Chapter 10: Acoustic Output; Alara Principle; Applying The Alara Principle - FujiFilm SonoSite Edge II User Manual

Chapter 10: Acoustic Output

This chapter contains safety information required by regulatory agencies pertaining to acoustic output. The
information applies to the ultrasound system, transducer, accessories, and peripherals.

ALARA principle

ALARA is the guiding principle for the use of diagnostic ultrasound. Sonographers and other qualified
ultrasound users, using good judgment and insight, determine the exposure that is "as low as reasonably
achievable. " There are no set rules to determine the correct exposure for every situation. The qualified
ultrasound user determines the most appropriate way to keep exposure low and bioeffects to a minimum,
while obtaining a diagnostic examination.
A thorough knowledge of the imaging modes, transducer capability, system setup and scanning technique is
necessary. The imaging mode determines the nature of the ultrasound beam. A stationary beam results in a
more concentrated exposure than a scanned beam, which spreads that exposure over that area. The
transducer capability depends upon the frequency, penetration, resolution, and field of view. The default
system presets are reset at the start of each new patient. It is the scanning technique of the qualified
ultrasound user along with patient variability that determines the system settings throughout the exam.
The variables that affect the way the qualified ultrasound user implements the ALARA principle include
patient body size, location of the bone relative to the focal point, attenuation in the body, and ultrasound
exposure time. Exposure time is an especially useful variable, because the qualified ultrasound user can
control it. The ability to limit the exposure over time supports the ALARA principle.

Applying the ALARA principle

The system imaging mode selected by the qualified ultrasound user is determined by the diagnostic
information required. 2D imaging provides anatomical information; CPD imaging provides information about
the energy or amplitude strength of the Doppler signal over time at a given anatomical location and is used
for detecting the presence of blood flow; Color imaging provides information about the energy or amplitude
strength of the Doppler signal over time at a given anatomical location and is used for detecting the presence,
velocity, and direction of blood flow; Tissue Harmonic Imaging (THI) uses higher received frequencies to
reduce clutter, artifact, and improve resolution on the 2D image. Understanding the nature of the imaging
mode used allows the qualified ultrasound user to apply the ALARA principle.
Prudent use of ultrasound requires that patient exposure to ultrasound be limited to the lowest ultrasound
output for the shortest time necessary to achieve acceptable diagnostic results. Decisions that support
prudent use are based on the type of patient, exam type, patient history, ease or difficulty of obtaining
diagnostically useful information, and potential localized heating of the patient due to transducer surface
temperature.
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