Question

Explain the functions of the various layers of a typical ultrasound transducer. Why are thicknesses of λ /2 and λ /4 used for the piezoelectric layer and the matching layer?

Answer 1

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)
•mayconsist of a single element or be a broadband transducer with multiple elements
•element thickness is determined by what resonance frequency is desired

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

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 minimizing reflection due to traversing different mediums (acoustic impedance)

5.housing

•electrical insulation and protection of the element
•includes a plastic case, metal shield and acoustic insulator

why are thickness lemda/2 and lemda/4 used see ...

However, the very low acoustic impedance and the reduced thickness suggest the possibility to use them as conventional matching layers in the low MHz frequency range. Typical frequency location of the first thickness resonance of these films is between 0.3 and 0.6 MHz, so they can be used as active quarter wavelength (λ / 4) matching layers in the low MHz frequency range (0.15–0.3 MHz).

The purpose of this paper is to present an air-coupled piezoelectric transducer with two active layers, one is a 1–3 connectivity piezocomposite made of a random distribution of piezoelectric fibers embedded in a polymeric matrix and the other one is a cellular polypropylene ferroelectret film .

The design presents the special feature that the ferroelectret film can be used as both active layer (mainly as receiver) and as impedance matching layer (λ / 4). the first step is to characterize the ferroelectret film in order to determine its acoustic impedance and thickness resonant frequency (λ / 4).

It is also necessary to determine how the properties of this film are affected when it is attached to the rest of the transducer. It consists on using an electrically conductive double sided adhesive tape (non-woven conductive scrim with acrylic adhesive, thickness 90 µm, density 1,100 kg / m^3 and contact electrical resistance <0.4 ohm) to glue to the ferroelectret film to the substrate.

The influence of this layer on the response of the ferroelectret film and on the whole transducers has also been considered. Then a 1–3 connectivity piezocomposite, whose first thickness resonance (λ / 2) is tuned to the λ / 4 thickness resonance of the ferroelectret, is selected.

Two intermediate matching layers are then introduced so that the impedance profile along the transducer cross-section provides the optimum energy output. The thicknesses are calculated so that the frequencies of the λ / 4 thickness resonances of these two layers are tuned to the resonant frequency of the ferroelectret (λ / 4) and the piezocomposite (λ / 2).