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The advantages of using composite piezoelectric transducers for medical ultrasound

By Cyprien Rusu 11 March 2020

It’s not a secret that piezoelectric crystals have been used very effectively in ultrasonic transducers to generate acoustic waves.

(If you don’t know what piezoelectricity is, check out this article!)

While the most basic transducer is just a PZT disk cabled to a source of voltage, many other types of transducer exist.

So called “composite piezoelectric” transducers have been used a lot in medical imaging in recent years. They are composed of individual PZT pillars (or rods) in a polymer matrix, like in the following image:

Piezoelectric Transducers

Why? That’s what we are going to discuss in this article.

But first …

What is a medical transducer used for?

The first application that comes to mind when we speak about medical transducers is of course ultrasound imaging. Anyone who is going to have a baby needs to know if the baby is in good health throughout the pregnancy. That’s why ultrasound transducers are extremely important!

Ultrasound

It is of course important to have a transducer that follows certain special requirements. Medical ultrasonic probes, for example, use piezocomposite elements along with an acoustic lens, several layers of acoustic matching layers, and backing elements to meet those requirements:

Piezocomposite Layers

Let’s talk about these requirements.

What are the requirements for the piezoelectric elements used in medical transducers?

First, for sensitive transducers, the piezoelectric element must efficiently convert between electrical and mechanical energy.

Sensitive Transducers

Second, the piezoelectric element must be acoustically matched to tissue so that the acoustic waves in the transducer and tissue couple well both during transmission and reception.

Ultrasound

Third, the electric properties must be compatible with the driving and receiving electronics.

Ultrasound

The relevant properties are, respectively:

  • The electromechanical coupling constant, Kt
  • The specific acoustic impedance, Z
  • The dielectric constant, εs

In the next section, we will go over what these properties are for conventional piezoelectric materials, piezoelectric polymers, and piezocomposite polymers.

Piezoelectric transducers

For sensitive transducers, it’s also important to pay attention to electrical ( tan δ) and mechanical (Qm) losses.

Many other technological requirements (shapeability, thermal stability, structural strength, etc.) must be met as well.

What are the primary requirements and how can these be met?

The primary requirements are:

  1. A high electromechanical coupling (Kt tends towards 1)
  2. An acoustic impedance close to tissue (Z tends towards 1.5 Mrayls)
  3. A reasonably large dielectric constant ( εs > 100)
  4. Low electrical ( tan δ < 0.05) and mechanical (Qm > 10) losses

How do conventional piezoelectric materials meet these needs?

Piezoelectric ceramics, such as lead zirconate titanate, lead metaniobate, and modified lead titanates, are the most popular choice for medical ultrasonic transducers.

Piezoelectric ceramics

Transducers made with these materials offer:

  • A high electromechanical coupling (Kt ~0.4–0.5)
  • A wide selection of dielectric constants ( εs ~ 100–2400)
  • Low electrical and mechanical losses ( tan δ < 0.02, Qm ~ 10–1000)

Their major drawback is that they have high acoustic impedance (Z ~ 20–30 Mrayls).

In order to use these conventional transducers, engineers have invented a technology using “matching layers” to couple these ceramics to tissue. This makes it possible to design transducers that are relatively broadband and sensitive.

How do piezoelectric polymers meet these needs?

Piezoelectric polymers, such as polyvinylidene difluoride and its copolymer with trifluoroethylene, provide a contrasting set of material properties.

Piezoelectric polymers

Transducers made with these materials offer a low acoustic impedance (Z – 4 Mrayls), which makes acoustic matching easy.

Their major drawbacks are:

  • Low electro-mechanical coupling (Kt < 0.3)
  • High dielectric losses ( tan δ ~ 0.15), which seriously degrade the sensitivity
  • Low dielectric constant ( εs ~ 10), which places severe demands on the transmitter and receiver

How do composite piezoelectric transducers meet these needs?

PZT

First, they provide material properties superior to both piezoceramics and piezopolymers!

Their advantage are:

  • Coupling coefficient can be larger than those of the ceramics (Kt ~0.6–0.75)
  • Acoustic impedance is much lower (Z < 10 Mrayls), almost reaching the range of piezopolymers!
  • They also provide a wide range of dielectric constants ( εs ~10–100)
  • They have low dielectric and mechanical losses

In short, composite piezoelectric materials have amazing characteristics and that’s why they are heavily used in medical ultrasound applications!

Note: Transducer arrays and piezocomposites look visually similar but they have a major difference:

  • In piezocomposites a single transducer block has multiple materials layered/placed next to each other and the pillars are part of the transducer, which has a single electrode pair
  • In a transducer array the pillars are individual transducers and are individually electroded

Comparing each type of piezoelectric material

The following table summarizes the main properties of each type of transducer.


*Acoustic impedance must be close to tissue (Z tends towards 1.5 Mrayls).

How to obtain the exact characteristics of piezocomposites?

Because composite piezoelectric transducers are very important for medical imaging, it is important to understand the precise characteristics of these complex composite structures to optimize the design of medical probe devices.

Simulation is the perfect tool to carry out the evaluation of complex piezoelectric devices using composite piezoelectric materials, backing, matching layer, and lens in full 3D.

We provide a step-by-step tutorial on our support website: Simulating a Piezoelectric Transducer (1-3 Composite Array).

See also our video:

Cyprien Rusu
Cyprien Rusu

Cyprien Rusu is our Director of Engineering for Asia at OnScale. He has a extensive background in FEM, Technical Marketing, Sales and Support. Cyprien recieved his MS in Civil Engineering from Tsinghua University. At OnScale he is a trusted advisor for our client base in Asia, while creating and providing OnScale training.