---

Short Wave Diathermy (SWD) - Complete Exam Guide

1. Definition

---

2. Physical Characteristics

The frequency 27.12 MHz is the most commonly tested value - memorize it!

---

3. Types of SWD

A. Continuous SWD (CSWD)

• Electromagnetic energy delivered continuously
• Produces significant thermal effects
• Used for chronic conditions requiring deep heating

B. Pulsed SWD (PSWD) / Pulsed Electromagnetic Field (PEMF)

• Energy delivered in short pulses with rest intervals between them
• Average power is low - minimal heat produced
• Primarily non-thermal (athermal) effects
• Used for acute conditions, inflammation, and tissue repair
• Also called Pulsed Shortwave Therapy (PSWT)

---

4. Methods of Application

Capacitive (Condenser) Method

• Uses two metal plate electrodes placed on either side of the body part
• The tissue acts as a dielectric (capacitor) between the plates
• Heats more superficial tissues (subcutaneous fat, superficial muscle)
• Electric field is most intense in poor conductors (fat)
• Creates an E-field (Electric field)

Inductive (Solenoid/Cable) Method

• Uses a coil/drum electrode placed near the body part
• Produces eddy currents (induced currents) within tissue
• Heats deeper tissues (muscle) preferentially
• Creates an H-field (Magnetic field)
• Works best on high water-content tissues (muscle)
• Two types:
  • Monode (drum electrode) - single inductive coil
  • Diplode - two drum electrodes

Key distinction: Capacitive heats FAT more; Inductive heats MUSCLE more

---

5. Physiological/Therapeutic Effects

Thermal Effects (CSWD)

• Increased local tissue temperature (up to 3-4°C rise in deep tissues)
• Increased metabolic rate and enzyme activity
• Increased blood flow and vasodilation
• Increased tissue extensibility (collagen becomes more plastic)
• Muscle relaxation - reduces spasm
• Pain relief (sedation of sensory nerve endings)
• Increased nerve conduction velocity

Non-Thermal (Athermal) Effects (PSWD)

• Enhanced cell membrane permeability
• Accelerated tissue repair and healing
• Reduction of oedema (especially pulsed)
• Anti-inflammatory effects
• Stimulation of macrophage activity
• Increased collagen synthesis

---

6. Indications

• Musculoskeletal conditions: osteoarthritis, rheumatoid arthritis (chronic phase), ankylosing spondylitis
• Soft tissue injuries: muscle strains, ligament sprains (subacute/chronic phase)
• Bursitis, tenosynovitis (subacute/chronic)
• Pelvic inflammatory disease
• Post-fracture rehabilitation
• Nerve injuries - neuritis, neuralgia
• Sinusitis (facial application)
• Wound healing (pulsed SWD)

---

7. Contraindications (very commonly examined!)

Absolute Contraindications

• Metal implants in the treatment area (total joint replacements, surgical plates, screws, IUDs)
• Cardiac pacemakers (and all implanted electronic devices)
• Pregnancy (especially over the abdomen/pelvis)
• Malignancy (tumors in the treatment field)
• Active hemorrhage / bleeding disorders
• Thrombosis / thrombophlebitis
• Impaired sensation / anaesthetic areas (risk of burns)
• Open wounds / infected areas (continuous SWD)

Relative Contraindications

• Obesity (excess fat may cause overheating)
• Edema (fluid-filled areas can concentrate heat)
• Copper-bearing IUDs
• Children (growing epiphyses)
• Menstruation (over lumbar/pelvic area)

---

8. Dosage (Schliephake's Classification)

---

9. Biophysics: How It Heats

1. Ionic oscillation - Charged particles oscillate back and forth rapidly, generating heat through friction (dominant in tissues with high water/ion content)
2. Dipole rotation - Polar molecules (like water) rotate to align with the oscillating field, generating heat

---

10. Differences: SWD vs. Microwave Diathermy vs. Ultrasound

---

11. Electrodes & Setup Tips

• Spacing rule: Electrode-to-skin distance should be equal on both sides for uniform field
• Electrodes should be parallel to the body part
• Avoid air gaps that are unequal (can cause hot spots)
• The patient should not touch metal during treatment
• Treatment time: typically 15-20 minutes

---

Quick Memory Points for Exam

• Frequency = 27.12 MHz, Wavelength = 11 m
• Capacitive = heats fat/superficial; Inductive = heats muscle/deep
• Continuous = thermal; Pulsed = athermal
• Dose I = no heat (acute); Dose IV = intense heat (chronic)
• 4 absolute CIs to always mention: metal implants, pacemakers, pregnancy, malignancy
• PSWD is the treatment of choice for acute inflammation and wound healing

---

---

CHAPTER A - SHORT WAVE DIATHERMY (Complete Answers)

---

1. DEFINITION

---

2. PRINCIPLE OF WORKING

1. Ionic oscillation - Ions (charged particles) in tissue fluids oscillate rapidly back and forth in the alternating field → friction → heat
2. Dipole rotation - Polar molecules (especially water) rotate to align with the rapidly reversing field → molecular friction → heat
3. Eddy currents (in inductive method) - Induced circulating currents flow through conductive tissues → resistive heating

---

3. INDICATIONS & CONTRAINDICATIONS

✅ Indications

❌ Contraindications

• Metal implants in treatment field (plates, screws, prostheses, IUDs)
• Cardiac pacemakers and implanted electronic devices
• Pregnancy (especially abdomen/pelvis)
• Malignancy (cancer in treatment area)
• Active hemorrhage / thrombosis
• Impaired sensation (anesthetic skin - burns risk)
• Wet dressings / plaster cast (heat concentration)

• Acute inflammation / edema (use pulsed SWD instead)
• Obesity (excess fat overheating)
• Menstruation (lumbar/pelvic treatment)
• Growing epiphyses in children
• Fever / systemic infection

---

4. BIO-PHYSICS OF DEEP HEATING USING SWD

A. Capacitor (Condenser Field) Method

• Body part is placed between two condenser plates - acts as a dielectric material
• An electric field (E-field) is set up between the plates
• Heat is generated by ionic oscillation and dipole rotation
• Tissues with HIGH resistance (low water content) = MORE heat → subcutaneous fat is heated preferentially
• Penetration: moderate depth
• Best for: joints with little overlying fat, sinuses

B. Inductance (Magnetic Field / Inductothermy) Method

• A coil/cable or drum electrode is placed near the body part
• Produces an alternating magnetic field (H-field)
• The magnetic field induces eddy currents in electrically conductive (high water content) tissues
• Tissues with LOW resistance (high water content) = MORE heat → muscle is heated preferentially
• Penetration: deeper than capacitive
• Best for: deep muscle heating

Key exam point: Capacitive → heats FAT | Inductive → heats MUSCLE

---

1. PRODUCTION OF SWD

Construction

A. Machine Circuit (Oscillator Circuit)

• Power supply (mains electricity converted to DC)
• Vacuum tube oscillator (or modern solid-state transistor oscillator)
• Tank circuit = Inductor (L) + Capacitor (C) in parallel
• The LC circuit oscillates at the resonant frequency: f = 1 / (2π√LC)
• This generates the 27.12 MHz high-frequency oscillating current
• The output is amplified and fed to the patient circuit

• Transformer, Rectifier, Capacitor bank, Inductor coil, Vacuum tube/transistor

B. Patient Circuit (Resonator Circuit)

• The patient becomes part of a secondary/resonator circuit
• Consists of: variable tuning capacitor + inductance coil + patient (load)
• The patient circuit must be tuned (resonated) to match the machine circuit frequency (27.12 MHz) for maximum energy transfer
• When resonance is achieved → maximum current flows through patient tissue → maximum heating
• Tuning is done by adjusting the variable capacitor until resonance is reached (indicated by a resonance meter/ammeter showing peak deflection)

---

2. Mechanism of Production of SWD

1. AC mains supply → stepped up/down by transformer
2. Rectified to DC
3. Fed to oscillator circuit (LC tank circuit) → generates 27.12 MHz oscillations
4. Oscillations amplified by vacuum tube / transistor
5. High-frequency current passed to patient circuit electrodes
6. Electromagnetic energy transmitted to tissue → converted to heat

---

3. Indications for Circuits to be "In Tune"

• When circuits are in tune (resonance): maximum energy transfer occurs → maximum therapeutic effect
• When out of tune: energy is reflected back → poor treatment, machine damage

• Ammeter/resonance meter shows maximum deflection
• Patient reports onset of warmth
• No interference/arcing at electrodes

---

4. Transmission of Shortwave into Tissues

• Electrode spacing - affects depth of penetration (wider spacing = deeper)
• Electrode size - larger electrodes = wider distribution
• Tissue impedance - determines how energy is distributed
• Frequency - 27.12 MHz penetrates deeper than microwave (2450 MHz)

---

2. TECHNIQUE / METHOD OF APPLICATION OF SWD

1. Preparation of Equipment

• Switch on the machine and allow 5 minutes warm-up time before use
• This stabilizes the oscillator circuit output

• Set the intensity to a low level
• Adjust the tuning/resonance control until the resonance meter shows maximum deflection (circuit in tune)
• Re-check tuning after any adjustment

• Test with a neon lamp held near the electrodes - it should glow, confirming energy output
• Check electrode cables for cracks or damage
• Ensure no metal objects are present in the field

---

2. Application of Treatment

A. Condenser Field Method (Capacitor Field Method)

• Two metal plate electrodes enclosed in felt/rubber spacers placed on either side of the body part
• The body part acts as the dielectric between the plates
• Electrodes connected to the machine via coaxial cables

B. Cable Method / Inductothermy

• A multi-turn insulated cable (solenoid) is wound around the body part (e.g., limb) in a spiral/helical arrangement
• OR a drum electrode (monode/diplode) is placed near the area
• The cable/drum acts as the primary coil; the body part = secondary coil
• Eddy currents are induced in the tissues

• Turns should be equally spaced
• Turns should not cross or touch each other (short circuit risk)
• Minimum 2-3 cm gap between cable turns and skin (to avoid surface burns)

---

3. Condenser Field Method - Details

A. Types of Electrodes

1. Metal plate electrodes - rigid, used in spacers/pads
2. Flexible electrodes - can conform to body contour
3. Drum/disc electrodes - circular rigid electrodes for flat surfaces

B. Size of Electrode

• Electrode size should be equal to or slightly larger than the treatment area
• If too small: concentration of field = hot spots = burns
• If too large: energy dispersed, ineffective
• Matching electrodes (same size) used for uniform field distribution

C. Electrode Spacing

• Field lines are more divergent / spread out
• Deeper penetration of energy
• More uniform heating across tissues
• Used when deep heating is required

• Field lines are more convergent / concentrated
• Superficial heating
• Risk of surface hot spots
• Less depth of penetration

Rule: The spacing between electrode and skin should be equal on both sides to ensure uniform field.

D. Electrode Positioning

• Both electrodes placed on the same side of the body part (side by side)
• Field passes superficially between the two electrodes
• Used for superficial structures (e.g., sinuses, superficial joints)
• Less depth of penetration

• Electrodes placed on opposite sides of the body part (facing each other)
• Field passes through the body part
• Used for deep structures (e.g., hip joint, shoulder, pelvis, spine)
• Greater depth of penetration
• Most commonly used for deep heating

---

Quick Revision Summary for Exam

---

---

CHAPTER B - MICRO WAVE DIATHERMY (MWD)

---

1. DEFINITION

It lies between infrared and radio waves in the electromagnetic spectrum.

---

2. BIO-PHYSICS OF MICROWAVE DIATHERMY

A. Dipole Rotation (Primary mechanism)

• Water molecules are electric dipoles (positive and negative ends)
• The rapidly alternating microwave field (2450 million times/sec) causes water molecules to rotate/oscillate to align with the field
• This molecular friction generates heat
• Since muscle has HIGH water content → muscle is heated most

B. Ionic Oscillation

• Ions in tissue fluids oscillate back and forth in the alternating field
• Friction between ions generates heat (secondary effect)

Tissue Penetration & Heating Pattern

• Depth of penetration: ~3 cm (less than SWD)
• MWD selectively heats muscle more than fat (opposite of capacitive SWD)
• Because microwaves are reflected at fat-muscle interfaces, there can be hot spots at tissue boundaries - a major concern

Wave Behavior in Tissue

• Reflection - some energy reflected at skin surface and tissue interfaces
• Refraction - waves bend at tissue boundaries
• Absorption - energy absorbed and converted to heat (most important)
• The absorption coefficient is highest in muscle

---

3. INDICATIONS & CONTRAINDICATIONS

✅ Indications

MWD is best suited for small, superficial areas (elbow, knee, shoulder, ankle) due to its limited beam size and penetration

❌ Contraindications

• Metal implants in the treatment field (plates, screws, prostheses)
• Cardiac pacemakers and implanted electronic devices
• Pregnancy (especially abdomen/pelvis)
• Malignancy in treatment area
• Eyes - extremely vulnerable to microwave heating (no blood supply to dissipate heat) → cataracts
• Testes - very sensitive (avascular, poor heat dissipation) → sterility
• Impaired sensation (anesthetic skin) - burn risk
• Thrombosis / active hemorrhage

• Edema / fluid-filled areas (hot spots)
• Obesity (surface burns risk)
• Growing epiphyses in children
• Wet dressings or plaster casts

Most important unique CI for MWD = EYES and TESTES

---

4. PRODUCTION OF MWD - THE MAGNETRON

What is a Magnetron?

Construction of the Magnetron

Components:
1. Cathode (central cylindrical rod) - heated by a filament
2. Anode (surrounding cylindrical block) - made of copper
3. Resonant cavities - holes/slots cut into the anode block (like a spoke wheel)
4. Strong permanent magnet - placed around the tube (N and S poles)
5. Antenna/probe - extracts microwave energy output
6. Coaxial cable - transmits energy to the applicator/director

Working of the Magnetron

1. Cathode is heated by the filament → emits electrons (thermionic emission)
2. High DC voltage is applied between cathode (-) and anode (+)
3. Electrons are accelerated toward the anode
4. The permanent magnet creates a strong magnetic field perpendicular to the electron flow
5. The magnetic field causes electrons to travel in curved/spiral paths (Lorentz force) rather than straight lines
6. Electrons interact with the resonant cavities in the anode as they spiral past them
7. This interaction causes electrons to give up energy to the cavities
8. The resonant cavities oscillate at 2450 MHz (determined by cavity dimensions)
9. Microwave energy is extracted via the antenna/probe and transmitted through a coaxial cable to the director (applicator)

Key Numbers - Magnetron

• Output frequency: 2450 MHz
• Wavelength: 12.25 cm
• Power output: typically 25-100 Watts

---

5. TECHNIQUE OF APPLICATION OF MWD

A. Patient Preparation

1. Explain the treatment to the patient - what they will feel (mild warmth), duration
2. Remove all metal objects - jewellery, watches, hair pins, belt buckles from the treatment area
3. Remove wet clothing or wet dressings from the area
4. Expose the treatment area - clothing should be dry and thin, or removed
5. Position the patient comfortably - lying or sitting, treatment part well-supported
6. Check skin sensation - test hot/cold discrimination before treatment; do NOT treat anesthetic areas
7. Inspect the skin - check for open wounds, recent scars, skin conditions
8. Screen the area if near eyes (use lead/microwave-absorbing screen) - eyes must be protected
9. Check for contraindications - pacemaker, pregnancy, metal implants

---

B. Selection of Treatment Applicator (Director)

• Match director size to the size of the treatment area
• Use a director that covers the area without beam spill onto sensitive structures
• Keep director at correct distance from skin (usually 5-10 cm - non-contact method)

---

C. Selection of Appropriate Power Level & Application of Treatment

• MWD is delivered in Watts (typically 10-80 W depending on area and condition)
• Power selected based on:
  • Body part size - larger area = higher power
  • Phase of condition - acute = low power; chronic = higher power
  • Patient's sensation - patient should feel mild, comfortable warmth only
  • Depth of target tissue

1. Switch on and warm up machine (1-2 minutes)
2. Select appropriate director/applicator
3. Position director at correct distance from skin (5-10 cm for most directors)
4. Director held at right angles to the skin surface for best absorption
5. Set power to a low starting level
6. Gradually increase power until patient reports mild comfortable warmth
7. Never leave the patient unattended
8. Monitor patient throughout treatment
9. Treatment duration: 15-20 minutes
10. At end: switch off, check skin for any erythema or burns

---

D. Dosage

---

E. Physiological & Therapeutic Effects

Physiological Effects (from heat):

1. Increased tissue temperature - up to 3-4°C rise in target tissues
2. Vasodilation and increased blood flow - hyperemia, increased O₂ and nutrient delivery
3. Increased metabolic rate - enzyme activity enhanced
4. Increased tissue extensibility - collagen becomes more pliable (better stretching)
5. Muscle relaxation - reduces spasm and tension
6. Pain relief - heat raises pain threshold; sedates sensory nerve endings; reduces bradykinin/substance P
7. Increased nerve conduction velocity
8. Reduced joint stiffness - synovial fluid becomes less viscous

Therapeutic Effects:

• Reduces muscle spasm and guarding
• Relieves chronic pain
• Reduces joint stiffness (before exercise)
• Improves range of motion
• Promotes healing in subacute/chronic stages
• Reduces periarticular stiffness
• Beneficial in sinusitis (local hyperemia)

---

F. DANGERS of MWD

Most unique dangers of MWD = Cataracts (eyes) and Sterility (testes) - always mention these!

---

Quick Revision - MWD Summary

---

CHAPTER C - ULTRASOUND THERAPY

---

1. DEFINITION

Key difference from SWD and MWD: Ultrasound is mechanical (sound) energy, not electromagnetic energy.

---

2. BIO-PHYSICS OF ULTRASOUND

Nature of Ultrasound Waves

• Ultrasound waves are longitudinal (compression) mechanical waves
• They travel as alternating compression and rarefaction zones through tissue
• They require a medium to travel (cannot travel through vacuum - unlike EM waves)
• They travel best through dense, homogeneous media (fluids, dense tissues)

Mechanisms of Action

A. Thermal Effects (Continuous US)

• Absorption of sound energy → molecular vibration → heat
• Heating is greatest at tissue interfaces (especially bone-periosteum, tendon-bone junctions)
• Temperature rise of 1-4°C at target tissue
• Effects: increased blood flow, reduced spasm, pain relief, increased collagen extensibility

B. Non-Thermal (Mechanical) Effects (Pulsed US)

1. Acoustic Streaming
   • Ultrasound causes unidirectional flow of fluid along cell membranes
   • Alters cell membrane permeability
   • Enhances transport of ions and molecules across membranes
   • Promotes tissue repair and healing
2. Cavitation
   • Formation and behavior of gas bubbles in tissue fluids due to pressure changes
   • Stable (non-inertial) cavitation: bubbles oscillate rhythmically → beneficial microstreaming around cells → enhanced healing
   • Unstable (inertial/transient) cavitation: bubbles collapse violently → tissue damage (to be avoided - caused by very high intensities)

---

3. INDICATIONS & CONTRAINDICATIONS

✅ Indications

❌ Contraindications

• Over the eyes
• Pregnant uterus / over fetus
• Over the heart / cardiac area
• Over malignant tumors
• Over the spinal cord (after laminectomy)
• Over the gonads (testes/ovaries)
• Thrombophlebitis (may dislodge clots)
• Pacemakers (over the chest/pacemaker site)
• Over bone epiphyses in children (growing plates)
• Impaired sensation - burn risk

• Acute hemorrhage
• Active infection
• Recent fractures
• Directly over metal implants (hot spots at metal surface)

---

4. PROPERTIES OF ULTRASOUND - REFLECTION, TRANSMISSION, ABSORPTION

A. Reflection

• When ultrasound waves reach an interface between two tissues of different acoustic impedances, some energy is reflected back
• Reflection is greatest at soft tissue-bone and soft tissue-air interfaces
• This is why coupling medium (gel) is essential - to eliminate the air gap between transducer and skin
• Reflection % depends on the difference in acoustic impedance between the two media

B. Transmission

• The portion of ultrasound energy that passes through the interface into the next tissue
• Transmission is greatest in homogeneous, fluid-rich media
• High transmission in: blood, muscle, water
• Low transmission through: bone (most is reflected), air (almost complete reflection)

C. Absorption

• The conversion of sound energy into heat as waves pass through tissue
• Absorption coefficient varies by tissue:

• Higher frequency (3 MHz) = more superficial absorption
• Lower frequency (1 MHz) = deeper penetration (up to 5 cm)

Exam key: 1 MHz = deep (5 cm) | 3 MHz = superficial (1-2 cm)

---

5. ULTRASONIC FIELD

Two Zones:

A. Near Field (Fresnel Zone)

• Region close to the transducer face
• Sound beam is roughly cylindrical (same width as transducer)
• Energy distribution is very uneven / turbulent - lots of peaks and troughs (hot spots and cold spots)
• Length of near field = r² / λ (r = radius of transducer, λ = wavelength)
• Treatment in near field = risk of uneven heating and hot spots

B. Far Field (Fraunhofer Zone)

• Region beyond the near field
• Sound beam diverges (spreads out)
• Energy distribution becomes more uniform
• Intensity decreases with distance

Beam Non-uniformity Ratio (BNR)

• Ratio of peak intensity to average intensity within the beam
• Ideal BNR = 1 (perfectly uniform) - never achieved in practice
• Acceptable BNR: ≤ 6:1 (FDA standard)
• High BNR = more hot spots = more risk

Effective Radiating Area (ERA)

• The area of the transducer that actually emits ultrasound (not the total face)
• ERA < physical transducer face size
• Important for calculating actual intensity delivered

---

6. COUPLING MEDIA

Properties of an Ideal Coupling Medium:

• Acoustically transparent (minimal absorption/reflection)
• Air-free (no bubbles)
• Non-toxic, non-irritant to skin
• Good viscosity (stays on skin without running)
• Inexpensive and readily available
• Should not damage the transducer

Types of Coupling Media:

Application Methods Using Coupling Media:

• Gel applied directly to skin
• Transducer moved in slow, overlapping strokes or circles over the area
• Most common method for flat/accessible areas

• Body part (hand, foot, ankle) submerged in degassed water
• Transducer held 1-3 cm away from skin, submerged in water
• Used for irregular surfaces (hands, feet) where direct contact is difficult
• Water acts as the coupling medium

• Water-filled bag placed between transducer and irregular skin surface
• Used for bony prominences

---

7. PRODUCTION OF ULTRASOUND

Piezoelectric Effect

• When mechanical pressure is applied to certain crystals → they generate an electric charge

• When an alternating electric current is applied to a piezoelectric crystal → the crystal vibrates/deforms at the same frequency as the current
• This vibration produces mechanical (sound) waves = Ultrasound
• This is the principle used in ultrasound therapy machines

Piezoelectric Materials Used:

• Natural: Quartz (original)
• Synthetic (most common today):
  • Lead Zirconate Titanate (PZT) - most widely used
  • Barium titanate
  • Lithium sulfate

Construction of the Transducer (Treatment Head):

Components:
1. Piezoelectric crystal (PZT disk) - the active element
2. Metal electrodes on both faces of the crystal
3. Backing material - absorbs energy from back face, prevents reverberations
4. Matching layer - on front face, improves acoustic coupling
5. Coaxial cable - connects to the generator
6. Transducer housing (stainless steel or aluminium face plate)

Working:

1. High-frequency AC generator produces current at 1 MHz or 3 MHz
2. Current applied to electrodes on the piezoelectric crystal
3. Crystal undergoes rapid expansion and contraction (vibrates)
4. Vibration of crystal face produces compression and rarefaction waves in the coupling medium
5. These mechanical waves = ultrasound → enter the tissue through coupling gel

Modes of Operation:

• Continuous mode: Crystal vibrates constantly → thermal + mechanical effects
• Pulsed mode: Crystal vibrates in bursts with off periods → primarily non-thermal (mechanical) effects; duty cycle = 20% or 50% typically

---

Complete Quick Revision Table

---

• Magnetron construction for MWD
• Piezoelectric effect for ultrasound production
• BNR and near/far field for ultrasound
• Cataracts and testes as unique MWD dangers
• 1 MHz vs 3 MHz depth difference