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image quality and minimize output intensity. There are several variables which affect the
way in which the output display indices can be used to implement the ALARA principle.
These variables involve:
Index values
•
Body size
•
Location of the bone relative to the focal point
•
Attenuation in the body
•
Ultrasound exposure time. Exposure time is an especially useful variable, because
•
it is controlled by the users.
Acoustic Output Limits
Applying ALARA
The system imaging mode of the operator selected that is depends on the user
information needed. Understanding the nature of the imaging mode used, the
transducer frequency, system setup values, scanning techniques, and operator
experience allow the sonographer to meet the definition of the ALARA principle. The
amount of acoustic output is 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, the potential localized heating of the
patient due to transducer 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. A high index reading does
not necessarily indicate the occurrence of a bioeffect; however, it must be taken
seriously. It is the operator responsibility to make every effort to reduce the possible
effects of a high index reading by limiting exposure time.
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. These controls can be divided into three categories: direct,
indirect, and receiver controls.
Using System Controls to Implement ALARA
Direct Controls
The system LU700 has no direct control for output, therefore the sonographer
must control exposure time and scanning technique to implement the ALARA principle.
To ensure that acoustic and thermal limits are not exceeded for all imaging modes, the
system LU700 is designed to automatically adjust output. The system does not exceed
a spatial peak temporal average intensity (I SPTA) of 720 mW/cm
I
≤ 720 mW/cm
•
SPTA
MI ≤ 1.9
•
TI ≤ 6.0
•
2
spta.3
2
for all imaging modes.
P30
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