Diagnostic ultrasound ( PDFDrive )

50 PART I Physics
controls whether the beam is scanned. Unscanned modes deposit
energy along a single path and increase the potential for heating.
he pulse repetition frequency (PRF) indicates how oten the
transducer is excited. Increasing the number of ultrasound bursts
per second will increase the temporal average intensity. PRF is
usually controlled by changing the maximum image depth in
B-mode or the velocity range in Doppler modes. Burst length
(also called “pulse length” or “pulse duration”) controls the
duration of on-time for each ultrasonic burst transmitted.
Increasing the burst length while maintaining the same PRF will
increase the temporal average intensity. he control of burst
length may not be obvious. For example, in spectral Doppler
ultrasound, increasing the Doppler sample volume length will
increase the burst length.
he selection of the appropriate transducer will also limit
the need for high acoustic power. Even though higher frequencies
provide better spatial resolution, the attenuation of tissue increases
with increasing ultrasound frequency, so penetration may be
lost. Perhaps most important are the receiver gain controls. he
receiver gain control has no efect on the amplitude of the acoustic
output. herefore before turning up the acoustic output intensity,
try increasing receiver gain irst. It should be noted that some
system controls actually interact with the acoustic output intensity
without direct control. Check to see whether the manufacturer
provides separate controls for receiver gain, time gain compensation
(TGC), and acoustic output intensity. he TGC can improve
image quality without increasing the output.
here is really no substitute for a well-instructed operator.
he indices and requirements of output display standards will
help only those willing to use and understand them. Real-time
display of the mechanical and thermal indices on diagnostic
scanners will help clinicians evaluate and minimize potential
risks in the use of such instrumentation. Physicians and sonographers
are encouraged to learn more about how to minimize
potential bioefects.
ULTRASOUND ENTERTAINMENT
VIDEOS
Of concern is the growing use of diagnostic ultrasound for the
nonmedical scanning of pregnant women to provide a fetal
“keepsake” video. Unfortunately, entertainment ultrasound is
promoted most vigorously in the second and third trimesters,
when bone calciication can increase thermal efects. Also, women
with the economic means to schedule multiple ultrasound imaging
sessions may be exposing both themselves and their fetuses to
even greater risk if ultrasound bioefects are shown to be additive,
or just by increasing the chances for a bioefect. If there is no
clinical beneit in such entertainment ultrasound, the beneitto-risk
ratio is clearly zero. In addition, because oten the
ultrasound equipment used is identical to diagnostic equipment
used by clinicians, the consumer may be unaware that no medical
information is being generated, interpreted, or referred to her
obstetrician. (It is recognized that release forms are signed to
the contrary.) he FDA 106 views this as an unapproved use of a
medical device and refers users to the AIUM statement regarding
keepsake videos.
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