BTL – 07p
operating manual & user's guide
page 11
II. USER'S
GUIDE
Mechanical waves with a frequency higher than 20,000 Hz are called ultrasound. Ultrasound is not generated
electronically for the application of therapy and is classified as mechano-therapy.
Mechanism of action
If a minimal air gap between emitter head and body surface is eliminated, the emitter head vibration will be more
easily transferred deeply into the tissues in the form of longitudinal waves. Every cell in the path of the ultrasonic
"beam" will begin to oscillate: a micro-massage is followed by a transformation from gel to sol (jelly structures
becoming liquefied) and transformation from mechanical to thermal power, with deep warming of tissues.
The amount of heat generated is dependent on the amount of energy that is absorbed. Other effects of ultrasound
(solution degassing, cavitation arising of cavities in humour, and local alkalization) are negligible with the small
intensities and ultrasound doses that are used for physical therapy.
The prophylaxis of ultrasound side-effects must include the knowledge that molecular oscillation occurs not only in
the course of the ultrasonic beam but, as a result of humour transportation to areas that are distant from the
application site, may possibly result in the restoration of former epistaxis or the acceleration of menstruation.
The quality of an ultrasonic beam and its distance from the emitter head determine its characterization as a close
or distant ultrasonic field. Great variation in ultrasound intensity caused by interferential effects and low beam
divergence is typical characteristics of a close ultrasonic field. The length of the close field is directly proportional
to the effective emitting surface of the emitter heads (ERA) and indirectly proportional to the frequency. For
example, the length of a close field with a 4 cm
2
ERA emitter head and a 1 MHz frequency is approximately 10
cm; the length of a close field with a 1 cm
2
ERA emitter head and a 1 MHz frequency is 2 cm.
The characteristics of a distant ultrasonic field are increasing beam divergence, gradual decrease in intensity and
almost no interferential processes.
Therapeutic effects take place mostly in a close field. Since the ultrasonic beam in a close field shows marked
interferential effects (interference of applied and reflected waves, both constructive and destructive), the result is
an uniform ultrasonic beam. The beam’s peaks of intensity (local increase in intensity caused by constructive
interference) may reach values many times higher than the preset value (the rise of stationary waves).
The Beam Non-uniformity Ratio (BNR) illustrates how often peaks of intensity exceed preset values. This value is
characteristic for an ultrasonic head when the frequency is fixed. A good quality head should not exceed 6. With a
preset intensity of 1 W/cm
2
, the intensity should not exceed 6 W/cm
2
in any part of the ultrasonic beam.
The BNR of older ultrasonic heads, as well as many of the newer ones (where the producer does not mention
the BNR), is often 20 or more!
The Effective Radiating Area (ERA) is always smaller than the actual surface area of the emitter head (the ERA is
determined by the size of the piezoelectric crystal or ceramic table that generates ultrasound by oscillating). The
dose of ultrasound (amount of power emitted to surface area) is therefore related to the ERA, not to the actual
surface area of the emitter head.
Refraction and reflection of ultrasound
follow the laws of wave physics. They originate from ultrasonic wave
transmission between tissues and the differences in vibration flow speed in these tissues. Therapeutic application
must include elimination of the air gap between the emitter head and skin (a 10
-9
m thick air gap causes 99 %
ultrasonic wave reflection because of the marked difference in the air and skin flow speed. With a pre-set intensity
of 1.0 W/cm
2
, the tissue receives only 10 mW/cm
2
- energy without any biological effect). Therefore, modern
ultrasonic heads have a built-in optic and/or acoustic unit that gives a signal and stops the time counter when
contact is insufficient.
Ultrasonic beam power increases (constructive interference) and decreases (destructive interference) occur as a
result of interference in the close field, which reaches its peak (up to 35 %) at the soft tissue-bone boundary.
It is necessary to move the emitter head continuously (see below) to prevent tissue lesions at
peaks of intensity.
The BTL-07p signals when there are stationary waves and stops ultrasonic power generation until stationary
waves are eliminated through more rapid movement of the emitter head.
As a result of reflection and constructive interference increases in local intensity and temperature may occur,
particularly in the periosteum-bone boundary. This increase can cause periosteum pain during treatment. When
this occurs, the intensity must be lowered immediately.
Ultrasound is primarily absorbed in deep tissues. Since these tissues contain virtually no thermoreceptors, it is
impossible to perceive a rise in local temperature. Nociceptive receptors are irritated and the patient feels pain
only if the local temperature exceeds 45
ο
C. Most experts agree that a short-lasting local temperature of 45
ο
C
is not dangerous.
As far as classic inflammation (edema, erythema, local temperature increase, pain, and non-function) is
concerned, additional heat production is contraindicated. In certain cases, only a pulse ultrasound (athermic) can
be applied.
In the peracute period of post-traumatic states (up to 24 - 36 hours), even pulse ultrasound application is
contraindicated (resulting vibrations obstruct capillary
proliferation and may cause
laggard bleeding).