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.
The coupling media used in this context include water, various oils, creams and gels. Ideally, the coupling medium should be fluid so as to fill all available spaces, relatively viscous so that it stays in place, have an impedance appropriate to the media it connects, and should allow transmission of US with minimal absorption, attenuation or disturbance. For a good discussion regarding coupling media, see Casarotto et al 2004, Klucinec et al 2000, Williams 1987 and Docker et al 1982. At the present time the gel based media appear to be preferable to the oils and creams. Water is a good media and can be used as an alternative but clearly it fails to meet the above criteria in terms of its viscosity. There is no realistic (clinical) difference between the gels in common clinical use (Poltawski and Watson 2007). The addition of active agents (e.g. anti-inflammatory drugs) to the gel is widely practiced, but remains incompletely researched. We are currently evaluating this intervention further.
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.
Absorption and Attenuation :
The absorption of US energy follows an exponential pattern - i.e. more energy is absorbed in the superficial tissues than in the deep tissues. In order for energy to have an effect it must be absorbed, and at some point this must be considered in relation to the US dosages applied to achieve certain effects (ter Haar, 1999, Watson, 2008, Watson and Young, 2008).us exponential
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 below 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
3 MHz 2.0 cm
1 MHz 4.0 cm
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).
Although cartilage and bone are at the upper end of this scale, the problems associated with wave reflection mean that the majority of US energy striking the surface of either of these tissues is likely to be reflected. The best absorbing tissues in terms of clinical practice are those with high collagen content – LIGAMENT, TENDON, FASCIA, JOINT CAPSULE, SCAR TISSUE (Watson 2000, 2008, Watson & Young, 2008, ter Haar 1999, Nussbaum 1998, Frizzel & Dunn 1982)
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).
Most machines offer the facility for pulsed US output, and for many clinicians, this is a preferable mode of treatment. Until recently, the pulse duration (the time during which the machine is on) was almost exclusively 2ms (2 thousandths of a second) with a variable off period. Some machines now offer a variable on time though whether this is of clinical significance has yet to be determined. Typical pulse ratios are 1:1 and 1:4 though others are available (see dose calculations). In 1:1 mode, the machine offers an output for 2ms followed by 2ms rest. In 1:4 mode, the 2ms output is followed by an 8ms rest period. The adjacent diagram illustrates the effect of varying the pulse ratio.
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.
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