Đầu dò Siêu âm Transducers Lecture ultrasound beams

Transducers  Ultrasound is produced and detected with a transducer, composed of one or more ceramic elements with electromechanical (piezoelectric) properties • The ceramic element converts electrical energy into mechanical energy to produce ultrasound and mechanical energy into electrical energy for ultrasound detection  Over the past several decades, the transducer assembly has evolved considerably in design, function, and capability, from a single-element resonance crystal to a broadband transducer array of hundreds of individual elements • A simple single-element, plane-piston source transducer has major components including the • piezoelectric material, • marching layer, • backing block, • acoustic absorber, • insulating cover, • sensor electrodes, and • transducer housing Piezoelectric Materials  A piezoelectric material (often a crystal or ceramic) is the functional component of the transducer • It converts electrical energy into mechanical (sound) energy by physical deformation of the crystal structure  ConverseIy, mechanical pressure applied to its surface creates electrical energy • Piezoelectric materials are characterized by a well-defined molecular arrangement of electrical dipoles (Fig 16-9)  An electrical dipole is a molecular entity containing positive and negative electric charges that has no net charge • When mechanically compressed by an externally applied pressure, the alignment of the dipoles is disturbed from the equilibrium position to cause an imbalance of the charge distribution  A potential difference (voltage) is created across the element with one surface maintaining a net positive charge and one surface a net negative charge • Surface electrodes measure the voltage, which is proportional to the incident mechanical pressure amplitude  Conversely, application of an external voltage through conductors attached to the surface electrodes induces the mechanical expansion and contraction of the transducer element  There are natural and synthetic piezoelectric materials • An example of a natural piezoelectric material is quartz crystal, commonly used in watches and other time pieces to provide a mechanical vibration source at 32.768 kHz for interval timing • This is one of several oscillation frequencies of quartz, determined by the crystal cut and machining properties  Slice thickness is typically the worst measure of resolution for array transducers • Use of a fixed focaI length lens across the entire surface of the array provides improved elevational resolution at the focal distance  Unfortunately, this compromises resolution due to partial volume averaging before and after the elevational focal zone (elevational resolution quality control phantom image shows the effects of variable resolution with depth  Multiple linear array transducers with five to seven rows, known as 1.5dimensional (1.5-D) transducer arrays, have the ability to steer and focus the beam in the elevational dimension  Elevational focusing is implemented with phased excitation of the outer to inner arrays to minimize the slice thickness dimension at a given depth (Fig 16-25)  By using subsequent excitations with different focusing distances, multiple transmit focusing can produce smaller slice thickness over a range of tissue depths • A disadvantage of elevational focusing is a frame rate reduction penalty required for multiple excitations to build one image  The increased width of the transducer array also limits positioning flexibility • Extension to full 2D transducer arrays with enhancements in computational power will allow 3D imaging with uniform resolution throughout the image volume IMAGE DATA ACQUISITION  Understanding ultrasonic image formation requires knowledge of ultrasound production, propagation, and interactions • Images are created using a pulse echo method of ultrasound production and detection • Each pulse transmits directionally into the patient, and then experiences partial reflections from tissue interfaces that create echoes, which return to the transducer  Image formation using the pulse echo approach requires a number of hardware components: • • • • • • the beam former, pulser, receiver, amplifier, scan converter/image memory, and display system  Ultrasound equipment is rapidly evolving toward digital electronics and processing, and current state-of-the-art systems use various combinations of analog and digital electronics Beam Formers  The beam former is responsible for generating the electronic delays for individual transducer elements in an array to achieve transmit and receive focusing and, in phased arrays, beam steering • Most modern, high-end ultrasound equipment incorporates a digital beam former and digital electronics for both transmit and receive functions  A digital beam former controls applicationspecific integrated circuits (ASICs) that provide transmit/receive switches, digital-toanalog and analog-to-digital converters, and preamplification and time gain compensation circuitry for each of the transducer elements in the array  Major advantages of digital acquisition and processing include the flexibility to introduce new ultrasound capabilities by programmable software algorithms and to enhance control of the acoustic beam Pulser  The pulser (also known as the transmitter) provides the electrical voltage for exciting the piezoelectric transducer elcnwnts, and controls the output transmit power by adjustment of the applied voltage • In digital beam-former systems, a digital-to analogconverter determines the amplitude of the voltage An increase in transmit amplitude creates higher intensity sound and improves echo detection from weaker reflectors  A direct consequence is higher signal-to-noise ratio in the images, but also higher power deposition to the patient User controls of the output power are labeled “output,” “power,” “dB,” or “transmit” by the manufacturer In some systems, a low power setting for obstetric imaging is available to reduce power deposition to the fetus A method for indicating output power in terms of a thermal index (TI) and mechanical index (MI) is usually provided (see section 16.1 1) [...]... with thicker elements • Resonance transducers transmit and receive preferentially at a single “center frequency.” Damping Block  The damping block, layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing • This component also dampens he transducer vibration in create an ultrasound pulse width a short spatial... applied to the element surfaces causes compression or expansion from equilibrium by realignment of the dipoles in response to the electrical attraction or repulsion force Resonance Transducers  Resonance transducers for pulse echo ultrasound imaging are manufactured to operate in a “resonance” mode, whereby a voItage (commonly 150 V) of very short duration (a voltage spike of ≈1 µsec) is applied, causing... 5-MHz ultrasound beam is 0.4 mm • The optimal matching layer thickness is equal to ¼λ = ¼ x 0.4 mm = 0 1 mm • In addition to the matching layers, acoustic coupling gel (with acoustic impedance similar to soft tissue) is used between the transducer and the skin of the patient to eliminate air pockets that could attenuate and reflect the ultrasound beam Nonresonance (BroadBandwidth) “Multifrequency” Transducers. .. instrumentation require a relatively narrowband transducer response in order to preserve velocity information encoded by changes in the echo frequency relative to the incident frequency  Continuous-wave ultrasound transducers have a very high Q characteristic • While the Q factor is derived from the term quality factor, a transducer with a low Q does not imply poor quality in the signal Matching Layer  The... Ultrasound transducers for medical imaging applications employ a synthetic piezoelectric ceramic, most often lead-zirconate-titanate (PZT) • The piezoelectric attributes are attained after a process of • Molecular... vibration (also known as “ring-down”) lessens the purity of the resonance frequency and introduces a broadband frequency spectrum • With ring-down, an increase in he bandwidth (range of frequencies) of he ultrasound pulse occurs by introducing higher and lower frequencies above and below the center (resonance) frequency  The “Q factor” describes the bandwidth of the sound emanating from a transducer as... causing the piezoelectric material to initially contract, and subsequently vibrate at a natural resonance frequency • This frequency is selected by the “thickness cut,” due to the preferential emission of ultrasound waves whose wavelength is twice the thickness of the piezoelectric material  The operating frequency is determined from the speed of sound in, and the thickness of, the piezoelectric material... attached to both surfaces, mechanical deformation occurs  The piezoelectric element is composed of aligned molecular dipoles  Under the influence of mechanical pressure from an adjacent medium (e.g., an ultrasound echo), the element thickness • Contracts (at the peak pressure amplitude), • Achieves equilibrium (with no pressure) or • Expands (at the peak rarefactional pressure), • This causes realignment