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.
Possible approaches to reduce the number of receiving channels without compromising image quality include subaperture processors and microbeamformers @cite_49 , whereby part of the beamformation is moved into the probe handle. However, this requires manufacturing expensive integrated circuits with high power consumption @cite_14 @cite_39 @cite_44 . Alternative strategies that have gained a lot of interest are based on using standard DAS with sparse arrays where some of the elements are removed, including deterministic designs such as vernier arrays and random designs @cite_61 @cite_29 @cite_56 @cite_15 @cite_9 @cite_58 @cite_2 @cite_33 @cite_17 . These works are concerned with designing a combined transmit receive effective aperture mostly for 3D imaging, whereas we propose methods that can be applied in both active and passive settings. Another approach is 2D row–column-addressed arrays for 3D imaging @cite_62 @cite_12 @cite_1 @cite_8 @cite_46 , in which every row and column in the array acts as one large element. However, this work is limited to 3D imaging and the use of large elements leads to a considerable increase in edge effects that limit image quality @cite_12 .
@cite_61 @cite_14 @cite_62 @cite_33 @cite_8 @cite_46 @cite_29 @cite_9 @cite_1 @cite_39 @cite_44 @cite_56 @cite_49 @cite_2 @cite_15 @cite_58 @cite_12 @cite_17
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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. 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 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 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.