Page 335 - Adams and Stashak's Lameness in Horses, 7th Edition

Diagnostic Imaging 301

ULTRASOUND

VetBooks.ir W. rich redding

INTRODUCTION density. The greater the differences in acoustic impedance
of the reflecting interfaces, the greater the intensity of
Over the last several decades, ultrasound technology the returning echo. Ultrasound waves are constantly
has dramatically improved. The current technology encountering changes in soft tissue that can affect prop­
found in linear array ultrasound systems has progressed agation of the sound wave, which cause scatter and a
rapidly and are now very well suited for musculoskeletal weakening of the return echoes. The brightness of the
examinations. Many of these high‐end systems have dot on the monitor screen correlates to the amplitude of
14–18‐MHz linear tendon probes and 8–10‐MHz the returning echo. Terms to describe the appearance of
microconvex probes with variable focusing capabilities an image relate to the tissue’s echo intensity or echo­
with multiple frequencies available in each probe. In genicity. The echogenicity of a structure or the degree to
addition, mainframe ultrasound platforms have been which a structure reflects sound waves determines the
reduced to the size of notebook‐sized computers as well brightness of objects on ultrasound. The strength of the
as advancements made in the miniaturization of elec­ reflective sound is displayed using a gray scale, where
tronics have reduced the quality differences between black indicates that no sound is reflected and white indi­
portable and stationary technologies. Ultrasonography cates that a large amount of the sound is reflected.
is now considered the imaging modality of choice to The amount of reflected ultrasound received by the
evaluate soft tissue injuries in the horse. probe depends on the angle of incidence (the angle that
Ultrasonography is a two‐dimensional real‐time the sound wave interacts with a structure) of the
imaging technique that utilizes the transfer and propa­ ultrasound beam transmitted by the probe. When the
gation of sound waves into soft tissue. 4,55,61,70,73,74,80 ultrasound beam is not perpendicular to a structure or
Ultrasound is defined as sound above the audible range. portion of a structure (such as a tendon), a portion of
Ultrasound waves behave as classic sound waves that the reflected ultrasound will be off angle and will not
operate at frequencies spanning 1–22 MHz. These sound return to the transducer. As a consequence, a darker
waves are mechanical waves that require some sort of (hypoechoic) area will be seen in the image. Thus, dur­
medium to allow the waves to form and travel. The ing scanning, it is essential to maintain the ultrasound
propagating medium determines how fast the sound beam as perpendicular as possible to the structure being
wave travels, how easily the waves can be formed, and imaged. The appearance of darker (hypoechoic) areas in
how well the traveling waves can remain together. the image that result from the ultrasound beam not
Ultrasound machines produce sound waves of longitu­ being perpendicular to the structure in question is
dinal orientation in which the elements of the medium designated anisotropism or off‐normal incidence. This is
are compressed and rarefied. The distance between the especially common when imaging tendons and ligaments.
start of one cycle of compression and rarefaction and Making sure that the ultrasound beam is perpendicular
the next is considered the wavelength, and most wave­ to the fibers of these structures can be accomplished by
lengths are 1 mm or less. Propagation speed of the ultra­ repositioning the probe, moving it closer to the tendon
sound wave is determined by the density and stiffness of or ligament edges, and by slowly moving the probe in
a given tissue with bone propagating at higher speeds, different angles while scanning. All this information is
while fluid‐filled structures propagate at medium speeds displayed as a cross‐sectional image developed by an
and air propagating at the lowest speeds. Because air has entirely different set of physical parameters of structures
molecules that are relatively far apart, sound travels (objects) than those measured by other imaging
relatively slowly (approximately 330 m/s) in air. In soft modalities.
tissue, the molecules are closer together allowing sound The piezoelectric crystals in the scan head determine
to travel faster with an average propagation velocity in the frequency of the sound wave. These crystals are man‐
soft tissues of around 1540 m/s. made and designed to vibrate at specific frequencies and
Ultrasound waves lose energy to the medium in the produce a consistent wavelength sound beam. These
form of heat through a process termed absorption. crystals also receive sound waves coming back from the
Absorption increases directly with distance and fre­ tissues and convert them to electrical energy. The wave­
quency. A transducer produces short bursts of specific length dictates the resolution and the energy contained
frequency sound waves that are transmitted into the by this sound beam. High‐frequency transducers have
patient and reflected back when it interacts with differ­ smaller crystals, the sound pulses are closer together, and
ent tissues and tissue interfaces. The transducer then the wavelengths are shorter. The shorter the wavelength,
detects the reflected sound waves, and these waves are the better the axial resolution, which is a measure of the
converted to electrical energy. A computer plots the time ability to show two interfaces as separate along the axis
the sound waves traveled along with the amplitude of of the beam. Axial resolution is determined by the wave­
the reflected sound waves. Echoes are produced at tissue length (pulse length), and the wavelength is determined
interfaces of different acoustic impedance. Acoustic by the frequency. Lateral resolution is the minimum dis­
impedance is a measure of how easily waves can be tance that two dots can be distinguished from one
formed and is dependent on sound velocity and tissue another in a plane perpendicular to the sound wave.