Physical Factors Involved in Ultrasonically Induced Changes in Living Systems: II. Amplitude Duration Relations and the Effect of Hydrostatic Pressure for Nerve Tissue
W. J. Fry, D. Tucker, F. J. Fry, V. J. Wulff
The Journal of the Acoustical Society of America 1951 · 10.1121/1.1906774
Abstract
The results of experiments with frogs under a hydrostatic pressure demonstrate that cavitation is not an important factor in the mechanism of production of paralysis of the hind legs of frog by ultrasonic (frequency one megacycle) irradiation over the lumbar enlargement region of the spinal cord. Experimental results indicate that a linear relation exists between the reciprocal of the minimum exposure time for paralysis and the acoustic amplitude. This result is readily described in terms of a one factor rate process. On the basis of this experimentally determined relation, it is shown that time rate of change of temperature cannot be correlated with the observations. It is concluded on the basis of a theoretical calculation that absorption of ultrasound at interfaces in the spinal cord does not result in minute hot regions. Further work on summation of subparalytic doses, spaced apart at various time intervals, indicates that the recovery process following exposure to a subparalytic dose of ultrasonic radiation may not be a monotonic function of time.
Abstract via crossref.
Exposures
Exposure 1: Intact frog, lumbar enlargement of the spinal cord, under atmospheric or 13-atmosphere hydrostatic pressure
Target: lumbar spinal cord — “lumbar enlargement of the spinal cord, intact frog, sound beam incident on the dorsal surface”
Device: custom-built
| Waveform | continuous | |
|---|---|---|
| Fundamental frequency (kHz) | 980 | ✓✓✓ |
| Pulse duration (ms) | not applicable | |
| Pulse repetition frequency (Hz) | not applicable | |
| Duty cycle (%) | not applicable | |
| Sonication duration (s) | 5.4, 7.8swept | ✓✓✓ |
| Free-field pressure (kPa) | 1,013, 1,216swept | ✓✓✓⚑ |
|---|---|---|
| Free-field Isppa (W/cm²) | not reported | |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | measurement | |
| In-situ pressure (kPa) | not reported | ⚑ |
| In-situ Isppa (W/cm²) | not reported | |
| In-situ Ispta (W/cm²) | not reported |
Frogs at atmospheric pressure or under 13 atmospheres hydrostatic pressure (sufficient to suppress cavitation) were irradiated on the dorsal surface over the lumbar enlargement with a single continuous sonication; the reciprocal of the minimum exposure time for paralysis increased linearly with acoustic amplitude, with a definite threshold in each case. In a separate summation experiment at 10 atmospheres acoustic pressure amplitude, an initial subparalytic dose of 5.4 s (paralysis time for a single irradiation at this level was 7.8 s) was followed by a second dose after a variable interval to study recovery.
Flags from extraction
exposures[0].free_field.pressure_kpa— List combines two explicitly stated free-field pressure-amplitude calibration points (about 12 atmospheres at 5.5 kV, used for the main amplitude-duration data of Fig. 2/3; and 10 atmospheres, held constant for the summation experiment of Fig. 4); the source quote given is only for the first.exposures[0].in_situ.pressure_kpa— Paper reports that the average acoustic pressure amplitude measured directly in the cord (via an inserted probe) is 0.8 of the free-field amplitude, but gives no absolute in-situ figure; per the no-derating rule this ratio is not converted into a number here.n_subjects— Paper states 'about 500 frogs' were needed for the main amplitude-duration dataset (Figs. 2-3); this excludes additional frogs used in the separate summation sub-experiments (stated only as '12 to 15 frogs ... per point').