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Chapter 8: Acoustic Output 119

Acoustic Output

Chapter 8: 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 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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