There are 3 basic MRI systems that are available to horse owners and it is important that one fully understands the advantages and disadvantages of each.

Low Field Standing

There is one MRI system, marked by Hallmarq from the UK that allows the acquisition of images in standing, heavily sedated horse. The magnet is a 0.3T (Tesla) permanent magnet mounted on and in the floor of an RF shielded room. Because these magnets are low field, the signal from the patient, (from which the images are generated) is weak and to compensate for this, image acquisition can take a long time, often many hours. In addition, the acquisition of images is complicated by patient motion, both voluntary movement but also just the normal rocking of a horse standing. These systems use motion correction software to help compensate for patient movement, but because of the low field configuration, prolonged scan time and patient motion, a diagnostic study can often be challenging to acquire. Because of the physical limitations of the technology, acquisition of thin slice high contrast images can be challenging and this can result in a suboptimal and potentially non diagnostic study in many patients. Studies are usually limited to the foot as there is less inherent motion in the foot than higher up the leg. Increasingly, clinicians are opting to anesthetize horses for studies in these low field magnets. This tends to negate the whole premise behind performing the study standing and in this scenario, one should consider a high field recumbent exam.

While the advantages of performing an MRI study on a horse sedated rather than under general anesthesia are intuitive, one must be aware that with current ‘standing’ technology, it is not possible to acquire images of the same quality as can be obtained in high field recumbent system.

Low Field Recumbent

These magnets are also permanent magnets in the order of 0.3T. Esoate is one such manufacturer in this market. These magnets are not configured for the standing horse and the patient must be under general anesthesia. General anesthesia offers the advantage of minimizing patient motion, and images from these machines are usually of good quality. However, compared to high field magnets, these systems take longer to acquire the image and imaging time is usually limited in a patient under general anesthesia. This often means less sequences can be acquired or images of the opposite leg, usually made for comparison, are forfeited in the interest of time.

High Field Recumbent

Magnets of 1T and more are considered high field. These are usually ‘superconducting’ magnets which means the magnetic field is maintained by passing an electric current through wire coils wrapped around the ‘bore’. Once ‘ramped’ up, the amount of electricity required to maintain the electric field is minimal as the system is cooled by cryogens to maintain a circuit with essentially no electrical resistance. These magnets, the most common in Veterinary Medicine is the Siemens Symphony 1.5T, are typically the same type of magnet in which you personally would be scanned in most any metropolitan area in the US.

There are many advantages of a higher magnetic field strength. The most important is that the higher the field strength, the more signal emanating from the patient that is available to generate images. Practically speaking, this means that better quality, thinner slice images of the region of interest can be acquired in a shorter time. From a diagnostic perspective, this means that there is less chance of a lesion going undetected. The disadvantage of a high field magnet is that, just with the low field recumbent magnet, the patient must be under general anesthesia. 

While there are risks associated with any anesthesia (even sedation), modern advances in drugs and equipment mean that equine anesthesia is now routine and considered very safe. High field magnets allow image acquisition faster than low field magnets and this results in reduced scan time. This means the patient is under anesthesia for less time and this reduces potential complications.

From a practical perspective, having accepted the need for anesthesia, it makes much more sense to image a horse in a high field magnet than a low field magnet. Image quality will be superior, more anatomy can be covered and the study can be completed in less time, thus minimizing potential anesthesia related complications.