| Stationary Vs. Non-Stationary Ultrasound An evaluation by Mark Schafer, Ph.D. |
I have evaluated a number of different ultrasound devices for therapy, and would like to comment on
the differences between “stationary” and “non-stationary” ultrasound, especially with regard to the Hill
Laboratories’ ultrasound system.
Therapeutic ultrasound systems must satisfy two potentially conflicting requirements: to heat targeted
tissue, while at the same time, preventing any unwanted overheating or “hot spots”. This is especially
difficult for conventional Continuous Wave (CW) ultrasound devices, especially those with relatively
small scanheads. CW systems have the possibility of creating a “standing wave”. This occurs if the
sound is reflected back towards the transducer from some structure within the body, for instance, from
bone. In this case, the reflected wave interacts with the transmitted wave to produce regions of high
ultrasound energy concentration, again leading to “hot spots”. Further, most of these systems have
relatively small ERA values (representing beams with a diameter of only several centimeters) and
moderate BNR values (indicating some level of concentration). Therefore these conventional
ultrasound devices must be continually moved around on the body to avoid the potential for
overheating the tissue.
Thus the practice of moving the scanhead around on the skin (“non-stationary” application) is really a
compensation for some of these technical limitations of conventional technology.
Unfortunately, moving the probe around in order to assure safety can have some negative
consequences for efficacy. Because the probe is continually moved around by the operator, any one
volume of tissue is only exposed to ultrasound for some portion of the time, in general as a ratio of the
ERA to the total scanning area. Even with a conservative assumption of a scanning range twice as
large as the scanhead, this would reduce the effective dose at tissue by 50 percent. Thus the “non-
stationary” approach has the potential, in fact, to reduce the effectiveness of the ultrasound therapy
treatment.
The Hill Laboratories Hands-Free system has been designed to address the specific concerns with
overheating the tissue by specific technical solutions. First, it has a large ERA of 65cm2, equivalent to a
beam diameter of 6.4cm (2.5 inches). Second, it has a BNR of less than 5.0, so that the beam is not
particularly concentrated (the practical lower bound is on the order of 3.5 for ultrasound transducers).
Finally, the system operates in a Pulsed Wave mode to reduce standing waves. The total power is up to
10 acoustic Watts, which is typical of many therapeutic systems. These features combine to permit the
safe operation of the device in a “hands free” condition.
Thus it should be concluded that the Hands Free approach, as implemented in the HF54/HF27,
provides the same level of safety and efficacy as other therapeutic devices. The issue of “stationary”
versus “non-stationary” ultrasound should really be viewed as a safety and efficacy concern for the
“non-stationary” systems: they have a higher potential for either overheating tissue because of their
beam hot spots or standing waves, or a potential for ineffectual treatment because their beams do not
properly encompass the entire tissue to be treated.
Respectfully,
Mark Schafer, Ph.D.
President, Sonic Tech, Inc.
Chairman of the Technical Standards Committee of the American Institute of Ultrasound in Medicine
(AIUM)
Chairman of the Technical Committee of the Ultrasound Section of the National Electrical Manufacturers
Association (NEMA)
Vice-President of the Ultrasonic Industry Association
Board Member for the IEEE Ultrasonics Society
Call Hill Laboratories Toll Free 1-877-445-5020
Email: info@hilltherapy.com
Email: info@hilltherapy.com
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