Philips HD11 XE - Page 31

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HD11 XE Getting Started
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31
Safety
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 ultra-
sound beam concentrates energy in a single location. A moving or scanned ultra-
sound 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 repeti-
tion frequency: focal depth, display depth, gate depth, 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 time-aver-
age intensity, the greater the likelihood of temperature increase and cavitation.
Pulse length or burst length or pulse duration is the output pulse duration in
pulsed Doppler. Increasing the Doppler gate length increases the pulse length.
Transducer selection indirectly affects intensity. Tissue attenuation changes with
frequency. The higher the transducer operating frequency, the greater the attenu-
ation of the ultrasonic energy. A higher transducer operating frequency requires
more output intensity to scan at a deeper depth. To scan deeper at the same out-
put intensity, a lower transducer frequency is required. Using more gain and out-
put beyond a point, without corresponding increases in image quality, can mean
that a lower frequency transducer is needed.