Types of Transducers
The simplest type of ultrasound
transducer is a simple mechanically steered transducer which
was employed in first generation ultrasound systems. This involved a single PZT
element that was mechanically steered throughan 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 lense 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 generatee 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 (see discussion of
temporal resoluation in advanced wave parameters to know).
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 ulatrsound 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, thesensitivity 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 wil
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 inbetween 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 ultrsound
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 acking materials.
sumber : http://echocardiographer.org
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