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
Examples of longwave (kilohertz) ultrasound devices
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.
Near and Far Fields
Ultrasound beam is divided into a NEAR and a FAR field. The extent of these fields is related to FREQUENCY and RADIATING AREA. The NEAR FIELD is ‘peaky’ with “Hot Spots” whilst the FAR FIELD is divergent but more uniform in nature. As the US frequency reduces, so does effective depth of the near field. At 1MHz, it is in the order of 10cm but at 47kHz, it is in the order of a few mm
This can account (in part at least) for the superficial heating effect experienced by many patients as the divergent beam means that the ultrasound energy will be more diffuse at depth with kilohertz US. It has been estimated (Ward & Robinson 1996) that the energy content of the kilohertz beam has reduced to 50% at 0.5cm and down to 10% at 2cm from the surface. The debate as to the appropriateness of these calculations continues (Robertson & Ward 1996, 1997, Bradnock 1995).
The Effects of kilohertz Ultrasound
There is very little (!) evidence as to whether kilohertz US does (or does not) have the same (or similar, or different) therapeutic effects as MHz ultrasound. The manufacturers claims are extensive and the anecdotal evidence is reasonably strong, but there is not much else to go on (by way of hard evidence).
The Heating (Heated??) Debate
One of the major disputes is centered on the heating effect of kilohertz US compared with traditional (MHz) US. The argument is centered around some in vitro experiments (e.g. Robertson & Ward 1995). The carried out a series of exposures to subaqueous US. 30 minute irradiation + 10 min extra recording using 6 thermocouples along the beam (3mm to 10.5cm) in ‘non living’ pig tissue.
The results show that at 45kHz, the maximum heating was at the surface layers and the maximum temperature increase was 0.4°C. At 1MHz, 5 out of 6 thermocouples demonstrated temperature increases and the largest temperature rise in superficial muscle layers.
Robertson and Ward conducted a similar experiment (1996), but with the treatment head in contact with the tissue. Similar heating pattern resulted with the kilohertz US producing little or no heating beyond 1-2cm depth. The maximum temperature change was of 18.4°C at first probe (@3mm) with the kilohertz energy. At 1MHz, the data showed heating throughout with changes of 12.2°C in superficial tissues, down to 1.2°C at 10.5cm.
In the clinical trial published by Bradnock 1995, patients with unilateral inversion ankle injuries were recruited and the trial compared MHz ultrasound with kHz (longwave) ultrasound with placebo US. The key outcome measures were pre and post treatment gait parameters and pain. The treatment was for 5 minutes duration, using a moving treatment head and ankle movement throughout the US treatment (which was thought to give improved results). The difference in gait was compared with expected norm (taken as symmetry). The gait was found to be asymmetrical in both groups pre treatment. Post treatment, the kilohertz US group demonstrated more improvement than traditional US group. The results were highly significant for STRIDE LENGTH, SWING SYMMETRY, CADENCE AND VELOCITY. There was (unfortunately) no longer term follow up (the measures were only taken pre and post a single treatment session).
There have been many criticisms of this trial, with the main issues coming down to : the current literature would not indicate an expected effect of US within a 5 minute single treatment session (though one could argue that it is impressive that the kHz US does appear to have such an effect). It is proposed that the kilohertz US effects are related more to heating and analgesia than they are to tissue repair and inflammatory changes, and therefore it might have been better to compare the treatment with another intervention that is intended to provide ‘immediate’ pain relief, and a comparison with a superficial heat treatment has been suggested. Further criticisms relate to the involvement of an individual with an intimate association with the manufacturer as the research lead and several other methodological issues (reported elsewhere).
Basso and Pike (1998) also carried out a clinical study on 38 patients with dorsally-displaced distal fractures who were prospectively studied to assess the clinical effects of low frequency (kilohertz) ultrasound treatment, started immediately after plaster removal.
Nineteen of the patients represented the control group and a double- blind protocol was followed. Assessment took place on the day of plaster removal and 2 and 8 weeks later. There was no significant difference in wrist motion and duration of follow-up between the treated and control patients.
Meakins and Watson (2006) carried out a laboratory study which aimed to compare the thermal effects of long wave (kilohertz) ultrasound and conductive heating (Hot Water Bottle - HWB) on ankle mobility using a non injured Achilles tendon model. These two modalities represent a clinical and a home variation of heat treatment.
A crossover design was used (n=18), with each of the interventions (US or HWB) being compared with its own control condition. Functional ankle mobility was assessed using the weight bearing lunge test (WBLT).
The results showed that the application of local superficial heat to non-injured Achilles Tendon (AT) increases functional ankle mobility. Analysis revealed a statistically significant difference in the change of ankle mobility between the application of US and its control session (p<0.0005) and between the application of HWB and its control session (p<0.0005). There was no statistically significant difference between the effects of the two treatments on changes in ankle mobility (p=0.125), though there was a trend for the HWB treatment to be more effective.
The graph illustrates that each treatment produced a much greater increase in mobility than its control condition (graph shows the mean and 95% confidence interval data), but it can be seen that there is not a lot of difference between the kilohertz US and the hot water bottle results – if anything the hot water bottle has the edge. This follows on from the criticisms of the Bradnock (1995) trial which suggested that the kilohertz US should be more realistically compared with a heat treatment than a MHz ultrasound treatment.
It was concluded that both kilohertz (longwave) ultrasound and a conductive heating intervention were shown to significantly increase functional ankle mobility under the conditions described. The demonstration that a local heat application (HWB), which can be self administered, is at least as effective as kilohertz US which has be applied in the treatment room may prove advantageous to patient self management under guidance.
Summary and Conclusion :
Longwave US has some different characteristics to traditional MHz ultrasound
In theory at least, it will penetrate FURTHER into the tissues
BUT a very high %age of the energy will be absorbed in the VERY superficial tissues
Leaving only a small proportion to reach the deeper tissues
There is very limited research evidence of effect or ‘better’ effect of kilohertz US compared with traditional (MHz) ultrasound
Anecdotally, it is suggested that it is ‘better’ – which may well turn out to be true – but is not yet evidenced
The existing evidence does not actually support these claims
There is a growing body of evidence relating to the use of kilohertz ultrasound in specialist wound debridement areas (e.g. Uhlemann et al 2003) but they are considered to be outwith the remit of this review which is primarily concerned with the application of this modality in the musculoskeletal and soft tissue arenas.
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