Chapter 2. Safety 2 – 23
sonographer in meeting the ALARA principle. The decision as to the amount of acoustic output
is, in the final analysis, up to the system operator. This decision must be based on the following
factors: type of patient, type of exam, patient history, ease or difficulty of obtaining diagnostically
useful information, and the potential localized heating of the patient due to probe surface
temperatures. Prudent use of the system occurs when patient exposure is limited to the lowest
index reading for the shortest amount of time necessary to achieve acceptable diagnostic
results.
Although a high index reading does not mean that a biological effect is actually occurring, it
should be taken seriously. Every effort should be made to reduce the possible effects of a high
index reading. Limiting exposure time is an effective way to accomplish this goal.
There are several system controls that the operator can use to adjust the image quality and limit
the acoustic intensity. These controls are related to the techniques that an operator might use to
implement ALARA and can be divided into three categories: direct, indirect, and receiver control.
2.5.1.2. Direct Controls
Application selection and the output intensity control directly affect acoustic intensity. There are
different ranges of allowable intensity or output depending on your selection. Selecting the
correct range of acoustic intensity for the application is one of the first things required during any
exam. For example, peripheral vascular intensity levels are not recommended for fetal exams.
Some systems automatically select the proper range for a particular procedure, while others
require manual selection. Ultimately, the user bears the responsibility for proper clinical use.
Samsung Medison systems provide both automatic and user-definable settings.
Output has direct impact on acoustic intensity. Once the application has been established, the
output control can be used to increase or decrease the output intensity. The output control
allows you to select intensity levels less than the defined maximum. Prudent use dictates that
you select the lowest output intensity consistent with good image quality.
2.5.1.3. Indirect Controls
The indirect controls are those that have an indirect effect on the acoustic intensity. These
controls affect the imaging mode, pulse repetition frequency, focus depth, pulse length, and
probe selection.
The choice of imaging mode determines the nature of the ultrasound beam. 2D mode is a
scanning mode, Doppler is a stationary or unscanned mode. A stationary ultrasound beam
concentrates energy on a single location. A moving or scanned ultrasound beam disperses the
energy over a wide area and the beam is only concentrated on a given area for a fraction of the
time necessary in unscanned mode.
The 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
given period of time. Several controls affect pulse repetition frequency: focal depth, display
depth, sample volume depth, color sensitivity, number of focal zones, and sector width controls.
The focus of the ultrasound beam affects the image resolution. To maintain or increase
resolution at a different focus requires a variation of output over the focal zone. This variation of
output is a function of system optimization. Different exams require different focal depths.
Setting the focus to 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-average intensity, the greater the
likelihood of temperature increase and cavitations. Pulse length, burst length or pulse duration is
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