Journal of Sport Rehabilifafion,1997,6,47-53
0 1997 Human Kinetics Publishers, Inc.
The Transducer Pressure Variable:
Its influence on Acoustic
Energy Transmission
Brian Klucinec, Craig Denegar, and Rizwan Mahmood
During the administration of therapeutic ultrasound, the amount of pressure at
the sound head-tissue interface may affect the physiological response to and
the outcome of treatment. Speed of sonification; size of the treatment area;
frequency, intensity, and type of wave; and coupling media are important parameters in providing the patient with 'an appropriate ultrasound treatment.
Pressure variations affect ultrasound transmissivity, yet pressure differences
have been virtually unexplored. The purpose of this study was to assess the
effects of sound head pressure on acoustic transmissivity. Three trials were
conducted whereby pig tissue was subjected to increased sound head pressures using manufactured weights. The weights were added in 100 g increments, starting with 200 g and finishing with 1,400 g. Increased pressure on
the transmitting transducer did affect acoustic transmissivity; acoustic energy
transmission was increased from 200 g (0.44 lb) up to and optimally at 600 g
(1.32 lb). However, there was decreased transmissivity from 700 to 1,400 g
(1.54 to 3.00 lb).
Therapeutic ultrasound may be the most commonly used modality by athletic trainers and physical therapists. Extensive research has been completed analyzing the effects of ultrasound on tissue temperature (3) and coupling agents (2,
4, 8, 10). It has been accepted that ultrasound treatments increase blood flow, decrease joint stiffness, decrease muscle spasms, resolve chronic inflammation, and
modulate pain (3, 7). However, to achieve the desired benefits, ultrasound treatments must be conducted using proper techniques and guidelines. Such guidelines
involve the selection of treatment time, intensity, treatment area, frequency, and an
eficient coupling agent.
Brian Klucinec was a student at Slippery Rock University at the time of the study and is
currently a student at the Krannert School of Physical Therapy, University of Indianapolis,
Indianapolis, IN 46227. Craig Denegar is with the Department of Orthopedics and Kinesiology, The Pennsylvania State University, University Park, PA 16802. Rizwan Mahmood is with
the Department of Physics, Slippery Rock University, Slippery Rock, PA 16057.
48
Klucinec, Denegar, and Mahrnood
When ultrasound is applied for a thermal effect, treatment time is important
to obtain a tissue temperature high enough to yield therapeutic results (3, 7). The
intensity, or dosage, is governed by the rate of tissue heating (1, 3,7). The size of
the area being treated should be appropriate for the effective radiating area (ERA)
of the sound head (3, 7). The speed at which the sound head is moved should
facilitate tissue heating while preventing "hot spots" caused by beam nonuniformity.
The frequency of the sound unit should be selected in relationship to the depth of
the target tissue (3, 9). Finally, a highly transmissive, low-attenuating coupling
agent should be selected (2, 4, 8, 10). These factors determine the optimal setup
for an ultrasound treatment. One factor that has been virtually unexplored is the
effect of pressure.
Pressure is defined as the force exerted per unit area (5). Typically, clinicians are unaware of the amount of pressure that they are applying, via the ultrasound transducer, to their patients. However, the pressure exerted through the sound
head may affect acoustic transmissivity and, therefore, the amount of sound energy delivered to the tissues. The purpose of this study was to explore the effects of
pressure on acoustic transmissivity.
Materials and Methods
In an effort to study the range of forces applied in a clinical setting, we recruited 16
athletic training students and faculty members, experienced in the use of ultrasound, who applied pressure to an Aquaflex gel pad (Parker Laboratories, Orange,
NJ) placed on a digital scale. They were asked to apply the same amount of pressure that they would typically apply during an ultrasound treatment. The mean
pressure was 544.32 g (1.2 lb) t 3 17.52 g (0.70 Ib). Therefore, we elected to apply
forces between 200 and 1,400 g. We found that applying 17,600 g (8 lb) of pressure, as suggested by Warren et al. (lo), damaged the gel pad. We also determined
the force per unit area exerted by the 5.0 cm2 sound head to be 45.45 g/cm2.
A 1 MHz Intelect 225P ultrasound unit (Chattanooga Corp., Chattanooga,
TN), with an ERA of 4.0 cm2and a beam nonuniformity ratio (BNR) of 3.5-4.0: 1.O,
was used to deliver acoustic energy to pig tissue. The unit was calibrated prior to
the first trial. Adetachable transducer having the same ERA was utilized to receive
the signal sent by the 1 MHz unit by patching its coaxial cable into an oscilloscope
via a Bus Net Connector (BNC). For recording purposes, a 100 Msls 450 Gould
digital storage oscilloscope (Gould Inc., Ilford, United Kingdom) was used to record
peak-to-peak values for each trial. For accuracy and consistency, a two-dimensional level was used to fix all movable instruments.
To perform the calibrating and experimental trials, a platform apparatus (Figure 1) was constructed of steel, aluminum, and Plexiglas. The foundation consisted of
a 50-lb aluminum block with 114-in. holes drilled into it at I-in. intervals. Four 114-in.
rods were fixed in these holes 7 in. apart and secured by nuts and washers. Two pieces
of Plexiglas were made to slide vertically along these rods, creating two horizontal
Transducer Pressure
Figure 1 - The constructed apparatus.
platforms. In the center of the Plexiglas platfonns, holes were drilled to house the
transducer heads. The bottom piece of Plexiglas was fixed and housed the receiving
sound head. The top piece of Plexiglas was compressible and housed the transmitting
transducer. The detachable sound head was fixed in a hand-tightened vise surrounded
by foam to protect the transducer. The ERA of the detachable sound head was then
made level and flush with the bottom piece of Plexiglas.
The transmitting transducer was placed in the hole drilled in the top piece of
Plexiglas, allowing the ERA to be parallel to the ERA of the receiving sound head.
We then fixed the transmitting sound head to the top plate of Plexiglas by creating
a harness for the transducer neck. This harness was made from two pieces of
thicker Plexiglas. One piece was fixed with bolts, while the other was allowed to
move by slots to adjust for tightness. Fixing the transmitting transducer created
only one movable piece throughout the duration of the experiment. A steel rod was
suspended over the ERA of the transmitting sound head to stabilize the weights
that were added on top of the sound head for each of the trials.
Klucinec, Denegar, and Mahmood
50
Three pilot trials were conducted to test the accuracy of the apparatus by
obtaining a consistent baseline of energy transmission following repeated setup
and dismantling of the testing apparatus. These trials were conducted using pig
tissue and Ultrasonic 100 ultrasound gel (Parker Laboratories, Orange, NJ). Each
trial was conducted by placing ultrasound gel at each interface to prevent reflection of the acoustic energy. The transmitting transducer was then lowered until it
contacted the tissue sample. The compressible piece, supported by springs, was
then made parallel to the bottom piece using the two-dimensional level. For data
recording, the ultrasound unit was turned on, set to a continuous duty cycle, then
manually adjusted to an intensity of 0.5 W/cm2.After each trial, the intensity was
turned down and the unit was shut off. The constructed apparatus was cleaned and
examined for debris that may have interfered with the test. During this study, ultrasound transmission was measured, by peak-to-peak voltage differences, at an intensity of 0.5 W/cm2 with weights being applied to the transducer. The weights
added were 200 to 1,400 g at 100-g increments.
Pig tissue was selected because the ratio of skin, fat, muscle, and bone is
similar to that of humans (6). The tissue sample had the dimensions of 12.7 by 8.9
cm with 0.95 cm of subcutaneous fat. The tissue was obtained fresh, was then
frozen, and finally was thawed in lukewarm water to room temperature. Room
temperature was recorded by a digital thermoprobe at 26.0 "C. Prior to beginning
of each trial, the Plexiglas, transducers, and tissue were cleansed with tap water.
This was done to free the instruments and tissue of debris that may have accumulated.
Three trials were conducted using a series of 100-g weights for each trial.
Care was taken to place an adequate amount of ultrasound gel at each interface to
prevent reflection and to eliminate the possibly of air at the interfaces. For each
trial, the movable segment of the apparatus was lowered until contact was made
with the tissue. This piece was then made parallel to the fixed bottom piece. The
ultrasound unit was turned on and manually set to an intensity of 0.5 W/cm2.After
each 100-g increment was added, the peak-to-peak voltage was read and recorded
immediately. Following each trial, the intensity was turned down and the unit was
shut off. The tissue sample was not subjected to acoustic energy for long periods
of time because heating would reduce the impedance of the tissue, causing voltage
fluctuation unrelated to the sound head pressure. A period of 10 min was allotted
between each trial to allow the tissue to return to normal shape after being compressed. This period would also allow the tissue to return to a baseline temperature
if any heating had occurred during the trial.
Results
Peak-to-peak voltages for the calibration trials are presented in Table 1 and for the
experimental trials in Table 2. The data show that pressure does affect acoustic
energy transmission. More acoustic energy was transmitted when weights up to
Transducer Pressure
51
Table 1 Peak-Peak Voltage Output With the Addition of the Specified Weight to
the Sound Head for Calibration Trials 1,2, and 3 at 0.5 W/cmZ
Weight (g)
Trial 1 (V)
Trial 2 (V)
Trial 3 (V)
Average (V)
Table 2 Peak-Peak Voltage Output With the Addition of the Specified Weight to
the Sound Head for Experimental Wals 1,2, and 3 at 0.5 W/cmZ
Weight (g)
Trial 1 (V)
Trial 2 (V)
Trial 3 (V)
Average (V)
Klucinec, Denegar, and Mahrnood
52
and including 600 g were added. However, weights at 700 g and above reduced the
amount of sound energy transmitted.
Discussion
Prior to this study, the effects of pressure on acoustic energy transmission had been
virtually unexplored. Warren et al. (10) reported that the density of the coupling
agent affected sound head pressure and acoustic transmissivity. Warren also suggested that 17,600 g (8 lb) of pressure is typically applied in the clinical contact
application of ultrasound (lo), but we found that significantly less pressure is typically applied. The relatively high standard deviation we obtained when assessing
applied pressures suggests inconsistency among clinicians in administering ultrasound treatments. This reinforces the idea that clinicians should be aware of the
force being applied to their patients.
We observed that increased sound head pressure affects acoustic transmissivity. We believe that exerting a firm amount of pressure at the sound head-tissue
interface maximizes the transmission of acoustic energy and thus the rate and extent of tissue heating.
16
0
data points
15
I
-
(rJ
.c-.
14
3'
13
12
0
400
800
1200
1600
Weights in grams
Figure 2 -Mass (g) versus average peak-to-peak voltage (V) for experimental trials 1-3.
Transducer Pressure
53
The relationship between mass (g) and peak-to-peak voltage output is depicted graphically in Figure 2. The reasons for the increase in acoustic transmissivity up to 600 g and the drop-off with greater pressures are not clear. It is possible
that lighter pressures may permit air to be trapped at the tissue interface, while
heavy pressures disperse the conducting gel. Also, it is possible that compression
of the tissue sample may have influenced the acoustic transmission. Sound head
pressure may have a different effect when sonating living tissue. Further investigation may provide the answers to these questions.
It is clear that frequency, dosage, treatment area, effective radiating area of
the sound head, beam nonuniformity ratio, duty cycle, and the speed at which the
sound head is moved affect the physiological response to treatment. Our results
suggest that the pressure applied through the ultrasound head may also affect transmissivity and, therefore, response to treatment.
We recommend that clinicians use a firm, constant pressure during ultrasound treatments. However, the clinician should exercise caution to not apply an
excessive amount of pressure, which could decrease acoustic transmissivity, make
the patient uncomfortable, damage the sound head crystal, or cause other problems that would interfere with treatment. Last, the clinician should be aware that
the results obtained in this study warrant further investigation, and that the amount
of pressure applied, via the sound head, may vary with each individual.
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