Therapeutic ultrasound is one of the most common deep heating modalities used by physical therapists, athletic trainers, and occupational therapists.1 Its thermal effects are used for treating soft tissue injuries2 and muscle spasm,3 restoring range of motion,4 increasing collagen extensibility,5 aiding in collagen alignment,6,7 and increasing wound strength.7 The non-thermal effects of ultrasound include: increased histamine release,1 increased phagocytosis,1,8 increased protein synthesis,9 enhanced tissue regeneration10–12 and wound healing,11 increased number of fibroblasts, and vascular regeneration.10–12 Therapeutic ultrasound uses high-frequency, (1 to 3 MHz) inaudible, acoustic vibrations to produce these thermal and non-thermal physiological effects. Unfortunately, despite its common use, therapeutic ultrasound is often misunderstood and misused.1,10,12 However, when used properly, it is an effective treatment method that can be applied to both normal and damaged tissue.13–16
Traditional ultrasound treatments are prone to clinician error (eg, treating too large a surface area and moving the soundhead faster than the recommended speed1), labor intensive, and time-consuming, requiring a clinician to manually move the ultrasound transducer over the target tissue and keeping the clinician occupied and unable to complete other tasks. Rich-Mar (Chattanooga, TN) addressed these problems by developing the AutoSound, a hands-free ultrasound alternative. The AutoSound works by activating and deactivating four rectangular transducer crystals that lie side by side.17 The first crystal turns on and then quickly turns off when the second crystal turns on. This process repeats down to crystal four, and then starts at crystal one again. The activation and deactivation of the crystals is equivalent to a clinician manually moving the ultrasound transducer at a speed of 4 cm/sec,18 the recommended speed of traditional soundhead movement.1 The firing pattern of the crystals is equivalent to manually moving the ultrasound transducer from one part of the treatment area to the other, picking the transducer up, and placing it back at the starting point.19 These crystals are housed in one unit that can be strapped on the body and left for the duration of the treatment.
The AutoSound could be a tremendous clinical asset, significantly adding to the time efficiency of the clinician if the machine works. Multiple studies19–21 have compared the AutoSound with traditional ultrasound in its ability to heat human muscles, and all have found that traditional ultrasound produced significantly greater temperature increases than the AutoSound. On further examination of these comparison studies,19–21 we discovered that each of the three used the 3-MHz frequency. Ultrasound delivered at a 3-MHz frequency is absorbed superficially in the tissues 1 to 2 cm deep, but may reach all the way to 3 cm deep,1,22,23 whereas ultrasound delivered at 1 MHz is absorbed in deeper tissues 2 to 5 cm deep.1,22
The intensity used in these studies is also important. Intensity is the rate at which ultrasound waves are being delivered to target tissues, per unit area of the transducer surface (expressed as W/cm2).24 The lower the intensity, the longer the treatment duration needs to be to achieve the desired results.22 Two of the previous studies20,21 on the AutoSound used an intensity of 1.5 W/cm2 for 10 minutes. The other19 used 1.0 W/cm2 for 8 minutes, even though treatments at a lower intensity should have a longer duration to produce the desired results. The purpose of this study was to compare intramuscular temperature changes in the gastrocnemius muscle produced by a 10-minute ultrasound treatment via the AutoSound and a traditional ultrasound treatment at a frequency of 1 MHz and an intensity of 1.0 W/cm2.
We recruited 16 healthy individuals for this study (6 males and 10 females, age = 22 ± 1.6 years; subcutaneous fat over the gastrocnemius muscle and skin of 6 males and 10 females = 0.85 ± 0.37 cm; weight = 72.5 ± 11.3 kg; height = 173.2 ± 8.4 cm) and gathered data using Doppler ultrasound (Logio P6; GE Health-care, Noblevlle, IN). Participants filled out a survey that screened them for inclusion and exclusion criteria during their signing of the consent form. Exclusion criteria were: a lower extremity injury within the past 2 months, a lower leg infection, open wound, rash, swelling, ecchymosis, decreased circulation, decreased sensation in the area being treated, and thrombophlebitis. Participants refrained from exercise 2 hours prior to each laboratory visit. All individuals provided written consent before their participation in the study. The study was approved by the university's institutional review board before participant recruitment began.
Traditional ultrasound was produced via the Thera-Sound Evo (Rich-Mar) delivered at a frequency of 1 MHz. Although Rich-Mar does not scan each crystal, they report a beam non-uniformity ratio of 5.5:1 and have an effective radiating area as close to the size of the soundhead as possible.17 However, Straub et al.25 scanned several Rich-Mar crystals and found them to be an effective radiating area of 3.85. The traditional ultrasound was performed using the TheraHammer (Rich-Mar), which houses a lead zirconate titanate crystal that is 2 cm2. Hands-free ultrasound was performed using the TheraSound Evo with the AutoSound attachment. The four crystals of the AutoSound are 1.5 by 2.5 cm each with 2 mm of dead space between each crystal. Thus, the treatment area of the AutoSound is approximately 14 cm2. The effective radiating area is not reported on the AutoSound. Both devices were new and calibrated before the study.
Temperature was measured using the ISO-Thermex (Columbus Instruments International, Inc., Columbus, OH) program. Temperature readings were received from an IT-21 thermistor (Physitemp Instruments Inc., Clifton, NJ). The thermistor was inserted via a 20-gauge catheter (BD Medical, Franklin Lakes, NJ). The depth of the inserted thermistor and adipose thickness were measured using Doppler ultrasound imaging (model LogiQ 55e; General Electric Company, Fairfield, CT). Adipose levels were taken from three locations on each participant: one directly above the thermistor, one on the far left of the frozen image, and one on the far right. All measurements were marked from the bottom of the skin to the top of the fascia surrounding the gastrocnemius, with all measurements averaged together. Imaging ultrasound was produced using the 12 L soundhead and a 12-MHz frequency.
A randomized cross-over experiment was performed. Participants reported to the laboratory twice, with at least 24 hours between visits. All participants were screened for contraindications via consent form, and those who were still eligible after the screening process reviewed and signed a consent form approved by the institutional review board. Once participants were officially enrolled in the study, they were randomly assigned by drawing a piece of paper out of an opaque cup to receive ultrasound treatment first via the AutoSound or by using traditional ultrasound.
Catheter and Thermistor Insertion. A single thermistor was inserted via catheter into the medial side of the individual's left lower leg, at an average depth of 2.25 ± 0.52 cm in the tissue (Figure 1). In accordance with other studies,1,22,23 the left leg was used for ease of a right-handed investigator to insert the probe. Patients lay prone on the treatment table during catheter insertion (and for the remaining time of the treatment) with their left lower leg exposed. The area of greatest girth on the patient's triceps surae muscle was visualized. A T-square was used to measure 2.25 cm anterior on the gastrocnemius muscle and a green dot was marked on the skin where the catheter would be inserted. The insertion site was cleaned with an iodine swab and allowed to air dry before the catheter was inserted. The catheter was horizontally inserted into the medial gastrocnemius muscle over the previously marked green spot. A thermistor was fed through the catheter and the catheter was removed, leaving the thermistor in place. Following the methods of another study,19 we used only one depth for the thermistors at 2.25 cm deep. Ultrasound treatment at 1 MHz ideally targets tissues 2 to 5 cm deep.22,23 Thermistor insertion depth was verified using Doppler ultrasound imaging (Figure 2). The individual collecting the data was a graduate student working on her master's thesis. She had been trained by a professor with 8 years of experience using this method with more than 200 participants. He supervised her on all treatments.
Temperature probe insertion on the medial side of the left gastrocnemius muscle. (Note the green dot shows the middle of the target tissue).
Verification of probe depth insertion via Doppler ultrasound imaging.
Ultrasound Treatment Area. The ultrasound treatment area was centered over the end of the thermistor for each participant. The treatment area of traditional ultrasound was marked using a template two times the size of the ultrasound head (approximately 4 cm2). Treatments performed via the AutoSound covered approximately 14 cm2 (two times the size of the sound-head, covering 4 crystals); whereas the treatment size of the Thera-Hammer was two times the size of the soundhead, covering two crystals.
Manual Ultrasound Treatment. After the temperature probes were inserted and depth verified, we applied ultrasound coupling medium to the treatment area. Five cubic centimeters (cc) of 100% Aquasonic ultrasound gel (Parker Labs, Fairfield, NJ) was applied in the manual treatment area. The thickness of the gel was approximately 5 mm and another 5 cc of ultrasound gel was applied halfway through the treatment (5 minutes).
AutoSound Treatment. After the temperature probes were inserted and depth verified, we applied ultrasound coupling medium to the treatment area. The coupling medium was a 10-mm thick hydrogel (Rich-Mar).
The ultrasound treatment using the manual technique was administered within the previously marked spot on the back of the gastrocnemius muscle for 10 minutes. Treatments were performed with a 2 cm2 transducer at a frequency of 1 MHz and an intensity of 1.0 W/cm2. Tissue temperature readings were recorded for each participant at baseline (once temperature had stabilized to the point that there was no more than 0.1°C change every 30 seconds) and every 30 seconds for the duration of the treatment. Ultrasound gel was used as the coupling medium in all traditional treatments.
AutoSound Ultrasound Treatment. Treatments with the AutoSound (Figure 3) were performed on the same leg as the traditional ultrasound treatment, once again over the area of greatest girth on the medial gastrocnemius muscle. Treatments were performed with a 2 cm2 transducer. Settings of 1 MHz and 1.0 W/cm2 for 10 minutes were used. The AutoSound was secured in place with 1-inch Powerflex tape (3M Health Care, St. Paul, MN). Ultrasound treatments were started after tissue temperature had stabilized to the point that there was no more than 0.1°C change every 30 seconds. Intramuscular temperatures were recorded every 30 seconds throughout the treatment session using the ISO-Thermex. A 10-mm thick gel pad (designed specifically for the AutoSound) was used as the coupling medium during all AutoSound treatments. The amount of energy that is absorbed with the gel pad has not been determined.
Ultrasound performed with the AutoSound (Rich-Mar, Chattanooga, TN) machine.
Thermistor Removal. At the conclusion of each treatment, the thermistor was removed from the participant's triceps surae muscle and a bandage was placed over the area for protection. The thermistors and catheters were sterilized using an Anprolene Gas Sterilizer (Model AN74i; Andersen Products, Inc., Haw River, NC).
A 2 × 2 repeated measures analysis of variance was used to determine interactions among the beginning and ending temperatures of each ultrasound unit. A hierarchal linear model was used to determine the rate of temperature change caused by each machine. In this model, a regression line was fit to the slope of temperature change for each individual. Individual slopes were then averaged together for an overall slope of the population. SAS 9.3 (2010 JMP Pro11; SAS, Inc., Cary, NC) was used for all statistical analysis, and alpha was set at a P value of less than .05.
The 2 × 2 repeated measures analysis of variance on temperature showed a statistically significant interaction between instruments and time (F = 23.72, P = .0002). On average, traditional ultrasound temperatures went from a starting temperature of 35.67°C ± 0.24°C to an ending temperature of 36.08°C ± 0.24°C. The mean tissue temperature before ultrasound performed with the AutoSound was 35.88°C ± 0.24°C and the ending temperature was 35.73°C ± 0.24°C.
Although traditional ultrasound using the Thera-Hammer reported changes between beginning and ending temperatures to be statistically significant (0.41°C ± 0.09 °C [P = .0016]), it may not be clinically different. There was no statistically significant change from beginning to ending temperature (tissue temperature decreased 0.16°C) with ultrasound performed with the AutoSound (P = .33). The hierarchal linear model revealed a statistically significant difference in the slopes between traditional ultrasound and the AutoSound (F = 124.17, P < .0001) with regard to the rate of heating. On average, traditional ultrasound increased tissue temperature 0.025°C ± 0.003°C/min (P < .0001). The AutoSound actually lowered tissue temperature 0.016°C ± 0.001°C/min (P = .95) (Table 1).
Summary of Baseline, Final, Total, and Rate of Temperature Change (Mean ± Standard Error)
We compared the heating of the AutoSound with traditional ultrasound delivered by the TheraSound Evo using the TheraHammer. We discovered that the AutoSound at 1 MHz did not raise the tissue temperature during the 10-minute treatment. These findings support previous research19–21 that the AutoSound does not heat as well as traditional ultrasound. Three studies have compared the heating of the AutoSound with traditional ultrasound at a frequency of 3 MHz,19–21 although ours is the first to test the AutoSound at 1 MHz. McCutchan et al.19 used the following parameters for their study: 3 MHz, 1.0 W/cm2, 8 minutes, assessing the tissue temperature at a depth of 1 cm. They found a 1.8°C increase in tissue temperature when the AutoSound was used and a 3.2°C increase when the Omnisound (Accelerated Care Plus, Reno, NV) was used. Like McCutchan et al.,19 we used an intensity of 1.0 W/cm2, but a longer treatment time of 10 minutes. In both cases, traditional ultrasound produced a significantly higher increase in tissue temperature when compared with the AutoSound.
The following parameters were used in the study by Gulick20: 3 MHz, 1.5 W/cm2, 10 minutes, tissue temperature probes 1 and 2 cm deep. The AutoSound increased the tissue temperature 5.1°C at 1 cm deep and 1.5°C at 2 cm deep. The Omnisound increased the tissue temperature 6.7°C at 1 cm and 4.0°C at 2 cm. Traditional ultrasound once again produced a significantly greater increase in tissue temperature when compared with the AutoSound. Although our settings varied from Gulick's20 in every other way, we also used a 10-minute treatment time.
Fincher et al.21 performed ultrasound using the AutoSound at 3 MHz and 1.5 W/cm2 for 10 minutes and traditional ultrasound via the 5 cm2 TheraHammer transducer on the AutoSound 7.6 Combo unit at a depth of 2.5 cm. The AutoSound increased temperature 2.05°C, whereas traditional ultrasound increased tissue temperature 4.53°C. Again traditional ultrasound produced significantly higher temperature increases than the AutoSound. Like Fincher at al.,21 we used the same ultrasound machine with different attachments for all ultrasound treatments, but used the 2 cm2 transducer for the traditional treatment instead of the 5 cm2 transducer that was used in the Fincher et al. study. The 2 cm2 transducer or the difference in frequency may explain why Fincher et al. received a 4.53°C change and a 2.05°C change with traditional ultrasound and the AutoSound, respectively, and we saw little change in temperature.
Straub et al.25 analyzed 66 crystals from six different ultrasound machines (Chattanooga, Dynatronix, Mettlar, Omnisound, XLTEK, and Rich-Mar). They found variability between the crystals. More specifically, the Rich-Mar had an effective radiating area of 4 cm2, whereas Straub et al.25 report an effective radiating area of 5 cm2. They reported that the spatial average intensity was not accurate. Johns et al.26 also showed variability in treatment parameters. The AutoSound did not increase tissue temperature to the same degree as traditional ultrasound in any of these cases.19–21 Research has found that heating varies from manufacturer to manufacturer.26–28 The Omnisound was used in two of these studies20,21 and may heat at a different rate than the TheraSound Evo because it seems to increase tissue temperature more than any other ultrasound machine it has been compared to.29,30 To eliminate variability between manufacturers, like Fincher et al.,21 we compared two devices manufactured by the same company (Rich-Mar).
At 3 MHz, the AutoSound produces poor temperature changes. A temperature increase of 1°C is considered mild heating and is used for increasing metabolism and reducing mild inflammation.1 A 2°C to 3°C increase is considered moderate heating and is indicated for increasing blood flow and reducing pain and muscle spasm.1 A 4°C increase is considered to be vigorous heating and is used to increase the extensibility of collagen fibers.1 According to this, participants in Gulick's study20 received vigorous heating (5°C increase at 1 cm) and mild heating (1.5°C increase at 2 cm) when the AutoSound was used at a frequency of 3 MHz. McCutchan et al.19 and Fincher et al.21 produced moderate heating (1.8°C at 1 cm and 2.05°C at 2.5 cm, respectively) when the AutoSound was used, again at 3 MHz. Heating may have occurred in these studies24–26 and not in ours due to the use of the 1-MHz setting. A 1-MHz ultrasound setting heats at one-third the peak temperature as 3 MHz.22 This is due to the crystal deforming at one-third the rate as a crystal at 3 MHz. Another reason might be that the beam diverges from the 1-MHz frequency, whereas the beam is collimated at the 3-MHz frequency.1 This might focus more energy on the temperature probe when 3 MHz is used, and not increase tissue temperature at 1 MHz.
At a 3-MHz frequency,19–21 the AutoSound may be clinically beneficial because it produces moderate1 heating. Most clinical practices target superficial tissues, so the AutoSound will produce moderate heating in the desired area and free the clinician to perform other tasks. However, at 1 MHz and 1.0 W/cm2, the AutoSound is not beneficial for clinicians or their patients in heating tissue. At 1 MHz, the AutoSound did not produce moderate or even mild heating. A 20-minute hot-pack treatment can raise tissue temperature 3.6°C at 1 cm and 0.8°C at 3 cm.31 Thus, the AutoSound at 1 MHz is probably no better than a hot-pack treatment at increasing muscle temperature. However, a hot-pack heats a much larger area than the AutoSound, making the hot-pack the treatment of choice when targeting superficial tissues.
In our opinion, the AutoSound did not raise the tissue temperature for the following reasons. There is a slight time lag between the firing of each successive crystal. This means that there is not always a crystal on, which could lead to a decrease in heating. There is also a slight amount of space (2 mm) between each of the four crystals in the AutoSound. This slight space between each crystal means that there is “dead space” where no heating occurs in the ultrasound unit. This may affect target tissue temperature change. The time delay from one crystal to the next and the fact that there is no heating under the dead space between adjacent crystals could be a reason the AutoSound does not appear to heat the tissue to the same degree as traditional ultrasound.
The gel pad used during AutoSound application may be too thick (10 mm). Ultrasound gel has been the coupling medium used during all traditional ultrasound treatments in the studies where traditional ultrasound was compared to the AutoSound.19–21 Studies have shown that ultrasound gel (such as that used during traditional ultrasound) is the most effective form of coupling medium at increasing tissue temperature when compared to 1- or 2-cm thick gel pads.32,33 However, the AutoSound uses a gel pad that is 10-mm thick. This may impair the ultrasound unit's ability to effectively deliver sound waves into the target tissue, especially in light of recent research34 that has shown that the Gel Shot (a 2- to 3-mm thick gel pad) is more effective than ultrasound gel when used at 1 MHz. Therefore, it is possible that a thinner gel pad could have been more effective and aid in increasing tissue temperature, but the gel pad currently used with the AutoSound may be too thick to see any positive effects.
The reason the AutoSound may not be effective in tissue heating has to do with the activation sequence and arrangement of the four crystals. A traditional ultrasound transducer uses one crystal. Ultrasound only produces significant heating when an area two times the size of the soundhead is used.1,22,35 Chudleigh et al.35 found that at 3 cm, a 10-minute, 1-MHz, 1.5 W/cm2 ultrasound treatment resulted in a 3.5°C increase in temperature when an area two times the size of the sound-head was treated. However, when an area six times the size of the soundhead was used, ultrasound increased the temperature only 0.57°C. Thus, a larger treatment area leads to a decrease in the amount of heating that takes place. During traditional ultrasound, an area the size of two crystals is heated. Although the AutoSound uses four crystals that lie side by side, this only mimics one crystal moving because only one crystal is activated at a time. This means that the AutoSound is technically covering an area four times the size of a soundhead (treating only 25% of the surface area at a time), instead of two times the size of the effective radiating area of the crystal (treating 50% of the surface area). Additionally, by activating the first crystal every time after crystal four has turned on and off, the AutoSound is mimicking picking the soundhead up and placing it back at the starting position. Although contact with the skin is maintained during AutoSound treatments, mimicking this pattern with traditional ultrasound would lead to loss of contact with the skin and a decrease in heating because the sound waves cannot be transduced into the tissue at this point. We suggest that the manufacturers of the AutoSound consider placing two large crystals (possibly 10 cm2) side by side in a new version of the AutoSound. Most likely, this would produce higher temperatures.
Our study has limitations. We used healthy individuals between 18 and 25 years of age to examine tissue temperatures in undamaged tissue. We assume that tissue temperature changes would be similar in an injured population over damaged tissue. Our results are also limited to the use of a 2 cm2 soundhead, a frequency of 1 MHz, and an intensity of 1.0 W/cm2. We suggest future research should be conducted on the AutoSound at 1 MHz and at a higher intensity than 1.0 W/cm2.
Implications for Clinical Practice
We successfully measured intramuscular temperature changes during ultrasound treatment with a traditional and a hands-free device. At an average depth of 2.25 cm, a 10-minute, 1-MHz, 1.0 W/cm2 ultrasound treatment did not produce desired heating with either machine. At 1 MHz, the AutoSound failed to increase the temperature of the triceps surae muscle, and the TheraHammer only minimally increased temperature. We suggest an alteration to the AutoSound where only two larger crystals are used so an area twice the size of the soundhead is treated. We also suggest employing the use of a thinner gel pad, approximately 3-mm thick, during AutoSound treatment.
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- Young SR, Dyson M. The effect of therapeutic ultrasound on angiogenesis. Ultrasound Med Biol. 1990;16:261–269. doi:10.1016/0301-5629(90)90005-W [CrossRef]
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Summary of Baseline, Final, Total, and Rate of Temperature Change (Mean ± Standard Error)
|MODE OF TREATMENT||BASELINE TEMPERATURE (°C)||FINAL CHANGE (°C)||TOTAL CHANGE (°C)||RATE OF CHANGE (°C/MIN)|
|Traditional||35.67 ± 0.24||36.08 ± 0.24||0.41||0.025 ± 0.003|
|AutoSound||35.88 ± 0.24||35.73 ± 0.24||−0.16||−0.016 ± 0.001|