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).
Ultrasound Dose Calculations
The most straightforward way to work out a particular dose of ultrasound for an individual patient is to use the combined available evidence and the flowchart below is based on just that. Following the flowchart is a piece of text that explains the process in some more detail.
Ultrasound for Fracture healing
Numerous recent papers have identified the benefits of using therapeutic ultrasound for both normally healing (fresh) fractures and those that demonstrate either a delayed union or non union (e.g. Mayr et al 2000, Busse et al 2002, Warden et al 1999). Ultrasound has been historically considered to be a contraindication is these circumstances, though the exact reason for this remains unclear. Given the volume and quality of the published evidence, it would be entirely inappropriate for fractures to remain on the contraindication list.
A recent systematic review and meta-analysis (Busse et al 2002) has carefully considered the evidence in respect to the effect of low intensity pulsed ultrasound on the time to fracture healing. They conclude that the evidence from randomised trials where the data could be pooled (3 studies, 158 fractures) that the time to fracture healing was significantly reduced in the ultrasound treated groups than in the control groups and the mean difference in healing time was 64 days.
Warden et al (1999) published a review paper concluded that from animal and human studies, the use of ultrasound could accelerate the rate of fracture repair by a factor of 1.6. The unit utilised for this work (Sonic Accelerated Fracture Healing System – SAFHS) delivers a low intensity (0.03 W cm-2) at 1.5MHz pulsed at a ratio of 1:4. Whilst this dose may be reproducible by standard therapeutic machines, the SAFHS device has a particularly low BNR and thus is considered to be safe to apply with a stationary treatment head, unlike conventional physiotherapy ultrasound machines. This could be an important factor as treatment was for 20 minutes daily, with the patient using the device rather than attending for therapy.
Ultrasound Gels and Coupling Agents
There has been a substantial debate over the years with regards the efficacy of several types of coupling media used with therapeutic ultrasound.
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.
Ultrasound in the Treatment of Apomorphine Nodules
Infused apomorphine – used in the treatment of Parkinson’s Disease – can cause formation of nodules and general hardening of tissue at the sites of administration, usually the lower abdomen or upper thigh. These tissue changes may make insertion of the infusion needle difficult, and may affect absorption of the drug. There are reports of the nodules being successfully treated with therapeutic ultrasound (US), but as yet there are no evidence-based guidelines for the optimal frequency or treatment duration. We have two sources of information for the guidance we give: anecdotal evidence from clinicians who have used ultrasound to treat nodules; and the findings of a pilot clinical trial we have conducted.