Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

C H A P T E R O N E
Introduction
GENERAL CONTRIBUTION OF RADIOLOGY TO VETERINARY
MEDICINE
The art and science of radiology became an integral part of veterinary medicine and surgery
shortly after the exposure of the first radiographic films. This subsequently led to the
publishing of the first English language text on the subject of radiology in canine practice.
1 Although many special radiographic procedures have been described since then, survey
radiography remains the standard for the majority of antemortem anatomical
diagnoses.
The computer’s ability to manipulate data has been applied to various imaging technologies,
resulting in new images such as those seen with digital subtraction fluoroscopy,
computed tomography (CT), magnetic resonance imaging (MRI), nuclear scintigraphy,
and diagnostic ultrasonography. This technological explosion has called for an expansion
of veterinary expertise beyond traditional diagnostic radiology to include all types of diagnostic
imaging. Of these newer technologies, ultrasonography has gained popularity most
rapidly among both veterinary specialists and small animal practitioners. Although future
developments in diagnostic imaging and the role for MRI, CT, and nuclear imaging remain
to be defined, it is clear that diagnostic radiology and ultrasonography will play an important
and expanding role in the practice of small animal medicine and will contribute to
improved health for pets.
ESSENTIALS OF RADIOGRAPHIC AND ULTRASONOGRAPHIC
PHYSICS
The physics of radiology and ultrasonography are complex, and merely mentioning the subject
to some individuals elicits a response that ranges from fear to boredom. Several textbooks
explain radiologic and ultrasonographic physics in detail. Because our purpose is to
emphasize radiographic and ultrasonographic diagnosis, these textbooks and articles should
be consulted if a more complete understanding of radiographic and ultrasonographic
physics is desired. 2-9 A minimal knowledge of the physics of the processes involved is helpful
for successful radiology procedures. X-rays are electromagnetic radiations with very short
wavelengths (high frequencies) and high energies. They are produced by bombarding a tungsten
target with a stream of energetic electrons. The resulting x-ray beam is a collection of
photons of different energies. These x-ray photons may pass through or be absorbed by a
substance, depending on their energy and the relative density, thickness, and atomic number
of the substance. In film-based medical radiology, after passing through body fluids, tissues,
and organs, the photons ionize silver, which is contained in the emulsion of a photographic
type of x-ray film. This may occur either directly, by interaction of the x-ray photon with the
silver emulsion, or indirectly, by interaction of the x-ray photon with a fluorescent intensifying
screen producing blue or green light that exposes the film. This pattern of ionized silver
(the latent image) becomes visible after the film is chemically developed and fixed. Digital
detectors in lieu of x-ray film may also be used to create images. In this circumstance, the
interaction of the photons that have passed through the patient with a computer-compatible
detector creates the latent image, which is then translated into a digital display on a computer
screen. The visible image as visualized using film or a computer is a composite picture
of the structures through which the x-rays passed before reaching the film. For the rest of this
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