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STATEMENT ON THE SAFE USE, AND
POTENTIAL HAZARDS OF DIAGNOSTIC ULTRASOUND
Prepared by the Safety Group of the British Medical Ultrasound Society. |
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Ultrasound is now accepted as being of
considerable diagnostic value. There is no evidence that diagnostic ultrasound
has produced any harm to patients in the four decades that it has been in use.
However, the acoustic output of modern equipment is generally much greater than
that of the early equipment and, in view of the continuing progress in
equipment design and applications, outputs may be expected to continue to be
subject to change. Also, investigations into the possibility of subtle or
transient effects are still at an early stage. Consequently diagnostic
ultrasound can only be considered safe if used prudently.
Thermal hazard exists with some diagnostic ultrasound equipment, if used
imprudently. A temperature elevation of less than 1·5°C is considered
to present no hazard to human or animal tissue, including a human embryo or
fetus, even if maintained indefinitely. Temperature elevations in excess of
this may cause harm, depending on the time for which they are maintained. A
temperature elevation of 4°C, maintained for 5 minutes or more, is
considered to be potentially hazardous to a fetus or embryo. Some diagnostic
ultrasound equipment, operating in spectral pulsed Doppler mode, can produce
temperature rises in excess of 4°C in bone, with an associated risk of
high temperatures being produced in adjacent soft tissues by conduction. With
some machines colour Doppler imaging modes may also produce high temperature
rises, particularly if a deep focus or a narrow colour box is selected. In
other modes, temperature elevations in excess of 1°C are possible, but are
unlikely to reach 1·5°C with equipment currently in clinical use,
except where significant self-heating of the transducer occurs.
Non-thermal damage has been demonstrated in animal tissues containing gas
pockets, such as lung and intestine, using diagnostic levels of ultrasound
(mechanical index values of 0.3 or more). In view of this, it is recommended
that care should be taken to avoid unnecessary exposure of neonatal lung , and
to maintain MI as low as possible when this is not possible. In other tissues
there is no evidence that diagnostic ultrasound produces non-thermal damage, in
the absence of gas-filled contrast agents. However, in view of the difficulty
of demonstrating small, localised, regions of damage in vivo, the possibility
of this cannot be excluded. The Mechanical Index, if displayed, acts as a guide
to the operator. The use of contrast agents in the form of stabilised gas
bubbles increases the probability of cavitation. Single beam modes (A-mode,
M-mode and spectral pulsed Doppler) have a greater potential for non-thermal
hazard than scanned modes (B-mode, Colour Doppler), although the use of a
narrow write-zoom box increases this potential for scanning modes.
June 2000
Reconfirmed by BMUS, 2009 |
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