Ultrasound transducers
the widening of the sound beam in the far field.
size of the transducer element(s).
nonuniform driving (excitation) of elements in an array to reduce grating lobes.
collection of active elements connected to individual electronic currents in one transducer assembly.
ability to distinguish two structures along a path parallel to the sound beam.
multiple transducer elements with individual wiring and system electronics.
occurs when two waves in phase with each other create a new wave with amplitude greater than the original two waves; in phase.
curved linear transducer containing multiple piezoelectric elements.
temperature to which a material is raised, while in the presence of a strong electrical field, to yield piezoelectric properties. If the temperature exceeds the Curie point, the piezoelectric properties will be lost.
material attached to the rear of the transducer element to reduce the pulse duration.
occurs when two waves out of phase with each other create a new wave with amplitude less than the two original waves; out of phase.
includes both axial and lateral resolution.
deviation in the direction of the sound wave that is not a result of reflection, scattering, or refraction.
aperture that increases as the focal length increases; minimizes change in the width of the sound beam.
variable receiving focus that follows the changing position of the pulse as it propagates through tissue; the electrical output of the elements can be timed to “listen” in a particular direction and depth.
piezoelectric component of the transducer assembly.
detail resolution located perpendicular to the scan plane; it is equal to the section thickness and is the source of the section thickness artifact.
region of the sound beam in which the diameter increases as the distance from the transducer increases.
distance from a focused transducer to the center of the focal zone; distance from a focused transducer to the spatial peak intensity.
concentration of the sound beam into a smaller area.
additional weak beams emitted from a multielement transducer that propagate in directions different from the primary beam.
all points on a wave front or at a source are point sources for the production of spherical secondary wavelets.
interference occurring when two waves interact or overlap, resulting in the creation of a new wave.
ability to distinguish two structures lying perpendicular to the sound path.
a ceramic piezoelectric material.
material attached to the front face of the transducer element to reduce reflections at the transducer surface.
region of the beam between the transducer and focal point, which decreases in size as it approaches the focus.
natural frequency of the transducer; it is determined by the propagation speed and thickness of the element in pulse ultrasound and by the electrical frequency in continuous wave.
conversion of pressure to electric voltage.
applying voltage pulses to all elements in the assembly as a group, but with minor time differences. Phased pulses allow multiple focal zones, beam steering, and beam focusing.
operated by applying voltage pulses to a group of elements in succession.
additional weak beams traveling from a single-element transducer in directions different from the primary beam.
dividing each element into small pieces to reduce grating lobes.
device that converts energy from one form to another.
transducer element, damping, matching layers, and housing; also known as probe, scan head, or transducer.
Piezoelectricity (piezoelectric effect)
• Piezoelectric principle states that some materials produce a voltage when deformed by an applied pressure.
• Various forms of ceramics and quartz are naturally piezoelectric.
• Lead zirconate titanate (PZT) is the most common manufactured piezoelectric element.
• PZT placed in a strong electric field while at a high temperature acts as an element with piezoelectric properties (Curie point).
• If the material exceeds the Curie point, the element will lose its piezoelectric properties (i.e., autoclave sterilization).
| COMPONENT | FUNCTION | DESCRIPTION | RELATIONSHIP |
| Piezoelectric element, also called:CrystalActive elementTransducer element | Converts electrical voltage into ultrasound pulses and the returning echoes back to electric voltageElectrical energy is applied to the element, increasing or decreasing the thickness according to the polarity of the voltage | Thickness of the element ranges between 0.2 and 1.0 mmPropagation speed of the element ranges between 4 and 6 mm/μsNatural Materials:Rochelle salt, quartz, and tourmalineManufactured Materials:Lead zirconate titanate (PZT), barium titanate, lead metaniobate, and polyvinylidene difluorideMixture of polymer and piezoceramic material (new)Single elements are in the form of a diskArray transducers contain numerous elements with separate electrical wiringContain a bandwidth of frequenciesImpedance is much greater than soft tissue | Propagation speed of the element is directly related to the operating frequencyThickness of the element is inversely related to the operating frequencyThickness is equal to half of the wavelengthImpedance is 20× greater than that of the skin |
| Damping, also called:Backing | Reduces the number of cycles in each pulseAn electronic means to suppress the crystal from ringingReduces pulse duration and spatial pulse length | Attached to the rear face of the elementMade of metal powder and a plastic or epoxyHigh absorption coefficientReduces sensitivity and Q-Factor Impedance in a way similar to that of the element | Increases the bandwidth and axial resolution |
| Matching layers | Reduce the impedance difference between the element and skinImprove sound transmission across the element–tissue boundary | Two layers are typically usedAqueous gel is a matching layer between the transducer face and the skin | Increase the transmission of sound into the bodyThickness equal to one fourth of the wavelengthImpedance of matching layer is in between those of the element and the skin |
| Transducer housing | Protects the components of the transducerProtects the operator and patient from electrical shockPrevents the transducer from outside interference | Covering for transducer componentsMade of metal or plastic | Damage to the housing can increase risk of electrical shock and decrease image quality |
Types of transducers
Continuous wave
• Produces a continuous wave of sound.
• Is composed of separate transmit and receiver elements housed in a single transducer assembly.
• Frequency of the sound wave is determined by the electrical frequency of the ultrasound system.
Pulse wave
• Transmits pulses of sound and receives returning echoes.
• Classified by the thickness and propagation speed of the element.
• Demonstrates a wide bandwidth and short pulse length.
• Linear, convex, and annular are types of transducer construction.
• Sequenced, phased, and vector are types of transducer operation.
• Produces a 2-cycle to 3-cycle pulse for gray-scale imaging and a 5-cycle to 30-cycle pulse for Doppler techniques.
• Minor or secondary beams traveling in directions different from the primary beam are termed side or grating lobes.
• Frequency of the sound pulse is equal to the operating frequency.
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| TYPE | DESCRIPTION | FOCUSING | BEAM STEERING |
| Convex sequenced array | Multiple elements arranged in a curved lineOperated by applying voltage pulses to groups of elements in successionPulses travel in different directions, producing a sector-shaped imageAlso called: curved array, convex array, curvilinear array | Electronic | Electronic |
| Intracavital | Mechanical, linear array, or phased array transducers mounted on probes designed to insert into the vagina, rectum, or esophagusCrystal is mechanically swept up and down to produce a 45 to 110 degree sector imageHigh frequency with rapid frame rates optimizing axial and lateral resolutionAlso called: endocavital, transcavital | Electronic | Electronic |
| Intraluminal | Extremely small crystal arrays are mounted on the end of a catheter designed to insert into a fetal, vascular, or anatomical structure (i.e., umbilical cord, artery, fallopian tube)High frequency (10 to 20 MHz)Also called: transluminal | Electronic | Electronic |
| Linear sequenced array | Straight line of rectangular elements about one wavelength wideOperated by applying voltage pulses to groups of elements in successionPulses travel in straight parallel lines producing a rectangular image.Also called: linear array | Electronic | Electronic |
| Linear phased array | Contains a compact line of elements about one-quarter–wavelength wideOperated by applying voltage pulses to most or all of the elements using minor time differencesResulting pulses can be shaped and steeredReceived echoes follow the changing position of the pulsePermits multiple focal zones | Electronic | Electronic |
| Mechanical | Uses a single element with a fixed focal depthProduces a sector image | Mechanical | Fixed |
| Sector | Each pulse originates from the same starting point |