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) ³

   • 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 ³

     • a thicker element produces a lower frequency oscillation while a thinner element produces a higher frequency oscillation ³

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 ³

   • ground electrode is on the front of the element ³

3. damping (backing) block

   • adhered to the back of the crystal (behind the positive electrode) ³

   • absorbs ultrasound energy directed backward and attenuates stray ultrasound signals from the housing ¹

   • dampens the resonant vibrations in the element which creates a shorter spatial pulse length ³; this allows for better axial resolution for imaging of organs and high bandwidth to receive reflected echoes ³.

4. matching layer

   • interface between the transducer element and the tissue ³

   • 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 ³

5. housing

   • electrical insulation and protection of the element ³

   • 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 inby mechanical or electronic means. In mechanical transducers, either oscillating or rotating wheels is used. In electronic transducers, there are two ways to generate ultrasound beam ³:

• 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.