IMAG 461 Unit 2 Lect 1 Transducers PDF

IMAG 461 Physics Unit 2  Chapter 3 TRANSDUCERS  Objectives  Described the construction of a transducer and the function of each part.  Explain how a transducer generates ultrasound pulses  Explain how a transducer receives echoes.  Describe a sound beam and list...

IMAG 461 Physics Unit 2  Chapter 3 TRANSDUCERS  Objectives  Described the construction of a transducer and the function of each part.  Explain how a transducer generates ultrasound pulses  Explain how a transducer receives echoes.  Describe a sound beam and list the factors that affect it.  Describe how sound beams are focused and automatically scanned through tissue cross sections.  Compare linear, convex, phased, and vector arrays.  Define detail resolution.  Differentiate between three aspects of detail resolution.  List the factors that determine detail resolution. Ultrasound transducers convert electric energy into ultrasound energy and vice versa  Piezoelectricity  Greek piezo to press  Some materials produce a voltage when deformed by an applied pressure.  Conversely, piezoelectricity also results in production of a pressure when an applied voltage deforms these materials.  Various formulations of lead zirconate titanate are used in the production of modern ultrasound transducer elements.  They are not naturally piezoelectric.  They are made that way by being placed in a strong electric field while they are at a high temperature.  If a critical temperature (the Curie point) subsequently is exceeded, the element will lose its piezoelectric properties  Single-element transducers take the form of disks.  Linear array transducers contain numerous elements that have a rectangular shape.  When an electric voltage is applied to the faces of either type, the thickness of the element increases or decreases, depending on the polarity of the voltage.  Capacitive micromachined ultrasonic transducers (CMUT’s)  Newer  Broader bandwith  Improved resolution  Better integration with newer instruments  Cost effective Comprised of 2 electrically conducting layers facing each other. One is fixed and the other is a flexible membrane. An insulating layer and a vacuum-sealed gap separate the two layers. When alternating current is applied to the layers, the flexible membrane vibrates toward and away from the fixed layer, producing an ultrasound pulse  Transducers are typically driven by 1 cycle of alternating voltage for sonographic imaging. This single cycle driving voltage produces a 2 or 3-cycle ultrasound pulse.  Longer driving voltages are used for Doppler techniques.  The driving frequency must be reasonably close to the operating frequency Operating frequency (resonance frequency) is determined by:  The propagation speed of the element (ct)  The thickness (th) of the transducer element.  The operating freq. Is such that the thickness corresponds to half a wavelength in the element material. Typical diagnostic ultrasound elements are 0.2 to 1 mm thick and have propagation speeds of 4 to 6 mm/μs. Thinner elements yield higher frequencies For wide-bandwidth transducers, voltage excitation can be used selectively to operate the same transducer at more than one frequency. The transducer is driven at one of two or three selectable frequencies by voltage pulses with the selected frequency.  Damping Material  Pulse repetition frequency is equal to voltage repetition frequency.  # of voltage pulses sent to transducer per second (determined by machine)  Damping material is attached to the rear face of the transducer elements to reduce the number of cycles in each pulse.  Damping reduces pulse duration and spatial pulse length, improves resolution, decreases efficiency and sensitivity.  Shortening the pulses broadens their bandwidth. Typical bandwidths for modern transducers range from 50% to 100%. An example of a 100% bandwidth is a 5 MHz operating frequency with a bandwidth of 5MHz, that is, from 2.5 to 7.5 MHz.  Matching Layers  Because the transducer element is a solid (having high density and sound speed), it has an impedance that is about 20x that of tissue.  Without compensation, this factor would cause about 80% of the emitted intensity to be reflected at the skin boundary. (most of the sound would not enter the body)  To solve this problem, a matching layer commonly is placed on the transducer face.  This material has impedance of some intermediate value between those of the transducer element and the tissue.  Analogous to the coating on eyeglasses or camera lenses that reduces light reflection at the air-glass boundary.  Many frequencies and wavelengths are present in short ultrasound pulses. This multiple matching layers improve sound transmission across the element-tissue boundary better than does a single layer.  Typically 2 layers Beams and Focusing  The transducer produces a sound beam with a width that varies according to the distance from the transducer face.  The intensity is not uniform throughout the beam.  Sometimes, significant intensity travels out in some directions not included in this beam. These additional beams are called side lobes. They are source of artifacts. Near and Far zones  Near zone length is determined by the size and operating frequency of the element.  The near-zone length increases with increasing frequency or element size, which is called aperture.  One near zone length is called the near zone, near field, or Fresnel Zone  The region that lies beyond the near zone is called the far zone, far field, or Fraunhofer zone. At the end of the near zone, the beam width is approximately equal to one half the width of the transducer element. At double the near zone length, the beam diameter is approximately equal to the diameter of the transducer element. An increase in frequency or aperture increases the near zone length.  Focusing  To improve resolution, diagnostic transducers are focused.  Sound may be focused by using curved transducer elements, a lens, or by phasing.  Focusing moves the end of the near zone toward the transducer and narrows the beam.  Beam width is decreased in the focal region and in the area between it and the transducer, but it is widened in the region beyond  Focal length – the distance from the transducer to the center of the focal region.  Focusing can only be achieved in the near zone  As the element or lens in increasingly curved, the focus moves closer to the transducer  The limit to which a beam can be narrowed depends on the wavelength, aperture, and focal length. Automatic Scanning  The transducer is responsible for emitting and receiving echoes and it is also for sending the pulses through the many paths required to generate a cross-sectional image.  This is called scanning, sweeping, and steering.  Electronic scanning is performed with arrays.  Sequencing  Applying voltage pulses to groups of elements in succession.  Real-time presentation requires scanning the beam across the transducer assembly several times per second.  The aperture is the size of the group of elements energized to produce on pulse.  Linear and convex arrays work in a similar manner.  If the pulsing sequence alternated groupes of three and four elements (e.g., elements 1 -3, then 1 – 4, then 2 - 4, then 2 – 5) the number of scan lines would be doubled, to 250, thereby increasing scanline density and improving the quality of the image.  Phased Array  Operated by applying voltage pulses to most or all elements (not a small group) in the assembly, but with small (