40 KHz Ultrasound Penetration

Much has been said about the relative merits of ‘short wave’ ultrasound at 1MHz & 3MHz in comparison with ‘long wave’ ultrasound at 40KHz. Anecdotal evidence from many users suggests that 40KHz ultrasound therapy provides much deeper penetration and is more effective, but some published research suggested that 40KHz ultrasound is less effective and provides less penetration particularly in comparison with 1MHz ultrasound. One possible explanation for these contradictory views is that there is little agreement about what effects are being sought from the use of ultrasound therapy. There has been an on-going disagreement for many years among physical therapists about the relative value of the heating or mechanical effects of ultrasound energy.

Published experimental work suggesting that 1MHz ultrasound provides the deepest penetration explored only the heating effect of ultrasound energy by inserting thermocouples into non-living pig tissue and measuring the heating effect at different tissue depths.

Experimental work carried out by researchers at The University of Rochester, N.Y, U.S.A. and published by The American Heart Association used a different approach which established the degree of attenuation in animal tissue using a hydrophone to measure the sound pressure level of a 40 KHz ultrasound signal passing through the rib cage of a pig. The measurements were carried out using ultrasound energy levels similar to those used in ultrasound therapy at 40 KHz and established that attenuation was generally around 3dB/cm depth of tissue. This was through a rib cage, but when measuring transmission through soft tissue (liver & muscle) it was concluded that there was very little attenuation and that depth of penetration was almost infinite.

From a practical point of view regarding the use of 40KHz ultrasound for use in ultrasound therapy, it is not clear what guidance this work offers, as the interests of the American Heart Association are the effect that 40KHz ultrasound has on the dissipation of blood clots, but this must have a great deal of direct relevance to it’s use in physical therapy.

The measurements made during this work suggest that although 40 KHz ultrasound energy will be reduced by 50%/cm of tissue with bone (a rib cage) soft tissue will not significantly reduce the level of transmission making it possible to carry out treatment of deep seated injuries.

Taking the rib cage as a worst case situation, the measurements demonstrate that an attenuation of 3dB (50%)/cm show that a doubling of the applied power will double the depth of penetration.

The above has been prepared in response to requests to provide technical guidance to users regarding the relationship between ultrasound power level & penetration depth and a comparison between long wave & short wave ultrasound. I have prepared this information by interpreting the findings of experimental work carried out at The University of Rochester being aware that their work was specifically intended to be relevant only to those with an interest in cardiovascular medicine. I have done this as an engineer with no training as a therapist purely to assist Veterinary Physiotherapists in the use of my long wave ultrasound therapy machines.

The above text is available in pdf form below.                           The original paper from Circulation, the journal of the American
                                                                                                                      Heart Association is available in pdf form below

Penetration of 40 KHz Ultrasound.pdf Penetration of 40 KHz Ultrasound.pdf
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