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Chapter: Medical Electronics : Recent Trends in Medical Insrumentation

Diathermy Units

Types of diathermy · Shortwave diathermy · Ultrasonic diathermy · Microwave diathermy · Surgical diathermy



Definition: Diathermy is the treatment process by which cutting, coagulation of tissues are obtained.


·        Application of high-frequency electromagnetic energy 

·        Used to generate heat in body tissues

·        Heat produced by resistance of tissues

·        Also used for non-thermal effects




·        Treatment can be controlled easily.


·        Use of appropriate electrodes permit the heat to be localized only in the region to be treated.


·        Amount of heat that is to be delivered can be adjusted accurately.


·        Inter lying tissues, muscles, bones, internal organs, etc, can be provided with heat by using high frequency


Physiologic Responses To Diathermy


·        Not capable of producing depolarization and contraction of muscles


·        Wavelengths too short


Physiologic Effects Are Those of Heat In General


·        Tissue temperature increase


·        Increased blood flow (vasodilation)

·        Increased venous and lymphatic flow

·        Increased metabolism

·        Changes in physical properties of tissues

·        Muscle relaxation

·        Analgesia


Diathermy Heating


Doses are not precisely controlled thus the amount of heating cannot be accurately measured


Heating= Current2 X Resistance


Types of diathermy


·        Shortwave diathermy

·        Ultrasonic diathermy

·        Microwave diathermy

·        Surgical diathermy




·        Power supply powers radio frequency oscillator (RFO)

·        RFO provides stable drift-free oscillations at given frequency

·        Power amplifier generate power to drive electrodes

·        Output resonant tank tunes in the patient for maximum power transfer


         Power output should provide energy to raise tissue temp to therapeutic range (40-45 deg c) (80-120 watts)


         Should exceed sar-specific absorption rate (rate of energy absorbed /unit area of tissue mass)


         Generates both an electrical and a magnetic field


SWD Electrodes


         Capacitor electrodes


         Inductor electrodes


         Selection of appropriate electrodes can influence the treatment



1.1 Capacitor Electrodes


·        Create stronger electrical field than magnetic field

·        Ions will be attracted or repelled depending on the charge of the pole


         Electrical field is the lines of force exerted on charged ions that cause movement from one pole to another


         Center has higher current density than periphery


         Patient is between elec trodes and becomes part of circuit


         Tissue is between electrodes in a series circuit arrangement

•   The tissue that offers the greatest resistance to current flow develop s the most heat


         Fat tissue resists curre nt flow


         Thus fat is heated in a n electrical field


         Typical with capacitor electrodes



Air Space Plates


         Two metal plates surroounded by plastic guard


         Can be moved 3cm within guard


         Produce high-frequency oscillating current


         When overheated discharges to plate of lower potential


         Area to be treated is pllaced between electrodes becoming part of circuit


         Sensation of heat in direct proportion to distance of electrode from skin


         Closer plate generates more surface heat


         Parts of body low in subcutaneous fat best treated

Pad Electrodes


         Greater electrical field


         Patient part of circuit


         Must have uniform contact (toweling)


         Spacing equal to cross-sectional diameter of pads


         Part to be treated should be centered


         Increasing the spacing will increase the depth of penetration but will decrease the current density


1.2 Induction Electrodes


Creates a stronger magnetic field than electrical field. A cable or coil is wrapped circumferentially around an extremity or coiled within an electrode


·        Passing current through a coiled cable creates a magnetic field by inducing eddy currents (small circular electrical fields) that generate heat


·        Patient in a magnetic field not part of a circuit

·        Tissues in a parallel arrangement

·        Greatest current flow through tissue with least resistance

·        Tissue high in electrolytic content respond best to a magnetic field


Cable Electrode


Two arrangements:

Pancake coils


Wraparound coils

Toweling is essential


Pancake coil must have 6” in center then 5-10cm spacing between turns


Drum Electrode

One or more monopolar coils rigidly fixed in a housing unit


May use more than one drum depending on area treated.

Toweling important.




·        It is used for curing the diseases of peripheral nervous system, skeletal muscle system and skin ulcers.


·        It is adopted when the short wave treatment has failed and it helps to achieve the localization of heart to the affected part.


·        The heating effect is produced in the tissues by the absorption of ultrasonic energy. The absorption effect is similar to that of a micro massage.


·        It is better than the manual massage because the micro massage provides a greater depth of massage without causing any pain to the patient.


·        Piezo-electric transducer is excited by the high frequency alternating current produced by the Rf oscillator.


·        Ultrasonic wave from the piezo electric transducer is used for the purpose of treatment.

·        It can be applied in continuous mode or pulse mode.


·        Frequency range of 800 KHz to 1MHz is suitable for the ultrasonic method of treatment





·        In this method the tissues are heated by the absorption of microwave energy. The frequency used is about 2450 MHz.


·        Better results are obtained by the microwave method and it is more advantageous than the short wave method.


·        There is no pad electrodes and flexible cable.

·        Microwave is transmitted into body and treat directly from the direction of unit.

·        Microwaves are produced with the help of magnetron

·        Proper cooling arrangements are made for the purpose of cooling the magnetron




·        Necessary precautions should be taken during this method of treatment

·        Excessive dosage causes skin burns and the skin should be dry as the waves are rapidly absorbed by water.



·        Patients with implanted pacemaker should not undergo this treatment


·        There are possibilities of over heating

·        Care should be taken while the treatment is made near the eyes.







·        Logic board is the main part of the unit which produces the necessary waveforms for cutting, coagulation and hemostasis modes of operation.


·        An astable multivibrator generates 500 kHz square pulses. The output from this oscillator is divided into a number of frequencies using binary counters.


·        These frequencies are used as system timing signals, A frequency of 250 KHz provides a split phase signal to drive output stages on the power output board.


·        Frequency of 250 Hz is used for cutting , after the high power amplification by push pull amplifier.


·        The output of the push pull amplifier is given to a transformer so that the voltage is stepped up and the output signal from the unit is well isolated.


·        The isolator switch provides an isolated switching control between the active hand switch and the rest of the unit.

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