Ultrasound transducer

An ultrasound transducer converts electrical energy into mechanical (sound) energy and back again, based on the piezoelectric effect. It is the hand-held part of the ultrasound machine that is responsible for the production and detection of ultrasound waves.

It consists of five main components:

1. crystal/ceramic element with piezoelectric properties
   • usually lead zirconate titanate (PZT)
   • may consist of a single element or be a broadband transducer with multiple elements
   • element thickness is determined by what resonance frequency is desired
     • equal to half the wavelength
     • a thicker element produces a lower frequency oscillation while a thinner element produces a higher frequency oscillation
2. positive and ground electrodes on the faces of the element
   • this allows for electrical connection
   • positive electrode is in the back of the element
   • ground electrode is on the front of the element
3. damping (backing) block
   • adhered to the back of the crystal (behind the positive electrode)
   • absorbs ultrasound energy directed backwards and attenuates stray ultrasound signals from the housing ¹
   • dampens the resonant vibrations in the element which creates a shorter spatial pulse length; this allows for better axial resolution but higher bandwidth
4. matching layer
   • interface between the transducer element and the tissue
   • allows close to 100% transmission of the ultrasound from the element into the tissues by minimising reflection due to traversing different mediums (acoustic impedance) ²
   • achieves this by consisting of layers of material with acoustic impedances that are between soft tissue and transducer material.
     • may consist of one or multiple layers
   • each layer is one-quarter wavelength thick
5. housing
   • electrical insulation and protection of the element
   • includes a plastic case, metal shield and acoustic insulator

Ultrasound transducers typically consist of 128-512 piezoelectric elements arranged in linear or curvilinear arrays. Each element is equal to or less than a ½ wavelength wide and transducer length is generally 5 to 15 cm. Each element is individually insulated.

Transducers can produce an ultrasound beam in two ways:

• linear array (also called sequential array)
• phased array

As a general rule, if the shape at the top of the images matches the shape at the bottom of the image it is a sequential array. If the shapes are different (e.g. rectangular at the top and curved at the bottom) it is a phased array.