The importance of standing computed tomography in lameness and orthopedics

The availability and utilization of computed tomography (CT) to aid lameness diagnosis and orthopedic surgery predates magnetic resonance imaging (MRI) and has dramatically increased in the last 5 years. In human orthopedics, CT is considered to be the best diagnostic modality for orthopedic problems and fracture repair. For horses, CT has been a valuable aid in diagnosing lameness in the distal limb, offering more information than radiographs. CT uses similar radiation to radiographs, but is faster, taking an average of few minutes to image a region. This allows the imaging of a significant portion of the horse’s limbs from distal radius/tibia to the foot. The new Standing CT systems for horses allows for imaging of both limbs simultaneously whereas other diagnostic modalities require separate acquisition of the contralateral limb. This facilitates a comparative evaluation of the anatomical area of interest and can help identify bilateral causes of lameness.

Historically, CT in horses has been a limited due to the need for general anesthesia and the risks associated with the anesthetic procedure. Now days, CT imaging in sedated, standing horses is now a possibility that provides three dimensional (3D) information that improves the possibility of obtaining a definitive diagnosis for orthopedic problems that cause lameness. Standing CT can provide more information regarding the morphology of bone and some soft tissue injuries due to the possibility of obtaining cross-sectional images in multiple planes. Musculoskeletal lesions (especially soft-tissue) can be further detected with the addition of contrast material (iodine based) through intra-vascular, intra-articular or intra-thecal routes. Based on the volume of information provided, it is essential that one understands anatomy in order to interpret all the CT images.

Radiographs and ultrasound have low sensitivity and specificity for the detection of early bone changes. In lameness cases, CT can provide valuable diagnostic information these other modalities often cannot provide. This makes standing CT a tremendously valuable tool for clinicians when diagnosing lameness in sport horses associated to the distal limb.

Imaging-assisted fracture repair is becoming part of the routine practice in orthopedic surgery. Fracture configurations are complex making it hard to characterize them radiographically. CT is a superior 3D imaging technique for pre-operative diagnosis and planning, intra-operative implant placement and post-operative evaluation of the repair. CT produces cross-sectional images with spatial separation of structures that assists in recognizing and following the number and direction of fracture lines in a comminuted fracture, facilitating a faster surgery and anesthesia time with less patient morbidity and minimal radiation exposure to surgery personnel.

Cone-beam CT scanners have been used in horses; however, movement of the horse during acquisition can affect the final image resulting in streaking artifacts, scatter and noise artifacts, and image distortion. Helical CT systems developed for imaging of human patients are currently being used to scan distal limb in horses. However, only one limb (non-weight-bearing) can be scanned at a time. A new multi-slice, helical, fan beam CT scanner design specifically for horses (Equina system; Asto CT Inc.) has been developed with a sliding gantry that can be tilted from 0° to 90° for horizontal or vertical scanning. For distal limb scanning, imaging from the distal portion of the radius or tibia, including the carpal or tarsal region, to the foot can be performed on both limbs simultaneously. The Equina Standing CT by ASTO was first installed in the University of Minnesota Leatherdale Equine Center in 2019. Since that time, more than 600 horses have been evaluated with this imaging modality in our hospital.