FujiFilm VisualSonics Vevo MD, User Manual

Page: 350 / 463

Direct, indirect, and receive 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.
Ultrasound users can monitor the MI and TI values on the left side of the clinical
monitor under
Transmit and implement the ALARA principle accordingly. For more
information on MI and TI, see "Related guidance documents" on page 418.
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 379.
Receive Controls
The receive controls are the gain controls. Receive controls do not affect output. They
should be used, if possible, to improve image quality before using controls that
indirectly affect output.
350 Appendices