Ultrasound (US) is a form of MECHANICAL energy, not electrical energy and therefore strictly speaking, not really electrotherapy at all but does fall into the Electro Physical Agents grouping. Mechanical vibration at increasing frequencies is known as sound energy. The normal human sound range is from 16Hz to something approaching 15-20,000 Hz (in children and young adults). Beyond this upper limit, the mechanical vibration is known as ULTRASOUND. The frequencies used in therapy are typically between 1.0 and 3.0 MHz (1MHz = 1 million cycles per second).
Sound waves are LONGITUDINAL waves consisting of areas of COMPRESSION and RAREFACTION. Particles of a material, when exposed to a sound wave will oscillate about a fixed point rather than move with the wave itself. As the energy within the sound wave is passed to the material, it will cause oscillation of the particles of that material. Clearly any increase in the molecular vibration in the tissue can result in heat generation, and ultrasound can be used to produce thermal changes in the tissues, though current usage in therapy does not focus on this phenomenon (Williams 1987, Baker et al 2001, ter Haar 1999, Nussbaum 1997, Watson 2000, 2008). In addition to thermal changes, the vibration of the tissues appears to have effects which are generally considered to be non thermal in nature, though, as with other modalities (e.g. Pulsed Shortwave) there must be a thermal component however small. As the US wave passes through a material (the tissues), the energy levels within the wave will diminish as energy is transferred to the material. The energy absorption and attenuation characteristics of US waves have been documented for different tissues (see absorption section).
WAVELENGTH - the distance between two equivalent points on the waveform in the particular medium. In an ‘average tissue’ the wavelength @ 1MHz would be 1.5mm and @ 3 MHz would be 0.5 mm.
VELOCITY - the velocity at which the wave (disturbance) travels through the medium. In a saline solution, the velocity of US is approximately 1500 m sec-1 compared with approximately 350 m sec-1 in air (sound waves can travel more rapidly in a more dense medium). The velocity of US in most tissues is thought to be similar to that in saline.
These three factors are related, but are not constant for all types of tissue. Average figures are most commonly used to represent the passage of US in the tissues. Typical US frequencies from therapeutic equipment are 1 and 3 MHz though some machines produce additional frequencies (e.g. 0.75 and 1.5 MHz) and the ‘Longwave’ ultrasound devices operate at several 10’s of kHz (typically 40-50,000Hz – a much lower frequency than ‘traditional US’ but still beyond human hearing range.
The mathematical representation of the relationship is V = F.l where V = velocity, F = frequency and l is the wavelength.
The US beam is not uniform and changes in its nature with distance from the transducer. The US beam nearest the treatment head is called the NEAR field, the INTERFERENCE field or the Frenzel zone. The behaviour of the US in this field is far from regular, with areas of significant interference. The US energy in parts of this field can be many times greater than the output set on the machine (possibly as much as 12 to 15 times greater). The size (length) of the near field can be calculated using r2/l where r= the radius of the transducer crystal and l = the US wavelength according to the frequency being used (0.5mm for 3MHz and 1.5mm for 1.0 MHz).
One quality indicator for US applicators (transducers) is a value attributed to the Beam Nonuniformity Ratio (BNR). This gives an indication of this near field interference. It describes numerically the ratio of the intensity peaks to the mean intensity. For most applicators, the BNR would be approximately 4 - 6 (i.e. that the peak intensity will be 4 or 6 times greater than the mean intensity). Because of the nature of US, the theoretical best value for the BNR is thought to be around 4.0 though some manufacturers claim to have overcome this limit and effectively reduced the BNR of their generators to 1.0.
A couple of recent papers (Straub et al, 2008 and Johns et al 2007) have considered some of the inaccuracies associated with current machines and Pye (1996) presents some worrying data with regards the calibration of machines in clinical use in the UK.
Ultrasound Transmission through the Tissues
All materials (tissues) will present an impedance to the passage of sound waves. The specific impedance of a tissue will be determined by its density and elasticity. In order for the maximal transmission of energy from one medium to another, the impedance of the two media needs to be as similar as possible. Clearly in the case of US passing from the generator to the tissues and then through the different tissue types, this can not actually be achieved. The greater the difference in impedance at a boundary, the greater the reflection that will occur, and therefore, the smaller the amount of energy that will be transferred. Examples of impedance values can be found in the literature e.g. Robertson et al 2007,Ward 1986.
The difference in impedance is greatest for the steel/air interface which is the first one that the US has to overcome in order to reach the tissues. To minimise this difference, a suitable coupling medium has to be utilised. If even a small air gap exists between the transducer and the skin the proportion of US that will be reflected approaches 99.998% which means that there will be no effective transmission.
Absorption and Attenuation
Because the absorption (penetration) is exponential, there is (in theory) no point at which all the energy has been absorbed, but there is certainly a point at which the US energy levels are not sufficient to produce a therapeutic effect. As the US beam penetrates further into the tissues, a greater proportion of the energy will have been absorbed and therefore there is less energy available to achieve therapeutic effects. The half value depth is often quoted in relation to US and it represents the depth in the tissues at which half the surface energy is available. These will be different for each tissue and also for different US frequencies.
The table gives some indication of typical (or average) half value depths for therapeutic ultrasound. (after Hoogland 1995). As it is difficult, if not impossible to know the thickness of each of these layers in an individual patient, average half value depths are employed for each frequency: 3MHz - 2.0cm; 1MHz - 4.0cm.
| || 1MHz|| 3MHz|
| Muscle|| 9.0mm|| 3.0mm|
| Fat|| 50.0mm|| 16.5mm|
| Tendon|| 6.2mm|| 2.0mm|
These values (after Low & Reed) are not universally accepted (see Ward 1986) and some research (as yet unpublished) suggests that in the clinical environment, they may be significantly lower.
To achieve a particular US intensity at depth, account must be taken of the proportion of energy which has been absorbed by the tissues in the more superficial layers. The table gives an approximate reduction in energy levels with typical tissues at two commonly used frequencies, and more detailed information is found in the dose calculation material
As the penetration (or transmission) of US is not the same in each tissue type, it is clear that some tissues are capable of greater absorption of US than others. Generally, the tissues with the higher protein content will absorb US to a greater extent, thus tissues with high water content and low protein content absorb little of the US energy (e.g. blood and fat) whilst those with a lower water content and a higher protein content will absorb US far more efficiently. Tissues can be ranked according to their relative tissue absorption and this is critical in terms of clinical decision making (Watson, 2008).
The application of therapeutic US to tissues with a low energy absorption capacity is less likely to be effective than the application of the energy into a more highly absorbing material. Recent evidence of the ineffectiveness of such an intervention can be found in Wilkin et al (2004) and Markert et al (2005) whilst application in tissue that is a better absorber will, as expected, result in a more effective intervention (e.g. Sparrow et al 2005, Leung et al 2004).
The effects of pulsed US are well documented and this type of output is preferable especially in the treatment of the more acute lesions. Some machines offer pulse parameters that do not appear to be supported from the literature (e.g. 1:9; 1:20). Some manufacturers describe their pulsing in terms of a percentage rather than a ratio (1:1 = 50% 1:4 = 20% etc). The proportion of time that the machine is ON compared with OFF is a relevant factor in dosage calculations and further details are included in the dose calculation support material.
Therapeutic Ultrasound & Tissue Healing
One of the therapeutic effects for which ultrasound has been used is in relation to tissue healing. It is suggested that the application of US to injured tissues will, amongst other things, speed the rate of healing & enhance the quality of the repair (Watson 2006). The following information is intended to provide a summary of some of the essential research in this field together with some possible mechanisms through which US treatments may achieve these changes. It is not intended to be a complete explanation of these phenomena or a comprehensive review of the current literature. It may, none the less, provide some useful basic information for clinical application.
The therapeutic effects of US are generally divided into: THERMAL & NON‑THERMAL.
In thermal mode, US will be most effective in heating the dense collagenous tissues and will require a relatively high intensity, preferably in continuous mode to achieve this effect.
Many papers have concentrated on the thermal effectiveness of ultrasound, and much as it can be used effectively in this way when an appropriate dose is selected (continuous mode >0.5 W cm-2), the focus of this paper will be on the non thermal effects. Both Nussbaum (1998) and ter Haar (1999) have provided some useful review material with regards the thermal effects of ultrasound. Comparative studies on the thermal effects of ultrasound have been reported by several authors (e.g. Draper et al 1993, 1995a,b, Leonard et al 2004) with some interesting, and potentially useful results. Further work continues in our research centre with a comparison of contact heating and longwave ultrasound (Meakins and Watson, 2006) and comparison of different US regiemes combined with US (Aldridge and Watson – in preparation)
It is too simplistic to assume that with a particular treatment application there will either be thermal or non thermal effects. It is almost inevitable that both will occur, but it is furthermore reasonable to argue that the dominant effect will be influenced by treatment parameters, especially the mode of application i.e. pulsed or continuous. Baker et al (2001) have argued the scientific basis for this issue coherently.
Lehmann (1982) suggests that the desirable effects of therapeutic heat can be produced by US. It can be used to selectively raise the temperature of particular tissues due to its mode of action. Among the more effectively heated tissues are periosteum, collagenous tissues (ligament, tendon & fascia) & fibrotic muscle (Dyson 1981). If the temperature of the damaged tissues is raised to 40‑45°C, then a hyperaemia will result, the effect of which will be therapeutic. In addition, temperatures in this range are also thought to help in initiating the resolution of chronic inflammatory states (Dyson & Suckling 1978). Most authorities currently attribute a greater importance to the non‑thermal effects of US as a result of several investigative trials in the last 15 years or so.
The non‑thermal effects of US are now attributed primarily to a combination of CAVITATION and ACOUSTIC STREAMING (ter Haar 1999, 2008 Baker et al 2001, Williams 1987). There appears to be little by way of convincing evidence to support the notion of MICROMASSAGE though it does sound rather appealing.
CAVITATION in its simplest sense relates to the formation of gas filled voids within the tissues & body fluids. There are 2 types of cavitation ‑ STABLE & UNSTABLE which have very different effects. STABLE CAVITATION does seem to occur at therapeutic doses of US. This is the formation & growth of gas bubbles by accumulation of dissolved gas in the medium. They take apx. 1000 cycles to reach their maximum size. The `cavity' acts to enhance the acoustic streaming phenomena (see below) & as such would appear to be beneficial. UNSTABLE (TRANSIENT) CAVITATION is the formation of bubbles at the low pressure part of the US cycle. These bubbles then collapse very quickly releasing a large amount of energy which is detrimental to tissue viability. There is no evidence at present to suggest that this phenomenon occurs at therapeutic levels if a good technique is used. There are applications of US that deliberately employ the unstable cavitation effect (High Intensity Focussed Ultrasound or HIFU) but it is beyond the remit of this summary.
ACOUSTIC STREAMING is described as a small scale eddying of fluids near a vibrating structure such as cell membranes & the surface of stable cavitation gas bubble (Dyson & Suckling 1978). This phenomenon is known to affect diffusion rates & membrane permeability. Sodium ion permeability is altered resulting in changes in the cell membrane potential. Calcium ion transport is modified which in turn leads to an alteration in the enzyme control mechanisms of various metabolic processes, especially concerning protein synthesis & cellular secretions.
MICROMASSAGE is a mechanical effect which appears to have been attributed less importance in recent years. In essence, the sound wave travelling through the medium is claimed to cause molecules to vibrate, possibly enhancing tissue fluid interchange & affecting tissue mobility. There is little, if any hard evidence for this often cited principle.
Ultrasound Application in relation to Tissue Repair
The process of tissue repair is a complex series of cascaded, chemically mediated events that lead to the production of scar tissue that constitutes an effective material to restore the continuity of the damaged tissue. The process is more complex than be described here, but there are several interesting recent papers and reviews including (Wener & Grose 2003, Toumi & Best 2003, Watson 2003, 2006, Hill et al 2003, Neidlinger-Wilke et al 2002, Lorena et al 2002, Latey 2001).
Employed at an appropriate treatment dose, with optimal treatment parameters (intensity, pulsing and time), the benefit of US is to make as efficient as possible to earliest repair phase, and thus have a promotional effect on the whole healing cascade. For tissues in which there is an inflammatory reaction, but in which there is no ‘repair’ to be achieved, the benefit of ultrasound is to promote the normal resolution of the inflammatory events, and hence resolve the ‘problem’ This will of course be most effectively achieved in the tissues that preferentially absorb ultrasound – i.e. the dense collagenous tissues.
The application of therapeutic ultrasound can influence the remodelling of the scar tissue in that it appears to be capable of enhancing the appropriate orientation of the newly formed collagen fibres and also to the collagen profile change from mainly Type III to a more dominant Type I construction, thus increasing tensile strength and enhancing scar mobility (Nussbaum 1998, Wang 1998). Ultrasound applied to tissues enhances the functional capacity of the scar tissues (Nussbaum 1998, Huys et al 1993, Tsai et al 2006, Yeung et al 2006). The role of ultrasound in this phase may also have the capacity to influence collagen fibre orientation as demonstrated in an elegant study by Byl et al (1996), though their conclusions were quite reasonably somewhat tentative.
The application of ultrasound during the inflammatory, proliferative and repair phases is not of value because it changes the normal sequence of events, but because it has the capacity to stimulate or enhance these normal events and thus increase the efficiency of the repair phases (ter Haar 99, Watson 2007, 2008, Watson & Young, 2008). It would appear that if a tissue is repairing in a compromised or inhibited fashion, the application of therapeutic ultrasound at an appropriate dose will enhance this activity. If the tissue is healing ‘normally’, the application will, it would appear, speed the process and thus enable the tissue to reach its endpoint faster than would otherwise be the case. The effective application of ultrasound to achieve these aims is dose dependent.
There are a growing number of ‘other applications’ for ultrasound energy ranging from tumour ablation – using High Intensity Focussed Ultrasound (or HIFU) though to stimulated related of encapsulated systemic drugs. These are largely beyond the scope of this review which is mainly concerned with tissue repair issues. There are however some useful applications to briefly consider. Shockwave Therapy (which is a variation on the ultrasound theme) is also covered by a wide range of literature. Well beyond the scope of this summary.
US for Fracture Repair – there is a wealth of research information in this area (summarised on the www.electrotherapy.org web pages if you want the detail). Essentially, the application of very low dose ultrasound over a fracture (whether healing normally or delayed or non union) can be of significant benefit. The main clinical issue is that the effective ‘dose’ is actually lower than most therapy machines can deliver – which is frustrating! Higher intensity ultrasound over a fracture can initiate a strong pain response – which is useful when it comes to using the modality to locate potential stress fractures.
LIPUS (Low Intensity Pulsed Ultrasound) is another variation on the standard US theme that is gaining ground. Like the fracture healing work cited above, it uses a much lower dose than ‘usual’ but in a broader arena – mainly soft tissue work. If low intensity US is effective in stimulating fracture repair, then what effect could it have on other soft tissue work? Several studies are emerging (keep you eye on Electrotherapy News for updates).
The use of US at trigger points has been used clinically for some time and is well supported from the anecdotal evidence. A recent study by Srbely et al (2007) raises some interesting points and demonstrates a measurable benefit (reviewed in Electrotherapy News Vol 3 Issue 4 if you want a quick update).
As a matter of (clinical) interest, the US treatment should be cleaned with an alcohol based swab (not just wiped with tissue) between treatments (Schabrun et al, 2006) to minimise the potential transmission of microbial agents between patients.
In addition to the reflection that occurs at a boundary due to differences in impedance, there will also be some refraction if the wave does not strike the boundary surface at 90°. Essentially, the direction of the US beam through the second medium will not be the same as its path through the original medium - its pathway is angled. The critical angle for US at the skin interface appears to be about 15°. If the treatment head is at an angle of 15° or more to the plane of the skin surface, the majority of the US beam will travel through the dermal tissues (i.e. parallel to the skin surface) rather than penetrate the tissues as would be expected.
Recent Related Research (from Pubmed)
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Evidence Based Practice
Learn about Evidence Based Practice in this month's members learn topic with book chapters from Practical Evidence-Based Physiotherapy 2012 & Evidence-Based Practice Across the Health Professions 2013