Unexpected tales abound in veterinary imaging

Veterinary imaging specialists are broadening their horizons, making greater use of MRI, CT, and nuclear scintigraphy to supplement information from radiography and ultrasound examinations. The number and complexity of diagnostic tests on dogs, cats, and horses are growing steadily, and even live sharks and elephants have been imaged.

Many considerations that apply to imaging small animals would be familiar to radiologists, and increased awareness of the scope of veterinary imaging might facilitate comparative research, said Christopher R. Lamb, a senior lecturer in radiology in the department of veterinary clinical sciences at the Royal Veterinary College (RVC) in North Mymms, U.K.

"There are many analogies between veterinary and pediatric imaging. The patients are of similar size; we cannot communicate with them easily; we sometimes have to restrain them in some way; and in both groups there are a lot of congenital anomalies that require imaging," Lamb told delegates at this year's U.K. Radiological Congress (UKRC) in Manchester. "I sometimes wonder if I'll come back in another life as a pediatric radiologist and use a lot of the skills I've acquired."

Dogs are scanned more often than any other species, probably followed by cats, horses, and rabbits in that order. Food animals and zoo species are imaged less frequently, although fertility workers might perform an ultrasound scan of a cow's genital tract.

"There are many instances where technology and animals don't necessarily go together well. You just have to look at a flood image from a gamma camera after a horse has given it a good kicking," Lamb said.

In one small-animal U.K. veterinary practice, 562 patients were x-rayed in 2002. The thorax, which accounted for 322 examinations, tends to be the most common site for x-rays of dogs and cats, because they often have cardiac problems, chronic coughs associated with bronchitis, and pneumonia. The abdomen was the second most frequently imaged site (231 patients), often due to ascites, hepatomegaly, and abdominal masses.

The extremities accounted for 133 examinations. Vets see many arthritic dogs, trauma cases, and associated fractures, Lamb said. Seventy-one radiographs imaged the pelvis. A vet might x-ray the pelvis of a clinically normal dog as a screening examination for hip dysplasia, although animals are generally scanned on an individual basis rather than as part of a screening program.

Conditions that are treatable in humans but potentially fatal in animals are a recurring frustration. The idiosyncrasies of the numerous breeds are also challenging, he said. Most King Charles spaniels, for example, have a Chiari type 1 malformation of the brain that can be seen only on MRI.

Eder and Valenta at the University of Vienna took the first radiographs of animals in 1896, the year after the discovery of x-rays. Throughout the 20th century, most vets worked with basic x-ray equipment, and that is still generally the case in small-animal practices, according to Lamb. The development of veterinary radiography has lagged behind medical radiography, but many referral centers and veterinary schools, of which there are six in the U.K., have installed digital radiography systems and a RIS or PACS for multimodalities.

ULTRASOUND AND CT

Ultrasound has had a substantial impact on veterinary medicine since the mid-1980s, when it began to be used widely on animals.

"Our patients usually need to be clipped for good ultrasound scanning. You cannot scan through the hair coat effectively unless you wet it down, which is rather inconvenient, so we tend to clip off the hair," Lamb said.

Ultrasound screening of the urinary tract is a common procedure in Persian cats, as about 40% of the breed contract polycystic renal disease. Ultrasound is also used to diagnose ectopic ureters in animals.

Two years ago, the RVC installed a secondhand PQ 5000 Picker single-slice CT scanner dating from 1999. The scanner bed was modified to accommodate a 700-kg horse, and the table was engineered so that the normal stepping motor in the patient couch could move the table, even with the additional weight.

Practitioners obtained a 73-cm-long scout image of an elephant's pelvic limb on this machine, but Lamb concedes that researchers at Berlin's Institute for Zoo Biology have far more expertise in imaging elephants. They perform obstetric ultrasound on elephants as part of a breeding program (Journal of Zoo and Wildlife Medicine 1998;29[2]:114-128). A sonographer stands on a stepladder to perform transrectal scans but must wear a helmet containing a tiny built-in screen to view the image, as the ultrasound machine is located a few meters away.

MRI is also available at the RVC. Vet MRI, part of the InHealth Group, provides a mobile service twice a week to scan patients with neurologic conditions.

CAREER SWITCH

Lindsey Day, a radiographer at Bristol's Queen Mother Hospital for Animals, who also spoke at the UKRC session, moved into veterinary work four years ago. She had previously worked at King's College Hospital and the Royal Brompton Heart and Lung Hospital, both in London, and the Princess Margaret Hospital in Swindon.

"The transition from 'human' to veterinary radiography can be easily made as long as you are aware of the differing needs of the patient," she said. "The role of the radiographer in maintaining image quality is very similar to that in a hospital."

Veterinary radiographers have their own terminology and methods, however. If a dog lies on his sternum, a dorsal-ventral view of the thorax is taken, but if he lies on his back, a ventral-dorsal view is taken. Veterinary radiographers also make frequent references to rostral, caudal, cranial, and dorsal-plantar views. Some body parts have different names in animals; e.g., quadrupeds have stifles, not knees.

"With a dog, we always start with a lateral view of the thorax, rather than an AP or a PA view," Day said. "This is because of the way we have to lift him onto the table. We can easily roll him into the lateral position, which protects his head and protects us in case he gets scared and tries to bite us. We always take two views of the thorax. Radiographers view the images as if the dog is standing up."

Veterinary radiographers commonly use muzzles, sedation, and a general anesthetic to obtain radiographs of animals. They take comparison views, especially in orthopedic cases where subtle differences that can cause severe lameness can be detected.

Radiographers use CT to look for spinal tumors and for fragmented coronoid processes, which are common in Rottweilers and Labradors.

High-resolution scans of the thorax are used to identify metastases and pneumothorax. Dogs' noses are another common examination site.

EQUINE SCANNING

Because the gantry is the same size as that of a regular scanner, CT can image only certain parts of a horse, such as the brain. The horse is sedated in a padded room or box, anesthetized, and hoisted onto the table. Blinkers are placed over the eyes and cotton wool inserted in the ears.

"It can be very stressful and time-consuming," Day said. "As well as looking at bones, we look for any tendon problems, and we scan both feet for comparison."

Handling horses requires considerable persuasion and restraint, said Dr. Rachel Murray, head of equine MRI and orthopedic research at the Animal Health Trust (AHT) in Newmarket, U.K.

"Our patients are athletes, and we are often dealing with the diagnosis of very subtle injuries. We may have an Olympic dressage horse that is failing to complete only one movement correctly or a showjumper that is starting to run out to the side at a fence, and we need to work out why that is happening," she told UKRC delegates.

Before making a diagnosis, a vet injects a local anesthetic, either perineurally or intra-articularly, to eliminate the horse's pain. If the vet suspects an incomplete fracture, the horse is not allowed to trot to the diagnostic center after injection of the anesthetic, Murray said.

Ultrasound is used widely for orthopedics, especially on the distal limb where there is little coverage over the ligaments and tendons. Vets are also increasingly using nuclear scintigraphy and MRI.

"To detect subtle abnormalities using scintigraphy, an understanding of radiopharmaceutical uptake patterns in normal horses of specific ages undertaking different work types is required," she said. "Analysis of images from normal horses indicates minimal effect of age after maturity but left/right differences at some sites, and an effect on work type on uptake patterns."

Murray's group has done extensive research on normal horses to determine typical behavior, and they apply that knowledge to detecting subtle injuries in horses in different sports.

Staff at the AHT used a 1.5T GE Signa EchoSpeed MR machine to scan up to and including the carpus/tarsus of more than 300 anesthetized horses over the past five years. Positioning the foot within the imaging portion of the magnet is more straightforward than positioning the proximal limb.

"Horses with small bottoms and long legs fit better into the scanner than horses with large bottoms and short legs," Murray said. "It takes quite a big team to put the horse into the magnet because of the pushing and pulling involved. All our horses get sprayed with show sheen before the examination so they slide straight into the machine."

MRI of the equine limb remains in its early stages but has already demonstrated the ability to show lesions undetectable with standard diagnostic methods. It has enhanced the understanding of pathology in the equine limb, Murray said.

In spite of the variety of their work, veterinary imaging specialists have one thing in common: a love of animals and a deep concern for their welfare.

"There is never a dull day for us, and you can get very attached to the patients because they are so cute and cuddly," Day said.

Philip Ward is editor of Diagnostic Imaging Europe.