There are a variety of different imaging modalities used every day in equine veterinary practice. They are used to assess the type and severity of injury in both bone and soft tissue structures of the musculoskeletal system. Most equine practices have radiography and ultrasonography equipment available. Larger clinics and referral centres often have more advanced imaging modalities such as gamma scintigraphy, computed tomography (CT) and magnetic resonance imaging (MRI).
Radiography is an imaging modality used frequently in equine practice to enable your vet to determine if there is any injury to the bones of the skeleton. It is performed by generating a small dose of ionizing radiation (x-ray) with an x-ray machine which is projected towards the area of interest on the horse. This generates an image on an x-ray film either by the more modern digital systems or by manually developing the film. Radiography is useful in the diagnosis of fractures, bone chips, changes in bone density (as seen with navicular syndrome), laminitis and to evaluate the origins and insertions of tendon and ligament structures on bone.
Radiography of certain parts of the horse are challenging to their size. Areas with well-developed overlying musculature are difficult for the x-ray beams to penetrate sufficiently to produce a diagnostic image, e.g. the pelvis. It has limited use in the diagnosis of soft tissue injuries as only bony structures can generally be viewed with x-rays. To fully evaluate soft tissue structures ultrasound and/or MRI may be required. Radiation safety is an important consideration for personnel working with x-ray equipment. Lead gowns, gloves and radiation badges are worn to protect against radiation exposure and monitor levels of exposure.
Ultrasound uses high frequency sound waves to evaluate tendon and ligament structures of the musculoskeletal system. It is the imaging modality of choice for identifying soft tissue injuries of the horse. It is particularly useful for imaging tendons of the lower limb which are frequent sites of injury in the horse. It can be used to identify foreign bodies lodged within soft tissue structures and to evaluate the margins of bone structures. With the right equipment and in the hands of a skilled clinician it can also be used to evaluate almost any soft tissue structure of the horse, in particular the abdomen and heart.
In order to obtain images of diagnostic quality your horse’s hair will need to be clipped, the skin cleaned and ultrasound gel applied to the area of interest. Your vet will be looking for enlargement of structures, evidence of fibre pattern disruption and fluid accumulation to determine potential sites of injury.
Often referred to as a bone scan, scintigraphy is used to image the equine skeleton. Large areas of the horse can be viewed in comparison to radiography. However, the amount of bone detail seen is less compared to radiography. Scintigraphy is performed by injecting the horse with a radioactive isotope which attaches to bone. Uptake of the isotope is greater in areas of active bone remodelling, i.e. in areas of bone injury. These areas of increased uptake are seen as hot spots (brighter areas) on gamma scans. Once the region of abnormal bone has been identified on scintigraphy, radiography can be used to visualise the area in greater detail.
Scintigraphy is used most readily in the following situations:
- Lame horses in which nerve blocks have failed to identify the source of pain.
- Horses which are not amenable to nerve blocks due to their temperament.
- Poor performance problems where the horse is not visibly lame on a particular limb.
- Racehorses with suspected pelvic fractures.
Following gamma scintigraphy, the horse must be isolated until its radiation levels have declined to within the recommended safety limits.
Computed tomography is an imaging modality used generally to image bone (and to a much lesser extent soft tissue) in much greater detail compared to radiography and scintigraphy. A CT scanner comprises an x-ray tube and radiation detectors. An image is generated by cells, called pixels, each of which is assigned a number and is used to produce the image on a screen.
It provides a three-dimensional image of the area of interest in contrast to radiography which only provides a two-dimensional image. Applications for CT include the examination of limbs for assessment of osteoarthritis, osteochondrosis lesions and fractures. It is also readily used in the diagnosis of diseases of the equine head.
The advantages of MRI in the diagnosis of equine lameness have become widely acknowledged. It is superior to the other imaging modalities due to the extraordinary detail of both bone and soft tissue structures it provides in a three-dimensional image. It has revolutionised the diagnosis of causes of foot pain in the horse, due to the ability to image soft tissue structures within the hoof capsule. MRI systems are now being installed in both referral and equine practices.
An image is generated by placing the area of interest into a magnetic field. There are two types of system available high field and low field according to the strength of the magnet. Although high field magnets generate better quality images, the horse must undergo general anaesthesia. The more popular low field systems can be utilised with the horse under standing sedation. This system is readily used for imaging of the foot and lower limb, imaging up to the knee and hock is possible but challenging and expertise is increasing in this field.
An endoscope is a fibre-optic camera up to 3m in length which enables direct visualisation of the upper respiratory, e.g. larynx and trachea, and gastrointestinal tracts, e.g. oesophagus and stomach, of the horse. It is particularly useful in the evaluation of poor performance problems and is readily used in the racehorse industry. Endoscopic examination can be used to identify disorders of the larynx with the horse at rest or during exercise with the latest exercise-adapted equipment. Endoscopy of the stomach (gastroscopy) is useful in the diagnosis of oesophageal obstruction and gastric ulceration.
Electrocardiography is the interpretation of the electrical activity of the heart over a period of time. This is achieved by using electrodes attached to the surface of the skin; the electrical activity is then recorded by an electrocardiography machine. The recording produced is called an electrocardiogram (ECG or EKG), or ECG tracing.
Electrocardiography is used to analyse the rate and regularity of a horse’s heartbeats; to monitor heart rate and rhythm during anaesthesia or drug therapy; and to assess the effect of training and exercise on the heart rate and rhythm.
An ECG will immediately show if your horse is suffering from any condition of the heart that causes the heart rate to be irregular.
Echocardiography, also called cardiac echo, is a sonogram of the heart. It uses standard two-dimensional, three-dimensional and Doppler ultrasound to create still and moving images of the heart.
Echocardiography is routinely used in the diagnosis, management and follow-up of cases with any suspected or known heart conditions. It is one of the most widely used diagnostic tests in cardiology as it can provide lots of useful information, such as the size and shape of the heart, pumping capacity, and the location and extent of any tissue damage. It is also used to assess the blood flowing through the heart using pulsed or continuous wave Doppler ultrasound. This allows assessment of both normal and abnormal blood flow through the heart. Collor Doppler ultrasound is used to visualise any abnormal communications between the left and right side of the heart, any blood leaking through the valves, and to estimate how well, or not, the valves open.