Miller D L, Thomas R M
Biology and Chemistry Department, Battelle Pacific Northwest Laboratories, Richland, WA 99352.
Ultrasound Med Biol. 1993;19(4):343-51. doi: 10.1016/0301-5629(93)90106-x.
Membrane damage resulting from ultrasonic gas body activation was investigated in leaves of the aquatic plant Elodea using pulse-mode exposures from 0.745-15 MHz. The frequency response was similar to that previously observed for continuous exposures. Cell death thresholds were higher for the pulse modes; for example, at 6 MHz the threshold was 166 W/cm2 spatial-peak, pulse-average (SPPA) intensity for 1 microsecond pulses and 1 kHz pulse repetition frequency (PRF) for 60 s exposures, compared to 12 W/cm2 for 60 ms continuous exposure. Increasing the PRF for 2.5 MHz, 3 microseconds pulses from 10 Hz to 5 kHz resulted in a gradual decrease in the threshold. Increasing the pulse duration from 1 microsecond to 30 microseconds and PRF from 33 Hz to 1 kHz only weakly influenced the threshold, for the constant 60 ms total on-time. Decreasing the pulse duration for single 6 MHz pulses increased the threshold up to 1,540 W/cm2 at 30 microseconds. The gas body activity was modeled with linear theory for oscillation of the gas channel walls, and intracellular shear stress, which causes lethal damage to the plasma membranes, was modeled with theory for acoustic microstreaming. Theory and observation indicate that the minimum shear stress threshold as a function of resonance frequency has essentially the same form as the Mechanical Index (MI) in the 0.5 to 20 MHz range. Specifically, for 3 microseconds pulses with 0.2-2 kHz PRF and 60 ms total on-time, the pressure-amplitude threshold divided by the square root of the frequency was approximately constant at 0.95 MPa/MHz1/2.
利用0.745 - 15 MHz的脉冲模式曝光，研究了超声气体激活对水生植物伊乐藻叶片造成的膜损伤。频率响应与先前连续曝光时观察到的相似。脉冲模式下的细胞死亡阈值更高；例如，在6 MHz时，对于1微秒脉冲和1 kHz脉冲重复频率（PRF）、持续60秒的曝光，阈值为166 W/cm²空间峰值、脉冲平均（SPPA）强度，而连续曝光60毫秒时的阈值为12 W/cm²。将2.5 MHz、3微秒脉冲的PRF从10 Hz增加到5 kHz会导致阈值逐渐降低。对于恒定的60毫秒总开启时间，将脉冲持续时间从1微秒增加到30微秒以及将PRF从33 Hz增加到1 kHz对阈值的影响较弱。将单个6 MHz脉冲的脉冲持续时间缩短会使阈值在30微秒时增加到1540 W/cm²。用气体通道壁振荡的线性理论对气体活性进行建模，并用声微流理论对导致质膜致命损伤的细胞内剪切应力进行建模。理论和观察表明，作为共振频率函数的最小剪切应力阈值在0.5至20 MHz范围内与机械指数（MI）基本具有相同的形式。具体而言，对于3微秒脉冲、0.2 - 2 kHz PRF和60毫秒总开启时间，压力振幅阈值除以频率的平方根在0.95 MPa/MHz¹/²时大致恒定。
