Transducer Anatomy - Unit 8

Introduction to Transducers

• A transducer is a device that changes one form of energy into another.
• Examples in daily life:
  • Engine: gas (chemical energy) -> motion (kinetic energy)
  • Light bulb: electrical energy -> light and heat
  • Muscles: chemical energy -> motion
• Ultrasound transducers are bi-directional transducers.

Basic Transducer Components

8.1 PZT Element

• Piezoelectric Effect:
  • Converts sound into electrical volts during reception.
  • Sound (pressure waves) interacts with piezoelectric material to produce voltage, creating ultrasound images.
• Reverse Piezoelectric Effect:
  • Converts electrical volts into sound waves during transmission.

Piezoelectric Materials

• Natural examples: quartz, topaz, cane sugar, tourmaline.
• Man-made materials: Lead zirconate titanate (PZT)
  • Not naturally piezoelectric – requires heat and magnetic field treatment.
  • Curie point: Over 500°F (300°C) – loss of piezoelectric properties at high temperatures.

Impedance and Resonance

• PZT has high impedance compared to skin.
• Mixing PZT with resin reduces impedance mismatch, improving bandwidth, sensitivity, and resolution.
• Frequency production:
  • Continuous wave transducers: electrical frequency = acoustic frequency.
  • Pulse wave transducers: frequency depends on PZT thickness and propagation speed.

Frequency and Wavelength Relationships

• Frequency is inversely related to element thickness.
• Direct relationship between frequency and propagation speed.
• Operating frequency formula: Frequency = Speed in element / (2 * Thickness of element)
• Wavelength and thickness are directly related.*

Transducer Components

Matching Layer

• Reduces impedance mismatch between PZT and skin.
• Typically 1/4 of the wavelength.

Backing Material

• Also known as damping material.
• Reduces PZT ringing, improving image resolution by shortening spatial pulse length.
• Effects:
  • Decreases sensitivity (misses low amplitude reflectors).
  • Increases bandwidth.
  • Lowers quality factor (Q-factor).

Bandwidth and Quality Factor

• Bandwidth = Highest frequency - Lowest frequency.
• Q-factor inversely related to bandwidth.
• High Q-factor = narrow bandwidth (pure tone).
• Low Q-factor = wide bandwidth (better for imaging).

Wire and Housing

• Wires connect each PZT element to the machine.
• Housing includes:
  • Electrical shield to prevent interference.
  • Acoustic insulator to absorb vibrations.

Transducer Care and Maintenance

• Regular cleaning and disinfection required.
• High-level disinfection for transducers used in body cavities or with open wounds.
• Storage to avoid contamination.
• Probe covers and sterile gel used in sterile procedures.

Conclusion

• Understanding each component and its function is crucial for optimal use and care of ultrasound transducers.