Temporal peak intensity as a critical parameter in ultrasound dosimetry

The in vitro effect of microsecond pulses of ultrasound is studied in mammalian cells using two different biologic endpoints. The frequency of sister chromatid exchanges (SCE) is unchanged in Chinese hamster ovarian cells insonated with a stationary transducer for 1-30 minutes at an SPTA intensity of 0.1 W/cm2. Raising the temporal average intensity to 2 W/cm2 by increasing the pulse repetition frequency alone still has no effect, whereas a change in temporal peak intensity produces an increase in SCE after insonation for only 1 minute. From experiments involving aggregation of platelets in suspension around acoustically active hydrophobic pores, pulse duration is found to be a more effective factor than the pulse repetition frequency. Results of the two studies show the biologic response to be strongly dependent on temporal peak intensity and pulse duration, but weakly dependent on duration of exposure. The temporal average intensity as a singular dosage specification is considered to be a poor indicator of potential nonthermal bioeffects associated with microsecond pulses.