Shortwave Diathermy

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167
7 Therapeutic Fields: Shortwave Diathermy
'Shortwave diathermy' refers to heating of deeply located tissue using electric or
magnetic fields which alternate at high frequency. The term 'shortwave diathermy', is
something of a misnomer as the contribution of waves, as such, to the treatment is
negligible. The physiological effects are a result of electric and magnetic fields
generated by the shortwave diathermy apparatus. Shortwave radiation plays little or
no role in the therapy.
The apparatus used by physiotherapists generates alternating electric and magnetic
fields with a frequency of 27.12 MHz. Since radio waves with frequencies in the range
10 MHz to 100 MHz are termed short waves the term has been, rather inappropriately,
applied to this therapeutic modality.
While shortwave diathermy
units do radiate waves with a
frequency of 27.12 MHz, this
is a side-effect. The
physiological effects are due
to the powerful electric or
magnetic fields generated by
the apparatus.
PRODUCTION OF THE FIELD
Shortwave diathermy apparatus consists of a
sinewave generator circuit which produces
alternating current with a frequency of 27.12
MHz and a resonant circuit which can be
tuned to exactly the same frequency. The
sinewave generator supplies energy to the
resonant circuit by transformer action. Figure
7.1 illustrates the arrangement.
Figure 7.1
Shortwave diathermy
apparatus (schematic).
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The sinewave generator consists of a power supply (chapter 5), an oscillator with
good frequency stability (chapters 2 and 5) and a power amplifier (chapter 5). The
power supply converts AC from the mains (of frequency 50 Hz) to DC which is needed
to power the equipment. It consists of a transformer (to convert the 240 V AC from the
mains to the voltage needed by the rest of the circuitry), and a rectifier to convert the AC
to DC. The DC is used to power a sinewave generator; a resonant circuit which
oscillates at 27.12 MHz and an amplifier, which boosts the current produced by the
resonant circuit to higher levels, as needed for patient treatment.
Electrical energy produced by the sinewave generator is coupled to the patient tuning
circuit by transformer action (figure 7.1). Two inductors are placed close together so
that energy produced by the power amplifier is transferred to the patient circuit. This
method of coupling ensures that DC in the apparatus is unable to reach the patient
and the risk of electric shock is minimized.
A variable capacitor, C, is included in the patient circuit so that the resonant frequency
of the patient circuit can be made equal to the frequency of the oscillator. This
ensures maximum efficiency of energy transfer (chapter 2) and reliable operation of
the apparatus. A power meter or indicator lamp shows when resonance is achieved
and maximum power is transferred. In older machines, the variable capacitor, C, was
manually adjusted with the operator adjusting a knob while observing the power
meter and adjusting for maximum power. Modern machines use electronic control of
the variable capacitor and are described as 'auto-tuning'. The principal advantage of
automatic tuning is that if the patient should move during treatment the machine will
adjust to keep the patient circuit in resonance. With manual tuning machines,
movement of the patient or electrodes can result in de-tuning and a drop in output of
the machine.
Any mains-frequency AC
produced by the apparatus is
also not conducted
appreciably to the patient
circuit as the resonant
frequency (27.12 MHz) is
vastly different to the mains
frequency (50 Hz).
The output of the apparatus is coupled to the patient via electrodes (in the capacitor
field technique represented in figure 7.1) or via an induction coil. The coil or
electrodes are connected directly to the output of the machine and the part of the
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patient to be treated is positioned in the electric or magnetic field. In figure 7.1, the
area highlighted in yellow is circuitry inside the machine.
The part of the patient to be treated would be positioned between the external
capacitor plates shown in figure 7.1. The plates are normally in the form of two metal
disks, each inside a clear plastic container or envelope. The electrical characteristics
of the patient's tissue affects the capacitance of the patient circuit, as does the
electrode size and spacing. For this reason it is necessary that the apparatus be
tuned (by adjusting C in figure 7.1) with the patient positioned in the field. Similarly, if
an induction coil is used rather than capacitor plates, tuning will be necessary. This is
because when the coil is wrapped around the part of the patient to be treated, the
inductance of the coil will depend on the number of turns of the coil and their radius.
When an induction coil is
used, the presence of
biological tissue in the field is
irrelevant but the tissue
volume to be treated will
influence the number of turns
of the coil and their radius.
MOLECULES IN AN ELECTRIC FIELD
In shortwave diathermy treatment a high frequency AC electrical signal is produced
and applied to the patient via an induction coil or electrodes. The high frequency
signal will produce a corresponding high frequency alternating electric or magnetic
field in the patient's tissue. We now consider what effect this has on the tissue.
Since an alternating magnetic field gives rise to an induced alternating electrical field
(as described in chapter 6) we first examine the effects of an alternating electric field
on the different molecules found in human tissue.
Charged Molecules
The conductivity of tissue is determined by the number of free ions in the tissue fluid.
In the presence of an electric field these ions will migrate along field lines and so
constitute an electric current. The process is not unlike electrical conduction in
metals. Metallic conduction results from the movement of free electrons. In
electrolytes the charge carriers are not electrons but ions; these are tens of
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170
thousands of times more massive than electrons.
Under the influence of the electric field ions will be
accelerated along field lines - but they will not travel far before
colliding with other molecules and losing their acquired
kinetic energy. The repeated sequence of accelerations and
collisions is the way in which electrical energy is converted to
heat energy, which is the random-motion energy of the
molecules. At the frequencies associated with shortwave
diathermy the field alternations are so rapid that the ions
oscillate about a mean position rather than undergoing any
large scale movement, but the alternations are not so rapid
that movement is prevented and heat generation is not
impaired.
Dipolar Molecules
Dipolar molecules such as water will orient themselves in an
electrical field and if the field is alternating this will result in
backwards and forwards rotation of the dipoles. In a liquid
the molecules are continually in motion (due to their thermal
energy) and are loosely associated with each other
(coupled); thus some of the rotational energy of the
molecules will be converted to heat energy by what can be
thought of as a frictional drag between adjacent molecules.
Non Polar Molecules
Though not normally polar these molecules will undergo a
distortion of their electron 'clouds'; that is, they will polarize in
an electric field. In an alternating field the electron clouds will
Figure 7.2
Response of molecules to a high
frequency alternating electric field.
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oscillate back and forth to each end of the molecule. Since this kind of motion does
not involve transport or rotation of the molecule as a whole it can only be coupled
indirectly with the gross molecular movement associated with heat energy.
Figure 7.2 summarizes, by illustration, the response of ions, polar molecules and
non-polar molecules to a high frequency alternating electric field. In each case there
is a net back and forth movement of charge: in other words, an alternating flow of
current.
REAL AND DISPLACEMENT CURRENT
From the previous discussion it is apparent that the different kinds of molecule in a
material will each respond differently to an applied electric field. The back and forth
movement of ions and the consequent collisions will result in a very efficient
conversion of electrical energy into heat energy. The rotational movement of polar
molecules provides a less efficient mechanism of energy conversion. The electron
cloud distortion of non-polar molecules represents the least efficient means of heat
production. Nonetheless each kind of molecule responds to an alternating electric
field in a way which results in movement of charges and hence an alternating current.
The difference is in the proportion of electrical energy converted to heat energy when
the alternating current is produced. With this in mind we distinguish real and
displacement current.
*
Real current is that associated with heat production. When real current flows
through a material the rate at which electrical energy is converted to heat energy
is given by Joule's law:
P = V.I
.... (1.4)
where V is the potential difference and I is the real current flowing through the
material. P is the power dissipated (in watts), in other words the amount of
electrical energy dissipated per second (1 watt (W) = 1 joule per second (J.s-1)).
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*
172
Displacement current is current flow which does not produce any heating.
In this case the power dissipated, and hence the heat generated, is zero.
Ionic materials are associated principally with real current and hence substantial
heat production. Polar substances are associated with both real and
displacement current and hence less heat production. Non-polar materials are
principally associated with displacement current and hence minimal heat
production.
An example which serves to illustrate the distinction between real and
displacement current is given in figure 7.3. Here we have a resistor and a
capacitor connected in series to a source of alternating current. In this case we
suppose that the capacitor is ideal - it comprises two metal plates separated by
a perfect insulator which can polarize and depolarize with no loss of electrical
energy to heat energy.
The magnitude of the current flowing in this circuit will depend on the voltage of
the AC source and the total impedance of the resistor/capacitor combination.
The actual impedance of the capacitor is calculated using equation 2.5. The real
current (Ir) flowing through the resistor will result in power dissipation according
to equation 1.4 and hence heat production in the resistor. The displacement
current (Id ) flowing through the capacitor (assumed ideal) gives no power
dissipation and hence no heat production as the material between the plates is
able to polarize and depolarize with no energy loss.
Figure 7.3
Real and displacement current
in an AC circuit.
In this case, then, the current flowing from the AC source appears as real current
in the resistor R and displacement current in the (ideal) capacitor C. Charges
move and heat is produced in the resistor while the charge movement
(displacement current) in the capacitor produces no heating. The two currents,
which are different forms of the same thing, are necessarily the same size.
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For a capacitor to be ideal the material between the plates must be an ideal dielectric
- a substance capable of polarizing in an electric field and depolarizing on its removal
without any dielectric absorption. In other words, with no conversion of electrical
energy to heat energy.
Biological materials, particularly those with high water and ion content are far from
being ideal dielectrics. When placed in an electric field the induced current will be a
combination of real and displacement current. The proportions of each kind of current
will depend on the proportions of ionic, polar and non-polar molecules.
We now consider biological tissue exposed to an electric or magnetic field which
alternates at a frequency of 27.12 MHz, the frequency licensed for use in shortwave
diathermy. As we have seen, shortwave diathermy may be applied using capacitor
plates (which produce an electric field) or an inductive coil (which generates a
magnetic field).
173
Most gases come close to
being ideal dielectrics, as do
some oils. Water being a
highly polar molecule, falls
short of this ideal and
dielectric absorption results
in significant heating at any
frequency below about 1010
Hz.
CAPACITOR FIELD TREATMENT
Consider first the situation depicted in figure 6.19(a), where an arm or leg is
positioned between two capacitor plates. Figure 6.19(a) shows the electric field
pattern, which is affected by refraction and termination of field lines. The total current
flowing through the tissue will be determined by the total impedance of the tissue plus
the air space between tissue and capacitor plates. Current will flow in the direction of
the field lines and the proportions of real and displacement current will depend on the
electrical properties of the particular tissue.
The amount of heating in any tissue layer will be determined by two factors: the field
intensity within the layer and the amount of real, rather than displacement, current.
Calculation of the proportions of real and displacement current in a particular tissue is
not difficult. Measured values of dielectric constant and conductivity are all that are
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needed. Calculation of the field pattern is much more difficult and has only been done
using simplified models: even simpler than the somewhat idealized geometries
shown in figure 6.19.
Useful qualitative pictures are nonetheless obtained by combining diagrams such as
those shown in figure 6.19, with calculated values of real and displacement current in
each tissue layer.
174
The conductivity determines
the amount of real current
flow, the dielectric constant
determines the amount of
displacement current.
At a frequency of 27.12 MHz the current flow in fatty
tissue and bone is approximately 50% displacement.
In muscle and tissues of high water content the
proportions are approximately 80% real current to 20%
displacement current.
Figure 7.4 shows a revised view of figure 6.19(a) which
takes into account the two kinds of current flow which
occur. In the air spaces the current flow is entirely
displacement current. In fatty tissue and bone the
current is assumed to be one half real current and one
half displacement current. For simplicity, muscle is
shown as having entirely real current.
Figure 7.4
Current type and directions in
a model for an arm or leg.
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When viewing diagrams such as these, bear in mind the simplifications made. The
pictures can be misleading if interpreted too literally. You should also bear in mind
that even a single tissue layer may be inhomogeneous at both the microscopic and
macroscopic level. An example of the complications introduced by tissue
inhomogeneity is seen with fatty tissue in the shortwave field.
Fatty Tissue
A practical limitation on the amount of heat which can be produced in deeply
located tissue is the heat production in fatty tissue. When using capacitor
plates the rate of heating of fatty tissue is always greater than that of the
underlying muscle tissue. Part of the reason is that fatty tissue is
inhomogeneous. The tissue is not a uniform distribution of cells but a
complex structure incorporating regions of high conductivity and dielectric
constant: the lymphatic and blood vessels.
The high conductivity and dielectric constant of the vessels will result in field
lines being focussed or channelled into them with a resulting high local field
intensity and corresponding high rate of heating in and near the vessels.
The phenomenon is illustrated in figure 7.5.
The localized high heat production will result in greater temperature
elevation of the vessels than the fatty tissue as a whole and a greater
sensation of heat than would be expected if the tissue layer was
homogenous.
Figure 7.5
Focussing of electric field lines in blood
and lymphatic vessels in fatty tissue.
INDUCTIVE COIL TREATMENT
We now consider application of the shortwave field with an induction coil. The
objective is to induce an electric field and hence a flow of current as a result of the
alternating magnetic field produced by the coil. In the example illustrated in figure 7.6
a cable carrying the shortwave frequency current is wrapped around a patient's lower
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limb. Figure 7.6(a) shows the inductive coil wound as a solenoid around the patient's
lower limb and figure 7.6(b) shows the current pathways in the different tissues.
The current pathways shown are predicted assuming that the alternating magnetic
field gives rise to an induced EMF in the patient's tissue. In this case the current will
follow circular paths parallel to the turns of the coil in figure 7.6(a). Note that in figure
7.6(b) the current through the fatty tissue is shown as half displacement current and
half real current while muscle is assumed to have real current only. As indicated
previously, this is only an approximation: while the proportion of real current in fatty
tissue is about 50%. in muscle it is about 80%.
If the coil in figure 7.6 had a large number of closely spaced turns and the coil
diameter was small compared to its length, then the magnetic field inside the
coil would be uniform and the induced EMF would be the same throughout the
tissue volume. Were this the case, the relative amounts of current flow in each
tissue would simply depend on the tissue impedance (which is determined by
the dielectric constant and conductivity).
Figure 7.6
Current flow induced in a limb by
inductive coil treatment.
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A complication is that with more widely spaced turns and a relatively large diameter,
the magnetic field inside the induction coil will be non-uniform. In an arrangement
like that shown in figure 7.6(a), the magnetic field would be strongest close to the coil
and decreasing in intensity towards the centre. The highest field intensity is thus in
the superficial tissues of the limb.
Another Kind of Coil
This means thaqt a greater
EMF will be induced in the
superficial tissues and
consequently there will be a
greater current flow.
Most manufacturers of shortwave diathermy apparatus offer accessories which
include a compact coil mounted in a plastic housing. This device is called a monode.
The monode is pointed at the part of the patient to be treated so that the coil is in a
plane parallel to the skin surface. With this arrangement (figure 7.7), currents are
induced which flow in circular paths parallel to the skin
surface.
The cable supplied with the shortwave machine can, of course,
also be wound into a spiral and positioned to produce a
similar distribution of induced current.
The spiral coil placed parallel to the skin produces more
superficial heating than the solenoidal coil (figure 7.6). This is
because the magnetic field intensity decreases rapidly with
distance from the coil, as the field lines diverge, spreading
apart and looping round to the opposite side of the coil. The
field spreading is similar to that which occurs at the ends of
the coils in figures 6.7 and 7.6. Magnetic field lines become
more separated, indicating a weaker magnetic field further
from the coil and consequently less induced EMF and less
induced current. Hence although the current induced in
muscle is mostly real current, the amount of current at depth is
much less than with a solenoid (figure 7.6).
Figure 7.7
Induced currents with a spiral coil mounted
parallel to the skin surface.
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Capacitative Effects
A practical complication which occurs with inductive coil treatment,
whether with a solenoid or a spiral coil (monode), is that in addition
to the currents induced by the magnetic field there is also a
pronounced electrostatic effect.
There is a certain capacitance between the loops of the coil. In fact
whenever a cable or wire is folded back on itself or coiled we have
produced a situation where there are two conductors separated by a
space; thus we have produced a capacitor. Although in the case of a
cable wound as a coil the capacitance is very small, the effect is
quite significant at MHz frequencies. The inductive coil behaves as
an inductor in parallel with a capacitor.
At the high frequencies used for shortwave diathermy the inductance
of the coil results in a high impedance to current flow in the cable
(equation 2.4). The capacitance associated with the coil presents a
lower impedance pathway for current to take (equation 2.5). In
consequence the induced current patterns are not as simple as
those shown in figure 7.6(b). The electric field between adjacent
turns (Figure 7.8(a)) results in current flow along the field lines
shown in blue. Because the electric field is stronger closer to the
coils, greater current flows and this adds to the current induced by
the magnetic field. The consequence is greater current flow in, and
greater heating of, superficial tissue (figure 7.8(b)).
The electric field between adjacent loops is similar to that between
two small electrodes (figure 6.1(c)). The field is most intense close
to the cable. A consequence is that there is a risk of burning the
superficial tissues with the electric field of the coil rather than
Figure 7.8
Electric field pattern (blue lines) between
adjacent turns of an inductive coil.
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heating deeper tissue with current induced by the alternating magnetic field.
A similar argument applies for a spiral coil. An electric field is produced between
adjacent turns within the loop. Close to the coil, the electric field is intense and
greater current flows. This adds to the current induced by the magnetic field so there
is greater current flow in, and greater heating of, superficial tissue.
Superficial heating due to the electric field can be minimized in three ways: (a) by
winding the turns of the coil close together, (b) by keeping the cable away from the
patient's skin using towelling and/or rubber spacer designed for this purpose and (c)
by using an electrostatic shield.
Electric field heating effects can also be minimized, in the case of a solenoid, by
positioning an earthed metal cylinder between the coil and the patient's limb. If a
monode is used, a flat metal plate between the monode and the patient's tissue
would be needed. The plate will screen-out the electric field while having little effect
on the magnetic field of the coil. The electric field inside the metal cylinder or behind
the metal plate would be almost nil because the metal is a good conductor and field
lines will terminate on its surface. Most metals are, however, transparent as far as
magnetic fields are concerned so the magnetic field is virtually unchanged. Some, but
not all, inductive coil applicators are supplied with an inbuilt electric field screen.
Screening is an important feature when depth efficient heating is required.
When the adjacent turns are
closer together, the electric
field is actually greater, but it
is also more localized to the
space between the turns of
the coil.
In summary, the options with inductive coil treatment are a coil wound around the part
of the patient to be treated or a flat coil (monode) positioned over the body part. The
difference is the depth efficiency of treatment. A solenoidal coil (figure 7.6) has greater
depth efficiency as far as tissue within the coil s concerned. A flat spiral coil (figure
7.7) has greater effect on superficial tissues.
With either method of application, there is the risk of excessive superficial heating due
to the electric field between adjacent turns of the coil or spiral. the risk is minimized by
spacing the coil or spiral away from the patient's superficial tissues.
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ELECTRODE PLACEMENT - CAPACITOR FIELDS
With capacitor field treatment, the therapist has more control over the field intensity in
different areas than with inductive coil treatment. We have discussed previously how
the combination of tissue layers in the part of the patient being treated alters the
shape of the electric field. The other factors influencing the field pattern involve the
placement of the electrodes. Each factor listed below must be taken into account in
the practical application of shortwave diathermy using capacitor field treatment.
*
The shape of the part of patient in the field. Compare Figure 6.19(a) with 6.19(b).
In addition, if the electrodes are placed over any prominence an undesirable
concentration of the field can result.
*
The arrangement of tissues layers in the treated structure. As discussed
previously, this factor plays a significant role in determining the final shape of the
field.
*
The size, spacing and orientation of the electrodes. Some examples of the
electric field in the absence of any object were shown in figures 6.2 and 6.3. We
consider below the effect when the patient is in the field.
Electrode Size
In general, it is preferable to use electrodes which are somewhat larger than the
structure to be treated. This results in the central, more uniform, part of the field being
used (figure 6.2).
The dielectric constant and conductivity of tissue are much higher than those of air
(table 4.2). Thus, with large electrodes, the field lines are bent towards the limb and
spreading of the field is minimised. The effect is illustrated in figure 7.9 where the
effect of the different tissue layers is ignored for simplicity.
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Compare these with figures 4.19 and 4.20. The use of
small electrodes results in an undesirably high field
intensity in the superficial tissues.
Unequal size electrodes (figure 5.10(c)) can be used to
selectively heat tissue located closer to one surface of a
limb. Large differences in electrode size, however, can
sometimes lead to difficulty in tuning or instability in
machine operation.
Electrode Spacing
The electrode spacing should normally be as wide as
possible. In this way the problems associated with a
non-uniform field pattern are minimised. The machine
itself, however, sets the limit on the maximum spacing
which can be used. As the electrodes are moved further
apart the capacitance of the two plates decreases. In
addition the field intensity (and consequently the rate of heating) will
decrease. A point will be reached where the machine can no longer be tuned
or insufficient power is available for adequate heating: this sets the limit on
the separation of the electrodes.
Figure 7.9
Effect of electrode size: (a) correct
electrode size (b) electrodes too small
(c) arrangement for selective heating.
By use of a wide spacing the electrical properties of the tissue have a smaller
effect on the overall field pattern and the electrical properties of air play a
greater role. Thus the field pattern is more uniform and less subject to
variation with movement of the patient in the field.
Figure 7.10 illustrates the effect of electrode spacing. In 7.10(a) the electrode
to surface distance varies considerably resulting in a local high field intensity
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in the limb. In 7.10(b) the electrode to surface
distance does not vary greatly and the field within
the limb is more uniform. Clearly, if a relatively
uniform field pattern is required the arrangement
shown in 7.10(b) is to be preferred.
The
arrangements shown in 7.10(c) and 7.11(c) are
both suitable if we wish to selectively heat one
surface of a limb. They would also be suitable for
heating a structure which is located close to one
surface of a limb or trunk - for example, the hip
joint.
Electrode Orientation
In the examples considered previously the
electrodes were placed parallel to each other in
order to obtain a relatively uniform heating pattern.
However if one part of the surface of a structure is
closer to an electrode, the field lines will be
concentrated in that region.
Figure 7.11 shows electrodes applied to the shoulder. Compare this with
figure 6.16. Electrodes which are parallel to each other as in figure 7.11(a) do
not give a uniform field because the air spacing varies considerably. The
dielectric constant and conductivity of each field-line pathway varies
considerably, resulting in variation in the field intensity. In figure 7.11(b) the
distance between the plates varies but the electrical characteristics of each
pathway are similar: thus the field is relatively uniform. Clearly the
arrangement shown in figure 7.11(b) is preferred when uniform heating is the
objective.
Figure 7.10
Effect of electrode spacing: (a) narrow
spacing, (b) wide spacing and (c)
unequal spacing.
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In all of the examples discussed previously the arrangement
of electrodes is contraplanar: that is, electrodes are placed
over opposite sides of a structure. Such an arrangement is
needed if deeply located tissue is to be heated.
In some circumstances it is preferable to use a coplanar
electrode arrangement. For example superficial structures,
such as the spine, which are too extensive for contraplanar
treatment may be treated in this way. Figure 7.12 shows a
coplanar arrangement of electrodes.
When using a coplanar arrangement it is very important to
ensure that the spacing between electrodes is greater than
double the skin to electrode distance. This results in the
majority of field lines passing through tissue rather than the
air space between the electrodes.
Figure 7.11
Effect of electrode orientation. (a) and
(c): incorrect orientation (b) correct
orientation.
Figure 7.12
A coplanar arrangement of electrodes.
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Even when using a contraplanar arrangement of electrodes
considerable heating occurs in the superficial tissues closest to the
electrodes. This effect can be minimized by using the cross-fire
technique of treatment shown in figure 7.13.
Half of the treatment is given with electrodes in one position (figure
7.13(a)). The electrodes are then moved so that the new electric field
is at right angles to the old one (figure 7.13(b)) and the treatment is
continued. In this way deeply located tissue receives treatment for
twice as long as the skin. The cross-fire treatment may be used, for
example, on the knee joint or pelvic organs. It is also particularly
useful for treating the walls of cavities within a structure, for example
the sinuses. Figure 7.14 shows the field pattern obtained with an
object of high dielectric constant which has an air-filled hollow at its
centre.
Figure 7.13
The cross-fire technique.
The field lines are concentrated in the dielectric resulting in uneven heating of the
walls of the cavity. Cross-fire treatment ensures that all of the cavity wall area is
treated.
HEATING OF TISSUE
Earlier we discussed qualitatively and in molecular terms, the heating effect of a high
frequency alternating electric field. We now consider heat production and temperature
rise and take a larger scale view of matter: a view at the level of tissue rather than
molecules.
Figure 7.14
A hollow dielectric between
capacitor plates.
We saw in chapter 1 that the power dissipated by a resistor, the rate at which
electrical energy is converted to heat energy, is given by equation 1.4:
P = V.I
.... (1.4)
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This expression relates the current, I, flowing through a resistor to the total power, P,
dissipated in the resistor. For resistors the current, I, is entirely real current and thus
produces heat. When we consider biological tissues we must distinguish between
real current and displacement current since only the real current results in heat
production. In additional, we are usually more interested in the rate of heating at a
particular point in the tissue rather than in the tissue as a whole. In this case a more
useful expression of equation 1.4 is equation 7.1.
Pv = E.ir
.... (7.1)
Here Pv is the power dissipated per unit volume of tissue at a particular point. The
units of Pv are thus watts per cubic metre. E is the field strength (in volts per metre)
and ir is the real component of current density (in amps per square metre) at that
point.
The power dissipated, Pv is equal to the rate of heat production. Hence, in order to
determine the rate of heating at a particular point in tissue we need to know the
electric field strength and the real current density. We begin by considering fields and
currents produced using capacitor field treatment.
Capacitor Field Treatment
Whether electrodes are positioned in a coplanar arrangement (figure 7.12) or in a
contraplanar arrangement (figures 7.9 to 7.11) the current flow in muscle will be
determined by the total impedance of the tissue combination plus the air space
between the tissue and capacitor plates.
Figure 7.15 shows electrical equivalent circuits for the two electrode/tissue
arrangements. The quantities Za, Zf, and Zm refer to the electrical impedances of air,
fat and muscle respectively.
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Figure 7.15
Electrode/tissue configurations and their
electrical equivalent circuits. (a) coplanar
arrangement, (b) contraplanar arrangement.
In figure 7.15(a) we ignore (displacement) current flow through the air directly between
the electrodes. We also ignore current flowing directly through the fatty tissue and
bypassing the muscle. If the electrode spacing is at least twice the electrode to tissue
spacing this will be a reasonable approximation. The impedance presented by each
alternate pathway will be sufficiently high to make these currents negligible.
In figure 7.15(b) we ignore current flow through the bone, directly around the fatty
tissue or through the air around the tissue. Again this is because these pathways
have very high impedance compared to the ones shown.
With these approximations the electrical equivalent circuits in 5.16(a) and (b) are the
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187
same. Each of the electrode/tissue arrangements are equivalent to a series
combination of impedances.
Just as with resistive circuits (described in chapter 1), when impedances are
connected in series the current in each impedance is the same. We thus have the
following relationship:
displacement
current
in air
=
displacement
+ real current
in fatty tissue
=
displacement
+ real current
in muscle
As mentioned earlier, the proportion of real current in fatty tissue is approximately 50%
while in muscle the proportion is about 80%. Thus the amount of real current flow in
muscle is 80/50 or about one and one half times greater than in fatty tissue.
Let us take the simple case where current spreading is minimal and estimate the
relative rate of heating in fatty tissue and muscle. We need to know both the real
current density and field strength in each tissue. The field strength is estimated
below.
The real current density in
muscle may be increased or
decreased if the electric field
lines converge or diverge.
This depends on the
tissue/electrode geometry see figure 6.19 for example.
When resistors are connected in series the current flow in each is the same but the
voltage across each resistor will, in general, be different. The largest resistor will
have across it the greatest potential difference. The equivalent statement for tissues
of different impedance is as follows:
When tissues are arranged in series the field intensity will be greatest in the tissue
with highest impedance.
Inspection of table 6.2 shows that muscle has a higher conductivity and dielectric
constant than fatty tissue: both figures are several times higher. Now a high
conductivity and dielectric constant means a low impedance. Combining the two
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188
figures from table 6.2 we calculate that fatty tissue has an electrical impedance some
ten times larger than muscle.
The rate of heating of each tissue is given by equation 7.1:
Pv = E.ir
.... (7.1)
The real current density in muscle is as we have seen, about one and a half times
greater than in fatty tissue, however the field intensity in fatty tissue is approximately
ten times higher. Hence the rate of heating of fatty tissue is predicted to be
approximately 10/1.5 times higher than muscle.
We thus have the general conclusion that if spreading or converging of the field is
minimal the rate of heat production in fatty tissue will be about seven times higher
than in muscle.
If the electrode/tissue configuration permits spreading of the field in muscle the
current density will be reduced and the rate of heating of muscle correspondingly
reduced. Conversely if the geometry produces convergence of the field lines in
muscle the current density will be increased and the relative rate of heating will be
increased accordingly.
Inductive Coil Treatment
With capacitor field treatment tissues are effectively in a series electrical arrangement.
The current flow in muscle is thus limited by the impedance of the fatty tissue layers.
When inductive coil treatment is used such is not the case.
Consider the coil and tissue arrangement shown in figure 7.7. Currents are induced
in the plane of the fatty tissue and in the plane of the muscle. The current loops are
complete electrical pathways in the one tissue. For this reason the current flow in
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SHORTWAVE DIATHERMY
189
muscle is not limited by the fatty tissue but depends only on the strength of the
induced electric field and the electrical characteristics of the muscle tissue. In other
words the induced currents flowing in each tissue layer are independent of each
other.
The real component of the current density, the current density which determines heat
production, is given by equation 6.10, which can be written:
ir = σ.E
.... (6.10)
Substituting this formula into equation 7.1 we obtain an alternate expression for the
power dissipated per unit volume:
Pv = σ.E2
.... (7.2)
Table 6.2 shows that the conductivity, σ, of muscle is some sixteen times greater than
that of fatty tissue. Hence, for the same induced electric field strength, both the real
current density and the power dissipated in muscle will be sixteen times greater than
in fatty tissue.
How large is the magnetically induced electric field? The intensity of the induced field
is determined by the rate of change of the magnetic field and the permeability, µ, of the
material. The permeability is close to one for biological materials (see table 6.3) so
fatty tissue and muscle are alike in this regard.
For the same strength of alternating magnetic field then, both fatty tissue and muscle
will have the same strength of induced electric field. Thus the rate of heating of
muscle in this situation will be about sixteen times greater than that of fatty tissue.
The intensity of the induced
electric field is determined by
the rate of change of the
magnetic field and the
permeability, µ, of the
material. It does not depend
on the electrical properties, σ
and ε, of the tissue.
In practice such a degree of selective heating is difficult to achieve. This is for two
reasons:
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190
*
Muscle is located beneath fatty tissue and so is further from the induction coil.
Thus the magnetic field is weaker in muscle and the strength of the induced
electric field is correspondingly smaller.
*
Fatty tissue, being closer to the induction coil may also experience an
appreciable electric field due to the capacitance between adjacent turns of the
coil. This effect was described earlier (see figure 7.8).
These two factors combine to increase the heating of fatty tissue relative to muscle so
that a sixteen to one advantage is rarely obtained. Nonetheless efficient selective
heating is achieved with close spacing of the turns of the coil and a sufficiently large
coil to patient distance. One would also expect good discrimination with applicators
which incorporate an electric field screen in front of the inductive coil.
For more information on
relative heating rates see the
book 'Therapeutic Heat and
Cold' J F Lehmann (ed)
(1982) chapters 6 and 10.
HEAT AND TEMPERATURE RISE
Having described the factors determining the rate of heating of tissue we now
consider the relationship between rate of heating and rate of increase of temperature.
The rate of heating per unit volume is given in terms of electric field intensity and real
current density by equation 7.1. Hence the amount of heat produced per unit volume,
∆Qv, in a time interval ∆t is given by equation 7.3.
∆Qv = E.ir.∆t
.... (7.3)
∆Qv has units of joules per cubic meter (J.m-3).
In considering the therapeutic effects of diathermy it is not the heat produced, as such,
which determines the physiological response but the resulting temperature rise.
Temperature is a key factor in determining the rates of chemical reactions and hence
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SHORTWAVE DIATHERMY
191
physiological processes.
The SI unit of temperature is the kelvin (symbol K). It is related to the perhaps more
familiar degree Celsius (oC) by the expression
oC
= K - 273.15
Notice that from this definition the size of the degree Celsius is the same as the kelvin.
In other words a change in temperature of five degrees Celsius is precisely the same
as a change of five Kelvin's. When we are talking about increases in temperature
brought about by diathermy treatment the terms kelvin and degrees Celsius can be
used interchangeably to describe the increase.
To convert from degrees
Celsius to Kelvin's, simply
add 273.15 to the Celsius
temperature.
When a fixed amount of heat is supplied to different substances the increase in
temperature of each will, in general, be quite different. The factor which determines
the resulting temperature increase is the specific heat capacity of the substance.
The specific heat capacity is defined as the amount of heat required to raise 1 kg of a
substance through one kelvin. The units of specific heat capacity are thus joules per
kilogram per kelvin. This can be measured experimentally by supplying a certain
amount of heat (∆Q) to a known mass (m) of the substance an measuring the
resulting temperature increase (∆T). The experiment must be arranged so that all of
the heat supplied is used to increase the temperature of the substance. If the loss of
heat is negligible then the specific heat capacity (c) can be calculated using equation
7.4:
∆Q
c=
.... (7.4)
m.∆T
When we consider the heating of tissues by diathermy, heat transfer between tissues
and to the bloodstream will have a large effect on the temperature distribution during
treatment.
Alternatively, when the
specific heat capacity of a
substance is known, equation
7.4 can be used to calculate
the temperature increase
resulting from the heat
supplied.
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192
Prior to the start of treatment the body tissues are in a state of dynamic equilibrium.
Cellular activity, metabolism and muscle contraction result in the steady production of
heat and the circulation of blood and tissue fluids provide an efficient means of heat
transfer. The net production of heat is balanced by net transfer of heat from the tissue
and a stable temperature is maintained.
Once treatment is started heat is produced in the tissue according to equation 7.3 and
the temperature starts to increase. An expression for the initial rate of increase in
temperature is obtained below.
Rearranging 7.4 we have ∆Q = m.c.∆T
Dividing this expression by volume we obtain:
∆Qv = ρ.C.∆T
.... (7.5)
where ρ is the mass per unit volume or density of the tissue.
Dividing 7.5 by ∆t gives:
∆Qv
∆T
= ρ.c.
∆t
∆t
.... (7.6)
where ∆Qv/∆t is the volume rate of heating (in Joules per cubic metre per second) and
∆T/∆t is the rate of increase in temperature (in Kelvin's per second).
This equation can be used to compare the initial rate of temperature increase in fatty
tissue with that of muscle. The densities of the two tissues are similar but the heat
capacity of muscle is some 50% greater than that of fatty tissue. Thus if the rate of
heating of each tissue is the same, the initial rate of temperature increase in muscle
will be only two thirds of that of fatty tissue. To produce the same initial rate of
increase in temperature in each tissue the rate at which heat energy is produced in
muscle must be 50% greater.
Note that this conclusion is a
general one. It applies not
just to shortwave diathermy
but to any diathermic
modality.
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SHORTWAVE DIATHERMY
193
An equation specifically applicable to shortwave diathermy is obtained by solving
equations 7.5 and 7.3. We then have: ρ.c.∆T = E.ir.∆t, which on rearranging gives:
∆T E.ir
=
∆t ρc
.... (7.7)
Equation 7.7 shows that the initial rate of increase in temperature (∆T/∆t) in shortwave
diathermy depends on four factors:
*
*
*
*
E, the field intensity at the point
ir, the magnitude of the real current density at the point
ρ, the density of the tissue
c, the specific heat capacity of the tissue
Once the temperature of any tissue has increased appreciably two things happen:
*
The body's temperature regulation mechanism responds. Blood vessels dilate,
circulation is increased and more heat is transferred from the tissue.
*
Heat is transferred by the blood and tissue fluids to adjacent cooler tissues.
Both of these effects lower the rate of increase in temperature. Eventually, the stage is
reached where the temperature ceases to increase. A new dynamic equilibrium is
achieved where the net production of heat is once again balanced by the net transfer
from the tissue.
Figure 7.17 illustrates the temperature variation during treatment. There is a transient
period during which the tissue temperature increases, followed by a steady state
where a constant (elevated) temperature is produced. The transient period for tissue
volumes of interest in physiotherapy is typically of the order of twenty to thirty minutes
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SHORTWAVE DIATHERMY
194
(see Lehmann (1982), chapter 10). Thus for treatment times of up to several minutes,
equation 7.7 gives a reasonable approximation to the real physical situation.
Application of equations 7.1 and 7.7 to quantitative prediction of
the rate of heating and rate of temperature increase in different
parts of tissue is difficult. The difficulty arises in the calculation
of the field intensity in a particular area. For a review of results
obtained using various approximations see A. W. Guy in J F
Lehmann (1982).
In patient treatment, shortwave diathermy remains something of
an art as well as a science. The physiotherapist must use a
knowledge of anatomy together with knowledge of the electrical
properties of tissues to determine the optimum placement of
electrodes or coil to give the required field pattern. Once the
field pattern is selected, the physiotherapist uses a knowledge
of the relative heating of the tissues and the patient's report of a
sensation of warmth to adjust the intensity of the applied field to
an appropriate level. With this procedure it is not possible to
accurately monitor dose or dose rate for the individual tissues.
Since this is a problem common to all diathermic modalities we
will defer further discussion of dosage until chapter eleven.
Physiological Effects
The therapeutic value of shortwave diathermy arises from the
physiological response of tissues to an increase in
temperature. A number of physiological responses are found:
*
Figure 7.16
A simple model for tissue temperature
variation during treatment.
at the cellular level an increase in temperature increases the rate of biochemical
reactions. Thus cellular metabolism is increased - there is an increased
demand for oxygen and nutrients and the output of waste products is increased.
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SHORTWAVE DIATHERMY
*
blood supply is increased. A number of factors determine this response. The
increased output of cellular waste products triggers dilation of the capillaries and
arterioles. The temperature increase itself causes some dilation, mainly in the
superficial tissues where heating is greatest. In addition, stimulation of sensory
nerve endings (again mainly in the superficial tissues) can cause a reflex
dilation.
*
a rise in temperature can induce relaxation of muscles. If there is abnormal
muscle activity caused by pain, for example, repeated treatment with shortwave
diathermy can be beneficial. The treatment helps to interrupt the vicious circle of
pain producing muscle activity which in turn produces more pain and so on. A
number of factors may contribute to relaxation: the direct effect of heat on muscle
tissue, the removal of any accumulated metabolites due to increased circulation
and the sedative effect of heat on sensory nerves.
*
the response of sensory nerves to heat is useful for the relief of pain generally.
Mild heating appears to inhibit the transmission of sensory impulses via nerve
fibres. In addition, when pain results from inflammation of tissue an increase in
the rate of absorption of exudate with increase in temperature can result in a
secondary pain-relief effect.
195
Some claims have been made that additional non-thermal effects can be produced
under the conditions used for therapy. As yet there is no clinical evidence for these
claims. Non-thermal effects seem to have been demonstrated using pulsed
shortwave treatment when the peak power level is significantly higher than used for
diathermy. The few published comparative studies indicate little or no nonthermal
effect at the low continuous power levels of conventional shortwave field treatment.
These points are considered further in chapter 8 following.
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196
EXERCISES
1
Figure 7.1 shows a schematic diagram of shortwave diathermy apparatus.
(a)
Briefly explain how the apparatus produces a high-frequency alternating
electric or magnetic field.
(b)
What is the function of inductors L1 and L2?
Why is the capacitor in the patient tuning circuit a variable one?
c)
2
3
4
5
(a)
Why is it necessary to tune shortwave diathermy apparatus with the patient
coupled to the machine?
(b)
What is the advantage of automatic versus manual tuning of shortwave
diathermy apparatus?
Figure 7.2 illustrates the response of ions, polar molecules and non-polar
molecules to a high-frequency alternating electric field.
(a)
Briefly describe the movement of each kind of molecule in the field.
(b)
How is the movement related to heat production in a material?
(c)
Which kind of movement is associated with greatest heat production and
which with least heat production?
(a)
What is meant by the terms 'real current' and 'displacement current'?
(b)
Consider the movement of ions, polar molecules and non-polar molecules
in an alternating electric field. Describe the relationship between each kind
of movement and real and displacement current.
(a)
Consider each of fatty tissue, muscle and bone in the shortwave diathermy
field. Is current flow in each tissue best described as real or displacement
current?
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SHORTWAVE DIATHERMY
(b)
On the basis of your classification which tissue would be associated with
maximum heat production?
(c)
Describe the complications (as far as prediction of heat production is
concerned) caused by the presence of Iymphatic and blood vessels in fatty
tissue.
6
Figure 7.4 shows current pathways in a model for an arm or leg. Describe
the principal factors determining the relative rate of heating of each tissue
layer.
7
A patient's lower limb is enclosed in a solenoidally wound coil as shown in figure
7.6.
(a)
Describe the motion of polar, non-polar and ionic molecules when a high
frequency alternating current flows through the coil.
(b)
Indicate (with a diagram) the direction of movement of molecules in the
limb.
8
If the solenoidally wound coil in question 7 was replaced by a pair of capacitor
plates (one above the knee, one below the sole of the foot), what would be the
new directions of molecular motion? Draw a diagram to illustrate.
9
Figure 7.8 shows the electric field associated with two adjacent turns of an
induction coil
10
(a)
what is the practical significance of this electric field in patient treatment?
(b)
how can the effects of this electric field be minimized?
197
For shortwave diathermy it is common practice to use electrodes which are
somewhat larger than the structure to be treated (figure 7.9). Explain in terms of:
(a)
the field pattern produced
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SHORTWAVE DIATHERMY
(b)
198
the pattern of heating of tissue.
What are the advantages and disadvantages of using unequal size electrode
(figure 7.9(c) )?
11
(a)
It is normal practice to space electrodes as far apart as possible (figure
7.10(b)) in shortwave diathermy treatment. Why is this the case?
(b)
What is the practical limitation on the electrode spacing which can be used?
(c)
Is there any advantage in positioning one electrode close to the patient's
tissue as shown in figure 7.10(c)?
12
Consider the electrode arrangements shown in figure 7.11. Explain why the field
intensity is non-uniform in diagrams (a) and (c). Under what circumstances will
the field intensity be uniform, as in (b)?
13
(a)
Draw a diagram showing a coplanar arrangement of electrodes over tissue
and the resulting field pattern.
(b) What are the advantages and disadvantages of coplanar electrode
arrangements?
(c)
14
What practical limit is there on the spacing of coplanar electrodes?
Consider the hollow dielectric between capacitor plates which is shown in figure
7.14.
(a)
Explain where heat production is greatest and why.
b)
What technique can be used to produce more uniform heating of the
dielectric? Explain.
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15
16
17
18
199
When coplanar electrodes are used for patient treatment the tissues can be
considered to be in series electrically (see figure 7.15(a)).
(a)
what approximations are implicit in this statement?
(b)
draw an electrical equivalent circuit similar to that in figure 7.15(a) for the
situation where the electrodes are close together.
(c)
how would bringing the electrodes closer together affect the relative heating
rate of muscle and fatty tissue? Justify your answer.
(a)
Explain why, in principle, it is easier to produce selective heating of muscle
with an inductive coil rather than capacitor field electrodes.
(b)
what practical constraints limit the selective heating of muscle with an
induction coil?
(a)
Explain the meaning of each of the terms in equation 7.6.
(b)
The initial rate of temperature increase in fatty tissue in an experiment is
found to be double that of muscle. Assume that the densities of each tissue
are the same and that muscle has a 50% greater heat capacity and
calculate the relative rate of heating of the tissues.
The relationship between heat production (∆Q) and current flow (I) in a conductor
is given by Joule's law: ∆Q = V.I.∆t where V is the potential difference across the
conductor and ∆t is the time interval for which current I flows.
(a)
Show how equation 7.3 can be obtained as an alternative form of Joule's
law.
(b)
An electric field intensity of 100 V.m-1 in a conductor results in a current
density of 50 A.m-2 . Use equation 7.2 to calculate the amount of heat
produced in a 30 second time interval. You may assume that the current is
entirely real.
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SHORTWAVE DIATHERMY
19
(a)
Explain the meaning of each of the terms in equation 7.7.
(b)
An electric field of intensity 200 V.m-1 in a material results in a real current
density of 50 A.m-2. The mass density of the material is 900 kg.m-3 and its
specific heat capacity is 4.0 kJ.kg-1.K-1. Calculate the initial rate of increase
in temperature (∆T/∆t) of the material.
20
A block of conducting material is placed in an electric field. The field intensity in
the material is 300 V.m-1 and the resulting real current density is 120 A.m-2. If the
material has a density of 1000 kg.m-3 and it has a specific heat capacity of 3.8
kJ.kg -1 .K -1 , calculate the initial rate of increase in temperature of the material
(using equation 7.7).
21
Equation 7.7 describes the initial rate of increase in temperature of tissue in
shortwave diathermy treatment. Describe the physiological response to the
initial temperature rise and the effect this has on the subsequent rate of increase
of temperature (figure 7.16).
200
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