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Acoustic output 10-1
CHAPTER 10
Acoustic output
This section contains information about the ALARA (as low as reasonably achievable)
principle, the output display standard, and acoustic power and intensity tables. 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 which 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;