Diagnostic ultrasound ( PDFDrive )

CHAPTER 29 Bioeffects and Safety of Ultrasound in Obstetrics 1035
• An increasing number of fetuses in the irst trimester, a
time of maximal susceptibility to external insults, are
exposed to ultrasound, particularly spectral Doppler. 26
• “Entertainment” ultrasound, scanning to obtain pictures
or videos of the fetus (fetal “keepsake” videos) without a
medical indication has burgeoned 27 despite calls for avoidance
of unnecessary exposure. 10,28,29
• Clinical users of obstetric ultrasound appear to have limited
knowledge and awareness of bioefects and safety. 30
hus the main goals of this chapter are as follows:
1. Summarize the literature on bioefects in experimental settings
as well as the available knowledge on bioefects in the
human fetus.
2. Analyze changes that occurred over time in energy levels of
ultrasound machines and the regulations involved.
3. Describe how manipulation of many instrument controls alters
acoustic energy and thus exposure.
4. Educate sonographers and physicians on how best to minimize
fetal exposure without sacriicing diagnostic quality.
INSTRUMENT OUTPUTS
Over the years, output of ultrasound instruments has increased. 31,32
Furthermore, many machine controls can alter the output, and
various machines can behave diferently when manipulating
similar controls. For example, keeping in mind that the degree
of temperature elevation is proportional to the product of the
amplitude of the sound wave times the pulse length and the
pulse repetition frequency, it becomes immediately evident why
any change (augmentation) in these characteristics can add to
the risk of elevating the temperature, a potential mechanism for
bioefects. hree important parameters under end-user control
are the (1) scanning/operating mode (including transducer
choice), (2) system setup and output control, and (3) dwell time.
Scanning Mode
When comparing modes, the spatial peak temporal average
intensity (ISPTA) increases from B-mode (average, 34 mW/cm 2 )
to M-mode to color Doppler to spectral Doppler (average,
1180 mW/cm 2 ). 33 Average ISPTA values are 1 W/cm 2 in Doppler
mode but can reach 10 W/cm 2 . Caution is therefore recommended
when applying this mode. Color Doppler has higher intensities
than B-mode but is still much lower than spectral Doppler, mainly
because of the mode of operation: sequences of pulses, scanned
through the area of interest (“box”). High pulse repetition frequencies
are used in pulsed Doppler techniques, generating greater
temporal average intensities and power than B-mode or M-mode
and thus greater heating potential. Also, because the beam needs
to be held in relatively constant position over the vessel of interest
in spectral Doppler ultrasound, temporal average intensity may
further increase. his is particularly concerning in irst-trimester
applications. In addition, transducer choice is important because
it will determine frequency, penetration, resolution, and ield of
view.
System Setup
Starting or default output power is another important ultrasound
parameter. Some manufacturers “boot” their machines
with high power, which supposedly produces a better image,
and the sonographer must act to decrease that power. Other
systems boot up with low power and, only if judged necessary,
the sonographer will increase that power. In, For example, the
Doppler signal in Fig. 29.1A was obtained with a high power,
as relected by an elevated TI of 2.9, whereas in Fig. 29.1B the
power was greatly reduced (TI = 0.6), and the image is still
diagnostic. Also, the examiner ine-tunes to optimize the image,
inluencing output but with no visible efect, except to change
thermal index (TI) and mechanical index (MI), as discussed in
Chapter 2.
A
B
FIG. 29.1 Effect of Changing Power Setting on Thermal Index of Bone (TIB) During Spectral Doppler Velocity Measurements of Umbilical
Artery. (A) The output power is high, and the thermal index (TI) is 2.9 (yellow box). (B) The power has been lowered; the TI is now 0.6, and the
tracing is still diagnostic.