Materials that convert sound into electricity and vice versa

Natural: quartz, rochelle salts, tourmaline

Synthetic: barium titanate, lead metaniobate, lead titanate, lead zirconate titate (PZT)

A common ultrasound transducer 

AKA ceramic AKA active element AKA crystal

Loses piezoelectricity if heated about Curie point of 360°C (680°F) - why we cannot sterilize transducers with heat

Case

Electrical Shield

Acoustic Insulator

Wire

Matching Layer

Active Element

Damping Element/Backing Material

Cylindrical tube that protects the internal components from damage

Insulates the patient from shock

Constructed of metal or plastic

Provides electrical connection between the crystal and system

Active element or crystal must have electrical contact

Increases the efficiency of sound energy transfer between the active element and the body

Positioned in front of the crystal; protects the active element

The portion of the probe in contact with the skin; evens out impedance between crystal and skin

1/4 wavelength thick

Impedance: PZT > matching layer > gel > skin

The piezoelectric crystal shaped like a coin

1/2 wavelength thick

Characteristics of the sound beam are related to the characteristics of the crystal

Damping Element 

AKA Backing Material

Bonded to active element to reduce the "ringing" of PZT by restricting the extent of deformation

Commonly made of epoxy resin impregnated with tungsten

Dampens the emitted sound pulse to shorten duration and length -> high sound absorption with acoustic impedance similar to PZT

Enhances axial resolution to increase image accuracy

Continuous wave transducers to do not require damping material

The minimal distance two structures must be separated in order for the system to resolve them as separate structures and produce two distinct echoes on the u/s image displayed on the system monitor

Measure the system's ability to resolve two structures very close together that are parallel to the sound beam's main axis

Decreased sensitivity: reduces vibration during reception and is less responsive to minute reflectors (less able to convert low-level sound reflections into meaningful electrical signals)

Wide bandwidth: increases the range of frequencies in the pulse

Low quality factor

The range of frequencies in a pulse

The main frequency emitted by the transducer

AKA center AKA primary AKA natural frequency

Unitless number representing the degree of damping; inversely related to bandwidth

Q = resonant frequency ÷ bandwidth (will be < 1)

Low = more damping

High = less damping

Dampening limits ringing

Short pulse length and duration

Wide bandwidth aka broadband

Low Q

Low sensitivity

Improved axial resolution

Sound is always being produced

Electrical frequency = acoustic frequency

Does not produce image

No backing material = narrow bandwidth pulses

Therapeutic ultrasound or continuous wave Doppler

Creates short duration electrical spike that excites PZT

Capable of producing images 

Produce wide bandwidth aka broadband pulses

Frequency determined by properties of the crystal:

• Propagation speed (directly related) -- 4-6 mm/μs
• Thickness (inversely related) -- 0.2-1 mm

Designed with varying thicknesses of crystal to have varying frequencies