Ultrasound Gels and Coupling Agents
Ultrasound Gel Transmissivity Studies
There has been a substantial debate over the years with regards the efficacy of several types of coupling media used with therapeutic ultrasound.
Some of the following material is abstracted from the Ultrasound handout (available from the DOWNLOAD page)
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 the same. 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.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 to body. 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 which will be reflected approaches 99.998% which in effect means that there will be no transmission.
The primary job of the coupling medium is to facilitate transmission of the ultrasound energy from the machine head to the tissues. Given an ideal circumstance, this transmission would be maximally effective with NO absorption of the US energy, nor any distortion of its path etc. This 'ideal' is almost impossible to achieve, but the type of coupling medium employed does make a difference.
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 2006) - see below.
Ultrasound Transmissivity through Coupling Agents (Gels) in the UK
We have recently completed a study of the commonly employed gels (in the UK), evaluating differences in their behaviour and characteristics. Using a carefull managed lab set up using a power balance, we tested a variety of the available gels, comparing their transmission characteristics with water.
The full data and experimental description can be found at :
Poltawski, L. and T. Watson (2007). "Relative transmissivity of ultrasound coupling agents commonly used by therapists in the UK." Ultrasound Med Biol 33(1):120-8.
The gels were tested at power levels and at frequencies that reflect commonly applied parameters in the clinical setting - some of the previous work has used parameters to do not reflect clinical practice.
The gel thickness was evaluated as it would reasonably be expected to make a difference to energy transmission. Testing with gel thicknesses betwee 0.2 and 6mm, we were able to identify a difference, but at the 'thinnest' two (0.2 and 1.5mm) we were unable to detect any clinically significant difference. Certainly increasing the thickness of the gel will diminish the ultrasound energy reaching the patient, and thus it is suggested that you should not make the gel any thicker than it needs to be in order to replace the air gap - the thicker the gel, the less ultrasound reaches the patient.
In terms of the main results, the relative transmission of various gels (compared with degassed water) are shown in the table below (full data available in the paper). It can be seen that the 'rank order' of te gels varies with the two US frequencies tested (1.1 and 3.4MHz).
At the end of the day, there are differences between the gels available from different manufacturers, BUT these differences are not clinically significant - the accuracy of the machines used in practice means the the very small differences between the gels are not significant. The Biofreeze was tested as an 'extra' - but the data suggests that there is a marked difference between the standard gels and Biofreeze.
Ultrasound Transmission through Wound Dressings
In a subsequent paper, we describe the relative transmission of different wound dressings for ultrasound energy. The use of ultrasound as a wound management intervention is well documented, and it is argued that IF a particular dressing is an effective transmitter of ultrasound energy, it would be possible to apply the therapy through the dressing (which has several significant clinical advantages). A full description of the rationale, method and detailed results are to be found in :
Poltawski, L. and T. Watson (2007). "Transmission of therapeutic ultrasound by wound dressings." Wounds 19(1): 1-12.
48 different dressing were tested, and the results are (as one might expect) complex, but there were certainly BIG differences in transmission characteristics between them - ranging from excellent (over 100% compared with water) through to zero (no ultrasound passed through the dressing material). Some of the dressings appeared to change both appearance and behaviour as the ultrasound was applied. Some dressing combinations were also evaluated (as this reflects current clinical practice).
It is very difficult to make general statements about dressing types and their overall characteristics. Dressings from the same 'group' (films, gels, alginates, foams) behaved very differently, and therefore it is not possible to say with any degree of certainty that the 'gel' or the 'film' dressings are better. It is suggested that each dressing type should be considered on an individual basis. Clearly, there are more than 48 types of dressings out there, but at least this data provides a significant insight into the key issues and provides information for those considering the use of ultrasound (therapeutic or diagnostic) on chronic wounds through a dressing material.
Many thanks to Leon Poltawski for his sterling efforts in making both of these trials scientifically sound and for developing a quality methodology.
Ultrasound Transmission through Drug Based Gels
Many therapists use gels which include an active compound (like ibuprofen) with suggested clinical advantage. There are many arguments why this approach might or might not be clinically effective, but we are trying to replicate the ultrasound gel transmission study with a range of NSAID impregnated gels. In general terms, one would expect less energy to reach the tissues with these gels (i.e. the transmissivity would be poorer than for 'standard' gels), and we will be publishing the results just as soon as they are available - watch this space.