FujiFilm SONOSITE X-PORTE, User Manual

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10-2 Acoustic output
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 means limiting ultrasound to situations in which it is medically useful and limiting
patient exposure to the lowest ultrasound output for the shortest time necessary to achieve acceptable
diagnostic results. Although there are no direct user controls for acoustic output, users can indirectly control
output by varying depth. 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. See
“Transducer surface temperature rise” on
page 10-10. In the event of a device malfunction, there are redundant controls that limit transducer power.
This is accomplished by an electrical design that limits both power supply current and voltage to the
transducer.
The sonographer uses the system controls to adjust image quality and limit ultrasound output. The system
controls are divided into three categories relative to output: controls that directly affect output, controls that
indirectly affect output, and receiver controls.
Direct, indirect, and receiver controls
Direct controls The system does not have a direct user control for output. Rather, the system has been
designed to automatically adjust output to ensure that acoustic and thermal limits are not exceeded for all
imaging modes. Since there is no direct user control for output, the sonographer should rely on controlling
exposure time and scanning technique to implement the ALARA principle.
The system does not exceed a spatial peak temporal average intensity (ISPTA) of 720 mW/cm2 for all
imaging modes. The mechanical index (MI) and thermal index (TI) may exceed values greater than 1.0 on
some transducers in some imaging modes. For either the Ophthalmic or Orbital exam, the acoustic output
is limited to the following values: ISPTA does not exceed 50 mW/cm2; TI does not exceed 1.0, and MI does
not exceed 0.23. Ultrasound users can monitor the MI and TI values on the right side of the clinical monitor
and implement the ALARA principle accordingly. For more information on MI and TI, see BS EN
60601-2-37:2001: Annex HH.
Indirect controls The controls that indirectly affect output are controls affecting imaging mode, freeze, and
depth. The imaging mode determines the nature of the ultrasound beam. Freeze stops all ultrasound output
but keeps the last image displayed on screen. Freeze can be used by the ultrasound user to limit exposure
time while studying an image and maintaining probe position during a scan. Some controls, such as depth,
show a rough correspondence with output, and may be used as a general means for indirectly reducing MI
or TI. See
“Guidelines for reducing MI and TI” on page 10-3.
Receiver controls The receiver controls are the gain controls. Receiver controls do not affect output. They
should be used, if possible, to improve image quality before using controls that directly or indirectly affect
output.