Diagnostic ultrasound ( PDFDrive )

1042 PART IV Obstetric and Fetal Sonography
credentialing in NT do not demonstrate compliance with the
recommended use of the TI in monitoring acoustic output. 177
An easy way to reduce exposure is to reduce the TI and MI,
using the appropriate controls, and/or reduce the dwell time.
he 1999 statement of the British Medical Ultrasound Society,
reconirmed in 2009, declares 178 :
For equipment for which the safety indices are displayed
over their full range of values, the TI should always be less
than 0.5 and the MI should always be less than 0.3. When
the safety indices are not displayed, T max should be less than
1°C and MI max should be less than 0.3. Frequent exposure
of the same subject is to be avoided.
he British Medical Ultrasound Society has strict recommendations
for maximum allowed exposure time (T max ), depending
on the TI (Table 29.1).
Miller and Ziskin 57 demonstrated a logarithmic relationship
between exposure duration and temperature elevation in producing
harmful bioefects in animal fetuses. For temperatures below
43°C, the exposure time necessary for every 1°C increase in
temperature was decreased by a factor of 4. Using a maximum
“safe” exposure time of 4 minutes for a temperature elevation
of 4°C, based on these calculations, the following maximal
exposure times are allowable with no apparent obvious risks:
128 minutes at 1°C, 64 at 2°C, 16 at 3°C, and only 4 minutes at
4°C. Precautions are, naturally, of particular importance in early
gestation 179,180 and for Doppler exposure. 181
General recommendations from major professional organizations
follow. It is strongly recommended to consult various safety
statements published on these organizations’ websites. 182-192
1. A diagnostic ultrasound exposure that produces a maximum
in situ temperature rise of no more than 1.5°C above
normal physiologic levels (37°C) may be used clinically
without reservation on thermal grounds. 193
2. A diagnostic ultrasound exposure that elevates embryonic
and fetal in situ temperature above 41°C (4°C above normal
temperature) for 5 minutes or more should be considered
potentially hazardous. 193,194 In this regard, maternal temperature
elevation (e.g., from viral disease) should be considered
because body temperature of the fetus will also be
increased above normal. 48
TABLE 29.1 Duration of Obstetric
Ultrasound as a Function of
Thermal Index
Thermal Index (TI)
Recommended Upper Limit
0.7 60 min
1 30 min
1.5 15 min
2 4 min
2.5 1 min
Modiied from British Medical Ultrasound Society (BMUS) Safety
Group. Guidelines for the safe use of diagnostic ultrasound
equipment. Ultrasound. 2010;18:52-59. 178
3. he risk of adverse efects is increased with the duration of
exposure (dwell time). 195
4. Based on available information, there is no reason to withhold
scanning in B-mode for medical indications. he risk
of thermal damage secondary to heating appears to be negligible.
193
5. M-mode ultrasound appears to be safe and not to cause
thermal damage (Fig. 29.5). 48
6. Spectral Doppler ultrasound may produce high intensities,
and routine Doppler examination during the embryonic
period is rarely indicated. 196
7. hree-dimensional (3D) and four-dimensional (4D) ultrasound
are based on two-dimensional (2D) B-mode imaging
with multiple 2D planes obtained and assembled (reconstructed)
into a volume. Hence the exposure is really to
B-mode and is, most likely, safe. Time of exposure has to be
watched to avoid long episodes of scanning to obtain the
“ideal” 3D volume (Fig. 29.6).
8. Education of ultrasound operators is crucial; the responsibility
for the safe use of ultrasound devices is shared between
the users and the manufacturers, who should ensure
the accuracy of the output display. 196
9. he AIUM advocates the responsible use of diagnostic
ultrasound and strongly discourages the nonmedical
use of ultrasound for “entertainment” purposes. he use
of ultrasound without a medical indication to view the
fetus, obtain a picture of the fetus, or determine the fetal
gender is inappropriate and contrary to responsible
medical practice. Ultrasound should be used by qualiied
health professionals to provide medical beneit to the patient.
28
10. Examinations should be kept as short as possible and with
as low MI and TI outputs as possible, but without sacriicing
diagnostic accuracy. Follow the as low as reasonably achievable
(ALARA) principle. 197
CONCLUSION
Diagnostic ultrasound has been used in medicine in general and
obstetrics and gynecology in particular for more than half a
century. No conirmed biologic efects have been described in
patients as a result of exposure to diagnostic ultrasound. Such
efects, however, have been described in animals, oten at exposure
levels higher than, but also occasionally equivalent to, those used
in clinical practice. Epidemiologic information available is from
studies performed on instruments with acoustic output much
lower than current machines. Oten, exposure data are insuicient
and number of subjects too small. Furthermore, “no reported
efects” does not mean “no efects,” and such biologic efects may
be identiied in the future. Prudent use of ultrasound in fetal
scanning, following the ALARA principle, is therefore recommended.
Based on known mechanisms, there is no contraindication
to the use of B-mode, M-mode, 3D/4D, and color Doppler
ultrasound, when clinically indicated. However, special precaution
is necessary when applying pulsed Doppler ultrasound, particularly
in the irst trimester.