The risk of exposure to diagnostic ultrasound in postnatal subjects: thermal effects

Abstract

This review evaluates the thermal mechanism for ultrasound-induced biological effects in postnatal subjects. The focus is the evaluation of damage versus temperature increase. A view of ultrasound-induced temperature increase is presented, based on thermodynamic Arrhenius analyses. The hyperthermia and other literature revealed data that allowed for an estimate of a temperature increase threshold of tissue damage for very short exposure times. This evaluation yielded an exposure time extension of the 1997 American Institute of Ultrasound in Medicine Conclusions Regarding Heat statement (American Institute of Ultrasound in Medicine, Laurel, MD) to 0.1 second for nonfetal tissue, where, at this exposure time, the temperature increase threshold of tissue damage was estimated to be about 18 degrees C. The output display standard was also evaluated for soft tissue and bone cases, and it was concluded that the current thermal indices could be improved to reduce the deviations and scatter of computed maximum temperature rises.

Figure 1

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

Figure 2

Temperature-time curves (Figure 1) plus the following threshold data: filled circle, cat brain; open triangle, cat brain (estimated; see section I); filled triangle, rabbit brain; filled square, rat brain; open diamond, rabbit muscle; open circle, dog prostate; open square, baby hamster kidney cells; dashed line, multiple tissue thresholds; and shaded line (just above the bold t₄₃ = 1 minute red line), multiple in vitro thresholds. Details are listed in Table 3.

Figure 3

Temperature-time curves (see Figure 2) plus a bold red line that represents a conservative boundary for nonfetal exposure durations, particularly for exposure durations less than 5 seconds.

Figure 4

Dimensions of the 33 rectangular aperture cases investigated.

Figure 5

Paired maximum steady-state temperature increase ΔT_max versus unscanned-mode TIS for 192 circular sources grouped by frequency under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 6

Paired maximum steady-state temperature increase ΔT_max versus unscanned-mode TIS for 192 circular sources grouped by f-number under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 7

Paired maximum steady-state temperature increase ΔT_max versus unscanned-mode TIS for 594 rectangular sources grouped by frequency under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 8

Paired maximum steady-state temperature increase ΔT_max versus unscanned-mode TIS for 594 rectangular sources grouped by f-number under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 9

Paired maximum steady-state temperature increase ΔT_max versus location (axial distance) of ΔT_max for circular sources grouped by f-number under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 10

Paired maximum steady-state temperature increase ΔT_max versus location (axial distance) of ΔT_max for rectangular sources grouped by f-number under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 11

Paired location of the geometric focus (ROC) versus normalized location (axial distance) of ΔT_max [(location of ΔT_max)/ROC] for circular sources grouped by f-number.

Figure 12

Paired location of the geometric focus (ROC) versus normalized location (axial distance) of ΔT_max [(location of ΔT_max)/ROC] for rectangular sources grouped by f-number.

Figure 13

Profiles of t_80% as a function of axial distance for the 2-MHz (red) and 7-MHz (blue) cases for 1-cm-diameter (top) and 4-cm-diameter (bottom) circular sources under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 14

Paired maximum steady-state temperature increase ΔT_max versus the proposed unscanned-mode TIS_new(1) (top) and TIS_new(2) (bottom) for rectangular sources grouped by f-number under the condition that the derated I_SPTA.3 is 720 mW/cm².

Figure 15

Temperature-time line that represents a conservative boundary for nonfetal exposure durations. This line appears as the red line in Figure 3.