Page 138 - Basic Monitoring in Canine and Feline Emergency Patients

turn, generates the US images projected onto the body are weakened and subsequently are darker/
monitor through the US machine’s software. have less resolution due to attenuation. Echoes
VetBooks.ir Acoustic characteristics returning from superficial structures are higher in
amplitude (better resolution) due to less attenua-
tion than that of deeper structures. To try to main-
tain a uniform echogenicity of the entire organ,
A tissue’s acoustic appearance is dependent upon
several factors; the most important factor is the tis- time-gain compensation, depth and frequency set-
sue’s acoustic impedance. Acoustic impedance deter- tings can be utilized to optimize an image.
mines the level of sound wave reflection and thus the The goal with US is to focus on one area of the
echogenicity of the tissues on the monitor. Ultrasound abdomen or one organ at a time. Thus, it is impor-
wave reflections are stronger at interfaces of tissues tant to adjust the depth of the image to focus on the
with variable acoustic impedances and weaker organ of interest. Ideally the depth of the US beam
between tissues with similar acoustic impedances. As should be adjusted so that the organ of interest is
density of a tissue increases, the level of acoustic captured within the top three-quarters of the US
impedance increases. Imaging tissues with mild vari- viewing window.
ation in acoustic impedance allows for examination The frequency should be at the highest frequency
of changes in echogenicity between tissues as well as allowable to still see the deepest part of the organ
intra-organ architecture comparison. If the differ- of interest. Remember that the higher the fre-
ence between acoustic impedance is large (such as quency, the lower the penetration of the US waves.
imaging from muscle to bone) then the US waves are Thus, a higher frequency will have higher imaging
reflected away and less detail is appreciated. quality for superficial structures.
The ultrasound terms for describing white, gray, The gain control electronically amplifies return-
and black areas are hyperechoic, hypoechoic, ane- ing echoes (i.e. controls how bright the images are)
choic, and isoechoic in comparison to their sur- and can be adjusted to create stronger echoes at all
rounding tissues. Hyperechoic imaging occurs when levels. Decreasing the gain results in an overall
nearly all the US waves are returned to the US darker image. This control should be set carefully. If
receiver, thus creating a bright white projection. This the gain is too high, the image is too bright and
is often seen with bone, stone, and air. Hypoechoic contrast resolution is decreased. If the gain is set too
imaging occurs when variable degrees of US waves low, the image will be dark and information may be
are reflected to the receiver; this creates an image lost. It is tempting to set the gain too high if per-
with varying shades of gray. Anechoic images occur forming US examination in a room that is not dark
when no US waves return to the receiver; this creates enough, so always take ambient light and its effect
a pure lack of projection, shown as black. Examples on the screen into account when adjusting the gain.
of anechoic areas in the body include fluids such as Many newer machines (especially those utilized
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urine, bile, and blood. Isoechoic images occur when for bedside AFAST /TFAST /VetBLUE/COAST )
tissues are the same shades of gray. Tissue echogenic- now have an image control function that will set
ity varies among organs and tissues. correct gain and time-gain compensation control
Normal and abnormal tissues are described as parameters automatically. Therefore, when using
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hypoechoic and hyperechoic to their normal state these machines for AFAST , TFAST , and VetBLUE,
or to other structures. Figure 7.1 shows the relative using standard abdominal settings and simply
echogenicity of tissues and other materials. A mne- adjusting the depth of the image will be sufficient
monic that the author uses to remember the order to obtain adequate images.
from hypoechoic to hyperechoic for core abdom-
inal organs is ‘My cat likes sunny places’: My, renal Artifacts
medulla; cat, renal cortex; likes, liver; sunny, spleen;
places, renal pelvis/prostate. Ultrasound artifacts create echo signals that do not
represent true tissue. Artifacts may be produced by
improper equipment operation but are also inher-
Setting the machine to obtain ent in ultrasound imaging techniques. Successful
a diagnostic image
ultrasound interpretation depends on the ability to
Control settings are utilized to regulate the inten- recognize the difference between real tissue signals
sity of echoes returning from varying depths. and imaging artifact. Please see Table 7.1 for a
Echoes coming from tissue interfaces deep in the description of different types of artifacts.

130 D.M. Hundley