Taking a Look Inside: Different Diagnostic Imaging Modalities Available for Horses

Nov 10, 2017 / Student Blogs

horse in 3T MRI

The first radiograph (x-ray) was performed in 1895 by Wilhelm Conrad Roentgen, the German scientist who discovered x-rays. Within months of Roentgen’s discovery, surgeons started to use the new technology to aid their diagnoses and treatments, including locating bullets in wounded soldiers.

Over the past century, our ability to produce high-quality radiographs has improved tremendously. In the mid- to late 1900s, imaging and computing technologies developed. CT or CAT scans, MRI, and ultrasound were direct results of computer advancements.

Today, all of these modalities are available in veterinary medicine to help diagnose and treat our furry, feathered, and even scaled companions. Here at the University of Illinois, our Equine Medicine and Surgery Services offer radiographs, ultrasound, MRI, CT, and nuclear scintigraphy to help diagnose and treat your equine partners.

With all of these choices, how do we decide which modality to use? This article will provide some insight into the advantages and disadvantages of the many imaging modalities utilized at the University of Illinois.

 

Radiographs (X-Rays)

Radiographs are generated by sending x-rays through tissue from one side of the body to the other. Some of these x-rays are absorbed by bone and other soft tissues, while others will pass through to a plate placed on the other side of the body. Bone, because of its density, generally does not allow x-rays to pass through, whereas soft tissues do allow for some x-rays to pass. Air, being the least dense, allows the most x-rays to pass. The difference in densities within the body creates a shadowgram when the x-rays reach the plate. The result is known as a radiograph. Radiograph machines range in sizes and strengths and can be performed both in the hospital and in the field, with our ambulatory service.

What are they good for?

Bone and mineralization. Radiograph is a quick, portable, and relatively inexpensive modality used for evaluating changes and locations of the bones. Diseases such as arthritis, laminitis, and fractures can often be diagnosed on radiographs. Radiographs can also be useful for locating abscesses within the hoof.

What are the pitfalls?

Radiographs do not offer excellent soft tissue detail and struggle to penetrate exceptionally thick tissues, such as an adult horse’s chest or abdomen. At U of I, in addition to our portable machines, the hospital utilizes a large radiograph machine that is capable of performing radiographs of majority of the adult equine spine and thorax and is sometimes used to look for sand in the abdomen.  Additionally, the x-rays utilized in making radiographs are a form of ionizing radiation. Fortunately, the amount of radiation your horse will receive from radiographs will not cause any harm in the future.

 

Ultrasound

Ultrasound is a real-time imaging modality that allows veterinarians to visualize the appearance of soft tissue structures up to approximately 20 cm in depth, depending on the probe. Ultrasound images are achieved by a transducer emitting high frequency sound waves and listening for their return. Different tissue densities cause the waves to return at differing strengths. A computer analyzes the return of the waves and produces a picture based on how quickly the waves have returned, and the characteristics of that wave. This is similar to echolocation used by some bats, whales, and dolphins to navigate and find food.

What is it good for?

Ultrasound provides good details of soft tissue structures, such as tendons and ligaments in the lower limb. Ultrasound is helpful for visualizing parts of the abdomen and the reproductive tract in mares and stallions. Ultrasound may also be useful in determining the presence of pneumonia in both adults and foals.

What are the pitfalls?

Ultrasound waves transmit poorly through air and bone. Therefore when using ultrasound to evaluate the lungs for pneumonia, we are only able to visualize disruptions of the lung tissue that involve the outer surfaces of the lung. Similarly, we are only able to evaluate the outer surfaces of bone and therefore is unlikely to be the first choice for a suspected injury to the bone. With horses, many of the intricate structures within the hoof capsule cannot be imaged with ultrasound due to the inability to “see through” the hoof capsule itself.

 

Nuclear Scintigraphy (Bone Scan)

Nuclear scintigraphy, or what is commonly referred to in horse lingo as a bone scan, is just that—a scan, and in horses, often of only the bones. To perform nuclear scintigraphy, a radioactive isotope is administered through an IV to the horse. This isotope travels in the blood throughout the body and concentrates in areas of increased blood flow, inflammation, and increased bone turnover. The isotope emits gamma radiation, and therefore, when the horse is placed in front of a gamma camera, the location of the isotope can be visualized. Images are gathered with the gamma camera at varying lengths of time after the initial injection, depending on the type of tissue the doctors are interested in.

What is it good for?

Nuclear scintigraphy is often used to detect very early stress fractures or to help identify the source of a non-specific lameness. Also, if a horse seems to have more than one source of lameness, a bone scan may be a good choice to get a “bird’s eye” perspective on the horse’s skeletal system as the entire body can be scanned. The images are gathered while the horse is sedated, so no anesthesia is required.

What are the pitfalls?

Nuclear scintigraphy images can sometimes be vague. Because the images produced represent areas within the body that have increased metabolism, nuclear scintigraphy serves as more of a guide, rather than a definitive diagnostic tool. Often further imaging, such as radiographs, will be necessary following the bone scan. Nuclear scintigraphy is not a quick imaging modality, and a full body bone scan in an adult horse can take several hours. Additionally, because the horse will still be emitting gamma radiation, we require the horse to stay overnight while the radiation levels decrease.

 

Computed Tomography (CT Scan)

CT utilizes a computer and thousands of x-rays sent through the body at many different angles to create cross-sectional, two-dimensional images. Then, these images can be reconstructed by a computer and viewed in multiple planes. Three-dimensional reconstructions are also possible to help guide diagnosis and treatment.

What is it good for? 

Like radiographs, CT is very good at detecting changes to the bone. Additionally, through the administration of intravenous (IV) contrast agents, certain soft tissue structures can be well visualized on CT. Additionally, lung disease, such as pneumonia in foals, can be readily visualized with CT imaging. CT images are acquired very quickly, with an average scan taking two to three minutes to perform. The advantage of CT over radiographs when evaluating a bone is the ability to view the bone in multiple planes, from multiple angles. For example, it is possible to see individual bones within a joint from multiple angles, whereas with radiographs, there is often a great deal of overlap in structures, especially within joints.

What are the pitfalls?

Most CT scanners, including ours at U of I, require the horse to be laying down. With adult horses, this means the use of general anesthesia and a special table that can accommodate the weight and size of the horse. In foals, sedation, rather than general anesthesia is sometimes an option, especially in sick foals. Additionally, the bore or opening of the CT scanner is size restrictive and therefore is limited to scanning the head, neck, and limbs of adult horses up to the level of the hock or carpus (knee).

 

Magnetic Resonance Imaging (MRI)

MRI utilizes computers and powerful magnets to create two-dimensional images. The way images are created sets MRI apart from other imaging modalities. MRI does not use radiation. Instead, MRI utilizes a strong magnetic field and a series of radiofrequency pulses to align the atoms of the body. The alignment of the atoms within the magnetic field, and how quickly they return to alignment following a pulse is used to create an image detailing the composition of the body’s tissues.

What is it good for?

MRI offers excellent details of soft tissue and can also indicate pathology and inflammation within bones. Therefore, MRI is the gold-standard choice for imaging the structures of the lower limb as well as the brain and spinal cord. MRI is the only way to truly evaluate the intricate structures within the hoof capsule, such as the navicular apparatus.

What are the pitfalls?

MRI machines come in a few different shapes, sizes, and strengths. The strength of the magnet is measured in the unit of tesla. (Not the car!) The greater the teslas (T), the higher the image quality.  Generally speaking, both standing and traditional (horse must be laying down) MRI scanners are available for imaging horses. While standing MRI machines only require sedation rather than general anesthesia, they have weaker magnets (fewer teslas) and any movement of the horse can cause artifacts (inaccuracies or imperfections in the image). At U of I, our MRI machine is a large-bore 3T scanner (pictured above) and is the only scanner of its kind in the Midwest. The scanner does require the horse to be recumbent and under general anesthesia. The combination of the lack of movement because the horse is anesthetized and the power of the magnet result in very high quality scans.

The time required to complete a set of MRI scans is longer than CT and this time differs with the strength and quality of the machine. At U of I, it takes approximately 45 minutes to scan one limb.  Finally, the bore or opening of the machine is size restrictive, similar to CT. At U of I most adult horses can be imaged from the hoof to mid-cannon bone in all four limbs as well as the head. With standing MRI, most horses can be imaged up to the fetlock region of all four limbs. However, images of the head are not possible with standing MRI machines commonly available for horses.

 

Ultimately, there is no one perfect imaging modality. Often, a combination of several imaging modalities along with a good physical examination and work-up is required to reach a diagnosis. Sometimes the imaging findings will be unremarkable, or appear normal. While this may be frustrating to clients, normal imaging is valuable as well because it eliminates possibilities and guides the diagnosis. An accurate diagnosis of the problem at hand is the first step towards selecting the best treatment plan that will set you and your horse on the path to recovery and success. Here at the University of Illinois, we have the imaging capabilities to do just that.

 —Dr. Jessie McAfoos