Therapeutic Ultrasound
Therapeutic Ultrasound
Sound
is the mechanical disturbance of an elastic medium such as air.
Vibrations which travel through the air or another medium are called as
sound. The frequency of hearing sound is 20Hz – 20,000Hz.
Ultrasound means the above the frequency of sound or the sound waves which are produced above the 20,000 Hz.
Production of Ultrasound:
For
a one mega Hz frequency machine vibrating source with a frequency of
one million pulses/s (million cycles/s) is needed. This is achieved by
using either quartz crystal or barium crystal. These crystals deformed
when subjected to various potential differences (Piezoelectric effect).
The basic components of Ultrasound apparatus are the following.
o Source if high frequency current
o Cable
o Link electrodes
o Barium or Quartz crystal
o Metal plate
Working:
Source
of high frequency current which is conveyed via co – axial cable to a
transducer circuit or ultrasound head. Inside the ultrasound head the
high frequency current is applied to a quartz or barium crystal through
link electrode, the meal plate being fused with the quartz crystal. Any
change in the shape of quartz crystal causes movement of the metal plate
which in turn produces ultrasound waves. Strict control of high
frequency current (1 MHz or 3 MHz) produces a steady or regular
ultrasound waves.
Parameters of ultrasound:
Intensity (watt):
The
unit of intensity when using ultrasound is watt. But this is a gross
measure of power transmitted by the treatment head. So, an average
intensity will be used. There are two types of intensities.
Space average intensity:
The average intensity over a specified area is given (watt/cm²). This is used for continuous ultrasound.
Time average/space average intensity:
It
can be given when ultrasound being applied in a pulsed mode and gives
average intensity over the whole treatment time for a specified area
e.g. if 0.5 watt/cm² is applied in a pulsed mode at 1 : 4 then in 1
second, the average intensity will be 0.1 watt/cm², if ultrasound is
continuous.
Reflection of ultrasound:
Sound waves obey
the laws of reflection and if an ultrasound beam travelling through one
medium passes to another medium (encounter) which will not transmit
this ultrasound beam, reflection will take place. For example, air will
not transmit the ultrasound waves. So in ultrasound treatment care
should be taken that air will not left between the treatment head and
the body surface to minimize reflection.
However, there will
be some reflection at interface that ultrasound beam encounter; this
will give rise to acoustic impedance. If acoustic impedance is low
transmission will be high and vice versa.
Transmission of ultrasound:
If
the ultrasound beam encounter between two media and is transmitted it
may be refracted deflection from original path when travelling from one
medium in which its velocity is low to a medium having high velocity, it
might be refracted from its normal path.
Significance of
refraction is that if x is a target refraction would cause the
ultrasound beam to miss it. Refraction will not occur when the incident
wave travelling along the normal direction, treatment should be given
with majority of wave travelling along the normal direction. Refraction
will not occur if the ultrasound head is used perpendicular to the body
surface.
Attenuation of ultrasound:
Reduction in the
intensity of ultrasound beam once it has left the ultrasound head is
called attenuation of ultrasound. Reduction may occur by the following
two processes.
· Absorption
· Scatter
Ultrasound
beam are absorbed and converted into heat and reduced its intensity is
called absorption. This occur when normal ultrasound beam is deflected
from its path by refraction i.e. air bubbles in the tissues. The overall
affect of these two as such that the ultrasound beam is reduced in
intensity, the deeper it passes, this give rise to the expression of
half values distance which the depth of the soft tissues that reduces
the ultrasound beam to half its surface intensity. The half value
distance for soft tissue is different for different frequencies i.e. for
1MHz is 4cm and for 3MHz is 2.5cm. In practice (treatment practice)
when treating deep structures consideration needs to be given to the
frequency and intensity of ultrasound.
Coupling media:
Ultrasound
cannot be transmitted through air therefore medium is used for it. 100
%waves cannot be transmitted but only %age can be transmitted through
skin having medium. The following are the media which transmit the
ultrasound waves.
Aqua sonic gel
|
72.6 %
|
Glycerol
|
67 %
|
Distal water
|
59 %
|
Liquid paraffin
|
19 %
|
Petroleum
|
0 %
|
Air
|
0 %
|
Air
will in fact reflect the ultrasound waves back into the treatment head
and this could set up the standing waves which might damage the crystal.
Physiological effects of ultrasound:
Thermal effects
Non thermal effects
Thermal effects:
The heat absorbed by the tissue depends upon the following.
a. Absorption of tissue:
Protein absorbs more heat. So, tissue having more protein will absorb heat.
b. Insonated tissues:
Those tissues which are exposed to ultrasound produce much heat which have power of increase circulation.
c. Number of treatment sessions
d. Continuous produce ultrasound
e.
Periosteum will reflect the ultrasound waves leading to standing waves.
It causes burning sensation in bony area. So avoid ultrasound on bony
prominence.
Non thermal effects:
i. Mechanical effects (micro-massage):
This
is where the longitudinal compression waves of the ultrasound produce
compression/rarefaction of the cells and effects the movement of the
tissue fluid in the interstitial space. This can help to reduce the
oedema combined with the thermal effect, the extensibility of the scar
and adhesions could be affected in such away to make stretching them
easier.
ii. Biological effects:
It helps to reduce inflammation.
·
Inflammation: ultrasound increases the fragility of lysosome and thus
enhances the release of their enzymes. These enzymes will help to clear
of debris (waste product) and allow the next stage to occur.
·
Proliferative stage: Fibroblast and myofibroblast may have calcium ions
driven into these injured area and the collagen fibers are formed. This
is called proliferative stage.
· Remodeling stage
Uses of ultrasound:
Recent injury and inflammation:
Mechanical
effect causes the removal of exudates. Thermal effects accelerate
protein synthesis which helps in the repair of the damage tissue.
Chronic scars:
The thermal effect of the ultrasound is also use for the removal of chronic scar by stretching the tissues.
Chronic oedema:
Mechanical effect and thermal effect of ultrasound help in the removal of oedema.
Dangers of ultrasound:
Burn:
If
the continuous ultrasound is used with stationary head will cause burn
in the tissue. When ultrasound is use o bony prominence, the waves
cannot be absorb by the Periosteum and reflect them causing burn in the
underlying tissues.
Damage to the equipment:
When the
head of ultrasound treatment remains in air will produce standing waves
which damage the treatment head crystal of ultrasound.
Contraindications of ultrasound:
· Thrombophlebitis
· Tumors
· Radiotherapy (up to 6 month) has devitalizing effect on the tissue so ultrasound is contraindicated
· Pregnancy
. Cardiac diseases
Phonophoresis (sonophoresis) (ultrasonosonophoresis)
Phono means sound and phoresis means migration of the ions through a membrane by the action of an electric current.
Phonophoresis
is defined as the movement of the drugs through skin into the
subcutaneous tissues under the influence of ultrasound.
Phonophoresis depends upon:
· Intensity of ultrasound
· Frequency of ultrasound
· Nature of drugs
In
1950s, they said that the drugs goes 4 – 5 cm deep to the skin while
recent study shows that drugs go 1 -2 mm deep into the skin.
 |
| Phonophoresis |
Indications:
· Bursitis
· Tendinitis
·
Medical interest of transcutaneous systemic delivery of substances such
as insulin that cannot be delivered effectively by mouth
· For superficial effects, cutaneous anaesthetic are delivered by frequency devices
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