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OPTIMIZING THERMAL ULTRASOUND 1
Ultrasound: Are Aquaflex Gel Pads Equally Effective as Gel Alone for Achieving Increased
Tissue Temperature within Optimal Parameters?
Texas A&M University- Corpus Christi
Clinical Question
OPTIMIZING THERMAL ULTRASOUND 2
When applying therapeutic ultrasound within optimal parameters, is using an aquaflex gel pad as
a medium equally effective as using gel alone for achieving increased tissue temperature?
P Young adults with no edema present or injury in prior 6 months at lateral posterior ankle
I Thermal therapeutic ultrasound with gel only as the medium
C Aquaflex gel pad
O Achieve vigorous heating for increasing joint ROM through effective stretching
Thermal ultrasound is a deep-heating modality that can be used for therapeutic purposes
including pain control, reducing chronic edema, reducing muscle spasms, releasing trigger
points, stretching collagenous tissue and increasing joint range of motion (Rubley & Touton,
2009; Bishop, Draper, Knight, Feland, & Eggett, 2004). In order for any therapeutic effects to be
achieved however, effective heating of the target tissue must take place. Therefore, it is
important to investigate and compare the methods of applying ultrasound through gel alone or
with an aquaflex gel pad because if using a gel pad as a medium does not have the potential to
produce the same vigorous heating as ultrasonic gel alone it is highly inappropriate to continue
using this method in the clinical setting.
According to Rubley and Touton (2009), previously, studies on thermal ultrasound have
generally focused on the deep-heating aspect independently of method; but as the study of
therapeutic modalities continues to progress, it is necessary to presently focus on what specific
aspects influence the effectiveness of deep-tissue heating received from therapeutic thermal
ultrasound. Before the outcomes of using gel alone compared to using an aquaflex gel pad as a
medium can be understood, it is relative to discuss the other main variables that have influence
as well which are frequency, intensity, and treatment duration. These factors can predict the level
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of heating the target tissue attains. 1C is mild, 2-3C is moderate and >4C is vigorous heating
(Rubley & Touton, 2009). After establishing the optimal parameters for applying therapeutic
ultrasound, the question, “Is using an aquaflex gel pad as a medium equally effective as gel alone
for achieving increased tissue temperature?” can be answered.
Bishop et al. (2004) reports that gel pads transmit 27% more ultrasound energy, which
theoretically should lead to a greater increase in tissue temperature. They used 18 college
students from 19-27 years old to test this theory. The 13 women and 5 men had no edema or
injury present at their lateral posterior ankle for the previous 6 months; the frequency was 3 MHz
with a 1.0 W/cm2 intensity and a treatment duration of 10 minutes while moving the head at a
speed of 4cm/s. Rubley and Touton (2009) also reported multiple studies set up with the same
parameters for testing vigorous heating. This is most likely due to the known ultrasound chart
that calculates the application of the heat, establishing these parameters as optimal for effectively
achieving vigorous heating of the tissue surrounding the ankle.
In the study performed by Bishop et al. (2004) the tissue temperature was measured every
30 seconds directly through a catheter inserted halfway between the lateral malleolus and the
Achilles tendon while the patient laid prone. They chose to use a 3 MHz frequency because the
aquaflex gel pad has optimal transmission at < 2.5 cm, which corresponds with superficial tissue.
Thus using an aquaflex gel pad with a 1 MHz frequency is inappropriate and ineffective. It can
be inferred they chose to use a 1.0 W/cm2 frequency in order to have the ability to perform a 10
minute treatment without overheating the tissue. In fact, Rubley and Touton (2009) report
patients commonly reported discomfort at 41C, likely due to the fact the general baseline for
tissue temperature is accepted at 36C thus making the discomfort usually seen once heating
passes the vigorous level of 4C. The effective radiating area (ERA) was 4 cm2, therefore the
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treatment area could cover a maximum of twice the ERA or 8 cm2. Bishop et al. (2004)
recommend using a 3:1 beam nonuniformity ratio and a metronome to pace movement at 4 cm/s.
It is important to note studying vigorous heating for achieving increased tissue temperature was
chosen because that is where the stretching window of 3 minutes for collagenous tissue leading
to an increase in joint range of motion (ROM) can be observed optimally.
Rubley and Touton (2009) point out that different methods can influence heating effects
differently. In the study performed by Bishop et al. (2004) using ultrasonic gel alone produced
the greatest increase in tissue temperature, however there was not a significance difference
between the increased tissue temperature observed from using gel alone or using an aquaflex gel
pad with a layer of gel on the top and the bottom of the pad as a medium. Although there was a
significant difference for increasing tissue temperature when using a gel pad with a layer of gel
on the top only and placing the gel pad itself directly on the skin; 8 out of the 18 patients also
reported discomfort with the later medium (Bishop et al., 2004) possibly resulting from air
pockets which can produce spikes in intensity from the energy not be completely absorbed.
Ultimately, the outcome saw using aquaflex gel pads as a medium for applying thermal
ultrasound was equally effective as using gel alone only if there was ultrasound gel coating the
top and bottom sides to the pad. Both methods yielded > 4C increased tissue temperature which
achieves vigorous heating (Bishop et al., 2004). Rubley and Touton (2009) suggest to observe
optimal therapeutic effects of thermal ultrasound, it is in the best interest of the clinician to be
familiar with not only the methods of application and appropriate parameters, but the ultrasound
unit as well.
In conclusion, to maximize treatment effectiveness it is important to know the ERA of
the unit and to not treat an area greater than twice that size. Correctly calculate what frequency
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and intensity are being used, specific for the target tissue, in order to stop treatment prior to or
once an increase of 4C is achieved for vigorous heating; also be familiar with the stretching
window of 3 minutes post-treatment for use with increasing joint ROM. The speed of the head
should be no more than 4 cm/s. According to Rubley and Touton (2009) high water content will
also contribute to maximizing energy transmission and will minimize the absorption. This is
important because this is likely the reason coating an aquaflex gel pad on both sides with
ultrasound gel provides the same effectiveness for increasing tissue temperature as gel alone;
when the skin is in contact with the gel it sets up the optimal therapeutic window referred to in
the Arndt-Schultz Principle and Law of Grotthus-Daper which overall state the energy cannot be
too great for the tissue otherwise it will be destructive but also, if the energy is absorbed too soon
the treatment is inadequate and no therapeutic effects will be seen. It is the clinician’s choice to
use an aquaflex gel pad or gel alone, although using an aquaflex gel pad alone may not be the
best method of application. When used in combination with ultrasonic gel on the top and bottom
sides, both methods are equally effective for increasing tissue temperature.
References
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Bishop, S., Draper, D. O., Knight, K. L., Feland, B., & Eggett, D. (2004). Human tissue-
temperature rise during ultrasound treatments with the aquaflex gel pad. Journal of
Athletic Training, 39(2), 126-131.
Rubley, M. D., & Touton, T. M. (2009). Thermal ultrasound: It’s more than power and time.
Athletic Training Today, 14(1), 5-8.
