time-average intensity, the greater the likelihood of temperature increase and
cavitation. Pulse length, burst length, or pulse duration is the output pulse
duration in PW Doppler. Increasing the Doppler sample-volume size increases
the pulse length.
Transducer selection indirectly affects intensity. Tissue attenuation changes with
frequency. The higher the transducer operating frequency, the greater the
attenuation 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 output intensity, a lower transducer frequency is required. Using more
gain and output beyond a point, without corresponding increases in image quality,
can mean that a lower frequency transducer is needed.
Receiver Controls
Receiver controls are used by the operator to improve image quality. These
controls have no effect on output. Receiver controls only affect how the
ultrasound echo is received. These controls include gain, TGC, dynamic range,
and image processing. The important thing to remember, relative to output, is
that receiver controls should be optimized before output is increased. For
example, before increasing output, optimize gain to improve image quality.
An Example of Applying the ALARA Principle
An ultrasound scan of a patient’s liver begins with selecting the appropriate
transducer frequency. After selecting the transducer and the application, which
are based on patient anatomy, adjustments to output power should be made to
ensure that the lowest possible setting is used to acquire an image. After the
image is acquired, adjusting the focus of the transducer, and then increasing the
receiver gain to produce a uniform representation of the tissue follows. If an
adequate image can be obtained with the increase in gain, then a decrease in
output should be made. Only after making these adjustments should you increase
output to the next level.
Having acquired the 2D display of the liver, Color can be used to localize blood
flow. As with the 2D image display, gain and image processing controls must be
optimized before increasing output.
51
iE33 User Manual
4535 616 23901
2
Safety
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