DK2985_C000 1..28 - AlSharqia Echo Club

Principles of Ultrasound 5the ratio of the transmitted ultrasonic power to the powerrequired to raise the temperature of the medium by 18C inthe same conditions. Mechanical index is a parameter thatreflects the potential for cavitation, or bubble formation inzones of rarefactions, which has been hypothesized as apotential mechanism for biologically harmful effects. Themechanical index increases together with the transmittedpower of the transducer and is inversely related to frequency.More specifically, it is given by the rarefactionalpeak pressure at a point along the ultrasound beam axiswhere the pulse intensity is maximum, divided by thesquare root of the transmitted frequency. For safetyreasons, it is more relevant to measure the mechanicalindex than the power of the source. It is common forinstruments to display mechanical index, as it allows theoperator to understand the effect of common operating parameters,such as the transmitting power on the mechanicalindex (Fig. 1.4). In a similar fashion, thermal index canoften be visualized on some instruments, and it is influencedby similar factors (Fig. 1.5).Depending on the type of clinical scan (obstetric,cardiac, peripheral, etc.), the maximum values of mechanicaland thermal indices are usually limited by theinstrument. Consequently, the potential for harmfuleffects is limited, and possibly nonexistent. However,epidemiological demonstrations of noncausality are verydifficult to obtain, and the existence of such risk shouldnot be ruled out.(A)(B)MI = 0.1-15 dBMI = 0.2-6 dB(C)(D)LARALVRVMI = 0.40 dBFigure 1.4 Transmitting power and mechanical index. (A–D) The mechanical index (MI) decreases when transmitting power isreduced [expressed in negative decibel (dB)]. A value of 0 dB means that the emitting source is at 100% of its power (LA, leftatrium; LV, left ventricle; RA, right atrium; RV, right ventricle).