Ultrasound-biophysics mechanisms

Abstract

Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.

Figures

Fig. 1

Schematic diagram of ultrasonic biophysics.

Fig. 2

(a) Acoustic spectrum and (b) medical ultrasound spectrum.

Fig. 3

Representations of longitudinal and shear waves: (a) longitudinal wave representation, (b) shear wave representation and (c) sine wave representation.

Fig. 4

Schematic representations of an acoustic waveform: (a) amplitude vs. distance and (b) amplitude vs. time.

Fig. 5

Schematic representations of continuous wave and pulsed wave ultrasound waveforms: (a) continuous wave representation and (b) pulsed wave representation.

Fig. 6

Measured 2.5-MHz center frequency pulsed wave ultrasound waveform.

Fig. 7

Temperature–time curves for 4 values of t43 (see Eq. (54) for which R = 0.5 for T>43 °C and R = 0.25 for T≤43 °C). The bolded t43 = 1 min line shows the lower exposure duration range (applicable to 1 min) of the March 26, 1997 AIUM Conclusions Regarding Heat statement (AIUM, 1997).

Fig. 8

Temperature–time curves (see Fig. 7) plus the following threshold data: filled-in circle, cat brain; open triangle: extrapolated cat brain; filled-in triangle, rabbit brain; filled-in square, rat brain; open diamond, rabbit muscle; open circle, dog prostate; open square, BHK cells, dashed line, multiple tissue thresholds; shaded line (just above bolded t43 = 1 min line), multiple in vitro thresholds. Details listed in Table 9.

Fig. 9

Dimensions of the 33 rectangular aperture cases investigated.

Fig. 10

Paired maximum steady-state temperature increase ΔTmax vs. unscanned-mode soft-tissue Thermal Index (TIS) for circular sources grouped by frequency under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 11

Paired maximum steady-state temperature increase ΔTmax vs. unscanned-mode soft-tissue Thermal Index (TIS) for circular sources grouped by f-number under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 12

Paired maximum steady-state temperature increase ΔTmax vs. unscanned-mode soft-tissue Thermal Index (TIS) for rectangular sources grouped by frequency under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 13

Paired maximum steady-state temperature increase ΔTmax vs. unscanned-mode soft-tissue Thermal Index (TIS) for rectangular sources grouped by f-number under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 14

Paired maximum steady-state temperature increase ΔTmax vs. location (axial distance) of ΔTmax for circular sources grouped by f-number under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 15

Paired maximum steady-state temperature increase ΔTmax vs. location (axial distance) of ΔTmax for rectangular sources grouped by f-number under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 16

Paired location of the geometric focus (ROC) vs. normalized location (axial distance) of ΔTmax (location of ΔTmax/ROC) for circular sources grouped by f-number.

Fig. 17

Paired location of the geometric focus (ROC) vs. normalized location (axial distance) of ΔTmax (location of ΔTmax/ROC) for rectangular sources grouped by f-number.

Fig. 18

t80% profiles as a function of axial distance for the 2-MHz (gray) and 7-MHz (black) cases for circular sources under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 19

Paired maximum steady-state temperature increase ΔTmax vs. the proposed unscanned-mode soft-tissue Thermal Indices (TISnew(1) and TISnew(2)) for rectangular sources grouped by f-number under the condition that the derated (0.3 dB/cm MHz) spatial-peak, temporal-average intensity ISPTA.3 is 720 mW/cm2.

Fig. 20

Peak rarefactional pressure rupture threshold of Optison™ as a function of frequency and pulse duration. PRF = 10 Hz.

Fig. 21

Summary of ED05 (5%) lesion occurrence thresholds in terms of pr(in situ) (MPa) (bars; left axis) and MI (lines, right axis). *denotes the studies that are evaluated under the same exposure conditions. Error bars are SEMs. These data, by groupings from left to right, are derived, respectively, from Zachary et al. (2001a) and O'Brien et al. (2001, , , .

Fig. 22

Summary of lesion occurrence thresholds in terms of pr(in situ) (MPa) (bars; left axis) and MI (lines, right axis) from studies not using our experimental techniques and statistical approach. These data, by groupings from left to right, are estimated, respectively, from Child et al. (1990), Raeman et al. (1993, , Frizzell et al. (1994), Holland et al. (1996), Baggs et al. (1996) and Dalecki et al. (1997c, d).

Fig. 23

Global summary of lesion occurrence thresholds in terms of in situ peak rarefactional pressure as a function of frequency for four species (see Figs. 21 and 22). The solid line is the threshold equation derived from pre-2000 data (AIUM, 2000).

Fig. 24

Global summary of lesion occurrence thresholds in terms of the Mechanical Index as a function of frequency for four species (see Figs. 21 and 22). The solid line denotes an MI of 1.9, the FDA regulatory limit (FDA, 1997).