Equine imaging update: Computed tomography

Computed tomography (CT) scanning has many uses in equine medicine and surgery. It can provide a great amount of information useful in the diagnosis and treatment of equine leg and foot lameness, including the evaluation of soft tissue injuries and fractures. It is also beneficial for evaluating equine head lesions such as skull fractures, sinus and dental problems, bone cysts and bone infections.

CT basics

CT is an anatomical imaging modality using X-rays and X-ray attenuation to produce a cross-sectional image. It allows practitioners to gain a better understanding of the structure of the imaged body tissue. It is a digital modality, so images can be computer-manipulated to enhance clarity, contrast and brightness and to zoom, rotate and measure. Additionally, specialized computer software allows images to be reformatted into different imaging planes.

While conventional radiography produces summed images of an object, tomographic scanners rotate the object around, dividing and organizing it into spatially consecutive image slices. Compared with common radiography, the elimination of superimposition and improved resolution are the major advantages of CT. Images are produced as the portion of the equine anatomy on the CT table moves through the circular tunnel of the CT gantry (Photo 1). An X-ray tube within the CT housing emits X-rays as it encircles the patient within the gantry tunnel. A detector array, opposite the X-ray tube, measures the X-rays that pass through the tissues, and computer-generated cross-sectional images are constructed from the data.

Photo 1: A horse positioned on a specialty CT table undergoing a CT scan. All horses are anesthetized for the procedure to prevent movement and injury.

Helical scanning now allows the patient to advance through the center of the gantry as it continues to rotate. This means that there is no waiting between image slices while the table is moved. The number of slices obtained per revolution depends on the number of CT detector rows, but it can range from 1 to more than 126. Each slice can vary in thickness from less than 1 mm to 15 mm. In general, it takes about one second per revolution, making some CT scanners ultrafast.

CT angiography

Originally designed for human medicine, computed tomography angiography (CTA) takes CT technology one step further. For this technique, contrast medium is used to visualize blood flow through vessels. CT then creates cross-sectional images, which are assembled by computer into three-dimensional pictures of the area being studied. Using CTA, vasculature of the equine distal limb can be assessed after traumatic injuries, thereby allowing equine surgeons to assess injury to blood vessels as well as to the bone.

This technique for CTA can be also be used to provide contrast enhancement of the soft tissues of the equine limb. This enhanced form of CT helps to clearly identify and evaluate tendon and ligament lesions. Contrast agents are used with both magnetic resonance imaging (MRI) and CT to define areas of increased perfusion and altered permeability, which allows for better margin definition of neoplastic and inflammatory lesions.

Advances in CT imaging

"As technology advances, several sites that perform CT scans have gone from a single-slice scanner to the rudimentary helical CT scanner and on to multi-slice helical scanning," says Sarah Puchalski, DVM, assistant professor in the Department of Surgical and Radiological Sciences at the University of California-Davis School of Veterinary Medicine. "Whereas before we worked off a single-slice, now we are capable of working off a 16-slice helical scanner — quite a big jump forward," says Puchalski. Multi-slice scanning results in better image resolution, and combined with increased software capabilities, produces improved three-dimensional and surface reconstructions (Photos 2 and 3).

Photos 2 : Three-dimensional reconstruction images can be rotated in any direction to provide surgeons with anatomical landmarks to minimize surgery time.

There are image-viewing and technique advances as well. "Although CTA was probably the big jump forward as far as techniques go, we are improving our abilities with contrast use in other ways," says Puchalski. "For example, arthrography uses contrast material in the joint to evaluate cartilage thickness."

Photos 3 : Three-dimensional reconstruction images can be rotated in any direction to provide surgeons with anatomical landmarks to minimize surgery time.

A clinic in the Netherlands now has the ability to scan equine stifles, diagnosing cruciate ligament injuries for which there has been no consistent technique until now. "That clinic is pushing the envelop on stifle arthrography, and we're hoping to have that capability soon at UC-Davis," Puchalski says.

Today, CT scanners are popping up all over the place. There is greater availability and accessibility for veterinarians and horse owners. The availability does vary with region, and within different U.S. regions the capabilities and equipment are different. In the Northeast, several different institutions have CT units available, including a couple of private practices. There are CT units at the University of Pennsylvania's New Bolton Center, North Carolina State University, the University of Florida, Auburn University, Mississippi State University and Michigan State University. While not all of these units are set up for scanning horses, several of the university practices that have CT available also have special tables to allow for equine scanning.

Adapting technology for the needs of the equine patient has been an impediment. "About 10 years ago, there were 'portable' CT scanners that could go into operating rooms, but they sort of fell out of vogue," says Puchalski. Now, that concept is being revisited. Currently, several equine practices are using portable "small-bore" scanners. Instead of having the horse move through the center of the CT gantry, the CT moves over the top of the leg. This is being used at the New Bolton Center and is especially valuable for preoperative planning of fracture repair.

These smaller units are a good alternative for private practices. They are relatively inexpensive and do not require a special table. Their only limitation is that they cannot be used to evaluate a horse's head.

Evaluating the equine limb

The primary use of CTA in today's equine practice is for tendon injury or foot lameness. "In addition to diagnostics, we are now performing CT-guided injections of treatment materials directly into the foot," says Puchalski. "This technique is especially useful for lesions that occur in the hoof capsule. However, there is definitely controversy over what to inject. Platelet-rich plasma, stem cells and biological agents that are thought to speed healing are being explored."

The portable machine at New Bolton Center is based in the surgery suite. Tom Schaer, VMD, runs the research laboratory there and does a lot of surgical research, using the CT unit for spinal surgery in small ruminants (sheep) and pigs.

"In the same building, we do our major equine orthopedic surgeries, and that is where I find it really exciting," says Dean Richardson, DVM, Dipl. ACVS. The equine surgical department uses this small CT unit to perform less invasive and more accurate fracture fixations on many types of lower-limb fractures. For example, they have repaired several coffin bone fractures that would have been much more difficult without the use of CT. As Richardson notes, "Because the CT is right there in the operating room, we can do intraoperative imaging to figure out precisely where to direct screws."

Other applications have included third tarsal bone slab fracture repair. "You can get the position of the screw absolutely perfect using CT in this type of fracture because you can define the real limits of the fracture — an impossibility with standard radiographs," says Richardson. "We are using CT to assist with navicular bone fractures, coffin joint arthrodesis, complex phalangeal fractures, etc., which would all be very difficult to do without this technology. It is pretty exciting to be able to add CT to other intraoperative imaging in order to further improve our fixations. I can wheel it into the operating room and scan a leg in 60 seconds! It takes some of the smarts out of fracture repair. I call it 'cheating.'"

Richardson notes, however, that at New Bolton Center a bigger unit is needed to be able to scan the upper limb, as well as for use on the skull of an adult horse.

Evaluating the equine head

Besides evaluation of the distal limb in horses, CT scans are now being used to assist in the diagnosis and treatment of soft tissue and bone abnormalities affecting the head. "We have used CT scans on horses with neoplasia of the head and mandibular disease," Puchalski says. "This is a very common and good use of CT."

Anthony Pease, DVM, MS, Dipl. ACVR, assistant professor of radiology and section chief of diagnostic imaging at Michigan State University's College of Veterinary Medicine, is involved in the continued development and use of CT for equine injuries and diseases of the head.

"Nowadays, there are two different methods that we can use for head CT in the standing horse," says Pease. "The first is a technique in which we have a sedated horse move through the CT scanner. That's basically what they're doing in Europe right now. The second is a process being used in Wisconsin in which they are creating the first standing CT scanner designed for imaging the equine head."

CT is ideal for evaluating bones of the head, avoiding the superimposition that is seen on a standard radiograph. A CT scan of the equine head produces cross-sectional images with a large amount of dental, sinus, brain and soft tissue detail.

Michigan State University just finished a project in which they looked at using CT scanning for horses with Cushing's disease. For this study, CT was used to identify pituitary enlargement in horses with Cushing's disease (Photo 4). "We could actually see the lesions very easily with the use of contrast material," says Pease. "It was the first study to measure the average size of an abnormal pituitary gland in the horse. All of the horses with Cushing's actually had large pituitary glands." Each scan was completed in about 10 minutes, which allowed the horses to be treated and rechecked later to monitor the size of the pituitary gland. "CT offers a way to monitor this disease noninvasively," Pease notes.

Photo 4: A CT image of the brain of a horse with Cushing's disease showing the large pituitary gland after contrast medium is administered.

Another reason to evaluate the equine head is in cases of trauma. With traditional radiography, diagnosis and treatment in these patients can be challenging. "The last thing that a veterinarian wants is to have a horse with a head injury under anesthesia for a long period of time. With advances in CT software, we can scan a horse's head, perform 3-D reconstructions and have all the information to the surgeons before they get the horse into surgery," Pease says. In these cases, MRI may provide more information, but this modality takes up to an hour. CT produces results much faster, which can make all the difference in cases of head trauma. That is not to say that the information gleaned from head CT is nondiagnostic.

"I've scanned horses with head trauma, tooth root abscesses and fractures of the jaw, and we have been able to identify many tumors, abscesses and infections. We're able to get a large amount of information from CT," says Pease.

While there are still daunting challenges posed by the equine patient, the future of CT in equine veterinary practice looks bright, with new applications for this imaging tool yet to be discovered.

Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background in horses, pets and livestock.

For a complete list of articles by Dr. Kane, visit dvm360.com/Kane.