Chapter 6: Safety 137
Safety
The 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 ALARA
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; DCPD 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 and direction 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.
a. Field strengths from fixed transmitters such as base stations for radio (cellular/cordless) telephones and land mobile
radios, amateur radio, AM and FM radio broadcast and TV broadcast cannot be predicted theoretically with accuracy.
To assess the electromagnetic environment due to fixed RF transmitters, an electromagnetic site survey should be
considered. If the measured field strength in the location in which the SonoSite TITAN ultrasound system is used
exceeds the applicable RF compliance level above, the SonoSite TITAN ultrasound system should be observed to
verify normal operation. If abnormal performance is observed, additional measures may be necessary, such as
re-orienting or relocating the SonoSite TITAN ultrasound system.
b. Over the frequency range 150 kHz to 80 MHz, field strengths should be less than 3 V/m.
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