P29
the examination. Using too high a level may not necessarily increase the quality of the
information, but it will expose the patient to unneeded ultrasound energy. The use of
ALARA is a way of implementing safety assurance. The threshold for diagnostic
ultrasound bioeffects is undetermined. Ultimately, the exposure time depends on the
person conducting the examination. Primarily, it’s our training, education, and
experience that determine how quickly we can obtain a useful image and thus the
length of the examination and the amount of exposure. So, the question is, “How much
time do we need to obtain the desired diagnostic information?” But there are also
some other factors that might affect the length of time that any particular tissue is
exposed. One is the mode, whether it’s a moving or a stationary beam; and another is
the choice of transducer. Other factors include the patient’s body characteristics, the
operator’s understanding of the controls on the system and how they affect output
levels, and, particularly, whether continuous wave or pulsed Doppler or color flow
Doppler is used. To achieve ALARA, we need thorough knowledge of the imaging mode,
transducer capabilities, system setup, and operator scanning techniques.
System capabilities include the following: mode, transducer capabilities, system setup,
and scanning techniques. Let’s talk about each.
First, the mode we select, such as M mode, B-mode, or Doppler, depends on what we’re
looking for. B-mode imaging gives anatomic information, while Doppler and color flow
Doppler modes give information about blood flow through vessels. M-mode gives
information about how anatomic structures move in time. If one wishes to use 3D/4D
ultrasound, one needs to remember that the 3D/4D image sets consist of series of B-
mode 2-dimensional (2D) acquisitions, which are then constructed by the computer into
3D/4D representations. Hence, whatever the settings are for B-mode 2D imaging will be
what determines the output. Time will be the most important variable because, on the
one hand, a 2D sweep will be fast and time limited, but prolonged exposure may result
from attempting to obtain the “best” set of images. Second, transducer capabilities
relate to the penetration depth of ultrasound in tissue at the frequency chosen,
resolution, and field of view that we can obtain with the selected transducer. Third,
system setup and control settings depend on where we start on the output scale and on
our knowledge of which combination of controls gets the best results. Fourth, the
scanning technique we use is based on our knowledge of anatomy and pathology, of
ultrasound physics, and of the equipment’s signal-processing features plus our
experience with a given scanning modality, such as sector, linear, and so forth. A
system’s recording and playback features let us reduce the exposure time to just the
time necessary to obtain a useful image. Analysis and diagnosis can be performed with
recorded images rather than lengthy live imaging sessions. The same can be said about
3D volumes, obtained by an examiner and analyzed by this examiner or someone else,
with no exposure to the patient, at the bedside, the reading room, the other side of town,
or another country. Without an output display standard, we must rely on that knowledge
to estimate a patient’s ultrasound exposure. With an output display standard, we have
a real-time indication of the exposure in terms of the potential for bioeffects. Either way,
we implement ALARA by minimizing the exposure level and duration while being sure to
obtain the necessary diagnostic information."
No set of rules can be formulated that would be sufficiently complete to dictate the
correct response to every circumstance. The qualified personnel can adjust to improve
