1348642949.5499Chapter7

SHORTWAVE DIATHERMY 167

While shortwave diathermyunits do radiate waves with afrequency of 27.12 MHz, thisis a side-effect. Thephysiological effects are dueto the powerful electric ormagnetic fields generated bythe apparatus.

Figure 7.1Shortwave diathermy

apparatus (schematic).

7 Therapeutic Fields: Shortwave Diathermy

'Shortwave diathermy' refers to heating of deeply located tissue using electric ormagnetic fields which alternate at high frequency. The term 'shortwave diathermy', issomething of a misnomer as the contribution of waves, as such, to the treatment isnegligible. The physiological effects are a result of electric and magnetic fieldsgenerated by the shortwave diathermy apparatus. Shortwave radiation plays little orno role in the therapy.

The apparatus used by physiotherapists generates alternating electric and magneticfields with a frequency of 27.12 MHz. Since radio waves with frequencies in the range10 MHz to 100 MHz are termed short waves the term has been, rather inappropriately,applied to this therapeutic modality.

PRODUCTION OF THE FIELD

Shortwave diathermy apparatus consists of asinewave generator circuit which producesalternating current with a frequency of 27.12MHz and a resonant circuit which can betuned to exactly the same frequency. Thesinewave generator supplies energy to theresonant circuit by transformer action. Figure7.1 illustrates the arrangement.


The sinewave generator consists of a power supply (chapter 5), an oscillator withgood frequency stability (chapters 2 and 5) and a power amplifier (chapter 5). Thepower supply converts AC from the mains (of frequency 50 Hz) to DC which is neededto power the equipment. It consists of a transformer (to convert the 240 V AC from themains to the voltage needed by the rest of the circuitry), and a rectifier to convert the ACto DC. The DC is used to power a sinewave generator; a resonant circuit whichoscillates at 27.12 MHz and an amplifier, which boosts the current produced by theresonant circuit to higher levels, as needed for patient treatment.

Any mains-frequency ACproduced by the apparatus isalso not conductedappreciably to the patientcircuit as the resonantfrequency (27.12 MHz) isvastly different to the mainsfrequency (50 Hz).

Electrical energy produced by the sinewave generator is coupled to the patient tuningcircuit by transformer action (figure 7.1). Two inductors are placed close together sothat energy produced by the power amplifier is transferred to the patient circuit. Thismethod of coupling ensures that DC in the apparatus is unable to reach the patientand the risk of electric shock is minimized.

A variable capacitor, C, is included in the patient circuit so that the resonant frequencyof the patient circuit can be made equal to the frequency of the oscillator. Thisensures maximum efficiency of energy transfer (chapter 2) and reliable operation ofthe apparatus. A power meter or indicator lamp shows when resonance is achievedand maximum power is transferred. In older machines, the variable capacitor, C, wasmanually adjusted with the operator adjusting a knob while observing the powermeter and adjusting for maximum power. Modern machines use electronic control ofthe variable capacitor and are described as 'auto-tuning'. The principal advantage ofautomatic tuning is that if the patient should move during treatment the machine willadjust 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 ofthe machine.

The output of the apparatus is coupled to the patient via electrodes (in the capacitorfield technique represented in figure 7.1) or via an induction coil. The coil orelectrodes are connected directly to the output of the machine and the part of the


patient to be treated is positioned in the electric or magnetic field. In figure 7.1, thearea highlighted in yellow is circuitry inside the machine.

When an induction coil isused, the presence ofbiological tissue in the field isirrelevant but the tissuevolume to be treated willinfluence the number of turnsof the coil and their radius.

The part of the patient to be treated would be positioned between the externalcapacitor plates shown in figure 7.1. The plates are normally in the form of two metaldisks, each inside a clear plastic container or envelope. The electrical characteristicsof the patient's tissue affects the capacitance of the patient circuit, as does theelectrode size and spacing. For this reason it is necessary that the apparatus betuned (by adjusting C in figure 7.1) with the patient positioned in the field. Similarly, ifan induction coil is used rather than capacitor plates, tuning will be necessary. This isbecause when the coil is wrapped around the part of the patient to be treated, theinductance of the coil will depend on 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 producedand applied to the patient via an induction coil or electrodes. The high frequencysignal will produce a corresponding high frequency alternating electric or magneticfield 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 fieldon 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 soconstitute an electric current. The process is not unlike electrical conduction inmetals. Metallic conduction results from the movement of free electrons. Inelectrolytes the charge carriers are not electrons but ions; these are tens of


thousands of times more massive than electrons.

Under the influence of the electric field ions will beaccelerated along field lines - but they will not travel far beforecolliding with other molecules and losing their acquiredkinetic energy. The repeated sequence of accelerations andcollisions is the way in which electrical energy is converted toheat energy, which is the random-motion energy of themolecules. At the frequencies associated with shortwavediathermy the field alternations are so rapid that the ionsoscillate about a mean position rather than undergoing anylarge scale movement, but the alternations are not so rapidthat movement is prevented and heat generation is notimpaired.

Dipolar Molecules

Dipolar molecules such as water will orient themselves in anelectrical field and if the field is alternating this will result inbackwards and forwards rotation of the dipoles. In a liquidthe molecules are continually in motion (due to their thermalenergy) and are loosely associated with each other(coupled); thus some of the rotational energy of themolecules will be converted to heat energy by what can bethought of as a frictional drag between adjacent molecules.

Non Polar Molecules Figure 7.2Response of molecules to a highfrequency alternating electric field.Though not normally polar these molecules will undergo a

distortion of their electron 'clouds'; that is, they will polarize inan electric field. In an alternating field the electron clouds will


oscillate back and forth to each end of the molecule. Since this kind of motion doesnot involve transport or rotation of the molecule as a whole it can only be coupledindirectly with the gross molecular movement associated with heat energy.

Figure 7.2 summarizes, by illustration, the response of ions, polar molecules andnon-polar molecules to a high frequency alternating electric field. In each case thereis a net back and forth movement of charge: in other words, an alternating flow ofcurrent.

REAL AND DISPLACEMENT CURRENT

From the previous discussion it is apparent that the different kinds of molecule in amaterial will each respond differently to an applied electric field. The back and forthmovement of ions and the consequent collisions will result in a very efficientconversion of electrical energy into heat energy. The rotational movement of polarmolecules provides a less efficient mechanism of energy conversion. The electroncloud distortion of non-polar molecules represents the least efficient means of heatproduction. Nonetheless each kind of molecule responds to an alternating electricfield 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 whenthe alternating current is produced. With this in mind we distinguish real anddisplacement current.

* Real current is that associated with heat production. When real current flowsthrough a material the rate at which electrical energy is converted to heat energyis given by Joule's law:

P = V.I .... (1.4)

where V is the potential difference and I is the real current flowing through thematerial. P is the power dissipated (in watts), in other words the amount ofelectrical energy dissipated per second (1 watt (W) = 1 joule per second (J.s-1)).


* 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 substantialheat production. Polar substances are associated with both real anddisplacement current and hence less heat production. Non-polar materials areprincipally associated with displacement current and hence minimal heatproduction.

An example which serves to illustrate the distinction between real anddisplacement current is given in figure 7.3. Here we have a resistor and acapacitor connected in series to a source of alternating current. In this case wesuppose that the capacitor is ideal - it comprises two metal plates separated bya perfect insulator which can polarize and depolarize with no loss of electricalenergy to heat energy.

The magnitude of the current flowing in this circuit will depend on the voltage ofthe AC source and the total impedance of the resistor/capacitor combination.The actual impedance of the capacitor is calculated using equation 2.5. The realcurrent (Ir) flowing through the resistor will result in power dissipation according

Figure 7.3Real and displacement current

in an AC circuit.

to equation 1.4 and hence heat production in the resistor. The displacementcurrent (Id) flowing through the capacitor (assumed ideal) gives no powerdissipation and hence no heat production as the material between the plates isable to polarize and depolarize with no energy loss.

In this case, then, the current flowing from the AC source appears as real currentin the resistor R and displacement current in the (ideal) capacitor C. Chargesmove 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.


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 1010Hz.

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 removalwithout any dielectric absorption. In other words, with no conversion of electricalenergy to heat energy.

Biological materials, particularly those with high water and ion content are far frombeing ideal dielectrics. When placed in an electric field the induced current will be acombination of real and displacement current. The proportions of each kind of currentwill depend on the proportions of ionic, polar and non-polar molecules.

We now consider biological tissue exposed to an electric or magnetic field whichalternates at a frequency of 27.12 MHz, the frequency licensed for use in shortwavediathermy. As we have seen, shortwave diathermy may be applied using capacitorplates (which produce an electric field) or an inductive coil (which generates amagnetic field).

CAPACITOR FIELD TREATMENT

Consider first the situation depicted in figure 6.19(a), where an arm or leg ispositioned between two capacitor plates. Figure 6.19(a) shows the electric fieldpattern, which is affected by refraction and termination of field lines. The total currentflowing through the tissue will be determined by the total impedance of the tissue plusthe air space between tissue and capacitor plates. Current will flow in the direction ofthe field lines and the proportions of real and displacement current will depend on theelectrical properties of the particular tissue.

The amount of heating in any tissue layer will be determined by two factors: the fieldintensity 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 isnot difficult. Measured values of dielectric constant and conductivity are all that are


The conductivity determines the amount of real current flow, the dielectric constant determines the amount ofdisplacement current.

Figure 7.4Current type and directions in

a model for an arm or leg.

needed. Calculation of the field pattern is much more difficult and has only been doneusing simplified models: even simpler than the somewhat idealized geometriesshown in figure 6.19.

Useful qualitative pictures are nonetheless obtained by combining diagrams such asthose shown in figure 6.19, with calculated values of real and displacement current ineach tissue layer.

At a frequency of 27.12 MHz the current flow in fattytissue and bone is approximately 50% displacement.In muscle and tissues of high water content theproportions are approximately 80% real current to 20%displacement current.

Figure 7.4 shows a revised view of figure 6.19(a) whichtakes into account the two kinds of current flow whichoccur. In the air spaces the current flow is entirelydisplacement current. In fatty tissue and bone thecurrent is assumed to be one half real current and onehalf displacement current. For simplicity, muscle isshown as having entirely real current.


When viewing diagrams such as these, bear in mind the simplifications made. Thepictures can be misleading if interpreted too literally. You should also bear in mindthat even a single tissue layer may be inhomogeneous at both the microscopic andmacroscopic level. An example of the complications introduced by tissueinhomogeneity is seen with fatty tissue in the shortwave field.

Fatty Tissue

A practical limitation on the amount of heat which can be produced in deeplylocated tissue is the heat production in fatty tissue. When using capacitorplates the rate of heating of fatty tissue is always greater than that of theunderlying muscle tissue. Part of the reason is that fatty tissue isinhomogeneous. The tissue is not a uniform distribution of cells but acomplex structure incorporating regions of high conductivity and dielectricconstant: the lymphatic and blood vessels.

The high conductivity and dielectric constant of the vessels will result in fieldlines being focussed or channelled into them with a resulting high local fieldintensity 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 temperatureelevation of the vessels than the fatty tissue as a whole and a greatersensation of heat than would be expected if the tissue layer was Figure 7.5

Focussing of electric field lines in bloodand lymphatic vessels in fatty tissue.

homogenous.

INDUCTIVE COIL TREATMENT

We now consider application of the shortwave field with an induction coil. Theobjective is to induce an electric field and hence a flow of current as a result of thealternating magnetic field produced by the coil. In the example illustrated in figure 7.6a cable carrying the shortwave frequency current is wrapped around a patient's lower


limb. Figure 7.6(a) shows the inductive coil wound as a solenoid around the patient'slower limb and figure 7.6(b) shows the current pathways in the different tissues.

The current pathways shown are predicted assuming that the alternating magneticfield gives rise to an induced EMF in the patient's tissue. In this case the current willfollow circular paths parallel to the turns of the coil in figure 7.6(a). Note that in figure7.6(b) the current through the fatty tissue is shown as half displacement current andhalf real current while muscle is assumed to have real current only. As indicatedpreviously, this is only an approximation: while the proportion of real current in fattytissue is about 50%. in muscle it is about 80%.

Figure 7.6Current flow induced in a limb by

inductive coil treatment.

If the coil in figure 7.6 had a large number of closely spaced turns and the coildiameter was small compared to its length, then the magnetic field inside thecoil would be uniform and the induced EMF would be the same throughout thetissue volume. Were this the case, the relative amounts of current flow in eachtissue would simply depend on the tissue impedance (which is determined bythe dielectric constant and conductivity).


This means thaqt a greaterEMF will be induced in thesuperficial tissues andconsequently there will be agreater current flow.

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 arrangementlike that shown in figure 7.6(a), the magnetic field would be strongest close to the coiland decreasing in intensity towards the centre. The highest field intensity is thus inthe superficial tissues of the limb.

Another Kind of Coil

Most manufacturers of shortwave diathermy apparatus offer accessories whichinclude 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 aplane parallel to the skin surface. With this arrangement (figure 7.7), currents areinduced which flow in circular paths parallel to the skinsurface.

The cable supplied with the shortwave machine can, of course,also be wound into a spiral and positioned to produce asimilar distribution of induced current.

The spiral coil placed parallel to the skin produces moresuperficial heating than the solenoidal coil (figure 7.6). This isbecause the magnetic field intensity decreases rapidly withdistance from the coil, as the field lines diverge, spreadingapart and looping round to the opposite side of the coil. Thefield spreading is similar to that which occurs at the ends ofthe coils in figures 6.7 and 7.6. Magnetic field lines becomemore separated, indicating a weaker magnetic field furtherfrom the coil and consequently less induced EMF and lessinduced current. Hence although the current induced inmuscle is mostly real current, the amount of current at depth is Figure 7.7

Induced currents with a spiral coil mountedparallel to the skin surface.

much less than with a solenoid (figure 7.6).


Capacitative Effects

A practical complication which occurs with inductive coil treatment,whether with a solenoid or a spiral coil (monode), is that in additionto the currents induced by the magnetic field there is also apronounced electrostatic effect.

There is a certain capacitance between the loops of the coil. In factwhenever a cable or wire is folded back on itself or coiled we haveproduced a situation where there are two conductors separated by aspace; thus we have produced a capacitor. Although in the case of acable wound as a coil the capacitance is very small, the effect isquite significant at MHz frequencies. The inductive coil behaves asan inductor in parallel with a capacitor.

At the high frequencies used for shortwave diathermy the inductanceof the coil results in a high impedance to current flow in the cable(equation 2.4). The capacitance associated with the coil presents alower impedance pathway for current to take (equation 2.5). Inconsequence the induced current patterns are not as simple asthose shown in figure 7.6(b). The electric field between adjacentturns (Figure 7.8(a)) results in current flow along the field linesshown in blue. Because the electric field is stronger closer to thecoils, greater current flows and this adds to the current induced bythe magnetic field. The consequence is greater current flow in, andgreater heating of, superficial tissue (figure 7.8(b)).

The electric field between adjacent loops is similar to that betweentwo small electrodes (figure 6.1(c)). The field is most intense closeto the cable. A consequence is that there is a risk of burning thesuperficial tissues with the electric field of the coil rather than

Figure 7.8Electric field pattern (blue lines) between

adjacent turns of an inductive coil.


When the adjacent turns arecloser together, the electricfield is actually greater, but itis also more localized to thespace between the turns ofthe coil.

heating deeper tissue with current induced by the alternating magnetic field.

A similar argument applies for a spiral coil. An electric field is produced betweenadjacent turns within the loop. Close to the coil, the electric field is intense andgreater current flows. This adds to the current induced by the magnetic field so thereis greater current flow in, and greater heating of, superficial tissue.

Superficial heating due to the electric field can be minimized in three ways: (a) bywinding the turns of the coil close together, (b) by keeping the cable away from thepatient'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, bypositioning an earthed metal cylinder between the coil and the patient's limb. If amonode is used, a flat metal plate between the monode and the patient's tissuewould be needed. The plate will screen-out the electric field while having little effecton the magnetic field of the coil. The electric field inside the metal cylinder or behindthe metal plate would be almost nil because the metal is a good conductor and fieldlines will terminate on its surface. Most metals are, however, transparent as far asmagnetic fields are concerned so the magnetic field is virtually unchanged. Some, butnot all, inductive coil applicators are supplied with an inbuilt electric field screen.Screening is an important feature when depth efficient heating is required.

In summary, the options with inductive coil treatment are a coil wound around the partof the patient to be treated or a flat coil (monode) positioned over the body part. Thedifference is the depth efficiency of treatment. A solenoidal coil (figure 7.6) has greaterdepth efficiency as far as tissue within the coil s concerned. A flat spiral coil (figure7.7) has greater effect on superficial tissues.

With either method of application, there is the risk of excessive superficial heating dueto the electric field between adjacent turns of the coil or spiral. the risk is minimized byspacing the coil or spiral away from the patient's superficial tissues.


ELECTRODE PLACEMENT - CAPACITOR FIELDS

With capacitor field treatment, the therapist has more control over the field intensity indifferent areas than with inductive coil treatment. We have discussed previously howthe combination of tissue layers in the part of the patient being treated alters theshape of the electric field. The other factors influencing the field pattern involve theplacement of the electrodes. Each factor listed below must be taken into account inthe 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 undesirableconcentration of the field can result.

* The arrangement of tissues layers in the treated structure. As discussedpreviously, this factor plays a significant role in determining the final shape of thefield.

* The size, spacing and orientation of the electrodes. Some examples of theelectric field in the absence of any object were shown in figures 6.2 and 6.3. Weconsider 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 thestructure to be treated. This results in the central, more uniform, part of the field beingused (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 andspreading of the field is minimised. The effect is illustrated in figure 7.9 where theeffect of the different tissue layers is ignored for simplicity.


Compare these with figures 4.19 and 4.20. The use ofsmall electrodes results in an undesirably high fieldintensity in the superficial tissues.

Unequal size electrodes (figure 5.10(c)) can be used toselectively heat tissue located closer to one surface of alimb. Large differences in electrode size, however, cansometimes lead to difficulty in tuning or instability inmachine operation.

Electrode Spacing

The electrode spacing should normally be as wide aspossible. In this way the problems associated with anon-uniform field pattern are minimised. The machineitself, however, sets the limit on the maximum spacingwhich can be used. As the electrodes are moved further

Figure 7.9Effect of electrode size: (a) correct

electrode size (b) electrodes too small(c) arrangement for selective heating.

apart the capacitance of the two plates decreases. Inaddition the field intensity (and consequently the rate of heating) willdecrease. A point will be reached where the machine can no longer be tunedor insufficient power is available for adequate heating: this sets the limit onthe separation of the electrodes.

By use of a wide spacing the electrical properties of the tissue have a smallereffect on the overall field pattern and the electrical properties of air play agreater role. Thus the field pattern is more uniform and less subject tovariation with movement of the patient in the field.

Figure 7.10 illustrates the effect of electrode spacing. In 7.10(a) the electrodeto surface distance varies considerably resulting in a local high field intensity


Figure 7.10Effect of electrode spacing: (a) narrow

spacing, (b) wide spacing and (c)unequal spacing.

in the limb. In 7.10(b) the electrode to surfacedistance does not vary greatly and the field withinthe limb is more uniform. Clearly, if a relativelyuniform field pattern is required the arrangementshown in 7.10(b) is to be preferred. Thearrangements shown in 7.10(c) and 7.11(c) areboth suitable if we wish to selectively heat onesurface of a limb. They would also be suitable forheating a structure which is located close to onesurface of a limb or trunk - for example, the hipjoint.

Electrode Orientation

In the examples considered previously theelectrodes were placed parallel to each other inorder to obtain a relatively uniform heating pattern.However if one part of the surface of a structure iscloser to an electrode, the field lines will beconcentrated in that region.

Figure 7.11 shows electrodes applied to the shoulder. Compare this withfigure 6.16. Electrodes which are parallel to each other as in figure 7.11(a) donot give a uniform field because the air spacing varies considerably. Thedielectric constant and conductivity of each field-line pathway variesconsiderably, resulting in variation in the field intensity. In figure 7.11(b) thedistance between the plates varies but the electrical characteristics of eachpathway are similar: thus the field is relatively uniform. Clearly thearrangement shown in figure 7.11(b) is preferred when uniform heating is theobjective.


Figure 7.11Effect of electrode orientation. (a) and

(c): incorrect orientation (b) correctorientation.

In all of the examples discussed previously the arrangementof electrodes is contraplanar: that is, electrodes are placedover opposite sides of a structure. Such an arrangement isneeded if deeply located tissue is to be heated.

In some circumstances it is preferable to use a coplanarelectrode arrangement. For example superficial structures,such as the spine, which are too extensive for contraplanartreatment may be treated in this way. Figure 7.12 shows acoplanar arrangement of electrodes.

When using a coplanar arrangement it is very important toensure that the spacing between electrodes is greater thandouble the skin to electrode distance. This results in themajority of field lines passing through tissue rather than theair space between the electrodes.

Figure 7.12A coplanar arrangement of electrodes.


Even when using a contraplanar arrangement of electrodesconsiderable heating occurs in the superficial tissues closest to theelectrodes. This effect can be minimized by using the cross-firetechnique of treatment shown in figure 7.13.

Half of the treatment is given with electrodes in one position (figure7.13(a)). The electrodes are then moved so that the new electric fieldis at right angles to the old one (figure 7.13(b)) and the treatment iscontinued. In this way deeply located tissue receives treatment fortwice as long as the skin. The cross-fire treatment may be used, forexample, on the knee joint or pelvic organs. It is also particularlyuseful for treating the walls of cavities within a structure, for examplethe sinuses. Figure 7.14 shows the field pattern obtained with anobject of high dielectric constant which has an air-filled hollow at its

Figure 7.13The cross-fire technique.

centre.

The field lines are concentrated in the dielectric resulting in uneven heating of thewalls of the cavity. Cross-fire treatment ensures that all of the cavity wall area istreated.

HEATING OF TISSUE

Earlier we discussed qualitatively and in molecular terms, the heating effect of a highfrequency alternating electric field. We now consider heat production and temperature

Figure 7.14A hollow dielectric between

capacitor plates.

rise and take a larger scale view of matter: a view at the level of tissue rather thanmolecules.

We saw in chapter 1 that the power dissipated by a resistor, the rate at whichelectrical energy is converted to heat energy, is given by equation 1.4:

P = V.I .... (1.4)


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 thusproduces heat. When we consider biological tissues we must distinguish betweenreal current and displacement current since only the real current results in heatproduction. In additional, we are usually more interested in the rate of heating at aparticular point in the tissue rather than in the tissue as a whole. In this case a moreuseful 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. Theunits 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 thatpoint.

The power dissipated, Pv is equal to the rate of heat production. Hence, in order todetermine the rate of heating at a particular point in tissue we need to know theelectric field strength and the real current density. We begin by considering fields andcurrents produced using capacitor field treatment.

Capacitor Field Treatment

Whether electrodes are positioned in a coplanar arrangement (figure 7.12) or in acontraplanar arrangement (figures 7.9 to 7.11) the current flow in muscle will bedetermined by the total impedance of the tissue combination plus the air spacebetween the tissue and capacitor plates.

Figure 7.15 shows electrical equivalent circuits for the two electrode/tissuearrangements. The quantities Za, Zf, and Zm refer to the electrical impedances of air,fat and muscle respectively.


Figure 7.15Electrode/tissue configurations and theirelectrical equivalent circuits. (a) coplanar

arrangement, (b) contraplanar arrangement.

In figure 7.15(a) we ignore (displacement) current flow through the air directly betweenthe electrodes. We also ignore current flowing directly through the fatty tissue andbypassing the muscle. If the electrode spacing is at least twice the electrode to tissuespacing this will be a reasonable approximation. The impedance presented by eachalternate 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 fattytissue or through the air around the tissue. Again this is because these pathwayshave very high impedance compared to the ones shown.

With these approximations the electrical equivalent circuits in 5.16(a) and (b) are the


same. Each of the electrode/tissue arrangements are equivalent to a seriescombination of impedances.

Just as with resistive circuits (described in chapter 1), when impedances areconnected in series the current in each impedance is the same. We thus have thefollowing relationship:

displacement displacement displacementcurrent = + real current = + real currentin air in fatty tissue in muscle

The real current density inmuscle may be increased ordecreased if the electric fieldlines converge or diverge.This depends on the tissue/electrode geometry -see figure 6.19 for example.

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 inmuscle 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 therelative rate of heating in fatty tissue and muscle. We need to know both the realcurrent density and field strength in each tissue. The field strength is estimatedbelow.

When resistors are connected in series the current flow in each is the same but thevoltage across each resistor will, in general, be different. The largest resistor willhave across it the greatest potential difference. The equivalent statement for tissuesof different impedance is as follows:

When tissues are arranged in series the field intensity will be greatest in the tissuewith highest impedance.

Inspection of table 6.2 shows that muscle has a higher conductivity and dielectricconstant than fatty tissue: both figures are several times higher. Now a highconductivity and dielectric constant means a low impedance. Combining the two


figures from table 6.2 we calculate that fatty tissue has an electrical impedance someten 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 timesgreater than in fatty tissue, however the field intensity in fatty tissue is approximatelyten times higher. Hence the rate of heating of fatty tissue is predicted to beapproximately 10/1.5 times higher than muscle.

We thus have the general conclusion that if spreading or converging of the field isminimal the rate of heat production in fatty tissue will be about seven times higherthan in muscle.

If the electrode/tissue configuration permits spreading of the field in muscle thecurrent density will be reduced and the rate of heating of muscle correspondinglyreduced. Conversely if the geometry produces convergence of the field lines inmuscle the current density will be increased and the relative rate of heating will beincreased 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 inducedin the plane of the fatty tissue and in the plane of the muscle. The current loops arecomplete electrical pathways in the one tissue. For this reason the current flow in


muscle is not limited by the fatty tissue but depends only on the strength of theinduced electric field and the electrical characteristics of the muscle tissue. In otherwords the induced currents flowing in each tissue layer are independent of eachother.

The real component of the current density, the current density which determines heatproduction, is given by equation 6.10, which can be written:

ir = .E .... (6.10)

Substituting this formula into equation 7.1 we obtain an alternate expression for thepower dissipated per unit volume:

Pv = .E2 .... (7.2)

Table 6.2 shows that the conductivity, , of muscle is some sixteen times greater thanthat of fatty tissue. Hence, for the same induced electric field strength, both the realcurrent density and the power dissipated in muscle will be sixteen times greater thanin fatty tissue.

The intensity of the induced electric field is determined bythe rate of change of the magnetic field and the permeability, , of the material. It does not depend on the electrical properties, and , of the tissue.

How large is the magnetically induced electric field? The intensity of the induced fieldis determined by the rate of change of the magnetic field and the permeability, , of thematerial. The permeability is close to one for biological materials (see table 6.3) sofatty tissue and muscle are alike in this regard.

For the same strength of alternating magnetic field then, both fatty tissue and musclewill have the same strength of induced electric field. Thus the rate of heating ofmuscle in this situation will be about sixteen times greater than that of fatty tissue.

In practice such a degree of selective heating is difficult to achieve. This is for tworeasons:


* 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 inducedelectric field is correspondingly smaller.

* Fatty tissue, being closer to the induction coil may also experience anappreciable electric field due to the capacitance between adjacent turns of thecoil. This effect was described earlier (see figure 7.8).

For more information onrelative heating rates see thebook 'Therapeutic Heat andCold' J F Lehmann (ed)(1982) chapters 6 and 10.

These two factors combine to increase the heating of fatty tissue relative to muscle sothat a sixteen to one advantage is rarely obtained. Nonetheless efficient selectiveheating is achieved with close spacing of the turns of the coil and a sufficiently largecoil to patient distance. One would also expect good discrimination with applicatorswhich incorporate an electric field screen in front of the inductive coil.

HEAT AND TEMPERATURE RISE

Having described the factors determining the rate of heating of tissue we nowconsider 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 realcurrent 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


To convert from degreesCelsius to Kelvin's, simplyadd 273.15 to the Celsiustemperature.

Alternatively, when thespecific heat capacity of asubstance is known, equation7.4 can be used to calculatethe temperature increaseresulting from the heatsupplied.

physiological processes.

The SI unit of temperature is the kelvin (symbol K). It is related to the perhaps morefamiliar 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 sameas a change of five Kelvin's. When we are talking about increases in temperaturebrought about by diathermy treatment the terms kelvin and degrees Celsius can beused interchangeably to describe the increase.

When a fixed amount of heat is supplied to different substances the increase intemperature of each will, in general, be quite different. The factor which determinesthe 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 asubstance through one kelvin. The units of specific heat capacity are thus joules perkilogram per kelvin. This can be measured experimentally by supplying a certainamount of heat (Q) to a known mass (m) of the substance an measuring theresulting temperature increase (T). The experiment must be arranged so that all ofthe heat supplied is used to increase the temperature of the substance. If the loss ofheat is negligible then the specific heat capacity (c) can be calculated using equation7.4:

c = Q

m.T .... (7.4)

When we consider the heating of tissues by diathermy, heat transfer between tissuesand to the bloodstream will have a large effect on the temperature distribution duringtreatment.


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 ofheat and the circulation of blood and tissue fluids provide an efficient means of heattransfer. The net production of heat is balanced by net transfer of heat from the tissueand a stable temperature is maintained.

Once treatment is started heat is produced in the tissue according to equation 7.3 andthe temperature starts to increase. An expression for the initial rate of increase intemperature is obtained below.

Rearranging 7.4 we have Q = m.c.T

Dividing this expression by volume we obtain:

Qv = .C.T .... (7.5)where is the mass per unit volume or density of the tissue.

Dividing 7.5 by t gives:Qvt = .c.

Tt .... (7.6)

where Qv/t is the volume rate of heating (in Joules per cubic metre per second) andT/t is the rate of increase in temperature (in Kelvin's per second).

Note that this conclusion is ageneral one. It applies notjust to shortwave diathermybut to any diathermicmodality.

This equation can be used to compare the initial rate of temperature increase in fattytissue with that of muscle. The densities of the two tissues are similar but the heatcapacity of muscle is some 50% greater than that of fatty tissue. Thus if the rate ofheating of each tissue is the same, the initial rate of temperature increase in musclewill be only two thirds of that of fatty tissue. To produce the same initial rate ofincrease in temperature in each tissue the rate at which heat energy is produced inmuscle must be 50% greater.


An equation specifically applicable to shortwave diathermy is obtained by solvingequations 7.5 and 7.3. We then have: .c.T = E.ir.t, which on rearranging gives:

Tt =

E.irc .... (7.7)

Equation 7.7 shows that the initial rate of increase in temperature (T/t) in shortwavediathermy depends on four factors:

* E, the field intensity at the point* ir, the magnitude of the real current density at the point* , 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 isreached where the temperature ceases to increase. A new dynamic equilibrium isachieved where the net production of heat is once again balanced by the net transferfrom the tissue.

Figure 7.17 illustrates the temperature variation during treatment. There is a transientperiod during which the tissue temperature increases, followed by a steady statewhere a constant (elevated) temperature is produced. The transient period for tissuevolumes of interest in physiotherapy is typically of the order of twenty to thirty minutes


(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 ofthe rate of heating and rate of temperature increase in differentparts of tissue is difficult. The difficulty arises in the calculationof the field intensity in a particular area. For a review of resultsobtained using various approximations see A. W. Guy in J FLehmann (1982).

In patient treatment, shortwave diathermy remains something ofan art as well as a science. The physiotherapist must use aknowledge of anatomy together with knowledge of the electricalproperties of tissues to determine the optimum placement ofelectrodes or coil to give the required field pattern. Once thefield pattern is selected, the physiotherapist uses a knowledgeof the relative heating of the tissues and the patient's report of asensation of warmth to adjust the intensity of the applied field toan appropriate level. With this procedure it is not possible toaccurately monitor dose or dose rate for the individual tissues.Since this is a problem common to all diathermic modalities wewill defer further discussion of dosage until chapter eleven.

Physiological Effects Figure 7.16A simple model for tissue temperaturevariation during treatment.

The therapeutic value of shortwave diathermy arises from thephysiological response of tissues to an increase intemperature. A number of physiological responses are found:

* at the cellular level an increase in temperature increases the rate of biochemicalreactions. Thus cellular metabolism is increased - there is an increaseddemand for oxygen and nutrients and the output of waste products is increased.


* blood supply is increased. A number of factors determine this response. Theincreased output of cellular waste products triggers dilation of the capillaries andarterioles. The temperature increase itself causes some dilation, mainly in thesuperficial tissues where heating is greatest. In addition, stimulation of sensorynerve endings (again mainly in the superficial tissues) can cause a reflexdilation.

* a rise in temperature can induce relaxation of muscles. If there is abnormalmuscle activity caused by pain, for example, repeated treatment with shortwavediathermy can be beneficial. The treatment helps to interrupt the vicious circle ofpain producing muscle activity which in turn produces more pain and so on. Anumber of factors may contribute to relaxation: the direct effect of heat on muscletissue, the removal of any accumulated metabolites due to increased circulationand 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 nervefibres. In addition, when pain results from inflammation of tissue an increase inthe rate of absorption of exudate with increase in temperature can result in asecondary pain-relief effect.

Some claims have been made that additional non-thermal effects can be producedunder the conditions used for therapy. As yet there is no clinical evidence for theseclaims. Non-thermal effects seem to have been demonstrated using pulsedshortwave treatment when the peak power level is significantly higher than used fordiathermy. The few published comparative studies indicate little or no nonthermaleffect at the low continuous power levels of conventional shortwave field treatment.These points are considered further in chapter 8 following.


EXERCISES

1 Figure 7.1 shows a schematic diagram of shortwave diathermy apparatus.

(a) Briefly explain how the apparatus produces a high-frequency alternatingelectric or magnetic field.

(b) What is the function of inductors L1 and L2?

c) Why is the capacitor in the patient tuning circuit a variable one?

2 (a) Why is it necessary to tune shortwave diathermy apparatus with the patientcoupled to the machine?

(b) What is the advantage of automatic versus manual tuning of shortwavediathermy apparatus?

3 Figure 7.2 illustrates the response of ions, polar molecules and non-polarmolecules 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 andwhich with least heat production?

4 (a) What is meant by the terms 'real current' and 'displacement current'?

(b) Consider the movement of ions, polar molecules and non-polar moleculesin an alternating electric field. Describe the relationship between each kindof movement and real and displacement current.

5 (a) Consider each of fatty tissue, muscle and bone in the shortwave diathermyfield. Is current flow in each tissue best described as real or displacementcurrent?


(b) On the basis of your classification which tissue would be associated withmaximum heat production?

(c) Describe the complications (as far as prediction of heat production isconcerned) caused by the presence of Iymphatic and blood vessels in fattytissue.

6 Figure 7.4 shows current pathways in a model for an arm or leg. Describethe principal factors determining the relative rate of heating of each tissuelayer.

7 A patient's lower limb is enclosed in a solenoidally wound coil as shown in figure7.6.

(a) Describe the motion of polar, non-polar and ionic molecules when a highfrequency alternating current flows through the coil.

(b) Indicate (with a diagram) the direction of movement of molecules in thelimb.

8 If the solenoidally wound coil in question 7 was replaced by a pair of capacitorplates (one above the knee, one below the sole of the foot), what would be thenew directions of molecular motion? Draw a diagram to illustrate.

9 Figure 7.8 shows the electric field associated with two adjacent turns of aninduction coil

(a) what is the practical significance of this electric field in patient treatment?

(b) how can the effects of this electric field be minimized?

10 For shortwave diathermy it is common practice to use electrodes which aresomewhat larger than the structure to be treated (figure 7.9). Explain in terms of:

(a) the field pattern produced


(b) 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 (figure7.10(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'stissue as shown in figure 7.10(c)?

12 Consider the electrode arrangements shown in figure 7.11. Explain why the fieldintensity is non-uniform in diagrams (a) and (c). Under what circumstances willthe field intensity be uniform, as in (b)?

13 (a) Draw a diagram showing a coplanar arrangement of electrodes over tissueand the resulting field pattern.

(b) What are the advantages and disadvantages of coplanar electrodearrangements?

(c) What practical limit is there on the spacing of coplanar electrodes?

14 Consider the hollow dielectric between capacitor plates which is shown in figure7.14.

(a) Explain where heat production is greatest and why.

b) What technique can be used to produce more uniform heating of thedielectric? Explain.


15 When coplanar electrodes are used for patient treatment the tissues can beconsidered 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 thesituation where the electrodes are close together.

(c) how would bringing the electrodes closer together affect the relative heatingrate of muscle and fatty tissue? Justify your answer.

16 (a) Explain why, in principle, it is easier to produce selective heating of musclewith an inductive coil rather than capacitor field electrodes.

(b) what practical constraints limit the selective heating of muscle with aninduction coil?

17 (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 isfound to be double that of muscle. Assume that the densities of each tissueare the same and that muscle has a 50% greater heat capacity andcalculate the relative rate of heating of the tissues.

18 The relationship between heat production (Q) and current flow (I) in a conductoris given by Joule's law: Q = V.I.t where V is the potential difference across theconductor 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 currentdensity of 50 A.m-2. Use equation 7.2 to calculate the amount of heatproduced in a 30 second time interval. You may assume that the current isentirely real.


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 currentdensity of 50 A.m-2. The mass density of the material is 900 kg.m-3 and itsspecific heat capacity is 4.0 kJ.kg-1.K-1. Calculate the initial rate of increasein temperature (T/t) of the material.

20 A block of conducting material is placed in an electric field. The field intensity inthe material is 300 V.m-1 and the resulting real current density is 120 A.m-2. If thematerial has a density of 1000 kg.m-3 and it has a specific heat capacity of 3.8kJ.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 inshortwave diathermy treatment. Describe the physiological response to theinitial temperature rise and the effect this has on the subsequent rate of increaseof temperature (figure 7.16).

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1348642949.5499Chapter7

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