Ultrasound waves are produced by a crystal that is encased in an applicator referred to as a soundhead. An important characteristic of the soundhead is the effective radiating area (ERA), which is the portion of the crystal within the sound-head faceplate that produces ultrasound waves. The ERA is usually slightly smaller than the crystal, but it is significantly smaller than the soundhead faceplate. To achieve adequate heating, it is recommended that the area treated is twice the size of the soundhead faceplate.1 In research settings, a template that is twice the size of the soundhead faceplate is typically used to define the treatment area.
Although clinicians likely do not use a physical template, we believe they replicate it by envisioning a template twice the size of the soundhead faceplate as the treatment area. Because the crystal and the ERA are substantially smaller than the soundhead faceplate, we believe that heating within this treatment area is not uniform. Therefore, the primary purpose of our study was to measure temperature at 3 locations within the ultrasound treatment area to determine whether uniform heating occurs. We believe that uniform heating will not occur because the template is much larger than the area covered by the ERA. Miller et al2 examined this hypothesis and found a 1.04°C intramuscular temperature difference between the center of the treatment area and a point near the periphery, suggesting that uniform heating does not occur. In their study, the peripheral thermocouple was placed at 1 to 3 mm from the edge of the template, which was outside the ERA, thus receiving no direct ultrasound energy. To further examine uniform heating, we specifically assessed heating at the center of the treatment template, the edge of the ERA, and the periphery of the treatment template. We believe the results would be similar whether using a 1- or 3-MHz ultrasound, similar to Miller et al.2 We chose to test at 1 MHz because we believe it is of greatest value due to the increased depth of penetration.
The sound energy delivered by the crystal is not uniform and this is defined by the beam nonuniformity ratio (BNR), which is the ratio of peak spatial intensity to average spatial intensity. Therefore, during treatment, it is important to keep the soundhead moving at a constant linear or circular motion so the same tissue does not constantly receive the peak intensity, which could cause discomfort or burns.1,3,4 The recommended soundhead velocity during application is 4 to 5 cm/s.1,3–7
A secondary purpose for our study was to assess the effect of soundhead velocity on intramuscular temperature change, as there is little empirical data to support the recommended velocity of 4 to 5 cm/s. We compared the recommended velocity to a slower and faster velocity, but we hypothesize that these different velocities will not affect temperature elevation.
We used a 3 × 3 repeated measures design with the following independent variables: location of treatment (center of treatment template, edge of ERA, or periphery of treatment template) and soundhead velocity (2, 4, and 6 cm/s). The dependent variable was change in intramuscular tissue temperature.
Twelve healthy college students (age, 24.25 ± 2.86 years; height, 171.29 ± 7.42 cm; weight, 81.46 ± 19.34 kg; and skinfold thickness, 25.08 ± 2.61 mm) volunteered to participate. We excluded participants with calf skinfold measurements outside of the 20- to 30-mm range, impaired circulation, ischemia, areas with sensory deficits, deep vein thrombosis, an active infection over the treatment site, any type of known cancerous tumors, any known fractures or stress fractures over the treatment site, or metal implants in the area. All participants signed an institutional review board–approved consent form prior to exclusion or inclusion in the study.
We used the Omnisound 3000 (Accelerated Care Plus, Sparks, Nevada) ultrasound unit for all treatments in this study. Holcomb and Joyce8 found the Omnisound 3000 to be more effective in raising intramuscular temperatures in tissues compared with a similar manufactured ultrasound unit. Omnisound is the only manufacturer that performs ERA and BNR scans on each transducer.9,10 Johns et al11 found the Omnisound was the only manufactured ultrasound device that did not show a significant difference between the reported and measured ERA values compared with 5 other devices.
We used a soundhead with a 5.0-cm2 crystal; the manufacturer reported the ERA to be 4.9 ± 0.2 cm2 and the BNR to be 3.5:1. For the calculations to determine placement of our temperature probes (described below) we considered the margin of error and used an ERA of 4.7 cm2. We used room-temperature ultrasound gel (Aquasonic Clear; Parker Laboratories Inc, Fairfield, New York) as the coupling medium. We measured intramuscular tissue temperature with an IT Series Flexible Thermocouple Probe (Physitemp Instruments Inc, Clifton, New Jersey) and collected data using the Iso-Thermex thermocouple thermometer (Columbus Instruments, Columbus, Ohio). Prior to each data collection, we placed the thermocouples in a steam autoclave (Ritter M9; Midmark Corp, Versailles, Ohio) and sterilized for 15 minutes at 132ºC under a 27.1 psi sterilization program with a 30-minute dry time.
We completed a series of simple calculations to find the precise location of the edge of the ERA, which was 1 of the 3 sites where we placed our thermocouples. We determined the radius of the soundhead to be 1.8 cm by measuring the diameter of the soundhead faceplate and dividing it by 2. We then determined the radius of the ERA using the formula A = ϖr2, where A was 4.7 cm2. The result was a radius for the ERA of 1.22 cm. The difference in the radius of the ERA and that of the soundhead faceplate was 5.8 mm, which provided the distance from the edge of the template to the edge of the ERA where we inserted 1 thermocouple. Figure 1 shows these calculations.
Figure 1. Birds-eye view of the treatment template formed by 2 soundhead faceplates, the covered effective radiating area (ERA), and locations within the treatment template where thermocouples were inserted. A = center of the template, B = edge of the ERA, and C = periphery of the template.
Study participants reported to the Athletic Training Research Laboratory to receive the ultrasound treatments. To ensure a homogeneous sample, a range of 10 to 15 mm of adipose and skin thickness was required, which was determined by taking half of a skin-fold measurement at the thickest portion of the calf. We recorded and maintained an ambient temperature of 22°C throughout each treatment session. The treatment area was the center of the triceps surae of the right limb. All procedures were completed in 1 laboratory session lasting approximately 2 hours.
The participants wore shorts and we positioned them prone on a padded treatment table for the duration of treatment. The treatment area was the posterior aspect of the thickest portion of the right triceps surae (defined as the location with the greatest distance measured from the tibia to the posterior calf). A rectangle with rounded ends 7.2 cm in length and 3.6 cm in width (the size of 2 soundheads placed side by side) was placed over the treatment area (Figure 1). To ensure uniform thermocouple placement between participants, we placed a transparent template that contained small holes for marking the 3 insertion points directly over the treatment template. We made a hole in the center of the template, at the edge of the ERA, and at the periphery of the template that was described earlier (Figure 1).
In preparation for insertion of the thermocouples, we shaved an area 20 cm in diameter of the thickest portion of the posterior aspect of the right calf and cleansed it with a povidone-iodine swab. We used aseptic techniques while inserting and removing thermocouples to ensure participant safety. We marked thermocouples 2.5 cm from the tip and then placed them in 21-gauge needles for implantation. Using a T-square with level to ensure a perpendicular insertion, we inserted the first needle directly into the posterior calf to the marked depth of 2.5 cm below the skin surface on the left edge of the ERA. We chose the depth of 2.5 cm because it is within the depth of the penetration range for the 1-MHz ultrasound.12 After the desired depth was reached, we removed the needle while the thermocouple remained in the muscle. We inserted the second thermocouple into the center of the treatment site at the same depth. We inserted the third thermocouple at the same depth at the edge of the treatment site on the right side.
After thermocouple implantation and before the onset of treatment, we secured the thermocouples and treatment template onto the skin to limit any movement. We then gave participants time to reach a baseline tissue temperature by resting in the laboratory. After maintaining a stable baseline temperature (defined as no change in tissue temperature greater than 0.1ºC in 1 minute), we delivered 3 treatments with the order counterbalanced with the soundhead moving at 2, 4, and 6 cm/s, respectively. To maintain consistency in velocity, we used a metronome to monitor the pace of the longitudinal movement of the soundhead. For each pass, a distance equal to the width of the soundhead (3.6 cm) was traveled, and the following metronome settings were used to achieve the 3 velocities: 33, 65, and 97 beats per minute, respectively. We used 20 mL of ultrasound gel as a coupling medium to allow the soundhead to move smoothly across the area and to help to prevent discomfort from friction and burning. Application parameters are shown in Table 1.
Table 1: Application Parameters Used With the Omnisound 3000 Ultrasound
Between applications, participants rested comfortably while tissue temperatures returned to baseline. At the conclusion of the third ultrasound application data collection, we removed the thermocouples, cleansed the treatment area with 70% isopropyl alcohol, and a placed a bandage over the injection site. The participants were given instructions on how to clean the treatment area and how to identify the signs and symptoms of infection.
We recorded temperatures at the 3 locations within the treatment template: the center of the template, the edge of the ERA, and the periphery of the template. These temperatures were recorded every 10 seconds during each of the 10-minute treatments at the soundhead velocities of 2, 4, and 6 cm/s.
At the conclusion of data collection, we assessed change in intramuscular tissue temperature that occurred during the 10-minute treatment for each velocity, at each location for all 12 participants. This was accomplished by subtracting the baseline temperature at each application from the temperature at 10 minutes. We used repeated measures analysis of variance to test for significant interactions and main effects. We used pairwise comparisons for post hoc tests in the case of a significant main effect. We set the significance level a priori at α = .05 for all statistical tests. We conducted all statistical tests using SPSS version 16.0 software (IBM Corp, Armonk, New York).
The mean baseline tissue temperature across all participants was 35.77°C ± 0.66°C. Significantly greater heating occurred at the center of the treatment area (4.38°C ± 0.08°C) than at the edge of the ERA (1.89°C ± 0.17°C) and at the periphery of the treatment template (0.72°C ± 0.03°C [F2,22 = 112.01, P < .001]). Table 2 shows the change in temperature for the three 10-minute treatment locations when the data were collapsed across soundhead velocity .
Table 2: Mean Ultrasound Temperature Increase (°C) for Each Treatment Location and Velocity
Soundhead velocity did not affect heating in any area (F2,22 = .061, P = .941). Table 2 shows descriptive statistics of overall intramuscular temperature changes at each location for each soundhead velocity.
We used Bonferroni post hoc tests to examine the nature of the significant main effect. The results revealed a significant difference in intramuscular tissue temperature change between the edge of the ERA and the center (P < .001), the edge of the ERA and periphery of the treatment template (P < .001), and the center and periphery of the treatment template (P < .001). Figure 2 shows the average temperature change among velocities at the various locations.
Figure 2. Mean temperature change at each location with standard error bars. a = Difference between the center and both the effective radiating area (ERA) and the periphery. b = Difference between the ERA and the periphery.
Our study was unique in that we compared intramuscular temperatures following a standardized ultrasound treatment at the center of the treatment template, at the edge of the ERA, and at the periphery of the treatment template. Miller et al2 conducted a similar study, but they did not calculate the location of the edge of the ERA; rather, they estimated this position by placing a probe 1 to 3 mm from the edge of the treatment template. We believe they underestimated this position, as we calculated it to be 5.8 mm from the periphery of the template. Miller et al2 reported a significant difference in intramuscular tissue temperature increases between the center of the template and the periphery of the treatment template, with increases of 2.62°C and 1.58°C, respectively. Thus, the periphery experienced a temperature increase of only 60.3% of that experienced in the center. Our results support the conclusions made by Miller et al2 that uniform heating within the treatment template does not occur, but our results are more pronounced. When collapsed across velocities, our results showed the edge of the ERA had a temperature increase of 43.2% and the periphery of the template had only 16.4% of the increase experienced at the center of the template.
A difference was observed in temperature increases at the various locations within the treatment area. On average across all velocities, the center of the treatment template increased by 4.38°C, the edge of the ERA increased by 1.89°C, and the periphery of the template increased by 0.72°C. According to the findings of Draper et al,12 tissue temperatures should increase by approximately 3.3°C in 10 minutes, using the parameters of our study. Although the center of the treatment template had a greater increase than expected, the edge of the ERA and the periphery of the treatment template did not experience vigorous heating, as temperatures did not increase 4°C.
Making comparisons to the literature is difficult because there is much variability in the heating capabilities of the ultrasound machines from different manufacturers,8,13 and the frequency, intensity, and duration of treatments vary widely, which greatly affects temperature increases.12 However, there are several studies7,12,14,15 that also used the Omnisound 3000 with a frequency of 1 MHz, an intensity of 1.5 W/cm2, a duration of 10 minutes, and measured temperature increases in the center of a similar-sized treatment area, so comparisons are possible. Only 1 study12 was found that also measured temperature at the 2.5-cm depth used in our study, and they reported a temperature increase of only 3.4ºC, which was less than our increase of 4.38ºC in the center of the treatment template. When those authors measured at the greater depth of 5 cm, the increase was only 3.1ºC. The BNR for their unit (1.8:1) was better than for our unit (3.5:1), thus providing more uniform heating with less peak intensity. Therefore, the only plausible explanation for this difference is that our unit had an ERA of 4.9 ± 0.2 cm2, whereas the unit used by Draper et al12 had an ERA of 4.1 cm2. We treated a larger area, which may have affected heat dissipation from the center of the treatment area where the measures were taken.
More recent studies have suggested other potential explanations for these differences.14,15 Straub et al14 found that there were large deviations between actual spatial average intensity and the intensity displayed on the ultrasound units. Consequently, even though we selected the same 1.5 W/cm2 intensity as did Draper et al,12 there is no guarantee that the actual intensity was the same. Johns et al15 conducted a Schlieren assessment and found that transducers that produced a more concentrated field cause a greater temperature increase than those transducers that produced a lesser concentrated field. Because we did not measure the concentration of the transducer field, we can eliminate this as a contributing factor.
The remaining study7 that used the Omnisound 3000 with a frequency of 1 MHz, an intensity of 1.5 W/cm2, duration of 10 minutes, and measured temperature increases in the center of a similar size treatment area measured temperature at the greater depth of 3 cm plus one-half the skinfold measurement. Draper et al12 reported a slightly lower temperature increase of 4.0ºC, which may be expected at the greater depth. However, Weaver et al7 reported an average increase of 5.1ºC while testing 3 different soundhead velocities. This larger increase could be due to a greater ERA of 5.0 cm2 and a lower BNR of 2.1:1, compared with the ERA of 4.9 ± 0.2 cm2 and the BNR of 3.5:1 of our unit.
As expected, our data revealed that soundhead velocity did not alter the heating effects during ultrasound treatment, as there were no significant differences in temperature elevation at any of the 3 locations. Draper16 suggested using a velocity of 4 cm/s because moving the soundhead too slowly may burn the patient; however, moving the soundhead too rapidly may cause the clinician to inadvertently treat too large an area. As the results indicate, as long as the intensity is appropriate and the treatment area is delineated, any of the 3 velocities tested are effective. Weaver et al7 reported similar increases in intramuscular tissue temperature, regardless of soundhead velocity, and their statistical analysis showed no significant difference among the velocities. The velocities they examined were 2 to 3, 4 to 5, and 7 to 8 cm/s, which varied slightly from the velocities used in our study.
The primary limitation of this study is that we were not able to specifically define the area within the template that experienced vigorous heating because measurements were made at 3 specific locations. For instance, we know that vigorous heating occurred in the center of the template but not at the edge of the ERA; however, we can only speculate about the area between these locations.
Significantly greater heating occurred at the center of the treatment area, with significantly less heating occurring at the edge of an area defined by the ERA and even less occurring on the periphery of the treatment area. Therefore, ultrasound should be applied to an area twice the size of the soundhead, with a smaller targeted treatment area centered in the application area.
Implications for Clinical Practice
The extent of heating with ultrasound may be affected by the exposure of tissue to ultrasound and the extent of heating. These results demonstrate that the area being heated significantly with ultrasound is very small; therefore, ultrasound should be used when deep heating for a small treatment area is indicated. If larger areas are being targeted, other deep heating modalities, such as diathermy, are recommended.
- Starkey C. Therapeutic Modalities. 3rd ed. Philadelphia, PA: FA Davis Company; 2004.
- Miller MG, Longoria JR, Cheatham CC, Baker RJ, Michael TJ. Intramuscular temperature differences between the mid-point and peripheral effective radiating area with ultrasound. J Sports Sci Med. 2008;7(2):286–291.
- Knight KL, Draper DO. Therapeutic Modalities: The Art and Science. Philadelphia, PA: Lippincott Williams & Wilkins; 2008.
- Cameron MH. Physical Agents in Rehabilitation: From Research to Practice. 3rd ed. Philadelphia, PA: WB Saunders; 2008.
- Draper DO, Harris ST, Schulthies SS, Durrant E, Knight KL, Ricard MD. Hot-pack and 1-MHz ultrasound treatments have an additive effect on muscle temperature increase. J Athl Train. 1998;33(1):21–24.
- Draper DO, Sunderland S. Examination of the law of Grotthus-Draper: does ultrasound penetrate subcutaneous fat in humans?J Athl Train. 1993;28(3):246–250.
- Weaver SL, Demchak TJ, Stone MB, Brucker JB, Burr PO. Effect of transducer velocity on intramuscular temperature during a 1-MHz ultrasound treatment. J Orthop Sports Phys Ther. 2006;36(5):320–325.
- Holcomb WR, Joyce CJ. A comparison of temperature increases produced by 2 commonly used ultrasound units. J Athl Train. 2003;38(1):24–27.
- Leonard JL, Merrick MA, Ingersoll CD, Cordova ML. A Comparison of intramuscular temperatures during 10-minute 1.0-MHz ultrasound treatments at different intensities. J Sport Rehab. 2004;13(3):244–254.
- Straub SJ, Johns LD, Howard SM. Variability in effective radiating area at 1 MHz affects ultrasound treatment intensity. Phys Ther. 2008;8(1)8:50–57 doi:10.2522/ptj.20060358 [CrossRef] .
- Johns LD, Straub SJ, Howard SM. Variability in effective radiating area and output power of new ultrasound transducers at 3 MHz. J Athl Train. 2007;42(1):22–28.
- Draper DO, Castel JC, Castel D. Rate of temperature increase in human muscle during 1 MHz and 3 MHz continuous ultrasound. J Orthop Sports Phys Ther. 1995;22(4):142–150.
- Merrick MA, Bernard KD, Devor ST, Williams MJ. Identical 3-MHz ultrasound treatments with different devices produce different intramuscular temperatures. J Orthop Sports Phys Ther. 2003;33(7):379–385.
- Straub SJ, Johns LD, Howard SM. Variability in effective radiating area at 1 MHz affects ultrasound treatment intensity. Phys Ther. 2008;88(1):50–57 doi:10.2522/ptj.20060358 [CrossRef] .
- Johns LD, Demchak TJ, Straub SJ, Howard SM. The role of quantitative Schlieren assessment of physiotherapy ultrasound fields in describing variations between tissue heating rates of different transducers. Ultrasound Med Biol. 2007;33(12):1911–1917 doi:10.1016/j.ultrasmedbio.2007.06.012 [CrossRef] .
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Application Parameters Used With the Omnisound 3000 Ultrasound
|Duty cycle||Continuous (100%)|
|Crystal size||5 cm2|
|Effective radiating area||4.9 ± 0.2 cm²|
|Beam nonuniformity ratio||3.5:1|
Mean Ultrasound Temperature Increase (°C) for Each Treatment Location and Velocity
|LOCATION||MEAN ± STANDARD DEVIATION|
|2 CM/S||4 CM/S||6 CM/S||COLLAPSED|
|Center||4.47 ± 0.97||4.33 ± 0.80||4.33 ± 1.18||4.38 ± 0.08|
|Effective radiating area||1.85 ± 0.73||1.87 ± 0.56||1.94 ± 0.91||1.89 ± 0.17|
|Periphery||0.75 ± 0.45||0.71 ± 0.44||0.70 ± 0.43||0.72 ± 0.03|