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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:
crystal/ceramic element with piezoelectric properties
- usually lead zirconate titanate (PZT)
- 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
- a thicker element produces a lower frequency oscillation while a thinner element produces a higher frequency oscillation
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
damping (backing) block
- adhered to the back of the crystal (behind the positive electrode)
- 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; this allows for better axial resolution but higher bandwidth
- 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) 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
- electrical insulation and protection of the element
- 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 in two ways:
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.