Since the threshold for diagnostic ultrasound bioeffects is undetermined, it is
the sonographer’s responsibility to control total energy transmitted into the
patient. The sonographer must reconcile exposure time with diagnostic image
quality. To ensure diagnostic image quality and limit exposure time, an ultrasound
system provides controls that can be manipulated during the exam to optimize
the results of the exam.
The ability of the user to abide by the ALARA principle is important. Advances
in diagnostic ultrasound, not only in the technology but in the applications of
that technology, have resulted in the need for more and better information to
guide the user. The output display indices are designed to provide that important
information.
There are a number of variables which affect the way in which the output display
indices can be used to implement the ALARA principle. These variables include
index values, body size, location of the bone relative to the focal point, attenuation
in the body, and ultrasound exposure time. Exposure time is an especially useful
variable, because it is controlled by the user. The ability to limit the index values
over time supports the ALARA principle.
Applying ALARA
The system imaging mode used depends upon the information needed. 2D and
M-mode imaging provide anatomical information, while Doppler, Color Power
Angio (CPA), and Color imaging provide information about blood flow. A scanned
mode, like 2D or Color, disperses or scatters the ultrasonic energy over an area,
while an unscanned mode, like M-mode or Doppler, concentrates ultrasonic
energy. Understanding the nature of the imaging mode being used allows the
sonographer to apply the ALARA principle with informed judgment. Additionally,
the transducer frequency, system setup values, scanning techniques, and operator
experience allow the sonographer to meet the definition of 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 transducer surface temperatures. Prudent use of the system occurs when
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Safety
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