Therapeutic Ultrasound Masterclass: Parameters, Physics + 20 MCQs

Therapeutic Ultrasound (UST) Exam Guide

💡 The Core Concept: Therapeutic Ultrasound uses high-frequency sound waves (mechanical vibration) produced by the Reverse Piezoelectric Effect to generate thermal and non-thermal effects in tissues. It requires a coupling medium (Gel/Water) because sound waves do not travel through air.

1. The Technical Arsenal (Key Parameters)

Frequency determines depth, and Intensity determines the dosage. Memorize this table for AIIMS/JIPMER.

Parameter	Details & Exam Values
Frequency	1 MHz: Deep tissues (2-5 cm penetration). 3 MHz: Superficial tissues (1-2 cm penetration).
Duty Cycle	Continuous (100%): Thermal effects (Chronic). Pulsed (20%): Non-thermal effects (Acute healing).
Intensity	Measured in W/cm² (SATA / SATP). Typical Range: 0.5 to 2.0 W/cm².
Coupling Media	Aquasonic Gel, Degassed Water, Glycerol. Avoid: Vaseline/Oils (block waves).

2. Mechanism of Action

A. Thermal Effects

Generated by friction at the molecular level. Occurs with Continuous Mode.

• Increased tissue extensibility (Collagen stretch).
• Increased blood flow and metabolism.
• Pain relief / Muscle relaxation.

B. Non-Thermal Effects (Mechanical)

Generated by the vibration itself. Occurs with Pulsed Mode.

• Cavitation: Formation of gas bubbles in tissue fluids.
  • Stable Cavitation: Good. Enhances cell membrane permeability.
  • Unstable (Transient) Cavitation: Bad. Tissue damage due to bubble collapse. Occurs at very high intensity.
• Acoustic Streaming: Movement of fluids along cell membranes, increasing cellular activity (protein synthesis).
• Micro-massage: Mechanical vibration effect.

3. BNR and ERA (AIIMS Exam Focus)

These are the most common technical questions in competitive exams.

• BNR (Beam Non-uniformity Ratio): The ratio of Peak Intensity to Average Intensity.
  • Ideal BNR: 1:1 (Impossible to achieve).
  • Acceptable BNR: Up to 6:1.
  • Significance: High BNR creates "Hot Spots" causing periosteal pain. Always move the sound head to prevent this.
• ERA (Effective Radiating Area): The area of the crystal that actually produces sound waves.
  • Exam Rule: ERA is always smaller than the physical size of the transducer head.

4. Special Techniques & Contraindications

Phonophoresis: Using Ultrasound to drive whole drug molecules (topical agents) through the skin. Common drugs: Hydrocortisone (Anti-inflammatory), Lidocaine (Analgesic).

⚠️ Absolute Contraindications

• Malignancy: Can spread metastasis.
• Thrombus/DVT: Can dislodge the clot (Embolism risk).
• Eyes: Causes Cavitation in fluid.
• Pregnancy: Over abdomen/pelvis/back.
• Plastic Implants/Cement: (e.g., TKR/THR with cement). Plastic heats up rapidly. Note: Metal implants are NOT a strict contraindication for US (unlike SWD), because sound reflects off metal, but caution is needed to avoid standing waves.
• Epiphyseal Plates: In growing children (disturbs bone growth).
• Pacemaker: Local application over the chest.

🏆 AIIMS "Golden Points" - Absorption Hierarchy

Ultrasound is absorbed best by tissues with high collagen/protein content and worst by tissues with high water content.

Absorption Order (High to Low):
Bone (Highest) > Cartilage > Tendon > Skin > Muscle > Fat > Blood (Lowest).

📝 20 High-Yield MCQs (Practice Now)

Click an option to check your answer immediately.

Q1. Which frequency is appropriate for treating a deep structure like the Piriformis or Hamstring belly?

A. 1 MHz B. 3 MHz C. 27.12 MHz D. 50 Hz

Rationale: Lower frequency (1 MHz) has a longer wavelength and penetrates deeper (2-5 cm). 3 MHz is absorbed superficially (1-2 cm).

Q2. The Beam Non-uniformity Ratio (BNR) should ideally be:

A. 1:1 B. 10:1 C. 8:1 D. 12:1

Rationale: The ideal BNR is 1:1 (uniform beam), though clinically 4:1 to 6:1 is acceptable. High ratios (>8:1) cause hot spots and tissue damage.

Q3. Which tissue absorbs the most Ultrasound energy?

A. Blood B. Fat C. Muscle D. Bone

Rationale: Ultrasound is absorbed best by tissues with high protein/collagen content and density. Bone absorbs the most, followed by cartilage and tendon.

Q4. Which phenomenon is responsible for the non-thermal effect of increasing cell membrane permeability?

A. Reflection B. Stable Cavitation / Acoustic Streaming C. Unstable Cavitation D. Refraction

Rationale: Stable cavitation (micro-bubbles) and acoustic streaming (fluid movement) mechanically alter the cell membrane, increasing permeability to ions like Calcium.

Q5. For an acute ankle sprain (Day 1), what are the recommended Ultrasound parameters?

A. Continuous mode, 1.5 W/cm² B. Continuous mode, 0.5 W/cm² C. Pulsed mode (20%), 0.5-0.8 W/cm² D. Pulsed mode, 2.5 W/cm²

Rationale: In acute conditions, heat is contraindicated. We use Pulsed Mode with low intensity to achieve non-thermal healing effects without exacerbating inflammation.

Q6. What is the Effective Radiating Area (ERA)?

A. The total surface area of the sound head B. The area of the crystal producing sound waves (smaller than head) C. The area of the treatment zone D. The size of the lesion

Rationale: ERA is always slightly smaller than the geometric area of the sound head because the crystal edges do not expand as much as the center.

Q7. When using the "Water Bath" method for irregular surfaces (e.g., fingers), the distance between the head and skin should be:

A. In direct contact B. 1 cm to 2 cm C. 5 cm to 10 cm D. 10 mm with gel on head

Rationale: In a water bath, the head should be held 1-2 cm away from the skin to allow waves to travel through the water coupling medium. Direct contact is not maintained.

Q8. Which of the following is a contraindication specific to Ultrasound over the laminectomy site?

A. Metal screws B. Scar tissue C. Exposed Spinal Cord (removed lamina) D. Muscle spasm

Rationale: If a laminectomy (bone removal) has been done, the spinal cord loses its bony protection. Ultrasound energy can cause cavitation in the CSF (Cerebrospinal Fluid), causing CNS damage.

Q9. Phonophoresis involves driving whole drug molecules using:

A. Acoustic energy B. Electric repulsion (Ions) C. Magnetic fields D. Heat conduction

Rationale: Phonophoresis uses ultrasound (acoustic energy) to increase skin permeability. Iontophoresis uses electricity to drive ions.

Q10. Why is the ultrasound head kept moving continuously?

A. To cover a larger area B. To prevent "Hot Spots" / Periosteal Burns C. To activate the crystal D. To mix the gel

Rationale: Because the beam is non-uniform (BNR), keeping the head stationary can cause standing waves and excessive intensity at specific points, leading to periosteal burns.

Q11. The crystal in the ultrasound head is typically made of:

A. Quartz (Natural) B. Lead Zirconate Titanate (PZT - Synthetic) C. Copper D. Silicon

Rationale: Modern transducers use synthetic PZT ceramics because they have superior piezoelectric properties compared to natural quartz.

Q12. High intensity or stationary application can lead to "Unstable Cavitation". Why is this dangerous?

A. It stops the machine B. Bubble collapse causes tissue damage C. It freezes the tissue D. It blocks sound waves

Rationale: Unstable (transient) cavitation involves the rapid expansion and violent collapse of gas bubbles, releasing high local pressure/heat that damages tissue.

Q13. Which condition implies a strict contraindication for Ultrasound over the lower leg?

A. Muscle Strain B. Deep Vein Thrombosis (DVT) C. Metal plate in tibia D. Chronic edema

Rationale: Mechanical vibration can dislodge a clot (thrombus) in DVT, sending it to the lungs (Pulmonary Embolism), which is life-threatening.

Q14. What is "Attenuation"?

A. Increase in intensity B. Loss of energy as the wave travels (Absorption + Scattering) C. Speed of sound D. Reflection of sound

Rationale: Attenuation is the decrease in energy intensity as the ultrasound wave passes through tissue, primarily due to absorption and scattering.

Q15. Why is Methyl Methacrylate (Bone Cement) or Plastic Implants a contraindication for US?

A. They absorb US rapidly and melt/overheat B. They reflect US completely C. They become magnetic D. They cause cavitation

Rationale: Plastics and synthetic cements absorb ultrasound energy very quickly (high attenuation), leading to rapid overheating and potential loosening of the implant.

Q16. To treat a superficial scar (skin depth), which frequency is best?

A. 1 MHz B. 3 MHz C. 50 KHz D. 27.12 MHz

Rationale: 3 MHz is absorbed rapidly in superficial tissues (1-2 cm depth), making it ideal for skin conditions and scar tissue.

Q17. The "Half-Value Depth" is:

A. The depth at which intensity reduces to 50% B. Half the total treatment time C. 50% of BNR D. The depth of the ERA

Rationale: Half-value depth is the distance the beam travels in tissue before its intensity is attenuated to half its original value. It is approx 3cm for 1MHz and 1cm for 3MHz.

Q18. Which medium is generally NOT suitable for Ultrasound coupling?

A. Degassed Water B. Aquasonic Gel C. Vaseline / Petroleum Jelly D. Glycerol

Rationale: Vaseline and oils contain air bubbles and have poor transmissivity for sound waves, blocking the energy from entering the skin.

Q19. The production of electric charge by applying mechanical stress to a crystal is called:

A. Piezoelectric Effect B. Reverse Piezoelectric Effect C. Inductive Effect D. Capacitive Effect

Rationale: Note the difference: Ultrasound *Production* uses the **Reverse** Piezoelectric effect (Electricity -> Vibration). The *Direct* Piezoelectric effect is Vibration -> Electricity. (Read the question carefully: "Applying mechanical stress" = Direct).

Q20. Which duty cycle produces the least heat (purely non-thermal)?

A. 100% B. 50% C. 20% D. 80%

Rationale: A 20% duty cycle (1:4 ratio) means the machine is ON for 2ms and OFF for 8ms. This long pause allows any heat generated to dissipate, resulting in purely non-thermal/mechanical effects.