User's Manual

Clarius Ultrasound Scanner - HD3 Scanners Safety Topics
revision 1 49
Controls indirectly affecting intensity:
• Pulse repetition frequency: The higher the PRF, the more output pulses per second,
increasing the temporal-average intensity.
• Focusing depth: Setting the scanner focus at the proper depth improves the resolution of
that structure, without the need to increase intensity to see it better.
• Pulse length: Generally, the longer the pulse, the greater the temporal-average intensity
value, which both raises the temperature in the tissue and slightly increases the likelihood
for cavitation.
• Dwell time: Scanned modes, such as B-Mode imaging, distribute the energy over a large
volume. In scanned modes (equipment keeps the beam stationary), the highest
temperature is frequently at the surface where the ultrasound enters the body.
Receiver Controls
The receiver controls have no output effect. The following receiver controls affect images only:
• Gain or time-gain control (TGC)
• Dynamic range
• Post-processing
User Responsibility
The various operating modes and output levels mean that more responsibility must be
assumed by the users. This is a point that is very often neglected: many assume that if an
instrument is “FDA cleared,” then there is no risk of bioeffects. This notion is inaccurate
because changing the mode of operation or manipulating controls has the potential to cause
major changes in output and hence in exposure. In other words, there is a shift in
responsibility for patient safety from the manufacturer to the user.
To obtain good diagnostic information, a high return signal amplitude is needed. This can be
attained either by higher output, similar to talking louder, or by higher receiver gain, similar to
a hearing aid with a volume control. You must attain the best diagnostic information with
minimal exposure to the patient. The threshold at which ultrasound energy causes bioeffects
for each individual patient is unknown, therefore, you must get the most information at the
lowest possible output level by adjusting the output intensity of the equipment.
As a general guideline:
1. Select the correct scanner frequency and application.
2. Start with a low output level.
3. Optimize the image by using focus, receiver gain, and other imaging controls.
4. If the image is still not diagnostically useful, increase output.