On April 24 at our Client Education seminar at the Virginia Horse Center in Lexington, Dr. Tabby Moore educated us about Equine Imaging Technology, its uses, advantages and limitations. She covered radiography, ultrasonography, scintigraphy (bone scan) and magnetic resonance imaging (MRI), explaining the various modalities that can be confusing for horse owners.
Equine Radiography – An x-ray beam is passed through the body part and a portion of the x-rays are absorbed or scattered by the internal structure and the remaining x-ray pattern is transmitted to a detector so that an image may be recorded.
Advantages of this modality: performed in the field, high detail, economical ($46/view).
Limitations: designed to image bone and is not as useful with other tissues such as cartilage, ligament, tendon, muscle, etc., not very good on large body parts like the thorax and abdomen, involves radiation.
Sometimes dye or probes are added to help highlight things, and it is often used for guided treatments and injections.
Equine Ultrasound – Ultrasound machines transmit high-frequency sound pulses into tissue using a probe. Sound waves travel into a body part and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone) and some of the sound waves get reflected back to the probe, while some travel on further until they reach another boundary and get reflected. The ultrasound machine consists of a large laptop, a probe and a coupling agent (gel or alcohol).
Advantages – Performed in the field, used primarily for non-bony tissues, it’s safe and economical ($75 – 250/study).
Limitations – Operator dependant on training, skill and equipment, angle of the beam relative to the structures, especially smaller finer structures.
Ultrasound is used to provide critical information in cardiology, gastrointestinal, musculoskeletal and reproduction issues.
Nuclear Scintigraphy (Nuclear Medicine or Bone Scan) is a chemical target tissue tracer, bone = (calcium and phosphorus), is chemically tagged to a radioactive isotope (technetium 99), Radiopharmaceutical (tech 99 + phosphorus) and is injected intravenously.
The injected material circulates through the entire body, going to areas where bone is remodeling. A gamma camera captures gamma radiation and the computer creates an image (scinitgram) with different color palettes.
Bone Scans may be warranted for diagnosis of occult or intermittent lameness, bone survey for multiple limb lameness, early detection of skeletal injury – fracture or for determining extent and severity of skeletal lesion – activity of radiographic lesions, old vs. new. It is useful to help when other modalities do not pinpoint a problem, but a horse exhibits poor performance, change in capabilities; very helpful with back pain cases.
Advantageous when there is localization of pain but inability to identify cause using radiography and ultrasonography, poor performance of ill-defined cause, suspected thoracolumbar or pelvic region pain and for evaluation of healing response and blood flow to bone.
Limitations – scintigraphy should not be a substitute for a comprehensive exam, as it has generally poor specificity, and usually follow-up radiographs and/or ultrasound are necessary. Osteochondrosis often do not produce a detectable change in the scan and regional anesthesia may cause a false positive on the soft tissue phase.
Disadvantages – must be done in hospital setting (NRC license), patient (horse) will be radioactive and must be isolated from the general public for 24-36 hours, expensive ($1000-1800/study).
Useful for evaluating stress fractures (cannon bone, radius, humerus, coffin bone), fractures of the hip/pelvis (done standing, no risk of anesthesia or recovery), back pain, sesamoiditis and degenerative joint disease.
MRI – produces very detailed images of the region of interest by generating “slices” of tissue in various geometric planes with different sequences that highlight various structures and tissues. Regions of interest are sliced into many parts and digitally regenerated. MRI can be performed with horse standing or recumbent, depending on the machine.
Advantages – MRI produces high quality images in any plane through the ROI. The images generated show the anatomy of bones, joints, muscles, ligaments and tendons at a level unsurpassed by any other modality.
Limitations – Must be done in hospital, limited to lower extremities and head (carpus/tarsus down), need to scan defined body part (6″ window), cost ($1800-2600/study)