Beam steering with pulsed two-dimensional transducer arrays

The major problem facing the development of 2-D arrays for imaging is the complexity arising from the large number of elements anticipated in such transducers. The authors have undertaken a theoretical investigation of the focusing and steering properties of pulsed 2-D arrays to characterize the parameters required for medical imaging, such as element size, spacing, and number of elements. Details of the computational methods employed are presented, as well as a discussion of the steered beam properties of wideband 2-D arrays. The effects of apodization and element cross-coupling on the beam properties of a 2-D transducer array are examined. The beam properties of various sparse arrays with elements randomly distributed over the aperture of the transducer are discussed.     

Determination of lamb wave dispersion data in lossy anisotropic plates using time domain finite element analysis. Part II: application to 2-2 and 1-3 piezoelectric composite transducer arrays

The use of finite element modeling, combined with optical generation and detection of Lamb waves in plate structures, was extended to encompass periodic ceramic-polymer materials typical of those encountered in 1-3 and 2-2 piezoelectric composite array transducers. The resultant dispersion data was employed to predict the occurrence of Lamb wave-induced cross talk in composite monolithic arrays. The finite element modeling method was then used to simulate the dispersion behavior of two array structures that were subsequently manufactured: a 1-D 45% volume fraction linear array coupon and a 2-D 35% volume fraction array coupon. Excellent agreement between theory and experiment was obtained using impedance measurements and laser scans of the surface displacement profile at selected frequencies. Regions of strong inter-element cross-coupling were identified and these are shown to correlate very well with the dispersion data obtained for the dual-phase plate material. This work is considered to provide a useful basis for the design of wideband monolithic composite arrays and minimization of guided wave propagation along the array substrate.     

Calculation of ultrasonic transducer signal-to-noise ratios usingthe KLM model

This paper describes a method for adding thermal and amplifier noise to a KLM model for a transducer element. The model is used to compare the magnitudes of various noise sources in a 5 MHz array element typical of those used for linear array imaging with and without an amplifier. Fundamental signal-to-noise ratio (SNR) issues of importance to array and amplifier designers are explored, including the effect on SNR of effective dielectric constant of the piezoelectric material, individual element size, changing the number of elements, and adding an amplifier to an element before and after a cable. SNR is considered both for the case in which the acoustic output is limited by the maximum rarefactive pressure which is considered safe for a particular application (Mechanical Index limitation) and the case in which acoustic output is limited by the maximum transmit voltage than can he delivered by the imaging system or tolerated by the transducer. It is shown that the SNR performance depends on many controllable parameters and that significant improvements in SNR can be achieved through proper design. The implications for 1.5-D and 2-D array elements are discussed     

Acoustical evaluation of a prototype sector-vortex phased-array applicator

A prototype sector-vortex phased-array applicator for ultrasound hyperthermia was constructed and acoustically evaluated. The array transducer consists of special lead-titanate ceramic elements of 16 sectors and two tracks attached on a element is driven by a complementary pair of power MOSFETs at 750 kHz. An annular focal field approximated by the Mth order Bessel function is theoretically predicted to be formed when the array elements are driven with a phase distribution that rotates M (相似文献   

二维阵列换能器声辐射力分布的计算分析

利用角谱理论得到了圆形活塞换能器阵元组阵后作用在平面悬浮物体上的声辐射力分布公式。通过数值仿真,分析了换能器频率、阵元间距以及阵元数目对声辐射力分布的影响。计算结果表明,换能器组阵使得声辐射力分布的指向性变窄,强度增强;随着换能器频率的提高、阵元间距的增大以及阵元数目的增多,声辐射力分布的主瓣更尖锐,但阵元间距的增大会使声辐射力分布的旁瓣增高。为了改善声辐射力的空间分布,采用伪逆矩阵算法,以能量增益为目标函数,通过调节换能器阵元表面振动速度的幅值和相位来形成多焦点的声辐射力分布,为阵列换能器声辐射力分布的调控和声悬浮稳定性的研究提供帮助。     

End-element anomalies in medical ultrasonic piezo-composite arrays

Piezoelectric composites are commonly used in medical diagnostic ultrasonic imaging arrays. The performance of the array elements at either end of the array can differ from that of array elements away from the ends. There is some general understanding about the origin of these effects (such as different acoustic impedance), and some standard compensatory designs exist (such as adding unused array elements further on the end than the last used array element). This work seeks to elucidate the origins of these end-element anomalies and to propose corresponding design changes. A commercially produced array with notable end-element anomalies is examined as a case study. Results from experiments and finite element analysis indicate that, in the presence of a stiffness discontinuity within the composite (such as poled elements adjacent to unpoled ones), a secondary wave that propagates laterally through the composite may be generated during the receive transduction. This wave appears to cause the anomalous behavior observed in the signals and metrics of the end elements. Changing the electrical loading of an element and poling the unused, previously unpoled elements are explored as anomalymitigating design alterations. The latter of these 2 initially appears to be the more effective solution.     

Multilayer piezoelectric ceramics for two-dimensional array transducers

In medical ultrasound imaging, 2-D array transducers have become essential to implement dynamic focusing and phase-correction in the elevation dimension as well as real-time volumetric scanning. Unfortunately, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance near resonance. These elements have poor sensitivity because their impedance is much higher than the electrical impedance of the transmit and receive circuitry. Sensitivity can be improved by using an N layer structure of PZT ceramic with the layers connected acoustically in series and electrically in parallel. For the multilayer ceramic (MLC), the damped capacitance is multiplied by a factor of N(2) and the electrical impedance by 1/N(2) compared to a single layer element of the same dimensions. A 3x43 phased-array transducer has been fabricated using 3 layer PZT-5H material. Each element had a thickness of 0.66 mm and an area of 0.37x3.5 mm. The MLC was manufactured using thick film technology with plated-through vias to electrically interconnect the electrode layers. The completed transducer was compared to a single layer control array of similar dimensions. With a light epoxy backing and a lambda/4 matching layer, the MLC array elements had an impedance of 100 Omega at series resonance of 2.25 MHz, compared to 800 Omega for the control elements. The lower impedance of the MLC elements resulted in a minimum round-trip insertion loss of 24.0 dB, compared to an 34.1 dB for the control array elements. These results were consistent with KLM modeling. B-scan images were made of cysts in a tissue-mimicking phantom and of the left kidney in vivo. The images clearly showed a higher signal-to-noise ratio for the MLC array compared to the control. As a result, 2-D arrays made of multilayer ceramics can be used to form images at a higher frequency and greater range than single layer arrays.     

Optimizing the radiation pattern of sparse periodic two-dimensionalarrays

A method for reducing the number of elements in a 2-D array while minimizing degradation of the beam forming properties is described. The method relies on selecting a different arrangement of elements when the array is transmitting energy and when the array is receiving energy. The transmit and receive aperture functions are chosen to minimize the difference between the effective aperture of the sparse array and the effective aperture of a desired dense array. In a companion paper [see ibid vol. 43, pp. 7-14, 1996], the design of sparse linear arrays using the effective aperture method was described. Here, we extend this method to the design of 2-D arrays. Comparisons of the radiation patterns of a dense 2-D array and sparse 2-D arrays with random and periodic element spacing are given. Using the effective aperture method, we show that the number of elements in a 64×64 2-D array can be reduced by more than six times, and the elements in a 128×128 array can be reduced by more than 12 times, with little effect on the beam forming properties of the arrays     

Large improvement of the electrical impedance of imaging and high-intensity focused ultrasound (HIFU) phased arrays using multilayer piezoelectric ceramics coupled in lateral mode

With a change in phased-array configuration from one dimension to two, the electrical impedance of the array elements is substantially increased because of their decreased width (w)-to-thickness (t) ratio. The most common way to compensate for this impedance increase is to employ electrical matching circuits at a high cost of fabrication complexity and effort. In this paper, we introduce a multilayer lateral-mode coupling method for phased-array construction. The direct comparison showed that the electrical impedance of a single-layer transducer driven in thickness mode is 1/(n2(1/(w/t))2) times that of an n-layer lateral mode transducer. A large reduction of the electrical impedance showed the impact and benefit of the lateral-mode coupling method. A one-dimensional linear 32-element 770-kHz imaging array and a 42-element 1.45-MHz high-intensity focused ultrasound (HIFU) phased array were fabricated. The averaged electrical impedances of each element were measured to be 58 Ω at the maximum phase angle of -1.2° for the imaging array and 105 Ω at 0° for the HIFU array. The imaging array had a center frequency of 770 kHz with an averaged -6-dB bandwidth of approximately 52%. For the HIFU array, the averaged maximum surface acoustic intensity was measured to be 32.8 W/cm2 before failure.     

Two-dimensional electroacoustic model of transducer array based on 1-3 piezocomposite materials

An analytical model is presented to achieve simultaneous prediction of the elementary electroacoustic response and directivity pattern of a one-dimensional (1-D) piezocomposite array. The theoretical approach was based on guided wave theory in a multilayered structure in which the 1-3 piezocomposite material is considered as a homogeneous piezoelectric plate. A matrix method was applied to simulate the displacement fields generated at the surface of the array when one element was excited with an electrical pulse. A test device was manufactured, then characterized through measurements of displacement performed with an interferometric laser probe when the array vibrated in air and in water. The experimental results are presented and compared with theory.     

穿孔管阻性消声器横向模态和声学特性计算与分析

应用二维有限元法计算穿孔管阻性消声器的横向模态,利用数值模态匹配法计算其传递损失,推导了相应的公式并编写了计算程序。对于圆形同轴穿孔管阻性消声器的传递损失,数值模态匹配法计算结果与三维有限元法计算结果以及实验值吻合良好,表明了二维有限元法计算穿孔管阻性消声器横向模态和数值模态匹配法预测消声性能的准确性。进而分析孔径、穿孔率、吸声材料的密度和穿孔管偏移对圆形直通穿孔管阻性消声器横向模态和消声特性的影响。结果表明,孔径减小、穿孔率增大,或者穿孔管偏移量增大均能使消声器有效的平面波区域变宽,高频消声效果变好,但中频消声效果变差;增加吸声材料的填充密度则能提高消声器中高频的消声量。     

A 2.5 MHz 2D array with Z-axis electrically conductive backing

The design, fabrication and initial testing of a prototype fully λ/2 sampled, 2500 element 2D phased array is presented. The array utilizes a unique Z-axis electrical conductivity backing layer to provide both acoustic attenuation and electrical interconnect for the signal channels. The electrical interconnect is designed to be in the acoustic shadow of the transducer elements so as to minimize the foot print of the array. A modular, demountable Pad Grid Array interconnect is used to connect to the backing of the array. Results are presented for measurements of the single element properties of electrical impedance, pulse echo waveform and spectrum, directivity, and cross talk     

Two-dimensional arrays for medical ultrasound using multilayer flexible circuit interconnection

The development of 2-D array transducers has received much recent interest. Unfortunately, fabrication of high density 2-D arrays is difficult due to the large number of electrical interconnections which must be made to the back side of the elements. A typical array operating at 2.2 MHz may have 256 or more connections within a 16.4 mm circular footprint. Interconnection becomes even more challenging as operating frequencies increase. To solve this problem, we have developed a multilayer flexible (MLF) circuit interconnect consisting of a polyimide dielectric with inter-laminar vias routing signals vertically and etched metal traces routing signals horizontally. A transducer is fabricated from an MLF by bonding a PZT chip to its surface and dicing the chip into individual elements, with the saw kerf extending partially into the top polyimide layer to ensure physical and electrical isolation of the elements. The KLM model was used to compare the performance of an MLF 2-D array to a conventional hand wired 2-D array. MLF and wire guide transducers were fabricated, each with 256 active elements, 0.4 mm interelement spacing, and 2.2 MHz center frequency. Vector impedance, pulse length, bandwidth, angular response, and cross-coupling were found to be comparable in both types of arrays. Using the MLF, however, fabrication time was reduced dramatically. More importantly, MLF technology may be used to increase 2-D array connection density beyond the limitations of current of hand wired fabrication techniques. Thus MLF circuits provide a means for the interconnection of current and future high frequency 2-D arrays.     

Input impedance matching of acoustic transducers operating at off-resonant frequencies

The input impedance matching technique of acoustic transducers at off-resonant frequencies is reported. It uses an inherent impedance property of transducers and thus does not need an external electric matching circuit or extra acoustic matching section. The input electrical equivalent circuit includes a radiation component and a dielectric capacitor. The radiation component consists of a radiation resistance and a radiation reactance. The total reactance is the sum of the radiation reactance and the dielectric capacitive reactance. This reactance becomes zero at two frequencies where the impedance is real. The transducer size can be properly chosen so that the impedance at one of the zero-crossing frequencies is close to 50 Ω, the output impedance of signal generators. At this off-resonant operating frequency, the reflection coefficient of the transducer is minimized without using any matching circuit. Other than the size, the impedance can also be fine tuned by adjusting the thickness of material that bonds the transducer plate to the substrates. The acoustic impedance of the substrate and that of the bonding material can also be used as design elements in the transducer structure to achieve better transducer matching. Lead titanate piezoelectric plates were bonded on Lucite, liquid crystal polymer (LCP), and bismuth (Bi) substrates to produce various transducer structures. Their input impedance was simulated using a transducer model and compared with measured values to illustrate the matching principle.     

Design of ultrasonic array elements for acoustic power considerations

In sound-transmitting applications such as therapeutic ultrasound, the acoustic power at a particular operating frequency is a critical figure of merit for transducer/array design. A design methodology for enhancing the acoustic power radiated from fluid-loaded piezoelectric array elements at a fixed frequency is developed in this paper. A gradient-based optimization algorithm is integrated within the finite element framework to guide the determination of the two design variables, the piezoelectric element thickness and the matching layer thickness, to optimize the acoustic power output. A method for avoiding explicit remeshing in the optimization iteration is presented. Optimized designs are determined numerically, and the effectiveness of the design method is confirmed by experimental measurements. The validated numerical analysis also shows that conventional design strategies using one-dimensional transducer analysis and rule-of-thumb matching layer or protection layer sizing rules may not give the best design for array elements in acoustic power applications     

A 30-MHz piezo-composite ultrasound array for medical imaging applications

Ultrasound imaging at frequencies above 20 MHz is capable of achieving improved resolution in clinical applications requiring limited penetration depth. High frequency arrays that allow real-time imaging are desired for these applications but are not yet currently available. In this work, a method for fabricating fine-scale 2-2 composites suitable for 30-MHz linear array transducers was successfully demonstrated. High thickness coupling, low mechanical loss, and moderate electrical loss were achieved. This piezo-composite was incorporated into a 30-MHz array that included acoustic matching, an elevation focusing lens, electrical matching, and an air-filled kerf between elements. Bandwidths near 60%, 15-dB insertion loss, and crosstalk less than -30 dB were measured. Images of both a phantom and an ex vivo human eye were acquired using a synthetic aperture reconstruction method, resulting in measured lateral and axial resolutions of approximately 100 μm     

相控线阵声源在套管井中的应用

在常规的单极子套管井测井中,由于过套管测量,即使在I、II界面均胶结好的状况下,测量的地层波信息也比较弱。利用相控线阵声源的声束偏转和能量聚焦技术,可大大提高声源的辐射强度和信噪比。采用FEM有限元软件建立了套管井和相控声源的二维模型,计算了点声源和相控线阵声源在套管井中的声压波形。计算结果发现当控制相控线阵阵元间延迟时间使其辐射主声束偏转角满足地层纵波发生临界折射时,可以明显增强地层波幅度,即使在I界面或II界面胶结不好时也可观测到地层波,因此利用相控线阵声源的强穿透性可以帮助我们在套管井胶结不好的情况下探测地层。     

一款宽带圆柱阵研究

圆柱阵换能器具有水平全向的优势,已广泛应用在水声探测领域。文章研究了一款宽带圆柱阵,利用匹配层技术拓宽圆柱阵阵元的带宽,通过有限元仿真优化单个换能器阵元带宽、发送电压响应和阻抗等参数。同时通过仿真以阵元错位密集方式形成圆柱阵,对阵元的个数及排列方式进行仿真优化,制作了一款宽带圆柱阵并进行了测量,圆柱阵直径为400 mm,高度为435 mm,圆柱阵的工作频段为20~30 kHz,频带内起伏3 dB,最大发送电压响应为160.5 dB,圆柱阵-3 dB水平波束宽度为360°。     

Electrical reflection coefficient and velocity shift for groove gratings

The electrical reflection coefficient r(e) and velocity shift (Deltanu/nu)(e) for grooves has never been formulated. All previous calculations for r(e) and (Deltanu/nu)(e) have used formulas valid for the reflection of a surface acoustic wave (SAW) from an array of conducting strips. However, the discontinuity in electrical boundary conditions leading to these formulas does not exist in groove gratings. A new expression for the electrical reflection coefficient and velocity shift for some material overlays is derived from both the variational principle and perturbation theory. The results for various substrates are compared to show the agreement between the two approaches. The implications of this new formulation for the design of grooved arrays on various materials are discussed, effectively resolving the discrepancy between theory and experiment.     

Thermoelastic expansion vs. piezoelectricity for high-frequency, 2-D arrays

Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated increased efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. In this paper we show that the radiated power from a black PDMS film is comparable to a 20 MHz piezoelectric two-dimensional (2-D) array element. Furthermore, we predict that a thermoelastic array element can produce similar acoustic power levels compared to ideal piezoelectric 2-D array elements at frequencies in the 100 MHz regime. We believe these results show that thermoelastic generation of ultrasound is a promising alternative to piezoelectricity for high-frequency, 2-D arrays.