Three-megahertz ultrasound delivered through a 100% ultrasound gel medium produced the highest temperature increase of the 3 coupling media groups.11–,13
Although the gel was superior in our study, the gel pads still produced a heating effect. Thus, the gel pad, whether 2 cm or 1 cm thick (with a thin coating of ultrasound gel on top to reduce friction), provides an adequate coupling medium for therapeutic ultrasound when indirect ultrasound is indicated.
Based on our findings, we believe that the thinner the medium, the greater the tissue heating. In fact, even though the 2-cm gel pad resulted in an increase of 6.5°C, the peak temperature was only 34.8°C. Because the treatment area was an extremity, we did not expect temperatures to reach 39°C–40°C; however, the thinner gel medium did reach 42°C.
In our study, the thickness of the gel pads was important. Casarotto et al7
investigated the thickness of coupling media with 4 commonly accepted coupling agents. White petroleum was compared with ultrasound gel, mineral oil, and degassed water at thicknesses of 0.3 mm and 0.5 mm. Their patients felt the transducer head became hotter with use of white petroleum. In general, when the thickness of the coupling agent increased, the transducer temperature increased, and the transmissivity (measured by the heat produced in the tissue) decreased.7
However, they used different coupling agents, so their results need to be taken with caution.
Other researchers have studied the thickness of ultrasound media. Poltawski and Watson9
reported that when the layer of couplant was increased from 0.2 mm to 0.6 mm, the mean transmitted power output from the transducer was reduced from 6.2 W to 5.85 W. Although this reduction in transmissivity was statistically significant, the authors suggested it might also become clinically significant with different treatment frequencies and settings.9
Our results also indicate that the transmissivity of the ultrasound was reduced as the thickness of the coupling media increased, but this finding may be due to the difference in our measurement techniques. Poltawski and Watson9
measured the wattage that passed through the coupling media; we chose to measure transmissivity by the temperature increase in the target tissue.
Our results differed from those of Bishop et al4
and Merrick et al6
in that we found a difference between using gel alone and using the gel pad. This may have been due to methodologic differences among the studies. We treated the Achilles tendon, whereas Bishop et al4
treated muscle. We applied ultrasound gel only to the top of the gel pad, whereas Bishop et al4
applied it to both the top and bottom of the pad. Merrick et al6
did not apply any gel to the gel pad and still found that gel alone and ultrasound gel pads were equivalent. The different results may have been due to the depth of the measurement (3 cm below the skin surface) and the tissue treated (muscle of the medial calf) used by Merrick et al.6
The rate of temperature increase per minute in tendon using 3 MHz ultrasound at 1 W/cm2
has been reported as 3.45 times as high as that of the heating rate of muscle tissue.14
Chan et al14
demonstrated that when treating the patellar tendon in an area 2 times the effective radiating area of the sound head at 3 MHz and 1 W/cm2
, temperature increased at a rate of 2.1°C per minute, compared with 1.2°C per minute for 4 times the effective radiating area.
Our results indicate that the temperature rise in the tendon was not as drastic as that noted by Chan et al.14
When using gel only and treating the Achilles tendon at the same settings, we found the temperature rise to be 1.33°C per minute. This value is 2.2 times the established 0.6°C per minute temperature rise found in human muscle tissue,15
compared with the 3.45 times greater value seen by Chan et al.14
We believe that the difference noted between these results may stem from the presence of bone in the treatment field in the Chan et al14
study, causing ultrasound energy to reflect back into the tissue and increasing the heating rate. In our study, no bone was present in the treatment field, preventing the potential reflection of ultrasound energy into the treatment tissue.
Merrick et al6
compared the same 2-cm-thick gel pad that we used with 100% gel and direct ultrasound. They reported no difference between the coupling media but noted that the thickness of the gel pad may have been a factor.6
They hypothesized that a thinner gel pad might have increased the temperature further. These different results than in our study may have been due to the treatment frequencies (1 MHz, 1.5 W/cm2
), duration (7 minutes), and tissue targeted (triceps surae) by Merrick et al.6
We found the mean temperature rises were 6.5°C ± 0.72°C for the 2-cm pad and 9.3°C ± 0.75°C for the 1-cm-thick pad. Thus, pad thickness may affect the length of the ultrasound treatment necessary to achieve a vigorous heating. We also found differences between the use of gel alone and either of the gel pads.
Our primary focus was to determine if peak temperatures differed during ultrasound treatments using 2 thicknesses of gel pads. If we look at the peak temperatures reached for therapeutic effects, only the ultrasound gel produced heating well above 40°C. The average temperature of this group was 42°C, high enough to be therapeutic but below the threshold for tissue destruction (45°C). This group started at a low baseline (30°C), which is not uncommon in a limb.
Our results contrasted with those of Bishop et al4
with regard to patient comfort. Of 18 participants, 8 reported that the treatment was uncomfortable when using the gel pad alone but not when the gel pad was coated on both sides with gel or the gel was used alone for a 10-minute treatment. Only 1 of our volunteers reported feeling uncomfortable and asked for the treatment to be stopped during the 10-minute treatment; this was in the gel-only group. Coating the gel pad only on the top with gel might have diminished discomfort during this procedure.
By far, the ultrasound gel produced the highest temperatures in our study. We did not objectively identify any differences in how the gel pads conformed to uneven areas. When the areas being treated are less contoured, we believe gel is the best option. However, in very contoured areas, where the sound head does not lie flat on the skin's surface, we think the 1-cm gel pad could be advantageous. The 1-cm gel pad may also be beneficial when treating open wounds, providing a sterile, bacteriostatic, conforming coupling medium to place over the wound. The gel pad may allow for reduced friction between the coupling media and the wound.
The cleanup required to remove gel from the wound may be eliminated by using the 1-cm gel pad. Presence of the pad may also prevent the sound head from coming too close to, or in direct contact with, the open wound and contaminating the gel, which may reduce the risk of cross-contamination among patients. Our results demonstrate that when indirect ultrasound is indicated, such as over a bony prominence or an open wound, the 1-cm-thick ultrasound gel pad provides sufficient coupling to achieve a heating effect.
The difference between the heating rates of the 2-cm and 1-cm pads may also be clinically significant: 6.5°C ± 3.12°C for the former and 9.2°C ± 2.97°C for the latter (). All temperatures, except for the single participant who ended the treatment early, were highest when the treatment ended at 10 minutes.
Mean temperature rise per minute for the 1-cm-thick and 2-cm-thick gel pads and gel only.
The results of this study are limited by the following factors: participants' age range (18 to 30 years), location of the treatment (posterior aspect of the human Achilles tendon), ultrasound machine (Omnisound 3000), depth of the treatment tissue (1 cm), length and settings for the treatment (3 MHz, continuous, 1 W/cm2, 10 minutes), size of the treatment area (twice the size of the sound head), speed of the movement of the sound head (approximately 4 cm/s), and the coupling agents (gel only, 2-cm-thick gel pad, 1-cm-thick gel pad).