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Technical Reports |
VJ Robertson, PT, PhD, is Professor, Teaching & Research Unit, Gosford Hospital, Bldg 14, Corner of Holden Rd and Racecourse Rd, Gosford, New South Wales 2250, Australia
Address all correspondence to Dr Robertson at: Val.Robertson{at}newcastle.edu.au
 | Why ERA Variability Is a Concern |
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The study by Straub et al was a thorough testing of 66 multifrequency transducers, from 6 manufacturers, used at a frequency of 1 MHz. The brands are well known, and the findings are relevant to clinicians. Straub et al calibrated their equipment prior to the study, as expected. This omission in many clinics could compound the extent of difference identified in the outputs of different brands of equipment. This highlights the need for regular calibrations of ultrasound equipment, as well as routine testing for electrical safety.2
A minor quibble—I am not convinced of the merit of discussions of old equipment studies in the present context. For example, studies conducted in 1982 and using Australian standards from 1969 are now irrelevant. Ultrasound equipment and manufacturing processes and standards have changed so much since then. Similarly, a reference to a 1994 study as being of "newly styled transducers" is outdated, given that they are now at least 14 years old.
The "elephant in the room" with the study by Straub et al is, of course, the relevance of output. As long as it is not too hot and does not burn patients, does variance in the output of therapeutic ultrasound really matter? Straub et al argue that it does, despite describing the effectiveness of ultrasound as "a topic of debate." Apologies to Straub and colleagues are due at this point. The focus of this commentary is the question of how much their findings matter, and not the technical details of their study.
There are 2 parts to my response to the question of how much variance in ultrasound output really matters. The first part notes the continuing problems with evidence supporting the use of ultrasound, and the second part relates to the clinical implications of the study by Straub et al, given what we know of how ultrasound is used.
| Evidence and Use of Therapeutic Ultrasound |
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One underlying problem is the continuing lack of evidence of a dose-response relationship for therapeutic ultrasound. That means that no one can accurately prescribe an appropriate dose or say how often ultrasound should be administered or for how long.7 By contrast, despite the relatively short period for which it has been used, a lot is already known about low-intensity pulsed ultrasound (LIPUS). The effects on fractures were very well described in a series of 1,317 human cases of nonuniting fractures published 8 years ago.8 Since then, research has shown the benefits of longer duration applications when treating osteochondral defects in rabbits.9 Research with femoral fractures in rats showed that the equipment physical therapists use, even with a higher BNR, also can produce faster rates of union.10 The implications for humans remain unknown at this stage, but we do know the parameters of a consistently effective dosage that is appropriate for treating a range of fractures and something of the implications of changing 2 of these parameters—duration of application and BNR.
Studies of LIPUS make its contributions to the treatment of various types of fractures and stages after fracture clear. Although its method of action has yet to be fully understood, that it is effective is well known. Parameters of an effective dosage have been identified. Time will tell whether further studies can better tailor the parameters to individuals with fractures.
The lack of an equivalent level of evidence supporting present uses of therapeutic ultrasound is telling. With more than 70 years of clinical uses of megahertz-frequency ultrasound behind us, perhaps it is time to acknowledge that, as used by most clinicians, it is not effective most of the time. Even convincing evidence of the relative benefits of ultrasound to facilitate soft tissue healing or stretching is still missing.
This does not mean that the performance of ultrasound equipment is unimportant. To the contrary, although there are benefits of any kind to therapists in continuing to use it,11 there are reasons to be concerned. Furthermore, time may well show that some existing equipment can be used similarly to that purpose-designed for LIPUS. In that case, we will need reliable knowledge of its output, particularly when the spatial-averaged temporal-averaged (SATA) intensity must be limited.
| Clinical Significance of Differences |
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The short answer is "yes," we should have concerns, and for 2 reasons: first, we have no idea of the number of adverse events from using ultrasound, and, second, there are existing reasons to be concerned about how it is applied.
The most common adverse events associated with therapeutic ultrasound are probably skin burns from applicators used in a manner that makes them excessively hot. Anecdotally, many physical therapists are aware that small, superficial burns do occur. However, in the absence of hard evidence, the extent and number are conjecture. In addition, not all therapists check, and, if an adverse event was identified, there is no central clearing house collating reports of them.
If the measured output of a brand or a model from a company is markedly at variance with the indicated output, the risk of patient damage must change. Therapists could modify the dosage to minimize the risk, but this would not address the broader concerns about how ultrasound is applied.
The equipment tested was multifrequency equipment. That means that different frequencies can be output via the same transducer, usually 1 and 3 MHz. This option raises issues of how ultrasound is applied.
Ultrasound at 1 and 3 MHz often is applied using the same dosage. This was apparent in the collapsing of dosage data provided in a recent survey of orthopaedic certified specialists.12 There is a considerable difference in the half-value depth of the 2 frequencies. In muscle, if the ultrasound frequency is 1 MHz, the half-depth value is 12 mm, and if the ultrasound frequency is 3 MHz, the half-depth value is 4 mm.13(p260) Similar ratios of half-depth values occur for other tissues. The very short wavelength of ultrasound at a frequency of 3 MHz means that it is much more easily absorbed, and in a much smaller volume of tissue, than is ultrasound at a frequency of 1 MHz. This higher rate of absorption is the other face of the low penetration depth.
Concerns with the dosages that therapists use clinically have recently been raised elsewhere.14 Additional reasons also are evident in the 2 studies that Straub et al described as providing evidence of differences in equipment from different manufacturers. One study15 tested 3 brands of 3-MHz transducers and measured changes in temperature at a depth of 1.6 cm in the medial head of the gastrocnemius muscle. The findings of that well-controlled study show considerable differences in the extent of heating produced by one brand. Straub et al indicated that this was partly due to differences in the SATA intensity and to the different surface areas of transducers, given that a standard dose covers twice that.16
The more probable explanation is much simpler. A SATA dosage of 1.5 W/cm2 was applied through all 3 transducers. The resultant total power applied with an ERA of 5 cm2 is 7.6 W (1.5 W/cm2 x 5 cm2), and if applied with an ERA of 6.7 cm2, is 29.4% higher at 10.05 W. Given that a much higher power was applied to a very similar volume of tissue under the same conditions, it is not surprising that the heating also was higher with the larger ERA. This is really the same problem as Straub et al reported: the size of the ERA "affects ultrasound treatment intensity." A different point, one relevant to discussions of LIPUS versus 1-MHz standard ultrasound equipment, is that transducers with an identical ERA produced very similar changes in tissue temperature over time, despite having different BNRs.
Similar problems existed in another study that Straub et al used to support their contentions regarding different brands of equipment. The other study,17 also well controlled, applied ultrasound at frequencies of 1 and 3 MHz and measured temperature changes at a depth of 2.5 cm in the medial head of the gastrocnemius muscle. The same SATA intensity of 1.5 W/cm2 was applied at both frequencies. The heating produced by the 3-MHz equipment was considerably greater even at 2.5 cm. The researchers reported being surprised by this, but it was to be expected. The power was the same but, when applied at the 3-MHz frequency, would have been absorbed in a volume approximately one third of that attained when using the 1-MHz frequency. Thus, excessive heating of the skin and a greater temperature rise at 2.5 cm was to be expected, even in perfused tissue. The same level of applied energy would have been absorbed in approximately one third of the volume of tissue. Factors such as the presence, shape, and depth of bone; structures containing air; or indwelling reflective metal objects will complicate knowing the distribution of energy in the tissues, but the same differential rates of absorption will still apply.
Both studies that Straub et al used raise issues of how widely the biophysical properties of ultrasound are known. This is consistent with an acknowledgment by Wong et al18 that some respondents in their study were possibly using excessively high spatially averaged intensities of ultrasound at a frequency of 3 MHz. Respondents in that study used very similar SATA intensities, regardless of the frequency. The need to decrease the intensity of ultrasound at a frequency of 3 MHz to approximately one third of that of ultrasound at a frequency of 1 MHz has been well described elsewhere.13(p286) Even an apparent assumption that the SATA intensities applied for ultrasound at frequencies of 1 and 3 MHz are interchangeable is a concern.
Misconceptions about dosage apparently abound, despite the biophysical properties of ultrasound being well described for many years. The magnitude of the problem is obvious if we accept the figures supplied by Straub et al (1-MHz frequency, SATA of 1 W/cm2, applied for 10 minutes, measured at 2.5-cm depth in tissue) and the heating rates of Draper et al.19 If the tested equipment was used at the 3-MHz frequency, with the same SATA intensities, the temperature increases could be nearly 6 times higher than indicated. In practice, this is not the case, as the tests that the authors conducted of the same equipment with the alternate frequency show.16 This is the nub of the problem. Even with calibrated equipment, there could be a considerable difference between the total power and the assumed power. Depending on the frequency selected and the knowledge of the user, this differential can be clinically very important.
My concern, therefore, is with users who seem unaware of the implications of using different frequencies of ultrasound. This does not bode well when using multifrequency equipment, especially when the power may differ considerably from what the set intensity implies. Combined with our lack of knowledge of the relative risk and extent of adverse events from using ultrasound, this provides grounds for concern with how ultrasound is used.
| Summary |
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We should all be concerned though with the findings of both equipment studies1,16 and consider carefully what they mean for us as a profession. The lack of a body of evidence clearly supporting the effectiveness of therapeutic ultrasound after approximately 70 years of use is one issue. This contrasts starkly with evidence of effectiveness of LIPUS for treating fractures. Despite its relatively recent introduction, an effective dosage has already been identified and some parameters systematically explored. By contrast, therapeutic ultrasound continues to be used, albeit for a range of reasons,11 many of which are seemingly inconsistent with us being an evidence-based profession.
Why else should the findings of Straub et al be of interest? The other reasons concern how therapeutic ultrasound is applied and how many users understand the implications of selecting one or the other frequency. Wong et al12,14 recently discovered that many certified orthopaedic specialists use ultrasound with a dubious range of dosages that do not necessarily account for the implications of changing frequency. This is possibly a widespread practice, but the finding of an unexpected degree of variance in the output of different transducers makes using ultrasound a greater hazard than usually expected.
In conclusion, the findings of the study by Straub et al should act as a reminder to all users of therapeutic ultrasound: frequent calibrations by appropriately qualified electromedical technicians are even more important than many may have guessed in limiting problems with the power. Having a very different ERA from what is expected can mean that a much greater power is applied than expected by a clinician. This can be compounded by having uncalibrated equipment. Clinicians also should take this opportunity to reconsider when and why they use therapeutic ultrasound, given the lack of evidence that it improves patients’ outcomes as used now.
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