GE Voluson 730, Инструкция по эксплуатации

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Safety
The rise in tissue-temperature under the influence of ultrasound energy is called
thermal effect .
The level of the temperature rise depends mainly on the following parameters: the irradiated quantity
of energy, the surface of exposure and the thermal characteristics of the tissue. Regarding the
thermodynamics, the AIUM-report comes to the following conclusions:
•
When only the temperature criteria is considered, an exposure to diagnostic ultrasound, leading to
a rise in temperature of 1°C above the normal physiological value can be made without limitations
in clinical examinations.
•
With fetal use a temperature rise in situ beyond 41°C is considered dangerous; the risk of harming
the fetus is increased with the duration of this temperature rise.
•
For fetal use the following intensities are considered to be safe:
•
SATA-intensity (in situ) below 200 mW/cm2 with beam widths of less than 11 wavelengths.
•
SATA-intensity (in situ) below 300 mW/cm2 with beam widths of less than 8 wavelengths.
Note that the thermal model of the AIUM does not take into account the influence of tissue blood
circulation.
Cavitation concerns the reaction of gas- or vapor bubbles or gas- or vapor accumulations present in
tissue or liquids. Two types of cavitation - transient and stable - have been described and investigated
in in-vitro and animal tests (Flynn HG, Physics of Acoustic Cavitation in Liquids, in Physical
Acoustics: Principals and Methods, edited by Mason, WP, Academic Press, New York, 1964, Vol. I/B,
Chap. 9, pp. 57-172).
Transient cavitation means dilatation and quick collapsing of a bubble as a reaction to one or more
ultrasound pulse beams. This quick collapse can lead to locally limited (in micrometer range) high
temperatures and pressures.
Stable cavitation concerns the repeated oscillation of a bubble. This bubble oscillation can have effects
on neighboring cells, especially due to transverse action acting on its membrane and due to
disturbance of the contained cytoplasma. Amplitude and frequency of the bubble oscillation are
dependent on the bubble size at the beginning and resonance frequency characteristics as well as on
frequency and pressure of the impinging ultrasound. With tissue of mammals the scientists came to
contradictory results regarding the ability to produce cavitation. This is possibly due to differences in
the occurrence of cavitation germs (bubbles). Very little is known about the factors determining the
presence or absence of micro bubbles, their chemical consistence and visco-elastic characteristics. The
results of cavitation tests also depend on ambient pressure, on the acoustic energy and the pressure
level. With the output power used in diagnostic ultrasound no cavitation was observed in vivo.
The AIUM-report reaches the following conclusions regarding cavitation:
•
Cavitation can occur with short pulses and produces potentially harmful biological effects.
•
A peak pressure of 10 MPa (3300 W/cm2) can lead to cavitation with mammals.
With the limited data available it is not possible to specify
threshold values for the pressure amplitude
at which - when using diagnostically relevant pulse lengths and pulse repetition frequencies -
cavitation occurs with mammals.
Some scientists observed that the peak rarefaction pressure (pr) of the transmitted disturbance is
related closer to occurrence of cavitation than to the overall measured peak pressure (due to
compression plus rarefaction). Other examinations showed that the peak rarefaction pressure of the
basic oscillation component of the disturbance could be among the three mentioned parameters the
most closely related to the cavitation.
Voluson
®
730 - Instruction Manual
2-10
105838 Rev. 3