Therapeutic Ultrasound Electrotherapy

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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 though there are devices which operate in the kHz range see comments on Low. Frequency Longwave Ultrasound at the end of this paper. 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. 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. where V velocity F frequency and is the wavelength. ULTRASOUND BEAM NEAR FIELD FAR FIELD AND BEAM NON UNIFORMITY. 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 Example of an Ultrasound Beam Plot. r2 where r the radius of the transducer crystal and. 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. Therapeutic Ultrasound Tim Watson 2015 Page 2, intensity will be 4 or 6 times greater than the mean intensity It is considered inappropriate to use a device. with a BNR value of 8 0 or more 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, Some recent papers Alvarenga et al 2010 Gutierrez et al 2010 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. 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 Docker et al 1982 Griffin 1980 Klucinec et al 2000 and Williams 1987 At the present time the gel based. media are preferable to the oils and creams Water is an effective 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, Bacterial Contamination of Ultrasound Treatment Heads and Gel Sources.
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 Spratt et al 2014 sampled ultrasound treatment heads and gel bottles in. the USA reporting over 50 of gel bottles with contamination some of which were positive for MRSA Some. 35 of the ultrasound treatment heads tested also demonstrated contamination though none with MRSA. The authors report that employment of adequate disinfection techniques was effective in significantly. reducing these levels they used Protex Parker Laboratories. Therapeutic Ultrasound Tim Watson 2015 Page 3,Ultrasound Application The Critical Angle. 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,ULTRASOUND 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, 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 1995,1 MHz 3 MHz,Muscle 9 0 mm 3 0 mm,Fat 50 0 mm 16 5 mm.
Tendon 6 2 mm 2 0 mm, 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. Depth cm 3 MHz 1 MHz,1 MHz 4 0 cm, These values after Low Reed are not universally 4 25 50. accepted see Ward 1986 and some research as yet 6, unpublished suggests that in the clinical environment 8 25. 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 and on the web pages www electrotherapy org. Therapeutic Ultrasound Tim Watson 2015 Page 4,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.
The physiological effects of ultrasound are almost. identical to those of Pulsed Shortwave and Laser,therapy the key difference however is that. ultrasound energy is preferentially absorbed in,different tissue to the other modalities as. summarised in the adjacent diagram,PULSED ULTRASOUND. 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,Therapeutic Ultrasound Tim Watson 2015 Page 5.
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, Mode Pulse Ratio Duty Cycle Some manufacturers describe their. Continuous N A 100 pulsing in terms of a percentage. Pulsed 1 1 50 rather than a ratio 1 1 50 1 4,1 2 33 20 etc An equivalence table is. 1 3 25 provided for convenience 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. Pulse Frequency, A point of confusion amongst many therapists is the frequency facility offered on some ultrasound machines.
The pulse ratio duty cycle is at say 1 4 20 but there is an option to alter the pulse frequency i e how. many ultrasound pulses are delivered per second This is achieved by adjusting the DURATION of the pulses. Typically these are at 2ms thus on a 1 4 ratio the machine is ON for 2ms and then OFF for 8ms It takes 10ms. There are myriad therapy ultrasound machines available from the small portable devices through to the multimodal machines which include ultrasound as one of the available options examples are illustrated below Dedicated Ultrasound machine EMS Physio Multi modal machine which includes Ultrasound DJO Chatanooga Portable Ultrasound device

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