P31
The equipment's mechanical index (MI) does not exceed values greater than 1.9 and
thermal index (TI) does not exceed values greater than 6.0.
Indirect Controls
The indirect controls are those that have an indirect effect on acoustic intensity.
These controls affect imaging mode, pulse repetition frequency (PRF), pulse length.
The choice of imaging mode determines the nature of the ultrasound beam. 2D is a
scanning mode; Doppler is a stationary or un-scanned 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 un-scanned mode.
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, time gain compensation (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. 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. In summary: Select the correct transducer frequency and application
for the job; start with a low output level; and optimize the image by receiver gain, and
other imaging controls.
Additional Considerations
Ensure that scanning time is kept to a minimum and that only medically required
scanning is performed. Never compromise quality by rushing through an exam. A poor
exam may require a follow
‑
up, which ultimately increases exposure time. Diagnostic
ultrasound is an important tool in medicine, and like any tool, it should be used
efficiently and effectively.
Output Display
There are two types of indices might be displayed: one is mechanical index (MI)
and the other is thermal index (TI). The mechanical index (MI) provides an
indication of the risk due to mechanical or nonthermal mechanisms. The thermal
index (TI) provides an indication of the risk of harm due to thermal mechanisms.
The mechanical index (MI) is continuously displayed over the range of 0.0 to 1.9,
in increments of 0.1.
