Transducer Basics
The simplest type of ultrasound transducer is a simple mechanically steered transducer which was employed in first generation ultrasound systems. This involved a single piezoelectric crystal (made of lead zirconate titanate (Pb[Zr(x)Ti(1-x)]O3) otherwise known as PZT) that was mechanically steered through an arc to generate sector. The transducer had a single focus which was achieved by either curvature of the surface of the PZT or through the application of a lens over the surface of the PZT (see schematic below)
However, the current generation of ultrasound systems use phased array transducer systems. A phased array consists of multiple PZT elements, generally arranged parallel to each other. The beam generated by the array of PZT elements is electronically steered through sequential activation of each element to generate a scanning sector. Similarly the beam is electronically focused by activating elements at the periphery of the array differentially to those in the center. Indeed, phased array transducers can have multiple focuses by transmitting two pulses per scan line with different focuses. This comes at the expense of temporal resolution or frame rate because it takes longer to scan each line
Before detailing the basic construction of a ultrasound transducer, there are a number of terms which describe parameters of the transducer which will be helpful to define. First, it is important to realize that any transducer actually generates not a single frequency but a range of frequencies around the central frequency. This is in part due to the fact that the PZT crystal continues to generate some ultrasound even after the electrical signal has been terminated, which is so called "ringing". The range of frequencies produced by the transducer is called the bandwidth. The quality factor is defined as the central frequency divided by the bandwidth. Finally, the sensitivity of the transducer is defined as the ability of the transducer to detect reflected ultrasound and generate an electrical signal.
The basic components of a modern ultrasound transducer are illustrated schematically below.
Transducers have a case to insulate them from electrical interference and a wire which receives and relays electrical inputs and outputs to the echo machine. The active element is made of PZT crystal and can be a single element or more commonly a phased array. Note the thickness of the PZT is generally 1/2 the wavelength of the ultrasound produced.
In front of the PZT is a thin layer of material on the surface of the probe called the matching layer. Note that the thickness of the matching layer is generally 1/4 the wavelength of the ultrasound produced. The purpose of the matching layer is to try to maximize transmission of ultrasound from the PZT to the patient. Recall that when there is a large difference in impedance between two media, most sound will be reflected and little will be transmitted at the interface. There is a large difference in impedance between the PZT and skin. The matching layer and ultrasound gel have an impedance in between that of the PZT and the skin to reduce the amount of reflection at this interface and improve transmission into the patient.
On many diagnostic ultrasound transducers, the PZT is mounted on a backing material (also sometimes called a damping material). The backing material has several effects on the properties of the transducer. The backing material has the effect of decreasing ringing of the PZT crystal. It therefore shortens the spatial pulse length and so improves axial resolution (this is why the backing material is used). The backing material also has the effect of widening the bandwidth and therefore decreasing the quality factor. Thus, diagnostic transducers are wide bandwidth, low quality factor transducers. By way of contrast, therapeutic ultrasound transducers typically do not have a backing material as they require a narrow bandwidth of frequencies around the central frequency. Similarly, dedicated continuous wave transducers such as the Pedoff probe have no need to improve axial resolution and therefore do not have backing materials.