Imaging techniques provide important pathologic and physiologic information necessary to treat specific conditions. Imaging can be divided into anatomic and physiologic methods. Anatomic imaging methods include radiology, ultrasonography, CT, and MRI. Physiologic imaging methods include scintigraphy and thermography. When diagnostic analgesia has failed to eliminate the lameness, the lameness is too subtle for localization by diagnostic analgesia, or the horse is not amenable to handling or injection, physiologic imaging techniques may help narrow the problem to a specific region. Anatomic imaging methods can then be used to evaluate those areas. Imaging may also help prevent injury. This requires early detection of the physiologic changes associated with injury. Although frequent use of an anatomic imaging method can detect change in one region, physiologic imaging allows assessment of the entire horse on a routine basis.
Anatomic Imaging Techniques
Radiologic techniques are the methods most commonly used to evaluate lameness in horses. Plain film radiography used to be the standard, but it has been replaced by computed radiography in equine practice. Computed radiography can be divided into indirect and direct. Indirect uses a special plate instead of film. The plate stores energy that is then read by a computer to produce the image. Direct digital radiography also uses a special plate, but the radiation is converted to a digital signal and sent directly to a computer. Direct digital radiography produces images faster, but both share the advantages of fewer retakes, a lower radiation requirement, and post processing techniques that eliminate contrast problems. Radiography requires multiple projections to evaluate any area. It allows assessment of bony tissues and reflects both acute and chronic changes. Occasionally, radiographic techniques that provide more information are needed. Contrast radiography provides information about articular cartilage and surfaces and is of particular value in determining whether subchondral cysts communicate with the joint and in delineating subcutaneous tracts. More recently, these techniques have been used to delineate changes in the navicular bursa and have produced 60% more information than plain radiography. Pathologic diagnoses are usually made by radiography in conjunction with clinical examination.
The goal of radiology, regardless of system used, is to examine the region sufficiently to fully evaluate the anatomic structure. Diagnostic films require preparation, positioning, and production. Preparation involves readying the object to be radiographed. In most cases, this requires the object to be clean and all foreign materials removed (eg, any iodine-based products on the limb will cause artifacts on the radiograph). For radiographs of the hoof, the shoe may need to be removed and the sulci packed, in addition to cleaning.
Positioning is critical; the object must be evaluated from a sufficient number of angles to ensure adequate evaluation. Minimally, this means two radiographs 90° apart. Many of the limbs require more views for adequate evaluation. Examination of those projections may necessitate further views to better assess any areas of interest. For instance, the equine foot, fetlock, and carpus require five projections, whereas the pastern and hock require four. The upper limbs of the horse require fewer projections. This is not because these are less complex areas; rather, the size of the horse makes it difficult to get more projections. Two views can usually be made of the elbow and the stifle. For the shoulder joint, usually only one view is possible. For the hip, anesthesia is usually required. However, digital radiography has made it possible to take standing hip projections on young horses and those with smaller muscle mass.
Production of good radiographs requires the correct exposure of the film. Proper kVp and mA settings, as well as proper focal film distance, are critical. Unfortunately, these factors vary and depend on the particular x-ray machine and the film or electronic system used. For ambulatory equine practitioners, another factor that must be considered is the electricity output in the barn where the images are taken—older barns may not have sufficient electrical output for the x-ray generator to make the desired exposure. New computed radiography systems that are battery operated can avoid these types of electrical problems.
Ultrasonographic examination can be used to assess any soft tissues. Like radiography, the area to be examined should be evaluated in two planes 90° apart. Selection of a probe should take into account the depth, contour, and location of the tissue to be examined. The deeper the tissue to be evaluated, the lower the wavelength of the probe used. The higher the wavelength, the greater the detail that can be achieved. For examination of superficial and deep flexor tendons or the suspensory ligament, a 7.5–10 MHz linear probe is best. Examination of complex anatomic areas such as the foot or pelvic region requires a convex linear probe. Examination of the pelvic region internally requires a rectal linear probe.
Ultrasonography is most useful in the evaluation of tendons and ligaments but can also be used to evaluate muscle and cartilage. In all cases, tissue fiber alignment and echogenicity are the factors used to determine anatomic disruption. Generally speaking, loss of fiber alignment and decreased echogenicity are signs of acute injury; increased echogenicity is generally thought to indicate chronic conditions. However, if any questions arise during the examination, the opposite limb or area can be examined to compare changes. For the novice ultrasonographer, it is a good idea to compare the right and left sides before making an ultrasonographic diagnosis.
Assessment of anatomic changes serves as the basis for any pathologic diagnosis, as well as being important in determining prognosis. For these purposes, radiography and ultrasonography are complementary. Radiography provides information regarding bony tissues, whereas ultrasonography provides information about the soft tissues that connect bone or provide support.
MRI and CT are high-detail anatomic imaging tools. Their use is becoming more common in equine lameness evaluations. MRI in particular has become quite popular. Two types of MRI are available: low-field and high-field magnets. High-field scanners produce a stronger signal and higher resolution pictures in a shorter time than low-field scanners. However, some low-field scanners can be used to examine the standing, sedated horse, whereas high-field scanners require the horse to be anesthetized. The standing units can only be used to evaluate from the carpus and hock distally. Because of the slower image acquisition with the low-field scanners, motion can be a problem. MRI provides sliced images of the anatomic region of interest. The slices are usually in three different planes: axial (transverse), sagittal (longitudinal), and dorsal. MRI of orthopedic disease is performed in several acquisition sequences. Each sequence displays different anatomic, physiologic, and pathologic information. The most common sequences are the proton density and the T1-weighted and T2-weighted images. Proton density provides the most anatomic detail. T1-weighted images highlight the structural characteristics of bone and soft tissues, whereas T2-weighted images emphasize the fluid characteristics of tissues and are sensitive for detecting synovial effusions, cysts, and edema. Special sequences can further clarify or highlight a lesion. For instance, fat-suppressed sequences are used to evaluate edema in high-fat signal areas such as the bone marrow.
CT is a technology that uses very small x-ray beams from many different angles around the body (called a slice) that are reconstructed by computer to produce an image. Because the images are in slices, there is less interference from surrounding anatomy. Therefore, the CT scanner provides the clearest images possible of the limbs, joints, nasal passages, skull, sinus cavities, and neck. These images improve the clinician’s ability to accurately define and identify the extent of abnormalities of these regions.
Physiologic Imaging Techniques
These techniques provide images that reflect physiologic processes. Unlike anatomic imaging, which reflects structure, physiologic imaging techniques assess metabolism or circulation. Thermography and scintigraphy allow examination of the entire horse. When combined with a thorough clinical examination, these methods help identify injuries that may otherwise go undetected.
Thermography is the pictorial representation of the surface temperature of an object. It is a noninvasive technique that measures emitted heat in the form of infrared radiation and helps detect inflammatory changes that may contribute to lameness. Relative blood flow dictates the thermal pattern; normal thermal patterns can be predicted based on vascularity and surface contour. Skin overlying muscle is also subject to temperature increase during muscle activity. Circulation is invariably altered in injured or diseased tissues. Thermographically, the “hot spot” associated with the localized inflammation generally is seen in the skin directly overlying the injury. However, diseased tissues may have a reduced blood supply due to swelling, vessel thrombosis, or tissue infarction. With such lesions, the area of decreased heat is usually surrounded by increased thermal emissions, probably due to shunting of blood. The American Academy of Thermology has published guidelines for the use of infrared thermography in veterinary medicine (view guidelines here) . The purpose of the guidelines is to provide criteria for the production of reliable and repeatable thermal images.
During scintigraphy, polyphosphonate radiopharmaceuticals are given IV. Their distribution is then measured by a gamma camera, which measures the radiation emitted from the radiopharmaceutical after it is distributed through the body. The polyphosphonates bind rapidly to exposed hydroxyapatite crystal, generally in areas where bone is actively remodelling. Because inflammation causes an increase in blood flow, capillary permeability, and extracellular fluid volume, inflamed tissues accumulate high levels of radiopharmaceutical during the soft-tissue phase of scintigraphy, allowing evaluation of soft-tissue injuries. During the bone phase, the radiopharmaceutical accumulates in areas of increased remodelling or vascularity. Because injured bone is remodelled more rapidly, scintigraphy helps detect lesions in bone and ligaments, particularly to identify enthesopathy (damage to the insertions of tendons and ligaments on bone).