Ultrasound intensity threshold

In a recent systematic study of the dependence of 20 kHz sonophoresis on ultrasound parameters, Mitragotri et al. showed that the enhancement of skin permeability varies linearly with ultrasound intensity and ultrasound on-time (for pulsed ultrasound, ultrasound on-time equals the product of total ultrasound application time and duty cycle), while is independent of the ultrasound duty cycle. Based on those findings, fhe authors reported that there is a threshold energy dose for ultrasound induced transdermal drug transport. Once the threshold value is crossed, the enhancement of skin permeability varies linearly with the ultrasound energy dose (J/cm ), which is calculated as the product of ultrasound intensity and ultrasound on-time. This result indicates that ultrasound energy dose can be used as a predictor of the effect of 20 kHz sonophoresis. The authors also indicated that it is important to determine the threshold energy dose for each individual sonophoresis system, for example, the real in vivo situation, because it may vary from system to system. Specifically, it may vary between different skin models, as well as with the ultrasound frequency and the distance of the transducer from the skin surface, etc. 

It can be seen that the effect of increasing ultrasound intensity is to promote a nonlinear increase in the deviation from the mean with an appearance consistent with the existence of a cavitational threshold [3]. 

The conditional clearing threshold of the ultrasound intensity / " for birefringence is dependent on the layer thickness as d [22, 25, 27],... 

Above the threshold the relation between the optical transmission m of the nematic layer and the ultrasound intensity J is nonlinear [10, 22, 25, 27] the transmission depends on the ultrasound intensity as m=sin (const.y ) and shows interference maxima and minima between ordinary and extraordinary light rays for monochromatic light [21, 25] and for small values of the intensity [21,... 

This early investigation of Lindstrom and Lamm was also particularly important in that it proved there had to be a threshold intensity of ultrasound before polymerisation took place. 

The presence of particulate matter — especially that of trapped vapour gas nuclei in their crevices and recesses — has also been found to lower the cavitation threshold. This is required under these conditions are of paramount importance in the clinical field, where ultrasound Is widely used in applications such as high-intensity focused ultrasound (HIFU),... 

A very important point occurs in the transmission of acoustic power into a liquid which is termed the cavitation threshold. When very low power ultrasound is passed through a liquid and the power is gradually increased, a point is reached at which the intensity of sonication is sufficient to cause cavitation in the fluid. It is only at powers above the cavitation threshold that the majority of sonochemical effects occur because only then can the great energies associated with cavitational collapse be released into the fluid. In the medical profession, where the use of ultrasonic scanning techniques is widespread, keeping scanning intensities below the cavitation threshold is of vital importance. As soon as the irradiation power used in the medical scan rises above this critical value, cavitation is induced and, as a consequence, unwanted even possibly hazardous chemical reactions may occur in the body. Thus, for both chemical and medical reasons there is a considerable drive towards the determination of the exact point at which cavitation occurs in liquid media, particularly in aqueous systems. Historically, therefore, the determination of the cavitation threshold was one of the major drives in dosimetry. 

The chemical effects of820-kHz diagnostic ultrasonic cavitation was studied and it was reported that the OH radical produced under cavitation in water could react with non-fluorescent terephthalic acid (TA) to produce fluorescent hydroxy tere-phthalate (HTA) [76-78]. Thus cavitation induced by ultrasound could be detected through measuring the HTA fluorescence value produced. When the sound intensity was above the threshold value of ultrasonic cavitation, the yield of HTA increased rapidly with the sound intensity and the yield increased approximately linearly with exposure time. When the sound intensity reached a certain value, the yield of HTA reached saturation. The relationship between the yield of HTA and sonication period was described and explained by an equation given by the authors. 

The intensity and frequency of the ultrasound play an important role. Instability begins just above a threshold value /. Sometimes domains appear first, and then... 

---