Patrick R. Gavin, DVM, PhD, DACVR/RO
Excerpted from Practical Small Animal MRI, (Gavin, Bagley)
Diagnostic imaging has always been a mainstay of the armamentarium for the veterinarian. Veterinarians have limited resources available as regards history and routine screening procedures. Therefore, diagnostic imaging has a major role in the workup of numerous veterinary patients.
An overreliance on diagnostic imaging has been observed by numerous clinicians; however, the move toward less invasive diagnostic procedures with a high precision of diagnosis has continued to drive this phenomenon. This paper deals with the advances in diagnostic imaging through the last 60 years.
Diagnostic radiology was invented in the late 1800s. The use of diagnostic radiology was rewarding primarily in the study of skeletal structures. However, due to the cost of the equipment, lack of education, and potential risks, the modality did not penetrate veterinary medicine until approximately the 1950s. Initially it was the colleges of veterinary medicine in North American that possessed the equipment to perform diagnostic radiographic examinations. There were no trained radiologist at that time and in some places the studies were often performed and interpreted by non-veterinarians. Clinicians did not know what to expect as they had no prior knowledge of the diagnostic modality. Clinicians were often asked if they wanted a V/D or lateral and would merely say “yes” and accept the outcome. Much was to be learned.
Diagnostic radiology advanced rapidly in veterinary medicine, and the first examinations for veterinary radiologists were performed by charter diplomats for the American College of Veterinary Radiology in 1965. Following this beginning veterinary radiology advanced rapidly. Diagnostic radiology was utilized in multiple species throughout colleges of veterinary medicine and in selected practices. By the early to mid- 1970s, advanced radiographic procedures including fluoroscopy and angiography were available, though primarily at colleges of veterinary medicine. The use of diagnostic radiology expanded with improved knowledge, especially with better understanding of its diagnosis of various pathologic conditions.
The use of diagnostic radiology abated somewhat with the advance of diagnostic ultrasonography; however, it has remained the stalwart of diagnostic imaging in the veterinary profession. At the current time, there is a major push to move from conventional analog film screen technology to computed and/or digital radiography. It is presumed that veterinary radiology will continue to follow the progression realized in human radiology.
NUCLEAR MEDICINE OR GAMMA SCINTIGRAPHY
The previously used term, nuclear medicine, fell out of favor with the antinuclear movement of the 1970s. Medical personnel were quick to adopt the softer terminology of gamma scintigraphy that facilitated its continued development as an imaging modality. While gamma scintigraphy has the advantage of visualizing physiologic and temporal pathologic changes, for the most part its greatest use in veterinary medicine has been static studies for the diagnosis of skeletal disease. The use of the modality for the diagnosis of skeletal disease is well documented. The challenges of using nuclear isotopes, radiation safety concerns, and time delays are well documented. Some studies have become rather routine in veterinary medicine. These include studies of the thyroid gland that have been published and have led to a better understanding of thyroid disease. While this modality has been present since the turn of the century, it became rather commonplace in veterinary medicine in the 1980s. Its involvement as a diagnostic modality has undergone little evolution in the last two decades.
Computed tomography (CT) was first utilized in the mid-1970s in veterinary medicine, primarily for the diagnosis of intracranial disease. The modality was modified for the study of large animal species shortly thereafter. CT has had a large expansion in the veterinary medical field. Virtually all colleges of veterinary medicine provide this diagnostic modality. In the last 10 years, extension into private veterinary practices has significantly expanded its availability. There are now numerous large specialties, and even general practices, with CT on site. Many units were purchased as used equipment, but many include state-of-the-art helical units.
CT uses the same basic physical principles as diagnostic x-ray, except it depicts the shades of gray in cross-section. It is also possible to better visualize different tissues and the pathologic change within them, if present. Therefore while the modality is similar to diagnostic x-ray, CT is superior to the two-dimensional radiographic projections. CT has led a renaissance in the understanding of three-dimensional anatomy and physiologic principles.
Ultrasonography became a clinical imaging modality in veterinary medicine in the late 1970s. It languished in veterinary colleges through much of the 1980s as the technology advanced. The initial technology of static B-mode machines was replaced by real-time machines that allowed an approximate 80% reduction in scanning time. The resolution and utility of the studies improved at the same time. However, diagnostic ultrasonography did not hit its stride and become mainstream in the United States until approximately the 1990s. Now, most large veterinary practices (and certainly referral practices) have diagnostic ultrasonography. This modality is also available in many smaller private practices. There have been numerous technologic advancements that have improved the quality of this modality. Increased availability of traveling diagnostic radiologists and/ or interpretation via teleradiology have improved diagnostic outcomes.
Other specialists utilizing diagnostic ultrasonography, including cardiologists and internists, have further fueled the expansion of this modality in veterinary medical practice. Currently, most ultrasonographic examinations are performed by licensed veterinarians. It is this author's opinion that in the future, many of these procedures will be performed by trained ultrasonographers and interpreted by radiologists, just as occurs in the human field. In the human field, there is a greater medical liability issue, and if physician radiologists can make it work, certainly veterinary radiologists can work in this format to further advance this modality's utility in the diagnosis of our veterinary patients.
MAGNETIC RESONANCE IMAGING
Magnetic resonance imaging (MRI) came into clinical utility in the mid-1980s. It was utilized in veterinary medicine primarily as a research tool in the 1980s and early 1990s. In the mid-1990s, some areas began to use MR as a routine clinical modality. The procedure was applied to large animal imaging a few years later. However, the attitude of "not invented here" plagued the inclusion of MRI for the diagnosis of veterinary patients at many sites in the early years. Many veterinary sites had antiquated equipment or equipment with poor reliability, which gave it the aura of an unreliable diagnostic modality. However, as more sites gained modern diagnostic equipment, the utility of the modality became apparent.
Following the change of the millennium, MRI became the modality of choice for the veterinary neurologist for the examination of disease processes involving the brain and spinal cord. Efforts to expand the use of the modality included corporate sponsorship of diagnostic facilities. At the time of this writing, this author is aware of more than 40 sites dedicated to MR imaging of animals using what would be considered modern stateof-the-art equipment. One limitation has been the non-availability of appropriately trained veterinary radiologists with expertise in this modality capable of providing accurate diagnoses of clinical conditions. Currently, the American College of Veterinary Radiology does not require training time minimums in MRI for their core curriculum, as not all training sites have this modality available. Therefore many veterinary radiologists, and others, must essentially undergo "on the job training" in the use of this modality.
There is a broad spectrum of equipment options. These options span from the currently available best, including machines capable of functional MRI, commonly utilized super conducting magnets, cost effective mid-field units, to even less expensive but less capable low field permanent magnets. It is this author 's opinion that equipment generally costs what it is worth. Therefore, equipment that is more expensive is of more diagnostic worth, and conversely, equipment that costs less has less diagnostic capability. The equipment purchase balance will be finding equipment that provides the utility required for the financial reality of the practice. There has been a rapid development of equipment in the last few years.