The standard technique used by commercial medical ultrasound systems to form B-mode images is delay and sum (DAS) beamforming. However, DAS often results in limited image resolution and contrast, which are governed by the center frequency and the aperture size of the ultrasound transducer. A large number of elements leads to improved resolution but at the same time increases the data size and the system cost due to the receive electronics required for each element. Therefore, reducing the number of receiving channels while producing high quality images is of great importance. In this paper, we introduce a nonlinear beamformer called COnvolutional Beamforming Algorithm (COBA), which achieves significant improvement of lateral resolution and contrast. In addition, it can be implemented efficiently using the fast Fourier transform. Based on the COBA concept, we next present two sparse beamformers with closed form expressions for the sensor locations, which result in the same beam pattern as DAS and COBA while using far fewer array elements. Optimization of the number of elements shows that they require a minimal number of elements which is on the order of the square root of the number used by DAS. The performance of the proposed methods is tested and validated using simulated data, phantom scans and in vivo cardiac data. The results demonstrate that COBA outperforms DAS in terms of resolution and contrast and that the suggested beamformers offer a sizable element reduction while generating images with an equivalent or improved quality in comparison to DAS.
Two-dimensional arrays are necessary for a variety of ultrasonic imaging techniques, including elevation focusing, 2-D phase aberration correction, and real time volumetric imaging. In order to reduce system cost and complexity, sparse 2-D arrays have been considered with element geometries selected ad hoc, by algorithm, or by random process. Two random sparse array geometries and a sparse array with a Mills cross receive pattern were simulated and compared to a fully sampled aperture with the same overall dimensions. The sparse arrays were designed to the constraints of the Duke University real time volumetric imaging system, which employs a wide transmit beam and receive mode parallel processing to increase image frame rate. Depth-of-field comparisons were made from simulated on-axis and off-axis beamplots at ranges from 30 to 160 mm for both coaxial and offset transmit and receive beams. A random array with Gaussian distribution of transmitters and uniform distribution of receivers was found to have better resolution and depth-of-field than both a Mills cross array and a random array with uniform distribution of both transmit and receive elements. The Gaussian random array was constructed and experimental system response measurements were made at several ranges. Comparisons of B-scan images of a tissue mimicking phantom show improvement in resolution and depth-of-field consistent with simulation results. A 2D array for acquiring 3D images is described. Interconnection methods are presented that connect elements of the array to integrated circuits (ICs) within the transducer. These ICs simultaneously process signals from ALL elements forming a fully sampled array. Comparisons are made with mechanical and sparse array solutions. Several signal processing options for use within the ICs are presented.  There is an increasing interest for sparse 2D arrays as an alternative to full gridded arrays used in commercial systems. This work presents an effective and straightforward solution to perform 3D ultrasound imaging by driving a reduced number of elements of a commercial 2D array. The selection of 128, 192 or 256 elements out of a 32×32 array was based on simulated annealing to produce radiated beam patterns with optimal tradeoff in terms of main lobe width (resolution) and side lobe level (contrast) over a (± 32°) volume. The aim of the present work is to experimentally validate these optimal configurations by performing 3D focused imaging on a phantom. The performance metrics included the lateral resolution and the contrast to noise ratio (CNR), measured on the images obtained by scanning a grayscale phantom. The results show that the optimized 256 elements array performs, as expected, the best among all the compared sparse arrays, presents the same resolution performance as the full array and a −6 dB loss of CNR while using 25 of the active elements. This paper investigates the effect of transducerintegrated apodization in row-column-addressed arrays and presents a beamforming approach specific for such arrays. Row-column addressing 2-D arrays greatly reduces the number of active channels needed to acquire a 3-D volume. A disadvantage of row-column-addressed arrays is an apparent ghost effect in the point spread function caused by edge waves. This paper investigates the origin of the edge waves and the effect of introducing an integrated apodization to reduce the ghost echoes. The performance of a λ 2-pitch 5-MHz 128 + 128 row-column-addressed array with different apodizations is simulated. A Hann apodization is shown to decrease imaging performance away from the center axis of the array because of a decrease in main lobe amplitude. Instead, a static roll-off apodization region located at the ends of the line elements is proposed. In simulations, the peak ghost echo intensity of a scatterer at (x, y, z) = (8, 3, 30) mm was decreased by 43 dB by integrating roll-off apodization into the array. The main lobe was unaffected by the apodization. Simulations of a 3-mm-diameter anechoic blood vessel at 30 mm depth showed that applying the transducer-integrated apodization increased the apparent diameter of the vessel from 2.0 mm to 2.4 mm, corresponding to an increase from 67 to 80 of the true vessel diameter. The line element beamforming approach is shown to be essential for achieving correct time-of-flight calculations, and hence avoid geometrical distortions. In Part II of this work, experimental results from a capacitive micromachined ultrasonic transducer with integrated roll-off apodization are given to validate the effect of integrating apodization into the line elements. This paper demonstrates the fabrication, characterization, and experimental imaging results of a 62+62 element λ 2-pitch row-column-addressed capacitive micromachined ultrasonic transducer (CMUT) array with integrated apodization. A new fabrication process was used to manufacture a 26.3 mm by 26.3 mm array using five lithography steps. The array includes an integrated apodization, presented in detail in Part I of this paper, which is designed to reduce the amplitude of the ghost echoes that are otherwise prominent for row-column-addressed arrays. Custom front-end electronics were produced with the capability of transmitting and receiving on all elements, and the option of disabling the integrated apodization. The center frequency and -6-dB fractional bandwidth of the array elements were 2.77 ± 0.26 MHz and 102 ± 10 , respectively. The surface transmit pressure at 2.5 MHz was 590 ± 73 kPa, and the sensitivity was 0.299 ± 0.090 V Pa. The nearest neighbor crosstalk level was -23.9 ± 3.7 dB, while the transmit-to-receive-elements crosstalk level was -40.2 ± 3.5 dB. Imaging of a 0.3-mm-diameter steel wire using synthetic transmit focusing with 62 single-element emissions demonstrated axial and lateral FWHMs of 0.71 mm and 1.79 mm (f-number: 1.4), respectively, compared with simulated axial and lateral FWHMs of 0.69 mm and 1.76 mm. The dominant ghost echo was reduced by 15.8 dB in measurements using the integrated apodization compared with the disabled configuration. The effect was reproduced in simulations, showing a ghost echo reduction of 18.9 dB. The fabrication of a dense (one-half wavelength element spacing) two-dimensional (2D) transducer array suitable for medical ultrasound imaging is unrealistic using existing technology. Consequently, there is interest in developing sparse 2D transducer arrays. In this paper, we present the results of a study looking at the broad-bandwidth radiation patterns of 72 different sparse 2D vernier arrays. Suppression of grating lobes is achieved by choosing a different arrangement of transmit and receive elements using an analogy with a vernier scale. The broad-bandwidth radiation patterns were investigated by simulating volumetric sector scan of a point target. We summarize these results by deriving a set of design curves that predicts the minimum number of elements, element spacing, and apodization required for a desired beam width and maximum secondary lobe. The results show that a sparse vernier array can be designed with significantly lower average and peak secondary lobes compared with a sparse random array with the same number of elements and aperture size.  This paper compares the imaging performance of a 128+128 element row-column addressed array with a fully addressed 16×16 2D array. The comparison is made via simulations of the point spread function with Field II. Both arrays have lambda-pitch, a center frequency of 3:5MHz and use 256 active elements. The row-column addressed array uses 128 transmit channels and 128 receive channels, whereas the fully addressed array uses 256 channels in both transmit and receive. The large size of the emulated row and column elements in the row-column addressed array causes ghost echoes to appear. The ghost echoes are shown to be suppressed when the sub-elements within each of the emulated row and column elements are apodized. The maximum ghost intensity is suppressed by 22:2 dB compared to using no apodization. With apodization applied, the full-width-at-half-maximum in the lateral direction for the fully addressed array is 2:81mm, and 1:01mm for the row-column addressed array. This shows that the detail resolution can be more than doubled using the row-column addressed array instead of the fully addressed array. The row column addressed array achieves a R 20 dB cystic resolution of 0:76mm, compared to 3:16mm for the fully addressed array. The significantly smaller R 20 dB -value for the row-column addressed array indicates that it can achieve a much higher contrast resolution than the fully addressed array. This paper will focus on the development of the second generation Sonic Window prototype, which will occupy a package the size of a deck of cards and be capable of forming C-mode ultrasound images in real-time. An array of 3,600 full receive channels—consisting of protection circuitry, a preamplifier, tuneable bandpass filter, sampler bank, and 8-bit ADC—is formed by flip-chip attaching twelve identical 300-channel custom ICs onto a double-layer flex circuit substrate. The receive channel pitch is 200 µm x 275 µm and the entire receive circuitry array has a form factor of only 1.9 cm x 1.8 cm. The flex circuit provides I O fanout to a beamforming PCB containing bias circuitry, programmable logic, a DSP, and LCD for image display. The receive circuitry array connects through a z-axis electrically conducting interface to a 60x60 element, 300-micron- pitch, fully-sampled transducer array inexpensively fabricated on a double-layer PCB substrate. Experimental test results from a test chip containing receive circuitry components are presented, along with a description of the 3,600-channel receive circuitry array development. The design, fabrication, and characterization of a 112 channel, 5 MHz, two-dimensional (2-D) array transducer constructed on a six layer flexible polyimide interconnect circuit is described. The transducer was mounted in a 7 Fr (2.33 mm outside diameter) catheter for use in real-time intracardiac volumetric imaging. Two transducers were constructed: one with a single silver epoxy matching layer and the other without a matching layer. The center frequency and -6 dB fractional bandwidth of the transducer with a matching layer were 4.9 MHz and 31 , respectively. The 50 spl Omega pitch-catch insertion loss was 80 dB, and the typical interelement crosstalk was -30 dB. The final element yield was greater than 97 for both transducers. The transducers were used to acquire real-time, 3-D images in an in vivo sheep model. We present in vivo images of cardiac anatomy obtained from within the coronary sinus, including the left and right atria, aorta, coronary arteries, and pulmonary veins. We also present images showing the manipulation of a separate electrophysiological catheter into the coronary sinus. Several sparse 2-D arrays for real time rectilinear volumetric imaging were investigated. All arrays consisted of 128 spl times 128=16384 elements with spl lambda spacing operating at 5 MHz. Because of system limitations, not all of the elements could be used. From each array, 256 elements were used as transmitters, and 256 elements were used as receivers. These arrays were compared by computer simulation using Field II. For each array, beamplots for the on-axis case and an illustrative off-axis case were obtained. For the off-axis case, the effects of receive mode dynamic focusing were studied to maintain the beam perpendicular to the transducer face. Main lobe widths, side lobe heights, clutter floor levels, and pulse-echo sensitivities were quantified for each array. The sparse arrays, including a vernier periodic array, a random array, and a Mills cross array, were compared with a fully sampled array that served as the "gold standard". The Mills cross design showed the best overall performance under the current system constraints.  Two-dimensional ultrasonic phased arrays are becoming increasingly popular in nondestructive evaluation (NDE). Sparse array element configurations are required to fully exploit the potential benefits of 2-D phased arrays. This paper applies the conformal mapping technique as a means of designing sparse 2-D array layouts for NDE applications. Modeling using both Huygens' field prediction theory and 2-D fast Fourier transformation is employed to study the resulting new structure. A conformal power map was used that, for fixed beam width, was shown in simulations to have a greater contrast than rectangular or random arrays. A prototype aperiodic 2-D array configuration for direct contact operation in steel, with operational frequency 3 MHz, was designed using the array design principle described in this paper. Experimental results demonstrate a working sparse-array transducer capable of performing volumetric imaging. One of the most promising techniques for limiting complexity for real-time 3-D ultrasound systems is to use sparse 2-D layouts. For a given number of channels, optimization of performance is desirable to ensure high quality volume images. To find optimal layouts, several approaches have been followed with varying success. The most promising designs proposed are Vernier arrays, but also these suffer from high peaks in the sidelobe region compared with a dense array. In this work, we propose new methods based on the principles of suppression of grating lobes to form symmetric and non-symmetric regular sparse periodic and radially periodic designs. The proposed methods extend the concept of sparse periodic layouts by exploiting either an increased number of symmetry axes or radial symmetry. We also introduce two new strategies to form designs with nonoverlapping elements. The performance of the new layouts range from the performance of Vernier arrays to almost that of dense arrays. Our designs have simplicity in construction, flexibility in the number of active elements, and the possibility of trade off sidelobe peaks against sidelobe energy. Three-dimensional imaging with 2-D matrix probes is one of the most exciting recent ultrasound innovations. Unfortunately, the number of elements of a 2-D matrix probe is often very high, and reducing this number deteriorates the beam properties. In this paper, we propose a new sparse-array design technique with irregular element positioning, which significantly reduces the number of active elements as well as the grating-lobe level. In particular, we introduce a new cost function for optimizing the weighting coefficients of the elements and a new annealing-based algorithm to compute the lowest cost solutions. Numerical simulations show substantial improvements over standard sparse arrays.  We have developed several methods of designing sparse periodic arrays based upon the polynomial factorization method. In these methods, transmit and receive aperture polynomials are selected such that their product results in a polynomial representing the desired combined transmit receive (T R) effective aperture function. A desired combined T R effective aperture is simply an aperture with an appropriate width exhibiting a spectrum that corresponds to the desired two-way radiation pattern. At least one of the two aperture functions that constitute the combined T R effective aperture function will be a sparse polynomial. A measure of sparsity of the designed array is defined in terms of the element reduction factor. We show that elements of a linear array can be reduced with varying degrees of beam mainlobe width to sidelobe reduction properties.