is one of the first things that occurs in any exam. For example, peripheral vascular
intensity levels are not recommended for fetal exams. Some systems automatically
select the proper range for a particular application, while others require manual
selection. Ultimately, the user has the responsibility for proper clinical use. The
ultrasound system provides both automatic (default) settings and manual
(user-selectable) settings.
Output power has direct impact on acoustic intensity. Once the application has
been established, the power control can be used to increase or decrease the
intensity output. The power control allows you to select intensity levels less
than the established maximum. Prudent use dictates that you select the lowest
output intensity that is consistent with good image quality.
Indirect Controls
The indirect controls are those that have an indirect effect on acoustic intensity.
These controls affect imaging mode, pulse repetition frequency, focus depth,
pulse length, and transducer selection.
The choice of imaging mode determines the nature of the ultrasound beam. 2D
is a scanning mode; Doppler is a stationary or unscanned mode. A stationary
ultrasound beam concentrates energy in a single location. A moving or scanned
ultrasound beam disperses the energy over an area and the beam is concentrated
on the same area for a fraction of the time as that of an unscanned mode.
Pulse repetition frequency or rate refers to the number of ultrasound bursts of
energy over a specific period of time. The higher the pulse repetition frequency,
the more pulses of energy in a period of time. Several controls affect pulse
repetition frequency: focal depth, display depth, sample volume depth, flow
optimization, scale, number of focal zones, and sector-width controls.
Focus of the ultrasound beam affects the image resolution. To maintain or
increase resolution at a different focus requires a variation in output over the
focal zone. This variation of output is a function of system optimization. Different
exams require different focal depths. Setting the focus at the proper depth
improves the resolution of the structure of interest.
Pulse length is the time during which the ultrasonic burst is turned on. The longer
the pulse, the greater the time-average intensity value. The greater the
iE33 User Manual
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