Chapter 1: ULTRASOUND BIOEFFECTS AND SAFETY: WHAT THE PRACTITIONER SHOULD KNOW

Definitions¹

1. Acoustic streaming: movement of tissue or fluid, resulting from the passage of alternating positive and negative pressures of the ultrasound wave. Can also result from movements of bubbles, as a result of changes in pressure.

2. ALARA principle: stands for As Low As Reasonably Achievable, a way to obtain the best, clinically relevant image while keeping ultrasound intensity and exposure as low as possible.

3. Cavitation: bubble activity, secondary to ultrasound insonation. The positive aspect of the ultrasound pressure wave causes compression of the bubble while the negative part, also called rarefactional, causes production of the bubbles or expansion of existing ones. Cavitation can be stable or inertial.

   • Stable cavitation: bubble activity where bubble does not collapse (see inertial cavitation, below) but is moving back and forth in the tissue or fluid, thus potentially causing the surrounding medium to flow (ie, stream, hence the term streaming).

   • Inertial (previously known as transient) cavitation: bubbles that are compressed and expanded but with each compressing (positive) component, causing the volume to diminish ever more, until collapse occurs. This collapse can generate tremendously elevated temperature and pressure for an extremely short time and over an extremely short space (called an adiabatic reaction). This can result in production of several more bubbles, local cell damage, and/or generation of free radicals.

4. Derating: action of multiplying a value measured in water with standard methods by a correction factor to account for the attenuation of the ultrasound field by the tissue traversed by the beam (usually 0.3 dB/cm/MHz).

5. Dwell time: the time during which the ultrasound beam impinges on a specific organ, body part, or entire organism.

6. Mechanical index (MI): expresses the potential for non-thermal (also known as mechanical) effects in tissues traversed by the ultrasound wave. Depends on the pressure and the frequency (= P/√f).

7. Output Display Standard (ODS): actual name—Standard for Real-Time Display of Thermal and Mechanical Acoustic Indices on Diagnostic Ultrasound Equipment. Introduced to make end users aware, in real-time, of the potential effects of ultrasound in tissues. See also mechanical index and thermal index.

8. Radiation force: force resulting from absorption of some of the energy of the acoustic wave by tissue and transformation into heat.

9. Scanned mode: refers to the ultrasound beam moving through the field, with energy distributed over a large volume, such as in B-mode and color flow Doppler.

10. Thermal index (TI): expresses the potential for temperature increase in tissues traversed by the ultrasound wave. It is given by the ratio of the power emitted by the transducer to the ultrasonic power required to raise tissue temperature by 1°C for the specific exposure conditions. This is a relative indication and does not necessarily correspond to the actual temperature increase. One of three thermal indices is displayed, based on whether soft tissue (TIS, mostly first and early second trimesters), bone (TIB, late second and third trimesters), or adult cranium (TIC) is being scanned.

11. Unscanned mode: the ultrasound beam is stationary with power concentrated along a ...