Page 336 - Adams and Stashak's Lameness in Horses, 7th Edition

302 Chapter 3

Lateral resolution is best in the focal zone and is depend­ roconvex probes have larger field of views and lower
ent on the width of the sound beam. The focal zones frequencies (lower resolution and deeper penetration).
VetBooks.ir tor at any depth/level of the image. Deeper to the focal toured (concave) and difficult to seat a flat‐face trans­
The microconvex probe is utilized when the skin is con­
optimize the beam and can be positioned by the opera­
ducer. The divergent beam allows the examiner to image
point, the ultrasound beam exponentially diverges, and
therefore the lateral resolution becomes progressively from a smaller skin contact point. These convex probes
inferior. Improving lateral resolution requires focusing can be more difficult to use because it is easier to inad­
the beam to the narrowest width possible. Axial resolu­ vertently change the beam angle especially when doing
tion is usually superior to lateral resolution. Images longitudinal assessments of fiber alignment. A macro­
should be obtained with the highest frequency probe convex curvilinear transducer is indicated for deeper
possible to obtain the best resolution of the structure of musculoskeletal examinations and for general abdomi­
interest. However, sound is attenuated at 1 dB/cm depth nal imaging, primarily focusing on the gastrointestinal
per megahertz (MHz). Higher frequencies are therefore tract. Many of these probes have multiple frequencies
attenuated at higher rates, which reduce the penetration available that allows the examiner to change the fre­
of the sound wave. Lower frequencies are attenuated at quency without needing to change the probe. Structures
lower rates, which allow them to penetrate deeper into within 5–7 cm of the skin should be evaluated with
tissue. The depth of a specific structure is then calculated transducers of a minimum of 7.5–10 MHz or higher.
based on the amount of time it takes for the transmitted Structures within 7–14 cm should be evaluated with
ultrasound beam to be reflected and received back by 5‐MHz transducers. Anything deeper than 14 cm will
the probe. The ultrasound travel time is calculated require lower frequencies such as 2.5–3.5 MHz. A
assuming that sound travels at the same velocity through phased‐array transducer has a relatively small rectangu­
all tissues. For any given image, sound can travel through lar, flat surface. In these transducers, the piezoelectric
fat, soft tissue, muscle, fluid, blood, and skin. These dif­ crystals are arranged in a rectangular shape and send out
ferent tissues have different acoustic impedances (the all the sound waves simultaneously. This allows imaging
speed that sound travels through the different tissues), of rapidly moving structures (like the heart or vessels
and the ultrasound image will reflect each tissue type as when using color flow Doppler), and it produces higher‐
a different shade of gray on the screen. contrast (black and white) images but with less resolu­
Over the past several decades, the transducer con­ tion compared with the linear and curvilinear transducers.
struction has evolved considerably in design, function, The flat surface of these probes makes it ideal for imag­
and capability from a single‐element resonance crystal to ing between the ribs.
a broadband transducer array of hundreds of individual Imaging software for available diagnostic ultrasound is
elements. Flat‐face linear (tendon) and convex (micro‐ constantly evolving, and recent introductions such as har­
and macro)variable focus probes are the most popular monic imaging, compound imaging, and extended field of
probes for musculoskeletal imaging. Transducers are view are now quite common. Harmonic imaging can be
typically described by the arrangement of the crystals in used to produce a clearer image by reducing artifacts. This
the probe. The specific arrangement creates the shape of requires a specialized probe, but it generates an image
the sound beam. A linear transducer has the piezoelectric from a fundamental frequency that is sent from the trans­
crystals organized in a linear fashion and emits sound in ducer. When this frequency interacts with an organ, it cre­
succession. Linear transducers are more durable, have ates a resonance frequency that is twice the fundamental
inherently better resolution, and have a flat, narrow face frequency. The benefit of this is that the resonance fre­
with approximately 4 cm of scanning surface. These quency that is used to generate the image is relatively high,
probes give superior images at tissue depths of 2 cm or giving better resolution, and only travels through the body
less due to less distortion and less artifact in the near once, so it is less prone to artifacts. Compound imaging is
field. Linear transducers can now be made to produce a the capability to use ultrasound waves sent from multiple
sound beam that is wedge shaped through a process angles to evaluate the same region. This is intended to
called virtual convexing. These transducers can provide a reduce artifacts and give a much more detailed view of a
high‐resolution image, but this technology compromises structure, although it is susceptible to blurring due to
the ability to image rapidly moving structures (like the motion. Extended field of view allows a complete tendon
heart). Standoff pads are available that improve contact to be imaged or provides the capability to image a struc­
with the skin increasing the footprint and moves the ture or lesion that is larger than the field of view obtained
superficial structures into the near‐field focal zone and by a given transducer. Three‐dimensional imaging and
away from the near‐field artifact. contrast medium ultrasound imaging are also available,
Early ultrasound systems utilized a sector transducer but at this time it has very limited use in the equine patient.
that transmitted a wedge‐shaped sound beam that was
created by mechanically rotating the crystals. These
probes were quite fragile, and the crystals could be easily ULTRASOUND TO EVALUATE TENDONS
dislodged by dropping the probe. They have been AND LIGAMENTS
replaced by curvilinear transducers that are similar in
design to the linear transducer except that the scan head Ultrasonography has significantly advanced the diag­
is slightly curved. These are called convex and come in nosis and management of a variety of musculoskeletal
microconvex and macroconvex based on the size of the injuries in performance horses. 21,24,34,41,72–74,80 Ultra­
transducer curved surface. Microconvex probes have sonographic imaging of the musculoskeletal system will
smaller field of view and higher frequencies (higher be discussed in this chapter, but the abdominal, tho­
resolution with lower depth of penetration), while mac­ racic, cardiac, and urogenital systems are also routinely