Biological Effects of Ultrasound: Wave Distortion, Heating, Cavitation, and Radiation Force

1. In the name of GOD

2. Chapter 10 Biological effects of ultrasound

3. Biological effects of ultrasound • In earlier years, it is believed that the power form diagnostic ultrasound is too low that nonlinear effects can not happen • As the requirement of better S/N, the peak and average intensity used increased and hence nonlinearity become important • The most important effects are: • Wave distortion • Heating • Cavitations • Radiation force • Streaming

4. Wave distortion • As the power level of acoustic wave increase, the sinusoidal wave distort • The reason is, the region of increased pressure, the propagation velocity is greater, causing the pressure peaks to catch up with the pressure troughs and hence the wave look like saw-toothed waveform and significant energy is transferred to higher harmonics • A list of typical acoustic output values in current use are:

5. Heating • A major portion of attenuated ultrasound in tissue results from conversion to heat • At low intensity level, the heat produced is rapidly diffused and no pronounced temperature rise • As the intensity rises, heat production is more pronounced and result temperature rise • Adverse biological effects results when temperature rises above 38.5º

6. Cavitation • The term used to describe the behavior of gas bubble in ultrasonic wave • Two types of cavitation: Transient and stable • Transient is when microbubbles suddenly growth and collapse in a liquid medium • The pressure increases in a half cycle causing bubble to collapse and disappear. • When the change of pressure is high, the radius increase markedly , reaching a peak well above the tolerance of the bubble • When the bubble collapse a very high amount of pressure generate (up to 80,000 ATM) and the temperature reaches 10,000ºK • A phenomena called sonoluminesence with picoseconds duration may occur

7. Stable cavitation describes a phenomena in which bubbles do not collapse • This type is more likely to occur in low intensity ultrasound • Cavitation can be suppressed by degassing, increasing viscosity or by increasing pressure applied to the system

8. Radiation force and Streaming • When ultrasound propagate in a fluid, transfer momentum to the medium via absorption and cause acoustic streaming in the direction of the beam • If a desecrate object is present, a radiation force is exerted on the object • If the object is a bubble, oscillation of the object can cause streaming • Non-linear phenomena can increase streaming • Streaming may induce shear wave on the borders

9. Ultrasound Bioeffects • Ultrasound bioeffects can be classified into: Thermal and non-thermal • Thermal effects: • Increasing temperature to a tissue can cause cell death. The relation is similar to hyperthermia • Review of the literature shows no lethal effects below 41 degree

10. Thermal index • The parameter selected by AIUM to consider attenuation, beam profile and tissue thermal properties of ultrasound propagating in soft tissue is Thermal Index (TI) and is defined: • W0 is the average emitted power of the source in water defined by the beam profile • Wdeg is the estimated power needed to raise the target tissue by 1ºC based on tissue thermal model • To take tissue attenuation into account W0 should be replaced by the derated power W0.3(z) at a distance z from the source

11. Assuming a constant attenuation coefficient of 0.3dB/cm.MHz (0.035np/cm), then: • fc is the ultrasonic central frequency • At z=0 the power is highest and is: • W0 is in mWatt and fc in MHz. TI for nonscan mode and Doppler is different • If TI is grater than 1 it must be displayed. • For TI less than 0.4 it is not necessary • For TI values less than1, the increment should not be greater than 0.2. • For TI values above 1 the increment should not be greater than 1.

12. Mechanical effects and Index • Cavitation and other nonthermal effects cause cell lysis, change in cell permeability, and lung damage • It is shown that cavitation threshold is found to be related to rarefractional peak pressure (negative going pressure), frequency, pulsing conditions, and the medium properties. • Mechanical index (MI) is defined by: • Pr0.3=derated peak rarefractional pressure in MPa by 0.3dB/cm-MHz • Zsp=axial distance where the derated pulse intensity integral (PII0.3)=∫ I0.3(z)dz is maximal • From figure, ISPPA=PII/pulse duration • Maximal allowable MI is 1.9 which is equivalent to ISPPA=190W/cm2

13. MI is a voluntary standard • It has several draw backs: 1)dose not consider examination time 2)Patient temperature 3) effects of stable cavitation and other nonthermal effects • MI developed in in vitro using gas containing polystyrene spheres

14. Data for bioeffects of ultrasound are inconsistent. • At the present time it is safe to conclude that: • In low MHz range, no adverse have been observed on non human biological tissues exposed in vivo under experimental ultrasound conditions if:

15. In summary (from figure below):1)For long exposure time, as long as ISPTA is below 100mW/cm2 no adverse effects observed2) For short exposure time the allowable intensity may be higher.As long as the imparted energy dose not exceed 50mJ/cm2, no adverse effects have been observed