Study of cellular response induced by frequency change of low intensity ultrasound

U937 human myelomonocytic cells were stressed at variable and fixed frequency by increasing and decreasing mode in a range between 400 kHz and 620 kHz at different PRF (Pulse Repetition Frequency). It was noted that at 10 Hz PRF decreasing mode was more efficient (a higher cell mortality occurred immediately after sonication), while the increasing mode was most efficient at 0.5 Hz. This data were compared with data at fixed frequency in extreme values and center of frequency range (400 kHz, 510 kHz, 620 kHz) at 10 Hz PRF, 180 s duration, 60 V voltage. At this tension the maximum intensity value is 65 mV/cm2 at 430 kHz. At fixed frequency we found that survival rate was always higher than at sweep frequency. On closer inspection, however we observed that there was a strong dependence by intensity: lowest values of survival rate corresponded to the highest power values. Furthermore a certain dependence of cell mortality from frequency was detected, but it was very detailed compared to that of the power.