Ultrasound Transducers Overview

Overview

This lecture covers ultrasound (US) transducers, focusing on their components, piezoelectric materials, their function, properties, and care.

Transducer Basics

• A transducer converts one form of energy into another.
• US transducers convert electrical energy to acoustic (mechanical) energy during transmission.
• During reception, US transducers convert acoustic (mechanical) energy back to electrical energy.

Piezoelectric Materials

• The piezoelectric effect is when certain materials create a voltage when mechanically deformed or pressured.
• The reverse piezoelectric effect is when these materials change shape when a voltage is applied.
• Piezoelectric materials (also called ferroelectric) include quartz, tourmaline, and Rochelle salts (natural), and PZT (man-made).
• PZT stands for lead zirconate titanate and is commonly used in US imaging transducers.

Transducer Components

• Case: Protects internal components and insulates the patient from shock.
• Electrical Shield: A thin metallic barrier inside the case that blocks electrical noise.
• Acoustic Insulator: Cork/rubber layer that isolates internal parts from the case.
• Active Element (PZT/ceramic/crystal): The piezoelectric crystal, one-half wavelength thick.
• Wire: Provides electrical contact for energy transmission and reception.
• Matching Layer: One-quarter wavelength thick; improves sound transfer between crystal and skin.
• Gel: Further decreases impedance mismatch, increasing transmission.
• Backing Material (damping element): Reduces PZT ringing, shortens pulse, enhances axial resolution.

Impedance Matching & Energy Transfer

• Sound energy transfer efficiency increases with better impedance matching (PZT > matching layer > gel > skin in decreasing impedance).
• Reflection occurs at boundaries with impedance differences.
• Matching layer and gel enhance energy transmission and image quality.

Damping Material: Pros & Cons

• Advantages: Increases image quality, shortens pulse, enhances axial resolution.
• Disadvantages: Decreased sensitivity, wide bandwidth, low quality factor (Q-factor).

Frequency & Bandwidth

• Operating (resonant) frequency is the primary frequency.
• Bandwidth is the range (highest–lowest) of frequencies in a pulse, measured in hertz (Hz).
• Imaging transducers: wide bandwidth, low Q-factor, short pulse.
• Non-imaging transducers: narrow bandwidth, high Q-factor, long pulse.

PZT Polarization & Care

• PZT is made by polarization—exposing the material to high electric field and heat (Curie temperature: 300-400°C or 600-700°F).
• Heating above Curie temperature causes depolarization (loss of piezoelectric properties).
• Transducers should be disinfected, not sterilized, to prevent depolarization.

Key Terms & Definitions

• Transducer — Device converting energy types.
• Piezoelectric Effect — Voltage generated when material is deformed.
• Reverse Piezoelectric Effect — Shape change with voltage application.
• PZT — Lead zirconate titanate, a man-made piezoelectric material.
• Matching Layer — Material matching impedance between PZT and skin.
• Bandwidth — Range of frequencies in pulse.
• Axial Resolution — Ability to distinguish structures along beam axis.
• Quality Factor (Q-factor) — Unitless, inversely related to bandwidth.
• Polarization — Process making PZT piezoelectric.
• Curie Temperature — Temperature where PZT is polarized.
• Depolarization — Loss of piezoelectric properties.

Action Items / Next Steps

• Label fig. 8.1 in the textbook (pg. 114).
• Review parts and functions of transducer components.
• Study advantages/disadvantages of damping materials.
• Practice identifying impedance order and Q-factor relationships.