Review of Radiographic Technique and Interpretation of the Equine Skull

Acquisition and interpretation of equine skull radiographs is notoriously difficult. However, by dividing the skull into anatomic regions and considering the radiographic appearance of various pathologic changes that occur in these regions, the process of evaluation becomes simpler. This report aims to review acquisition of skull radiographs and radiographic appearance of various diseases in order to aid the equine practitioner in evaluation of skull radiographs. Authors’ address: Department of Environmental and Radiological Health Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, 300 West Drake, Fort Collins, CO 80523; e-mail: [email protected].

1. Introduction

Obtaining diagnostic quality radiographs of the equine skull and interpreting the findings can be an intimidating task. The variation in degree of attenuation of x-rays by materials within the skull, from air in the sinuses to enamel in teeth, requires finesse with exposure technique. Also, different structures being evaluated require somewhat individualized radiographic views. To obtain diagnostic radiographs, it is helpful to have a compliant patient during the radiographic examination. Once good quality radiographs have been made, there is the next hurdle of interpretation. The complexity and overlap of the anatomic structures within the equine skull result in this being a challenging area of interpretation for everyone from the general practitioner to experienced radiologists. However, by breaking the skull down into several anatomic parts, applying general principles of radiographic interpretation, and learning to recognize the typical radiographic appearance of common pathologies affecting the equine head, accurate evaluation of equine skull radiographs becomes less daunting. 

2. Radiographic Technique

Portable x-ray machines and standard plates are sufficient for most skull radiographs. If available, large plates are helpful for including more structures in the image, and in-house x-ray machines with greater mA capabilities are useful for denser structures such as the cranial vault and dorsoventral views. Sedation is generally recommended, except in cases in which trauma to the skull results in a neurologic status unsafe for sedation. Not only does sedation limit motion artifact and reduce exposure to personnel by means of fewer retakes, it also generally causes the horse to lower its head, easing the acquisition of images. The horse’s standard halter should be replaced with a rope halter during image acquisition to reduce superimposition and metal artifacts on the radiograph. Standard safety equipment for personnel includes lead gowns, thyroid shields, and dosimeters for everyone in the room as well as lead gloves and/or long plate handles to minimize hand exposure for the plate holder.
 

Proper labeling of the radiograph is essential for accurate interpretation. For dorsoventral views, markers are needed to distinguish left from right. For dorsoventral or ventrodorsal oblique views, markers should be placed to demonstrate the laterality of the structures highlighted. For example, in a left dorsal–to–right ventral oblique view of the nasal cavity, the right maxillary and frontal sinuses and right side of the nasal cavity will be highlighted dorsally; therefore, a right marker should be placed on the top of the plate (Fig. 1). The opposite is true for a left dorsal–to–right ventral oblique view. Mitchell markers, which contain beads that fall to the bottom of the marker, are helpful for determining how the plate was positioned relative to the ground. When looking for fluid lines in the sinuses, the fluid lines should be in the same plane as the beads in the Mitchell marker.

The areas of the skull to be examined can be divided into four sections: (1) incisive region (rostral skull), (2) nasal cavity, maxillary cheek teeth, and paranasal sinuses, (3) horizontal mandible, and (4) caudal skull and cranial vault, which includes temporomandibular joint and tympanohyoid articulation. Table 1 provides a brief summary of the radiographic views recommended for each anatomic section. For a more complete description of radiographic technique, the reader is directed to texts such as Techniques of Veterinary Radiography or Textbook of Veterinary Diagnostic Radiology.^1,2 

3. Common Pathologic Findings by Anatomic Region

Incisive Region Fractures

One of the most common reasons for radiography of the rostral skull is for evaluating fractures. Fractures in this region are particularly common in younger, curious horses that are mouthing stall doors or other fixed objects and become startled, pulling back and causing a fracture of the alveolar bone³ and often extending into the incisive bone, maxillary bone, or incisive portion of the mandibular bone. The most useful radiographic projections in this case are intra-oral views, because they eliminate the overlap of incisors (Fig. 2). The affected incisors are generally displaced rostrally. Fractures of the rostral mandible can extend caudally to the diastema and to the level of the mandibular premolars, which, if incomplete, can be difficult to fully visualize radiographically.²

Neoplasia

Neoplasias of the rostral skull are rare and generally benign. [...]