by Brittany Silvers

How to Know Which Diagnostic Imaging Technique to Use

How to Know Which Diagnostic Imaging Technique to Use

When it comes to animal health, there are many different diagnostic technologies utilized to help assess various conditions. However, when you have a sick or injured animal and have to make a quick decision, it can be overwhelming to try to determine what is best for your animal and what all the scientific jargon and acronyms mean. So, in this article we are going to look at the different diagnostic imaging techniques, their uses, and potential risks.

Equine Ultrasound 

Ultrasound scanning (or sonography) uses sound waves that are out of the range of the human ear. During an ultrasound, the transducer (the handheld device used to scan) emits the sound waves. 

These waves are able to travel through soft tissues and fluids, however, will be reflected by more dense tissue, which is how the image is created. 

Fluids and less dense tissues will appear black or dark gray, and the denser the tissue, the whiter the image becomes. 

Because it utilizes sound waves rather than radiation, ultrasonography is considered a very safe technique (1). 

When is Ultrasound Commonly Used in Horses? 

Most commonly, horse ultrasound technology is utilized to view reproductive structures (2). This is useful when determining ovulation of mares for breeding, as well as monitoring the fetus throughout the pregnancy. 

Additionally, ultrasonography can be utilized to view tendons and soft tissue structures, which can be beneficial in detecting injuries or determining the cause of lameness in horses. 

Ultrasonography is also a beneficial tool for assessing health of the internal organs, including the gut. When a horse is exhibiting symptoms of colic or other gut health problems, such as hindgut ulcerations, ultrasounds can be utilized to visualize areas of the intestines. 

Through ultrasound images, it may be possible for a veterinarian to determine the cause of colic, or to confirm the presence of hindgut ulcers, which would be difficult to do without this technology (3, 4). 

Ultrasounds can be used to examine small animals for similar issues and, along with other uses, can also be used to detect foreign objects that may have been ingested by curious cats or dogs (5). 

Overall, ultrasounds are a safe option, usually requiring little to no sedation, that can be utilized to diagnose a wide range of conditions in both large and small animals.

Horse X-Ray

 An X-Ray (or a radiograph) is an image that is formed through the use of electromagnetic waves. This type of diagnostic imaging produces a 2-dimensional gray scale image. 

In contrast to ultrasound, which registers waves that have been reflected by tissues, a radiograph machine emits the electromagnetic waves on one side of the structure being imaged, and a detector on the other side absorbs the waves that have not been absorbed by the structure. 

The least dense tissue appears black because the waves pass through and onto the detector and the densest tissue appears white because the structure absorbs the waves and they do not make it to the detector (Think: Shining a flashlight at the wall and placing your hand between the light and the wall. Your hand will cast a shadow because your hand absorbed the light waves, this is similar to a radiograph, except the “shadow” will appear as light colors and the “light” that passed through will appear dark) (6). 

Because this technique utilizes radiation, it is considered higher risk than ultrasonography, however the amount of radiation the body is exposed to is actually quite small (7). 

Radiography is considered a non-invasive technique to utilize in animals, however, does usually require some extent of sedation, which may have associated risks.

When is X-Ray Commonly Used in Horses? 

Radiographs (X-rays) are used primarily to image bones within the body, because the radiation waves will pass through most soft tissue and fluids (like tendons or arteries) (8). 

This is commonly used when determining the cause of lameness in horses by investigating bones in the leg. It is also utilized to determine if a horse has any structural issues prior to purchase. These images can give a good snapshot of the bone, however, may not always be the most sensitive method, potentially not picking up small issues. 

Additionally, in some locations, such as California, abdominal radiographs are relatively common to diagnose enteroliths.

X-Rays of a horse’s extremities can be performed using a handheld radiograph machine and cassette, meaning they are mobile, can sometimes be brought to the client, and can be used to obtain a practically endless number of different views. 

However, any movement may cause inconsistencies in image quality, so unless the animal is willing to stay very still on its own (and for the safety of the technician holding the cassette, likely in a dangerous area around or between the legs), sedation is usually necessary. 

In companion animals, x-rays are typically used to diagnose broken bones, changes in bone, pregnancies, or to identify foreign objects in the body (9).

CT Scan for Horses 

Computerized Tomography (CT) scan uses the same electromagnetic waves as an x-ray, however rather than taking one scan it takes a series of images and compiles them to create either a 2-D or 3-D image that is more detailed than a traditional x-ray. 

Additionally, unlike an x-ray, CT scans can give detailed views of not only bones but also arteries, soft tissues, organs, tumors and more (10). These images are sensitive to movements of mere millimeters, so the animal must usually undergo anesthesia, which is riskier than normal sedation (11).  

When Would a Horse Undergo a CT Scan? 

CT imaging is a superior method for evaluating the skull, because the structure is very complex. A more complete image can be obtained, thus making a diagnosis much more accurate. Additionally, for horses, a CT system has been developed for the skull that allows a horse to be sedated and remain standing rather than under anesthesia. 

CT scans of the equine limbs are much better than conventional x-rays for diagnosing lesions in both bone and soft tissues. Currently, equine limb scans must still be done under anesthesia, however CT systems that would allow for the horse to remain standing are in the works and would be revolutionary in diagnostic imaging. 

In small animals, CT scans are commonly used to assess cancer and tumor spread. Additionally, the diagnosis of nasal disease is one of the most frequent uses of the CT scan in companion animals, along with a long list of other uses. Similarly, to horses, small animals must undergo anesthesia, which can have potential side effects in some animals (12)

Horse MRI

Magnetic Resonance Imaging (MRI) is slightly more complicated. MRI machines are made up of large magnets, and the magnetic field aligns water in the body. Radio waves emitted from these re-aligned water molecules cause faint signals, which then are used to produce the image. 

The image produced from this machine can either be a cross-sectional view (like a slice of a loaf of bread) or a 3-dimensional image. An MRI provides very detailed images of everything from bones and joints to internal organs to blood vessels, making it a very versatile tool (13). 

An MRI does not utilize radiation, so it is generally rather low risk aside from the potential risks associated with anesthesia and sedation.  

When is MRI Used in Horses? 

While this is considered the gold-standard of diagnostic procedures, it is not always a readily available technique, and it can be rather expensive, meaning it is probably something you would consider only after having exhausted all other options. 

The most common use for MRIs in horses is to evaluate lameness, as most MRI machines are only large enough to image a limb from the knee or hock down, or the head and the first few vertebrae. 

MRI is a great tool to assess any issues within the hoof, because it is a complex structure involving many soft tissues, but these soft tissues within the hoof wall cannot be imaged with an ultrasound. 

Animals must be anesthetized to undergo a high-field MRI because any movement can negatively affect the image, and scans can take anywhere up to 2-3 hours to complete (14). However, development of new low-field magnetic standing MRI machines for lower-limb imaging has allowed for an alternative where the horse can undergo sedation and remain upright (15). 

In companion animals, an MRI is used more like it is in humans to look at many different problems including brain and spinal cord issues. Companion animals must undergo anesthesia to have an MRI image taken, as they must remain completely still. For certain diseases, MRIs are considered the only imaging technique that can provide an accurate diagnosis (16).

Summary of Equine Diagnostic Imaging

 Overall, each imaging technique has its benefits and limitations. Generally speaking, you will want to start with the least invasive diagnostic methods and only resort to more invasive methods when inconclusive results are received. 

Additionally, in general, the more extensive the procedure, the more expensive it will be. It is important to talk with your veterinarian and establish the risk vs. reward for your specific animal when choosing the proper imaging technique. 

Your vet will be able to explain why they think a specific imaging technique would be best for the specific issue in your animal, however, hopefully now you feel empowered to understand a little bit more about each technique and the situations they are generally best used in. 

If you are using diagnostic imaging techniques with your horse or companion animal, there is probably some associated stress going on too, which could negatively affect their gut microbiome. To avoid any extra health problems, it is worthwhile to supplement with high-quality equine probiotics (or canine/feline probiotics) for ultimate gastrointestinal protection. 



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