Pediatric emergency medicine trisk 1120
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FIGURE 131.1 On the left is a low-frequency, phased array probe commonly used in cardiac or
abdominal applications. On the right is a high-frequency, linear probe used for procedures and
identifying superficial structures.
ULTRASOUND BASICS
Ultrasound Physics
A complete review of ultrasound physics is beyond the scope of this chapter.
Knowledge of the basic principles, however, will lead not only to a higher
comfort level with machine operation, but also to a greater facility with image
acquisition and interpretation.
Ultrasound refers to sound waves with frequency greater than 20,000 Hz (0.02
MHz, the upper range of audible sound). Frequencies used in diagnostic and
procedural ultrasound generally range from 2 to 15 MHz. The general principle of
diagnostic ultrasound is the pulse-echo effect. Sound waves are generated from
the transducer and sent into a medium (the body). The transducer then “listens”
for the return, or echo, of that sound. The emitted sound wave encounters
different tissues, with different densities and different distances from the surface
of the skin. Some of the sound is reflected back to the transducer footprint. Once
the transducer “hears” a returning echo, the ultrasound system calculates the
distance of an object from the transducer based on the amount of time it took the
echo to return. The intensity of the returning sound wave determines the
grayscale assignment on the image. For example, fluid does not reflect sound
waves at all, thus a fluid-filled bladder will appear black or anechoic (i.e., the
transducer did not “hear” any sound waves reflected back). The diaphragm,
however, is highly reflective and appears bright on the screen (Fig. 131.2 ). Each
pixel on the screen is generated in this fashion, thus creating an overall image.
FIGURE 131.2 A: A full bladder. Note that fluid is anechoic (black). B: Morison pouch. Note
the bright reflection of the diaphragm adjacent to the liver (arrow ).
Basic Controls
Ultrasound machines, upon first glance, can appear daunting. There are many
buttons and knobs, and the inexperienced user can often be disheartened. Keeping
in mind the basic sonography concepts, however, and understanding that these
controls simply allow one to adjust and optimize the image, should prevent even
the most novice sonographer from feeling intimidated.
Gain refers to the intensity of the returning echoes on the display screen.
Adjusting the gain essentially changes the brightness without improving the
quality of the image. Depth of the image can be adjusted as well. For superficial
structures, decreasing the depth allows for greater image quality, larger images of
the structure under scrutiny, and less wasted space on the screen. Increasing the
depth allows for visualization of deeper structures (i.e., farther away from the
skin and transducer). The freeze button allows the sonographer to hold a still
image, usually for purposes of studying an image, performing measurements, or
saving. On most machines, several seconds of memory are saved and can be
toggled forward and backward to find a desired image seen moments before.
Other control panel functions on ultrasound machines vary widely, but often
include color, Doppler, motion-mode (M-mode), focus and tissue harmonics. As
the practitioner gains more experience, these machine capabilities will become
more familiar and allow for more advanced applications.
General Scanning Techniques
One of the greatest advantages to ultrasound is that it is dynamic, with the ability
to capture images in multiple imaging planes from multiple orientations. This also
can lead to confusion when reviewing and interpreting images. For that reason,
standard and consistent orientation should be used when performing scans.
Generally speaking, the standard ultrasound examination is performed with the
sonographer on the right side of the patient’s bed. The probe should be held in the
right hand and ultrasound system adjustments made with the left hand during
scanning.
All transducers have some marking that correlates with a dot (or some other
identifier) on the monitor screen (Fig. 131.3 ). By convention, the transducer
marker is always kept to the patient’s right in transverse views or head in sagittal
and coronal views and the dot on the screen is located at the top, left side of the
monitor (with the exception of cardiac scanning). Adhering to this principle
allows for uniformity of images and makes interpretation and review more
seamless. Objects that are closer to the probe marker appear closer to the dot on
the monitor, and vice versa.
