By Christopher J. Joyce, PhD, ATC, CSCS
Therapeutic ultrasound (US) is one of the most common physical agents used in rehabilitation. Although in use for the past 50 years, its popularity as a treatment modality continues to increase.1,2 Ultrasound is most commonly used to treat pain and musculoskeletal and soft tissue injury, as well as healing wounds and reducing scar tissue.2,3 It can be used to generate both thermal and mechanical changes in tissues to promote and accelerate healing.
A variety of issues come into play when purchasing and using an US unit. These units come with a variety of options, some that add value to the machine and others that are just bells and whistles. The cost associated with the purchase and upkeep of US units can be relatively expensive, thus it is important to fully understand the function and use of this modality.
ULTRASOUND BASICS
Audible sound waves occur at frequencies between 16 kHz and 20 kHz, whereas therapeutic US waves have frequencies ranging from 75 kHz to 3 MHz.3 Ultrasound is produced via the reverse piezoelectric effect.1 Essentially, when an alternating electrical current is passed through a piezoelectric crystal, the crystal expands and contracts at the frequency of the electrical signal.1,3 Thus, the oscillations of the crystal create the US waves that are transmitted to the tissue being treated.
Piezoelectric crystals can be made of quartz (natural or artificial), ceramics, or other types of material,4 and the size and quality of crystals can vary greatly. The effective radiating area (ERA) conducts US from the transducer to the tissue. The ERA transmits at least 5% of the generated US waves.3,5 The ERA is always smaller than the transducer faceplate, therefore the ERA should be very close to the sound head in size.3,6 This is important when the size of the treatment area is taken into consideration. The treatment area should be no larger than two times the size of the transducer head.3,7 However, some larger sound heads may have only a 5 cm2 crystal.
The most common measure of piezoelectric crystal quality is the beam nonuniformity ratio or BNR. The ultrasound waves created by the crystal are not uniform across the surface of the US head; there are varying areas of intensity that create peaks and valleys in the US beam. The BNR is the ratio of the spatial peak intensity to the spatial average intensity.5,8 Areas of high intensity increase the likelihood of developing “hot spots” within the treatment area, thus the lower the BNR, the more uniform the US beam. Ultrasound units with BNR ratios between 2:1 and 6:1 appear to be clinically acceptable,3,5,8 although some US units may have a BNR as high as 8:1.3 Since BNR is an indication of the quality of the US beam, units with a lower BNR are typically more expensive than units with a greater BNR.
For ultrasound to be an effective agent, the US wave must be transmitted from the unit to the tissue via a conducting medium. The most common transmission mediums include US gel, mineral oil, lotions, gel pads, and water. Ultrasound gels and pads appear to be the best conducting mediums5; however, most gels and creams have not been clinically tested for the quality of their conduction capability. Ultrasound treatments conducted when the target tissue is immersed in water are often used for areas with irregular surfaces where it is difficult to maintain contact on the treatment area. However, immersed ultrasound is not as effective as ultrasound applied directly to the tissue via gel.3,5
Most US units produced today deliver acoustic energy at frequencies between 1 MHz and 3 MHz, although some units may have the capacity to deliver US at other frequencies. The frequency of the US wave is an important treatment consideration because the sound wave frequency will affect the depth within the tissue that the US is focused. As US frequency increases, more energy is absorbed in the tissue. Ultrasound delivered at lower frequencies tends to travel through superficial tissue and is absorbed at a greater depth. Thus, superficial structures (1 cm-2 cm deep) should be treated with higher frequency US such as 3 MHz, whereas lower frequencies should be incorporated when the target tissue is deeper (2 cm-5 cm).
Another consideration with ultrasound is the type of tissue being treated, or what tissue the US must travel through to reach the target. Ultrasound energy is absorbed at different rates by different tissues and this is related to both the water and protein content of the tissue.5 Skin and adipose tissue absorb less acoustic energy than muscle, tendon, and ligament. Nerve tissue and bone absorb the greatest amount of US energy. Therefore, you should consider not only the depth of the tissue to be treated, but also the type of tissue when determining treatment parameters.
You can, through different means, manipulate the dose of US delivered. One way to vary the amount of energy is to change the actual time that the US is delivered. To generate thermal effects in the tissue, US is typically delivered continuously, so that US waves are being delivered 100% of the time. However, you can choose to interrupt delivery of the US energy, which is referred to as pulsed ultrasound. With pulsed US, the acoustic waves are delivered in intermittent “on” and “off” cycles or the duty cycle. The duty cycle is usually reported as a ratio of “on” time to the total time a pulse is delivered.9 If the duty cycle is set at 25% and the total pulse time is 10 milliseconds, the US unit is delivering energy for 2 milliseconds and is off for 8 milliseconds. The higher the duty cycle, the more energy will be delivered. Conversely, the lower the duty cycle, the less energy will be delivered.
The intensity of the US energy can also be manipulated. The intensity determines the amplitude of the US wave and is measured in watts. The higher the amplitude, the greater the amount of acoustic energy delivered. The intensity is also a function of the area where energy is delivered, thus the intensity is measured in watts per square centimeter (W/cm2).3,5 Therapeutic US is typically administered at intensities between .75 W/cm2 and 3 W/cm2 .3,5,7,9 The tissue being treated, frequency, duty cycle, treatment time, and the desired effects of the ultrasound will influence the intensity of the treatment.
THE RIGHT UNIT FOR YOU
There are many manufacturers of US units and most of these companies have several different units to choose from. One of the first questions you should consider is in what capacities the unit will be used and what types of conditions you will be treating. This can help simplify the decision-making process when looking for a unit that will best meet your needs and those of your patients.
Since the BNR is an indication of the quality of the piezoelectric crystal, this is an important factor, although recent research suggests that a lower BNR may not always produce better therapeutic effects.10 As mentioned previously, machines with lower BNR are typically more expensive, so you need to weigh the value of a lower BNR and more uniform treatment versus the cost of the unit.
The range of frequencies a US unit can produce is another issue. Units that deliver US only at one frequency will significantly limit the conditions and tissue that can be treated. Most units can deliver US at both 1 MHz and 3 MHz, and newer units may also have 2 MHz as a frequency option. Ultimately, the more frequencies the unit can deliver, the greater the flexibility of the machine; however, this usually comes at a higher cost.
A unit’s ability to deliver pulsed US at a variety of duty cycles is another characteristic a practitioner should consider when selecting a model. All units should produce continuous US; the variety will come in the duty cycle settings that are available. Some units will have fixed duty cycles while others will allow the practitioner to set the duty cycle, typically ranging from 5% and up. As with other options, the greater the flexibility in duty cycle settings, the greater the cost of the unit.
Another consideration is the number of channels available on the unit. Some models have multiple channels, allowing for one machine to administer multiple treatments at the same time. This option may be useful in a busy clinic or athletic training room where client load is sufficient to justify the need for multiple channels. Otherwise, this option may not be useful to the clinic where multiple US units with one channel may be more effective for patient needs.
Many newer units have factory-installed software that has preselected parameters and also has the ability to read the treatment time and intensity changes. Some units can adjust either time or intensity as the other is manipulated. For instance, if you adjust the treatment time, the machine will automatically adjust the intensity to account for the change. While this is an intriguing function, many clinicians are not comfortable with the unit manipulating treatment parameters. In many cases, the software can be programmed to save individual treatment parameters and protocols that the clinician inputs into the software. These options can make treatment setup and delivery easier; however, the clinician must decide whether such advanced options are worth the cost.
Many clinicians choose to have multiple sound head sizes for their US units. This increases the number of treatment areas that you can access to utilize US and it can allow for treatment of larger areas. Sound heads come in sizes ranging from 1 cm2 to 10 cm2 . However, the clinician needs to keep in mind that the sound head size may not be a reflection of the true ERA. A 10 cm2 sound head may still have only a 5 cm2 ERA, thus only half of the US transducer would actually be producing US waves. The sound head should have an ERA approximately double the size of the treatment area.9 If the sound heads are changed often, carefully inspect the cable connection between the sound head and the cable. Over time, frequent changing of heads may lead to excessive wearing of the cable connections. Furthermore, if the sound head will be used for immersed US, you should make sure that it is insulated.
Some manufacturers are now making US heads that have remote controls for treatment time and intensity. This allows clinicians to make changes in parameters without having to direct their focus away from the treatment and patient. If this option is desirable, you should consider placement and fit of the controls on the sound head, and determine ease of use and comfort.
Unit portability is important. Is the unit powered only by AC current or is battery operation an option? If the unit is portable and will be traveling with you, is the external case durable? Does it come with a travel container? Are the cables and plugs sturdy?
The manufacturer warranty is also an essential component of any US unit purchase. Questions such as warranty duration, parts covered, and who handles repairs or warranty work are crucial. Also, you need to find out how often the unit needs to be calibrated and who can inspect and calibrate the unit without voiding the warranty. Calibration is essential to verify that the dosages of US are accurate and appropriate.11
Perhaps the best way to decide what ultrasound unit may be the best fit is to try several different manufacturers and models. Most manufacturers or vendors will lend US units to practitioners on a trial basis. Take advantage of such offers to find the unit that best meets your specifications, expectations, and patient needs.
Therapeutic ultrasound is a useful tool in the treatment and rehabilitation of a variety of injuries, illnesses, and conditions, providing both thermal and nonthermal benefits to facilitate tissue healing. You must be knowledgeable in the principles and the usage of ultrasound to guarantee proper treatment and patient safety.
Acknowledgement
I would like to thank Joel W. Beam, EdD, ATC, for his assistance and input on this article.
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