Longwave (Kilohertz) Ultrasound

Ultrasound, as a therapy, involves the application of sound energy at a frequency above the normal hearing range (approximately 16Hz - 20,000 Hz). Mechanical energy beyond this range is not audible, though the nature of the energy does not change – it is still a mechanical (pressure) wave. Therapeutic Ultrasound frequencies usually between 1 and 3 MHz (millions of cycles per second) whereas Longwave ultrasound in the 40 – 50 kHz range (tens of thousands of cycles per second). Whilst this is clearly still beyond the normal upper limit for audible sound, it is not as far removed as traditional (MHz) ultrasound. It is probably preferable to refer to this particular form of ultrasound therapy as kilohertz ultrasound (as a way to distinguish it from the more normal MHz (Megahertz) ultrasound therapy. For additional information relating to the ‘physics’ of ultrasound, the basic ultrasound page/handout should have the basic information that you need.

It is suggested that due to its lower frequency and therefore greater wavelength, the energy will penetrate further into the tissues and thus ‘reach’ deeper tissues and have effects that traditional ultrasound is unable to achieve. This may or may not in fact be the case, and the biggest problem lies with the lack of specific research into ‘longwave’ or ‘kiloherts’ ultrasound.
One of the major effects of this different frequency is that there is claimed to be a difference in the effective penetration depth. By employing a LOWER FREQUENCY, the wavelength will be greater (assuming the velocity in tissue is approximately constant). The relationship between sound wave frequency, tissue velocity and wavelength is denoted thus :
v = f.l
At 3MHz, the wavelength (l) will be in the order of 0.5mm
At 1MHz the wavelength will be in the order of 1.5mm
At 45kHz the wavelength will be in the order of several 10’s of cm’s (around 30cm at 45kHz)
The effective penetration depth is also related to frequency. It is known that 1MHz and 3MHz are absorbed at different rates in the tissues and therefore have different penetration depths. The penetration depth of kilohertz US is expected to be in excess of 20 times greater than MHz ultrasound.