Arrays For Medical Imaging Research Articles

Dual-Axis Hanle Magnetometer Based on Atomic Alignment with a Single Optical Access

Optically pumped magnetometers are serious candidates to replace SQUIDs, and thus avoid the need for cryogenics, in applications like magnetoencephalography. In building magnetometer arrays for imaging, a compact yet multiaxis architecture is highly desirable. Here the authors present a scheme based on atomic alignment that only requires one optical access point, bearing both a pump beam and a probe beam at a small angle to it, and that allows measurement of two magnetic field components. Measuring the third component should be possible via partial depolarization of the pump beam. Such a compact scheme opens interesting perspectives for magnetometer arrays for medical imaging.

Robust Waveform Design of Ultrasound Arrays for Medical Imaging.

Sound speed is an effective parameter in designing an optimal beamformer. In conventional ultrasound imaging systems, the beamformer is designed assuming a fixed value of speed, whereas the speed in a tissue is not known precisely and also may fluctuate by a great value. The errors in estimating sound speed may lead to a severe degradation in the reconstructed image, as mainlobe width and sidelobe level of the beampattern are sensitive to the speed variations. In this paper, we consider the design of a transmit beamformer, which is robust to the speed variations. The problem is formulated as a convex optimization problem versus the covariance matrix of the excitation waveforms to obtain a beampattern with predefined mainlobe width and a minimum sidelobe level for all possible variations of speed. Then, by eigen-analysis of the obtained covariance matrix, a set of nonidentical single-carrier short-pulses for the excitation waveforms were designed. Various simulations indicate that the proposed method can yield a robust beampattern whose mainlobe width and sidelobe level almost remain constant by 10% speed variations. In contrast, the beampatterns obtained by nonrobust methods suffer extensive changes.

A finite element tool for the analysis and the design of capacitive micromachined ultrasonic transducer (cMUT) arrays for medical imaging

The fabrication technology of capacitive micromachined ultrasonic transducers (cMUTs) is now mature enough to exploit the potential of this new generation of electro‐acoustic transducers in the field of diagnostic medical imaging and non‐destructive evaluation. Converting a demonstrator into a commercial product often requires many design and process refinements, and sometimes significant modifications to the fabrication process. Because each design‐fabrication‐and‐test cycle is time‐consuming and costly, the development of a modeling tool for the computer simulation of cMUTs is an essential aid to reduce the time to market and costs. Much effort has been put by researchers to develop more and more accurate and smart simulation tools, that advanced from the standard equivalent circuit modeling with lumped parameters to more powerful techniques based on the finite element method (FEM). This paper aims to give an overview of the most ordinary modeling approaches used for the simulation of cMUTs. It is shown that the pulse‐echo operation of cMUT arrays for medical imaging applications can be analysed by a set of FEM models developed using a commercial software. The important aspects of the cMUT operation, including electro‐mechanical‐acoustical coupling and nonlinear effects in transmission, are taken into account.

A Monolithic Three-Dimensional Ultrasonic Transducer Array for Medical Imaging

This paper presents the first monolithic multidirection-looking ultrasonic imager for minimally invasive medical diagnosis. In contrast to the traditional planar ultrasonic imagers that can only view in one direction, this 3-D array is able to view in multiple directions by using seven planar imagers integrated on a hexagonal silicon prism. Each facet on the prism is integrated with a planar 1- or 2-D capacitive-micromachined ultrasonic-transducer imager array for viewing in a specific direction. Each facet is connected by a flexible dielectric membrane, which is monolithically fabricated with the transducers. The dielectric membranes also support the thin-film electrical interconnects between the arrays on different facets. The substrate is folded into a hexagonal prism after completion of the transducer microfabrication process. With this architecture, a one flip-chip bonded or monolithically integrated front-end electronic circuit will be able to manage all the imagers on the 3-D array. The number of bonding wires for a connection to external electronics can therefore be reduced. Imager prisms, which are ranging from 1 to 4 mm in diameter and 2 to 4 mm in length, and positioned to view in seven directions, have been prototyped. Preliminary testing shows that the imager transducers behaved consistently before and after the assembly process. Applications of this 3-D imager array include capsule ultrasound endoscope, intravascular ultrasound, and other internal imaging needs.

Building CMUTs for imaging applications from top to bottom

This paper focuses on an innovative fabrication process of a capacitive micromachined ultrasonic transducer (CMUT) array for medical imaging. Our approach consists in inverting the function of each layer to build the CMUT capacitive cell starting from the membrane, made of silicon nitride, up to the bottom electrode, which is supported by an appropriate acoustic backing to absorb any ultrasound energy transmitted towards the backside of the array and to prevent the ringing of the pulse-echo signal. The fabricated devices exhibited a large bandwidth (>100%) and an increased sensitivity, in respect to the classical CMUT devices. Using the fabricated devices as linear probes, echographic images are obtained.

Development of a 35-MHz piezo-composite ultrasound array for medical imaging

This paper discusses the development of a 64-element 35-MHz composite ultrasonic array. This array was designed primarily for ocular imaging applications, and features 2-2 composite elements mechanically diced out of a fine-grain high-density Navy Type VI ceramic. Array elements were spaced at a 50-micron pitch, interconnected via a custom flexible circuit and matched to the 50-ohm system electronics via a 75-ohm transmission line coaxial cable. Elevation focusing was achieved using a cylindrically shaped epoxy lens. One functional 64-element array was fabricated and tested. Bandwidths averaging 55%, 23-dB insertion loss, and crosstalk less than -24 dB were measured. An image of a tungsten wire target phantom was acquired using a synthetic aperture reconstruction algorithm. The results from this imaging test demonstrate resolution exceeding 50 microm axially and 100 microm laterally.

Capacitive micromachined ultrasonic transducer (CMUT) arrays for medical imaging

Capacitive micromachined ultrasonic transducers (CMUTs) bring the fabrication technology of standard integrated circuits into the field of ultrasound medical imaging. This unique property, combined with the inherent advantages of CMUTs in terms of increased bandwidth and suitability for new imaging modalities and high frequency applications, have indicated these devices as new generation arrays for acoustic imaging. The advances in microfabrication have made possible to fabricate, in few years, silicon-based electrostatic transducers competing in performance with the piezoelectric transducers. This paper summarizes the fabrication, design, modeling, and characterization of 1D CMUT linear arrays for medical imaging, established in our laboratories during the past 3 years. Although the viability of our CMUT technology for applications in diagnostic echographic imaging is demonstrated, the whole process from silicon die to final probe is not fully mature yet for successful practical applications.

Laser micromachining of SrTiO 3 single crystal

Laser micromachining of piezoelectric materials has many advantages over other etching techniques for the fabrication of ultrasound transducer linear arrays for medical imaging. It can achieve high aspect ratios and high etch rates without the use of complicated photolithography techniques. We have investigated a laser projection etch technique to make linear arrays in single crystal (0 0 1) SrTiO 3 substrates as a model of epitaxial piezoelectric thick film heterostructure. Feature sizes of 17.5 μm were obtained with depth to width aspect ratios of 4:1. The effect of laser fluence on etching was studied and it was found that straighter sidewalls and flatter trench floors were achieved as laser fluence increases. On the other hand, higher laser fluence caused increase in heat affected zone by post-pulse plasma and made the top surfaces rougher because of the accumulation of evaporated materials. Clean top surfaces of the features were achieved by deposition and subsequent lift off of a YBa 2Cu 3O 7 sacrificial layer. In addition, the phases of recast layers on the sidewalls were characterized by four-circle X-ray diffraction with 2-D area detector before and after removed with a wet chemical etch solution. It was found that the use of the wet etchant could remove the thin polycrystalline recast layers.

Application of single crystal relaxor ferroelectric materials to medical imaging arrays

Single crystal relaxor ferroelectrics have been shown to possess extremely high electromechanical coupling coefficients, in some materials approaching 95% in length extensional mode. This high coupling allows the design of electroacoustic transducers with better sensitivity or bandwidth than can be achieved with conventional piezoceramic materials. These improved properties come at the expense of lower depoling temperatures, increased material fragility, poorer property uniformity, and increased cost. These issues preclude the adoption of single crystals as a direct drop-in replacement for conventional piezoceramics. Most issues can be addressed through refined crystal growth and transducer design or processing methods. This paper reviews the application of single crystals to transducers for medical imaging, including: characterization of material properties including the complete matrix of elastic, dielectric, and piezoelectric coefficients; design of high bandwidth transducers; modeling of transducer operation, including 1-D and finite element approaches; development of alternate fabrication processes; and measurement of the transducer performance improvement which has been obtained to date. Progress toward the goal of fabricating a single transducer which can operate effectively over the range of multiple conventional probes will be described.

Investigation of cross-coupling in 1-3 piezocomposite arrays.

Plate waves inside the piezoelectric layer are much involved in the elements cross-coupling in transducer arrays for medical imaging. In this work, such waves are analyzed in 1-3 piezocomposite materials on the basis of conventional guided modes formalism in which the piezocomposite is considered as a homogeneous medium. Cross-coupling measurements have been made on two different transducer arrays using network analyzer and a laser interferometric probe. It is shown how the analysis in terms of symmetrical Lamb waves gives an interesting qualitative interpretation, explaining most of the cross-coupling amplitude variations with frequency. Results show that the 0th and 3rd symmetrical Lamb waves are mainly involved in coupling inside composite plates. The S0 mode is responsible for the inter-element coupling, whereas the S3 mode widens the effective width of the excited element.

Interdigital pair bonding for high frequency (20-50 MHz) ultrasonic composite transducers.

Interdigital pair bonding is a novel methodology that enables the fabrication of high frequency piezoelectric composites with high volume fractions of the ceramic phase. This enhancement in ceramic volume fraction significantly reduces the dimensional scale of the epoxy phase and increases the related effective physical parameters of the composite, such as dielectric constant and the longitudinal sound velocity, which are major concerns in the development of high frequency piezoelectric composites. In this paper, a method called interdigital pair bonding (IPB) is used to prepare 1-3 piezoelectric composite with a pitch of 40 microns, a kerf of 4 microns, and a ceramic volume fraction of 81%. The composites prepared in this fashion exhibited a very pure thickness-mode resonance up to a frequency of 50 MHz. Unlike the 2-2 piezoelectric composites with the same ceramic and epoxy scales developed earlier, the anticipated lateral modes between 50 to 100 MHz were not observed in the current 1-3 composites. The mechanisms for the elimination of the lateral modes at high frequency are discussed. The effective electromechanical coupling coefficient of the composite was 0.72 at a frequency of 50 MHz. The composites showed a high longitudinal sound velocity of 4300 m/s and a high clamped dielectric constant of 1111 epsilon 0, which will benefit the development of high frequency ultrasonic transducers and especially high frequency transducer arrays for medical imaging.

Electromechanical properties of thin strip piezoelectric vibrators at high frequency

A method was developed and used to determine the electromechanical properties of high frequency (>20 MHz) piezoelectric strip vibrators. A nonlinear regression technique was employed to fit the impedance magnitude and phase as predicted by Mason’s model to measured values. Results from experimental measurements on 30 MHz array elements supported by an attenuative backing indicated degraded performance when compared to values predicted from the electromechanical properties measured at low frequency. This degradation may be attributed to damage incurred during fabrication and grain size effects, with a fine grain sized material providing superior relative performance. This technique may be used in the evaluation and comparison of different fabrication processes and materials for high frequency medical imaging arrays.

2–2 piezoelectric composites with high density and fine scale fabricated by interdigital pair bonding

Interdigital pair bonding (IPB) is proposed as a means to fabricate piezoelectric composites used for ultrasonic transducers and arrays for medical imaging and nondestructive evaluation, especially in the high frequency range. This letter presents experimental results of prototype 2–2 piezoelectric composites made by IPB. Two composites with different lateral dimensions were fabricated using 70 and 40 μm thickness dicing saw blades. The widths of ceramic stripe and epoxy kerf for composite A are 53 and 12 μm, respectively, and are 36 and 4 μm for composite B. The thickness mode and lateral modes for both composites were investigated by impedance spectrum analysis. The kt for both samples is 0.64. The current designs permit the fabrication of composites for transducers operated up to approximately 50 MHz using conventional dicing saw blades. The extension of IPB to fabrication of 1–3 composite is discussed.

Coarse-fine ultrasound transducer array for medical imaging

The ultrasound transducer array defines an average row spacing along the elevation for receiving transducer elements that differs from an average row spacing along the elevation for transmitting transducer elements. In one embodiment a relatively "coarse" row spacing is used for reception and a relatively "fine" row spacing is used for transmission. The differing spacings lead to different transmit-elevation and receive-elevation beam-patterns. In one configuration the number of transducer elements, and correspondingly the number of ultrasound processing channels, is reduced. In an alternative configuration the beam-pattern "footprint" is increased without increasing the number of transducer elements or processing channels. The array includes dedicated transmit-only elements and/or dedicated receive-only elements, together with coincident, time-shared transmit/receive elements. The coarse-fine spacings enable improved elevational focus without substantially increasing side lobe levels in a focal plane and far field portion of a resulting beam-pattern.

Simulation of B-scan images from two-dimensional transducer arrays: Part I — Methods and quantitative contrast measurements

Recently, theoretical investigations of the beamforming capability of two-dimensional (2-D) transducer arrays have characterized the array parameters required to steer a symmetrically focused ultrasound beam up to 45° off-axis. These investigations have also shown that the number of elements in a steered 2-D array can be dramatically reduced by using a sparse set of elements, randomly distributed throughout the aperture of the transducer. The penalty paid for the use of a sparse array is the development of a “pedestal” sidelobe in the beam profile, the amplitude of which increases as the number of elements in the array decreases. In this paper the potential of 2-D arrays for medical imaging is assessed by simulating B-scan images of spherical lesions, both cystic and scattering, embedded in a large random scattering volume. Similar contrast characteristics over a range of cyst sizes are demonstrated for a dense 2-D array and a sparse array with 1/8th the number of elements, both operating at 5 MHz. A 32nd order sparse array is shown to perform at a reduced level, producing unacceptable artifactual echoes within images of cysts. The 8th order sparse array pattern has been fabricated on a fixed-focus poly(vinylidene difluoride) transducer using photolithographic techniques. Experimental images from this transducer are used to verify some of the theoretical predictions made in this paper. Comparisons between simulated B-scan images from linear and 2-D phased arrays are presented in a companion paper.

Simulation of B-scan images from two-dimensional transducer arrays: Part I--Methods and quantitative contrast measurements.

Recently, theoretical investigations of the beamforming capability of two-dimensional (2-D) transducer arrays have characterized the array parameters required to steer a symmetrically focused ultrasound beam up to 45 degrees off-axis. These investigations have also shown that the number of elements in a steered 2-D array can be dramatically reduced by using a sparse set of elements, randomly distributed throughout the aperture of the transducer. The penalty paid for the use of a sparse array is the development of a "pedestal" sidelobe in the beam profile, the amplitude of which increases as the number of elements in the array decreases. In this paper the potential of 2-D arrays for medical imaging is assessed by simulating B-scan images of spherical lesions, both cystic and scattering, embedded in a large random scattering volume. Similar contrast characteristics over a range of cyst sizes are demonstrated for a dense 2-D array and a sparse array with 1/8th the number of elements, both operating at 5 MHz. A 32nd order sparse array is shown to perform at a reduced level, producing unacceptable artifactual echoes within images of cysts. The 8th order sparse array pattern has been fabricated on a fixed-focus poly(vinylidene difluoride) transducer using photolithographic techniques. Experimental images from this transducer are used to verify some of the theoretical predictions made in this paper. Comparisons between simulated B-scan images from linear and 2-D phased arrays are presented in a companion paper.

Simulation of B-scan images from two-dimensional transducer arrays: Part II — Comparisons between linear and two-dimensional phased arrays

Two-dimensional (2-D) arrays have been proposed as a solution to the degradation in medical ultrasound image quality occurring as a result of asymmetric focusing properties of linear phased array transducers. The 2-D phased transducer array is also capable of electronically steering the symmetrically focused ultrasound beam throughout a three-dimensional volume. In a companion paper the potential of 2-D transducer arrays for medical imaging has been investigated using simulated B-scan images. In this paper, the advantages of 2-D over linear transducer arrays is demonstrated by simulating images of spherical cysts embedded in a large scattering volume. The large elevation beamwidth in the nearfield of a 5 MHz linear phased transducer array results in a severe reduction in the image contrast measured between a 4 mm diameter cyst and the surrounding scattering media. By employing a 2-D array with symmetric focusing, the contrast between the cyst and surrounding scatterers is significantly improved. The use of additional elements in the elevation direction of a linear array is also investigated. In this case the additional elements are included only to focus, but not to steer the ultrasound beam. Using the contrast characteristics of a 4 mm diameter cyst, it is shown that relatively few elevation elements are required to significantly improve the nearfield imaging capability of the linear array.

Fabrication and characterization of transducer elements in two-dimensional arrays for medical ultrasound imaging

Some of the problems of developing a two-dimensional (2-D) transducer array for medical imaging are examined. The fabrication of a 2-D array material consisting of lead zirconate titanate (PZT) elements separated by epoxy is discussed. Ultrasound pulses and transmitted radiation patterns from individual elements in the arrays are measured. A diffraction theory for the continuous wave pressure field of a 2-D array element is generalized to include both electrical and acoustical cross-coupling between elements. This theory can be fit to model the measured radiation patterns of 2-D array elements, giving an indication of the level of cross-coupling in the array, and the velocity of the acoustic cross-coupling wave. Improvements in bandwidth and cross-coupling resulting from the inclusion of a front acoustic matching layer are demonstrated, and the effects of including a lossy backing material on the array are discussed. A broadband electrical matching network is described, and pulse-echo waveforms and insertion loss from a 2-D array element are measured.

A new vibration mode for medical imaging arrays

Abstract The possibility of designing medical imaging arrays using a new transducer vibration mode is analyzed. The vibration mode, that we call ‘‘width mode'', is studied theoretically and experimentally and compared to the classical ‘‘array mode''. Starting from the constitutive equations for piezoelectric materials and with proper boundary conditions, the propagation velocity for the new mode is first found for the case of mono-dimensional propagation approximation. Then an approximate theory for vibrating systems which possess only two degrees of freedom is used in order to express dispersion curves for the propagation velocity for the two modes of interest. Theoretical results are then compared with experimental data obtained for PZT5-A and PZT4 ceramic materials. The new oscillation mode shows some advantages such as a smaller degree of wave velocity dispersion, a smaller absolute value of wave velocity propagation and a lower electrical impedance. Plots of input electrical admittance show that with...