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 Waves :
FREQUENCY - the number of times a particle experiences a complete compression/rarefaction cycle in 1 second. Typically 1 or 3 MHz.
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).
As an example, a crystal with a diameter of 25mm operating at 1 MHz will have a near field/far field boundary at : Boundary = 12.5mm2/1.5mm » 10cm thus the near field (with greatest interference) extends for approximately 10 cm from the treatment head when using a large treatment head and 1 MHz US. When using higher frequency US, the boundary distance is even greater. Beyond this boundary lies the Far Field or the Fraunhofer zone. The US beam in this field is more uniform and gently divergent. The ‘hot spots’ noted in the near field are not significant. For the purposes of therapeutic applications, the far field is effectively out of reach.
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.