Ultrasound has been used as a therapeutic technique in physical medicine for nearly 65 years. By 1955, the Council on Physical Medicine and Rehabilitation of the American Medical Association recommended the technique as an adjunctive therapy for the treatment of pain,1–3 soft tissue injury and joint dysfunction including osteoarthritis,4,5 periarthritis,6 bursistis,7 tenosynovitis,8 and a variety of musculoskeletal syndromes. Other applications such as acceleration of wound healing,9 treatment of scar tissue,10 and treatment of sports injuries11 have been reported.
Ultrasonic therapy relies on mechanical vibration of tissue to cause thermal and mechanical effects, typically using a frequency of 1 or 3 MHz. The electrical output from the ultrasonic generator is converted into mechanical vibration through a transducer generally made of synthetic crystals such as barium titanate or lead zirconate titanate.12 The mechanical vibration produces an acoustic wave that travels through the tissue and is absorbed in the process. The rate of absorption, and thus the thermal effect, is based on the tissue type encountered, the frequency of the ultrasound beam, and the intensity (W/cm2) of the ultrasonic output.13,14
Ultrasound devices are not perfectly uniform, but have small peaks and valleys. Beam non-uniformity ratio (BNR) indicates the variability of intensity within an ultrasound beam.12 The BNR is the ratio of spatial peak intensity (the highest intensity point of the beam) to spatial average intensity (the average intensity across the whole beam).12,15,16
Johns et al.17 determined that beam profiles between transducers within the same manufacturer can significantly vary relative to the transducer's effective radiating area (ERA) and special average intensity. The ERA is the surface area that transmits a sound wave from the transducer to the tissues.12 It is unknown whether these variations in beam profiles alter treatment outcomes. The purpose of our study was to measure intramuscular tissue temperature during therapeutic ultrasound treatments between different transducers of the same manufacturer.
Sixteen participants were recruited to participate in the study (8 men and 8 women, age = 24.0 ± 2.3 years, height = 178.1 ± 9.5 cm, mass = 75.0 ± 12.4 kg). Participants were screened for the inclusion and exclusion criteria before enrollment. Participants were included in the study if they were healthy males or females between the ages of 18 and 35 years. Participants were excluded if they had a history of ecchymosis, infection, edema, and/or injury to the lower extremity in the past 6 months. Participants were also excluded if they had any of the following contraindications or precautions for therapeutic ultrasound: acute inflammation, infection, open wound, peripheral cardiovascular disease, lack of lower extremity sensation, malignancy, or metal implants.
All participants provided informed consent before enrollment and the study was approved by the institutional review board at the principal investigator's university.
Microprobe MT-23/5 needle thermocouples (Physitemp Instruments, Clifton, NJ) were plugged into an electrothermometer (Iso-Thermex; Columbus Instruments, Columbus, OH) to record tissue temperature. The reliability and validity of the electrothermometer has been described previously.18 Prior to using the needle thermocouples, each was tested in a 37°C water bath and compared with a thermometer certified by the National Institute of Standards and Technology, based on methods described previously.19 Across all needle thermocouples, the mean absolute thermocouple-calibrated National Institute of Standards and Technology thermometer difference was 0.14 ± 0.15 (range: −0.07 to 0.23).
We used the Omnisound 3000 Pro ultrasound device (Accelerated Care Plus, Reno, NV) with five 5-cm2 different transducers. Table 1 lists the transducers used in the study and their respective characteristics. General manufacturer specifications indicate that all of the transducers' power outputs varied less than ±10%, including measurement error during pre-market calibration.
Characteristics of 5-cm2 Ultrasound Transducers
Each participant made five visits, separated by at least 24 hours, to the laboratory. On the first visit, participants were assigned to the 1- or 3-MHz frequency by a random draw (n = 8 assigned to each frequency). At the five different visits, ultrasound treatments were provided with the five different ultrasound transducers. The same procedures were used for each visit to the laboratory, but a different ultrasound transducer was used each time. Participants were asked not to exercise at least 2 hours prior to their study appointment.
At each visit, participants lay prone on a treatment table. A small carpenter square was used to measure 2.5 or 1 cm down the medial calf from the posterior surface for 1- or 3-MHz treatment groups, respectively. The distance was marked with a dot indicating the insertion site of the needle temperature probe. The area was shaved and skin prepared using an iodine swab.
Before the needle thermocouple insertion procedures and to ensure aseptic procedures, the thermocouples were first cleaned with a protein-dissolving detergent (Enzol; Johnson and Johnson, Irvine, CA) for 1 minute. Following the cleaning, the thermocouples were placed in a 1.5% glyderaldehyde disinfectant (MetriCide; Metrex Research, Romulus, MI) for at least 12 hours. Right before use, the thermocouples were briefly rinsed with sterile water.
A 23-gauge needle thermocouple was inserted horizontally in the calf from the medial side (Figure 1). Musculoskeletal imaging ultrasound (LogiQ; General Electric Company, Fairfield, CT) was used to visualize the insertion of the needle thermocouple and verify the depth of the probe into the calf. The thermocouple's receiving end was plugged into the Iso-Thermex electrothermometer and the computer was set to collect data points every 1 minute. We waited 5 minutes for tissue temperature to stabilize and used this measurement as the baseline temperature. A 10-cm2 (treatment area = 2 × ERA) ultrasound treatment area was marked over the posterior calf. A 10-minute ultrasound treatment was administered at the randomized frequency and a 1.0 W/cm2 intensity. The ultrasound transducer was moved in a linear fashion at approximately 4 cm/sec, as visually estimated by the investigator based on the 10 cm2 treatment area. The same investigator provided all treatments. After the 10-minute treatment, the needle thermocouples were removed and the area was cleansed with isopropyl alcohol. An adhesive fabric bandage with triple antibiotic ointment was applied over the insertion site.
Therapeutic ultrasound treatment over the triceps surae muscle at 1.0 W/cm2. Treatment area equals 2× transducer size (10 cm2).
To normalize the data, we calculated the temperature change from baseline. To determine statistical differences between the transducers at 1- and 3-MHz frequencies, we used two separate repeated measures analysis of variance (1 and 3 MHz). We also analyzed the overall heating at the conclusion of the treatment (change in temperature at the 10-minute point) by using a one-way analysis of variance. We used a Tukey–Kramer post-hoc analysis. We also used linear regression analyses to determine whether there was a correlation between ERA or BNR and temperature change at the end of the treatment (10-minute point) for each frequency. All data were analyzed using JMP Pro 13 software (SAS Institute, Inc., Cary, NC) and a P value of less than .05 was considered statistically significant.
The actual probe insertion depth measured via imaging ultrasound was 2.29 ± 0.56 cm with the target depth being 2.5 cm. There was no difference in probe depth between each transducer's trial (F4,51 = 0.750, P = .563). There was a significant difference between transducers during the overall 1-MHz ultrasound treatment (transducer main effect) (F4,28 = 4.486, P = .0063). There was a significant difference between transducers' rate of heating over the course of the 1-MHz ultrasound treatment (transducer × time interaction) (F40,350 = 2.437, P ≤ .001). At the conclusion of the 10-minute ultrasound treatment, there was a significant difference between transducers (F4,35 = 3.292, P = .0216) (Figure 2A).
Mean ± 1 standard error change in intramuscular heating temperature of different 5 cm2 transducers at the conclusion of a 10-minute (A) 1-MHz or (B) 3-MHz ultrasound treatment. *Significantly different between light and dark marked transducers at P < .05.
There was a significant but weak correlation between end of treatment temperature change and ERA (P = .002, R2 = 0.232) (Figure 3A) and BNR (P = .010, R2 = 0.161) (Figure 3B).
Linear regression correlation and 95% confidence interval between change in intramuscular heating temperature and (A) 1-MHz effective radiating area, (B) 1-MHz beam non-uniformity ratio, (C) 3-MHz effective radiating area, and (D) 3-MHz beam non-uniformity ratio.
The actual probe insertion depth was 1.13 ± 0.18 cm with a target depth of 1 cm. There was no difference in probe depth between each transducer trial (F4,58 = 1.215, P = .301). There was a significant difference between transducers during the overall 3-MHz ultrasound treatment (transducer main effect) (F4,28 = 3.299, P = .0249). There was a significant difference between transducers' rate of heating over the course of the 3-MHz ultrasound treatment (transducer × time interaction) (F40,340 = 3.338, P ≤ .001). At the conclusion of the 10-minute ultrasound treatment, there was a significant difference between transducers (F4,34 = 3.081, P = .0287) (Figure 2B).
There was a significant but weak correlation between end of treatment temperature change and ERA (P = .027, R2 = 0.126) (Figure 3C). There was no significant correlation between end of treatment temperature change and BNR (P = .553, R2 = 0.010) (Figure 3D).
Intramuscular heating with therapeutic ultrasound is a function of frequency, intensity, and characteristics of the ultrasound transducer. Therapeutic ultrasound heating rates have been determined with a single transducer using different frequencies and intensities.14 Others have determined the heating rate differences between transducers from different manufacturers.20,21 We found that ultrasound transducers within the same manufacturer produce different intramuscular heating rates.
Beam profile differences of ultrasound transducers within the same manufacturer have been previously reported. Johns et al.17 found that spatial average intensity varied up to 17% when the device was set at 1.2 W/cm2 and 50% between transducers from the same manufacturer when set at 1 and 3 MHz, respectively. The varying spatial average intensity produced by different beam characteristics would ultimately affect the heating rates of the different transducers. For example, in our results the lowest and highest change in temperature at the end of the ultrasound treatment resulted in a 102% (0.62°C vs 1.92°C) and 83% (2.38°C vs 5.76°C) difference for the 1- and 3-MHz treatments, respectively. Small differences in transducer spatial average intensity between transducers can lead to large differences in intramuscular temperature over time. As treatment time is increased, the differences in intramuscular temperature due to varying spatial average intensity outputs between transducers will be exacerbated.
Differences in transducer heating rates may lead to poor clinical outcomes if intramuscular heating goals are used. For example, clinicians have used therapeutic ultrasound to heat tissues more than 4°C from baseline or 40°C absolute temperature to produce tissue extensibility for range of motion with positive outcomes.22 However, poor intramuscular heating rates due to transducer variations could limit the clinician's ability to reach the tissue temperature-based goal. It is nearly impossible for clinicians to know which type of transducer they have acquired. To help clinicians using temperature-based goals, future research should determine whether other clinical measurements, such as a subjective sensation of warmth or objective Doppler ultrasound measurements, correlate to tissue temperature changes.
Therapeutic ultrasound treatments delivered with an intensity of 1.0 W/cm2 have produced heating rates of 0.2°C/min and 0.6°C/min for 1- and 3-MHz frequencies, respectively.14 Two of the five transducers we tested reached similar heating rates as previously reported with the same device. A weak correlation existed between tissue heating and ERA, indicating a small trend of devices that produced less tissue heating due to smaller ERAs. The smaller ERA would have led to less energy delivery. For example, during the 1-MHz treatment, transducers A and C had a 20% (2,760 vs 2,260 J) energy delivery difference. Lower ultrasound energy rates may have added to the lower tissue temperature change seen with these transducers.
The BNR describes the peak variation in intensity of the ultrasound beam produced by the transducer. A lower BNR produces a more uniform beam, hypothesized to produce more consistent heating throughout the treatment field. All of the transducers in our study have a relatively good BNR. There was a very weak correlation (1 MHz) or no correlation (3 MHz) between intramuscular temperature change and BNR. Transducer movement during an ultrasound treatment has been hypothesized to smooth out areas of peak intensity across the treatment field, negating a slightly higher transducer's BNR ability to significantly change intramuscular heating.17 We hypothesize that beam characteristics other than BNR have a stronger role in intramuscular temperature changes.
Our study was performed with the Omnisound 3000 Pro ultrasound device and transducers, which has been reported to have higher heating rates than other devices.20,21 It is unknown how different therapeutic ultrasound transducers from other manufacturers would affect heating rates. We suggest that future research continue to aid clinicians' understanding of heating characteristics of different ultrasound devices and transducers.
We measured intramuscular tissue temperature at a standard depth of 1 and 2.5 cm, which does not account for subcutaneous tissue and muscle depth variations. Subcutaneous fat thickness has not been significantly correlated with intramuscular temperature,23 but different thicknesses could affect muscle depth. Future research should determine how heating rates of different ultrasound devices and transducers affect clinical outcomes in specific pathological conditions, instead of healthy young adult participants (age range: 18 to 35 years).
Therapeutic ultrasound manufacturers often calibrate the ultrasound transducer to an individual device that will be sold on the market. In our study, we used one ultrasound device for the treatments of all five transducers. It is unknown how switching transducers on the ultrasound device throughout this study affected its calibration. The manufacturer stated that the transducers were properly calibrated before the study began.
Implications for Clinical Practice
We reconfirmed that transducers within the same manufacturer may have significantly different heating rates. The U.S. Food and Drug Administration only requires that a manufacture's average ERA and BNR be reported across multiple transducers in a batch, but individual transducer characteristics would be beneficial for clinicians to know to understand each individual transducer's tissue heating potential. If transducer characteristics are not reported for each individual transducer and frequency, clinicians should at least ensure proper calibration of ultrasound devices at producing correct output intensities.
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Characteristics of 5-cm2 Ultrasound Transducers
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