A horse’s head can account for up to 10% of its total body weight. It is a complex structure composed of 34 different bones and houses the brain, eyes, nose and mouth. Unfortunately, the large size and complicated anatomy can make diagnosing conditions of the equine head challenging.
Injury and illness related to the equine head are often identified through diagnostic imaging. Due to the complex nature of the equine head, advanced technologies such as advanced ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) can provide information that is more detailed than conventional X-rays.
Computed tomography and MRI are cross-sectional imaging techniques, which means that they create images in different planes (or “slices”) through the body part of interest. With X-rays, anatomical structures can appear superimposed on one another, which makes it difficult to view specific features. Cross-sectional images avoid complications related to superimposition of structures in complex regions. The slices can also be combined to produce three-dimensional images, which provide additional anatomic information.
In recent years, these technologies have become more available and affordable, enabling veterinarians to document and learn from more cases. The best approach depends on factors such as what is being imaged, whether the injury or illness is recent or ongoing, as well as time, logistic and financial considerations.
Ultrasound is routinely utilized at UC Davis to evaluate the equine head and provides valuable information regarding both soft tissue and bone. Structures most commonly evaluated include the salivary glands and ducts, lymph nodes, temporomandibular joint (TMJ), tongue and eyes. Ultrasound-guidance can be safely used to remove items such as salivary stones, wires or foxtails or to biopsy lymph nodes or masses with a minimally invasive approach.
Ultrasound can be performed in the standing horse and is more affordable than CT or MRI. In some cases, ultrasound findings are sufficient for diagnosis and treatment planning. In others, it may indicate the need for further imaging such as CT or MRI.
Computed Tomography (CT)
Computed tomography is useful for evaluating areas with complex anatomy that are challenging to assess with radiographs and ultrasound. Although commonly the method of choice for bone, CT can also evaluate soft tissue structures. It is very useful to evaluate the equine head, particularly structures such as the teeth, sinuses, hyoid apparatus, and tongue. It can be used to identify invasive masses, displaced teeth, and conditions of the eyes and ears. Thanks to the development of larger bore scanners, CT can also be used to image the equine neck for bone abnormalities that can interfere with the spinal cord.
In comparison to other imaging modalities, CT is fairly rapid; a full head scan only takes a couple of minutes. It is also often more readily available and affordable than MRI.
The use of CT in the equine clinic has advanced quickly, especially with the advent of instruments that permit CT imaging of horses standing under sedation as opposed to under general anesthesia.
Magnetic Resonance Imaging (MRI)
Whereas CT is particularly useful for imaging bone, MRI is the gold standard for imaging soft tissues. By measuring differing magnetic properties, MRI is able to provide not only anatomic information (size and shape of structures), but also physiologic information (early changes before alteration of size and shape).
In comparison to CT, MRI scans take longer at 40-50 minutes. The increased time and higher cost of the instrument itself make MRI a more costly approach.
Although MRI can be employed in a horse under standing sedation for evaluation of limb lameness, currently equine head MRI requires horses to be under general anesthesia.
While there are pros and cons to each modality, these approaches should be considered complimentary to one another. They are all capable of performing detailed examinations of the equine head. In fact, more than one approach may be utilized depending on the nature of the case. It is important to note that insurers can have varying policies when it comes to imaging, so it is wise to review the existing coverage prior to signing off on a diagnostic procedure.
One of the most significant advances in equine imaging is the ability to perform diagnostics on standing sedated horses. Previously, imaging required horses to undergo general anesthesia, which adds to the cost, requires additional staff and equipment, and in rare cases, results in adverse reactions to anesthesia drugs. The ability to utilize these technologies on standing horses under sedation allows for more routine use and provides more options for patients that are not able to undergo anesthesia.
This article originally appeared in the UC Davis Spring 2023 Horse Report.