Equine Diagnostic Imaging: Equine Diagnostic Imaging: X-rays, Ultrasound, CT & MRI
Diagnostic imaging plays an important role in investigating lameness and musculoskeletal problems in horses, but not every imaging technique provides the same information. In this blog we explore why.
Radiographs, ultrasound, nuclear scintigraphy, CT and MRI each have different strengths and limitations. Some provide detailed structural information, while others can help identify areas of increased biological activity. The most advanced imaging technique is not necessarily the most appropriate one. The clinical examination helps determine which structures need to be investigated and which imaging modality is most likely to answer the clinical question.
This is why diagnostic imaging is most valuable when it is targeted and interpreted alongside the horse's history, clinical examination and gait findings. Quite often multiple imaging modalities are required to get the complete picture on the are in question as they provide different information.
X-rays: assessing bone and joints
Radiographs, commonly referred to as X-rays, remain one of the most widely used forms of equine diagnostic imaging. They are particularly useful for assessing bone and joint-related changes, including fractures, osteoarthritis, bone remodelling, changes at joint margins and some developmental abnormalities.
Digital radiography also allows high-quality images to be obtained quickly in the field, making it an important part of many lameness investigations.
However, radiographs provide a two-dimensional representation of three-dimensional anatomy, and their ability to evaluate many soft-tissue structures is limited. A normal X-ray therefore does not necessarily mean that there is no musculoskeletal injury present.
X-ray imaging is limited in areas that have a large mass such as the trunk and pelvic region and as such, it's use is limited in these areas, leaving us to rely on on other tools, namely ultrasound, to provide information.
Ultrasound: more than tendons and ligaments
Ultrasound is often associated with tendon and ligament injuries, but its applications in equine musculoskeletal medicine extend considerably further. Depending on the anatomical region, ultrasound can be used to assess as well muscles, joint margins, synovial structures, origins of ligaments off the bone and accessible bone surfaces. It can also be particularly useful in regions where we need more information about the soft tissues surrounding a joint or other structure.
Unlike a static radiograph, ultrasound provides real-time imaging. Structures can be examined in different planes. It is also very useful to image deep tissue structures such as the back and the pelvis.
Ultrasound also has an important role in image-guided procedures. Visualising the relevant anatomy in real time allows accurate placement of a needle for procedures such as joint and tendon sheath injections and treatment of deeper anatomical structures such as the neck, back and pelvis.
Ultrasound is also very valuable to examine areas where X-rays can't, in the horse, such as the back and the internal pelvis, to look at the sacroiliac, lumbar sacral, and nerve root structures.
Radiography and ultrasound should therefore not necessarily be thought of as competing techniques. In many cases, they provide complementary information about the same anatomical region.
Why might we use more than one imaging technique?
Different imaging modalities show different tissues and pathology. For that reason, one technique may answer part of the clinical question while another provides additional information.
For example, radiographs may identify changes affecting bone or joint margins, while ultrasound of the same region can provide information about associated soft tissues, synovial structures and accessible joint margins.
In some cases, radiography and ultrasound together provide enough information to reach a diagnosis and formulate a treatment plan. In others, the findings may indicate that referral for advanced imaging such as nuclear scintigraphy, CT or MRI is warranted. The appropriate modality depends on whether we are trying to identify an active area of bone turnover, define complex bony anatomy, or assess bone and soft tissues in greater detail. The aim is not to perform every available test. It is to progressively answer the clinical questions raised during the investigation.
When do we need CT?
Computed tomography, or CT, uses X-rays to produce cross-sectional images of the body. These can be reconstructed to provide detailed three-dimensional information about the region being examined.
One of CT's major advantages is the ability to assess complex bony anatomy without the superimposition of structures that occurs with conventional radiography. This can make CT particularly useful when an anatomical region cannot be adequately assessed with standard X-rays.
In equine medicine, CT is increasingly used to investigate areas including the head, cervical spine and selected regions of the limbs. The development of standing CT systems also means that some examinations can be performed in sedated horses without the need for general anaesthesia.
CT is not automatically the next step in every investigation. Referral for CT is considered when the clinical question and anatomical region make the additional information it provides worthwhile.
To complicate things however, CT is expensive and not widely available yet in Australia and some centres have only smaller machines that can do the limbs and head but not the lower neck or the trunk/pelvis.
What is nuclear scintigraphy?
Nuclear scintigraphy, commonly referred to as a bone scan, is a functional imaging technique that can help identify areas of increased bone activity. A small amount of radioactive product is administered into the vein and binds to areas of increased bone turnover. A gamma camera is then used to detect the distribution of the radioactive product throughout the skeleton and these areas of increased uptake show up as "hot spots".
Unlike radiographs or CT, which primarily provide structural information about bone, scintigraphy can highlight areas of increased bone metabolism. This can be particularly useful when investigating subtle or difficult-to-localise lameness, suspected stress-related bone injury, or regions that are difficult to evaluate comprehensively with conventional radiography.
Another advantage is the ability to assess large areas of the horse, which can be valuable when the clinical examination has not localised the problem to one region, or it is also very helpful in horses that are not able to be nerve blocked.
However, increased radiopharmaceutical uptake identifies an area of increased bone activity rather than providing a specific diagnosis. An area identified on scintigraphy will therefore often require correlation with the clinical findings and further targeted imaging, such as radiography, ultrasound, CT or MRI.
If a horse however has more of a soft tissue problem, findings may not be apparent on a scintigraphy. As such, a negative result on the scinitgraphy does not mean there is "no clinical problem".
What does MRI tell us?
Magnetic resonance imaging, or MRI, provides detailed information about both soft tissues and bone and can identify pathology that may not be visible with conventional x-rays or accessible with ultrasound.
It can be particularly useful when lameness has been localised to a specific region but initial imaging has not adequately explained the clinical findings.
The equine foot is a good example. Nerve blocks may localise pain to the foot, while radiographs are normal or show abnormalities of uncertain clinical significance. Many structures contained within the hoof capsule cannot be comprehensively evaluated using ultrasound. MRI allows these structures, as well as the bone, to be assessed in considerably greater detail.
As with CT, MRI is most useful when there is a specific clinical question to answer, rather than simply because it is a more advanced imaging technique.
Why doesn't every abnormality on an image cause lameness?
This is one of the most important considerations when interpreting diagnostic imaging. Finding an abnormality does not automatically establish that it is the source of the horse's pain. Imaging may identify changes associated with previous injury, adaptation or degenerative change that are not responsible for the horse's current clinical signs. This becomes particularly important when interpreting findings in older horses or horses with a long competition history, where multiple abnormalities may be present.
This is why imaging findings need to be considered alongside the horse's history, clinical examination and gait assessment.
Where appropriate, nerve blocks can help localise the source of pain before imaging. This allows us to investigate the structures most likely to be clinically relevant rather than simply identifying abnormalities and assuming they explain the lameness. The reverse is also important. Normal radiographs do not exclude musculoskeletal pain. Soft-tissue injury, early bone pathology and abnormalities within complex anatomical regions may require another imaging modality to identify.
The right image starts with the right clinical question
Good diagnostic imaging begins before the first image is taken. During a lameness investigation, the clinical examination, objective gait assessment and, where appropriate, nerve blocks can help identify the affected limb or region and localise the likely source of pain.
Once the problem has been localised, the veterinarian can select the imaging modality most likely to provide useful information about the structures involved.
If you would like to understand more about this process, read What Happens During a Lameness Examination?
Sometimes radiographs provide the answer. Sometimes ultrasound is more appropriate. Sometimes both are required, and occasionally the findings indicate that referral for scintigraphy, CT or MRI or is warranted.
Ultimately, the objective of equine diagnostic imaging isn't simply to find something abnormal. It is to determine whether the imaging findings explain what we are seeing clinically and use that information to make better decisions about treatment, rehabilitation and management.
Frequently Asked Questions
What is the best diagnostic imaging technique for lameness in horses?
There is no single best imaging technique for every horse. The appropriate modality depends on where the problem has been localised, which structures are suspected and the type of pathology being investigated. Radiographs and ultrasound may be sufficient in many cases, while CT or MRI may be recommended for selected problems.
What can ultrasound detect in horses?
Musculoskeletal ultrasound can assess many tendons, ligaments and muscles as well as joint margins, synovial structures, entheses and accessible bone surfaces. Its usefulness depends on the anatomical region and whether the structure can be adequately accessed with ultrasound.
Can a horse still have an injury if its X-rays are normal?
Yes. Radiographs provide excellent information about many bony structures but have limitations, particularly when assessing soft tissues and complex anatomy. A horse can therefore have clinically significant musculoskeletal pathology despite normal radiographs.
Why would my horse need ultrasound after having X-rays?
The two techniques provide different information. Radiographs predominantly evaluate bone and joint-related changes, while ultrasound can provide additional information about soft tissues, synovial structures, joint margins and accessible bone surfaces. Using both may provide a more complete assessment of an anatomical region.
When does a horse need an MRI or CT scan?
CT or MRI may be recommended when the clinical examination has localised the problem but conventional imaging cannot adequately answer the diagnostic question, or when the anatomical region is better evaluated using cross-sectional imaging. The choice depends on the suspected pathology and structures involved.
What is a bone scan in horses?
A bone scan, or nuclear scintigraphy, identifies areas of increased bone activity using a radioactive tracer and gamma camera. It can be particularly useful when investigating difficult-to-localise lameness, stress-related bone injury or anatomical regions that are difficult to assess with conventional radiography. Areas of increased uptake usually require interpretation alongside the clinical examination and may require further targeted imaging.
Investigating a lameness or performance concern?
A thorough clinical assessment can help localise the source of the problem and determine whether diagnostic imaging is required and which modality is most appropriate for your horse.
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