30 percent and suggested that this was comparable to conservative management.
A subsequent presentation by Michael Schramme reviewing the results of multiple papers for deep digital flexor tendon injury confirmed this, with 30 percent a typical success rate.
Elisabetta Giorio won the prestigious “Paper of the Year” award for this publication, an award only partly tarnished by the fact that the judge was a co-author.
