Direct Controls; Indirect Controls - FujiFilm SonoSite Edge II User Manual

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.
The system has been designed to ensure that temperature at the face of the transducer will not exceed the
limits established in Section 42 of EN 60601-2-37: Particular requirement for the safety of ultrasound
medical diagnostic and monitoring equipment. 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 controls

2
The system does not exceed a spatial peak temporal average intensity (ISPTA) of 720 mW/cm for all
imaging modes. (For either the Ophthalmic or Orbital exam, the acoustic output is limited to the following
2
values: ISPTA does not exceed 50 mW/cm ; TI does not exceed 1.0, and MI does not exceed 0.23.) The
mechanical index (MI) and thermal index (TI) may exceed values greater than 1.0 on some transducers in
some imaging modes. One may monitor the MI and TI values and adjust the controls to reduce these values.
See "Guidelines for reducing MI and TI" on page 10-3. Additionally, one means for meeting the ALARA
principle is to set the MI or TI values to a low index value and then modifying this level until a satisfactory
image or Doppler mode is obtained. For more information on MI and TI, refer to 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. Tissue attenuation is directly related to transducer
frequency. The higher the PRF (pulse repetition frequency), the more output pulses occur over a period of
time.
10-2 Acoustic Output