Dual Apodization Research Articles

Sidelobe Control for Bistatic SAR Imaging

This letter presents coherent and incoherent nonlinear apodization methods for bistatic synthetic aperture radar (SAR) imaging. The methods are developed on the principle of nonlinear apodization for monostatic SAR imaging and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\omega \,\,-\,\,k$ </tex-math></inline-formula> relationship representing the bistatic region of support. This relationship plays an important role in designing 2-D windows for the apodization methods. The simulation results show that the presented apodization methods work efficiently with bistatic SAR imaging. In the experiment with a coherent dual apodization (CDA) method presented in this letter, the sidelobe attenuates very fast, and the peak sidelobe is reduced about 16 dB, while the resolutions in azimuth and range are maintained.

Gated Transmit and Fresnel-Based Receive Beamforming With a Phased Array for Low-Cost Ultrasound Imaging.

Low-cost ultrasound imaging systems are desired for many applications outside of radiology and cardiology departments. By making ultrasound systems smaller and lower cost, the use of ultrasound has spread from these mainstays to other areas of the hospital such as emergency departments and critical care. To further miniaturize and reduce the cost of ultrasound systems, we have investigated novel Fresnel-based beamforming methods to reduce front-end hardware requirements. Previous studies with linear and curvilinear arrays demonstrated comparable imaging performance using Fresnel-based beamforming versus delay-and-sum (DAS) beamforming. In this work, we extend Fresnel-based beamforming to phased arrays with beam steering. To accomplish this in transmit mode, we introduce a technique called a gated transmit beamformer where multicycle bursts are gated using multiplexers. In receive mode, a 64-element 2.5-MHz phased array is broken up into four 16-element subapertures, and each subaperture performs Fresnel beamforming before a final beamforming step is done. Timing errors are inevitable with Fresnel-based beamforming leading to higher sidelobe and clutter levels. To suppress sidelobe and clutter contributions, we also combine this with our previous technique, dual apodization with cross correlation (DAX) to improve contrast. Field II simulations are performed to evaluate spatial resolution and contrast-to-noise ratio and compared to standard DAS beamforming. Fresnel-based and gated transmit beamforming is also implemented using synthetic aperture data from tissue-mimicking phantoms. Lastly, a hardware proof-of-concept (PoC) Fresnel beamformer was designed, assembled, and evaluated with images from tissue-mimicking phantoms and initial in vivo images.

Dual apodization with cross-correlation combined with robust Capon beamformer applied to ultrasound passive cavitation mapping.

Passive acoustic mapping (PAM) has received increasing attention in recent years and has an extremely widespread application prospect in real-time monitoring of ultrasound treatment. When using a diagnostic ultrasound transducer, such as a linear-array transducer, the initially used time exposure acoustics (TEA) algorithm will produce high-level artifacts. To address this problem, we recently proposed an enhanced algorithm for linear-array PAM by introducing dual apodization with the cross-correlation (DAX) method into TEA. But due to that the delay and sum beamformer used to create RX1 and RX2 is non-adaptive, the remaining X-type artifacts cannot be completely suppressed, yielding unsatisfactory image quality. This study aims to propose an improved version by combining DAX and robust Capon beamformer (DAX-RCB). Different from the delay and sum beamformer in the DAX-TEA algorithm, in the proposed version, the two sets of channel signals from a pair of complementary receive apodizations are beamformed by the RCB method, which may make passive cavitation images much less sensitive to X-type artifacts. The performance of the DAX-RCB algorithm is validated by simulations and in vitro experiments and compared with the initially used TEA algorithm and the previous DAX-TEA and RCB algorithms. Four indexes, including passive energy beam (PEB) size, image signal-to-background ratio (ISBR), energy estimation ratio (EER), and computing time, are used to evaluate the algorithm performance. Consider an example of the 8-8 alternating pattern (a pair of complementary apodizations are obtained by extracting eight elements every eight elements), the experimental results show that the A-6dB area (2D PEB size) of the proposed DAX-RCB is significantly reduced by 11.0 and 6.8mm2 when compared with TEA and DAX-TEA and is not significantly reduced when compared with RCB, the ISBR is significantly improved by 19.6, 10.8, and 5.6dB compared with TEA, DAX-TEA, and RCB, and the EER of DAX-RCB is over 90%. The simulation tests indicate that the DAX-RCB algorithm is also applicable to the image enhancement in the double-source scenario and the high-level noise scenario but at a risk of low energy estimation. The improvement of algorithm performance is accompanied by an increase in the computing time. The proposed DAX-RCB consumes 113.3%, 29.5%, and 17.8% more time than TEA, DAX-TEA, and RCB. The proposed DAX-RCB can be considered as an effective reconstruction algorithm for passive cavitation mapping and provide an appropriate monitoring means for ultrasound therapy, especially for cavitation-mediated applications.

An IEEE Single Precision Floating Point Arithmetic-Based Apodization Architecture for Portable Ultrasound Imaging System

Portable ultrasound systems are useful in point-of-care diagnostic. Improving the image contrast and spatial resolution over a large range of imaging depths while maintaining the minimum area and power are the desiderata in case of ultrasound systems. Adaptive apodization is used for obtaining high contrast and spatial resolution ultrasound images; however, the implementation of adaptive apodization in hardware consumes large area and power. In this paper, an adaptive algorithm is presented for obtaining good image contrast and resolution using low computational complexity. In the proposed apodization algorithm, minimum and maximum variance criteria and normalized cross-correlation are used to enhance the image contrast and spatial resolution over a large range of imaging depths. A resource optimized, IEEE single precision arithmetic-based architecture of the proposed method is also presented in this paper. Field II toolbox and the experimental data taken from the plane-wave imaging challenge in medical ultrasound are used for evaluating the performance of the proposed methods. Simulation and experimental results demonstrate that the proposed method provides better resolution coupled with the improved contrast performance than conventional dual apodization with cross-correlation methods. The architecture of the proposed method consumes 358.66-mW dynamic power and 1122.1 k NAND-2 equivalent gates are found to be in UMC 90-nm CMOS standard cell library. Implementation of the proposed method requires the calculation of inverse square root. A quadratic approximation-based inverse square root calculation method and its datapath to support IEEE single precision arithmetic are also presented. The datapath of the inverse square root calculator reduces the memory requirement up to 70% which need to be adopted in pursuit of higher performances.

An Optimum Imaging Scheme for IVUS Arrays: Eccentric Cylinder Wave Compounding.

With the development of integrated circuit (IC) technologies, complex transmitting and receiving circuits can be integrated into miniature intravascular ultrasound (IVUS) catheters, making it possible to adopt better synthesizing schemes for better imaging. Eccentric cylinder wave compounding should be an optimum synthesizing scheme for the small size cylinder shaped catheter. Eccentric cylinder waves centered at different points are emitted, signals are collected after each emission, and images can be synthesized with easy post processing. Detailed analyses about resolution and grating lobes were made; the optimum eccentric distance was determined. Simulations were done to examine the resolution, signal-to-noise ratio, and resistance to crosstalk and nonuniformity of arrays. Dual apodization and magnitude-based deconvolution were applied to further improve the results.

Passive cavitation mapping using dual apodization with cross-correlation in ultrasound therapy monitoring

Recently, passive acoustic mapping (PAM) has been successfully applied for dynamic monitoring of ultrasound therapy by beamforming acoustic emissions of cavitation activity during ultrasound exposure. The most widely used PAM algorithm in the literature is time exposure acoustics (TEA), which is a standard delay, sum, and integrate algorithm. However, it results in large point spread function (PSF) and serious imaging artifacts for the case where a narrow-aperture receiving array such as a standard B-mode linear array is used, therefore degrading the quality of cavitation image. To address these challenges, in this paper, we proposed a novel PAM algorithm namely dual apodization with cross-correlation (DAX)-based TEA, in which DAX was originally used as a reconstruction algorithm in medical ultrasound imaging. In the proposed algorithm, two sets of signals were beamformed by two receive apodization functions with alternating elements enabled, and the cross-correlation coefficient of the two signals served as a weighting factor that would be multiplied to the sum of the two signals. The performance of the proposed algorithm was tested on simulated channel data obtained using a multi-bubble model, and experiments were also performed in an in vitro vessel phantom with flowing microbubbles as cavitation nuclei. The reconstructed cavitation images were evaluated quantitatively using established quality metrics including full width at half maximum (FWHM), A−6dB area, and signal-to-noise ratio (SNR). The results suggested that the proposed algorithm significantly outperformed the conventionally used TEA algorithm. This work may have the potential of providing a useful tool for highly accurate localization of cavitation activity during ultrasound therapy.

A Hybrid Nonlinear Method for Array Thinning

A nonlinear method for array thinning is proposed. The method is based on hybrid usage of genetic algorithm (GA) and nonlinear apodization. The method proposes a special layout, which consists of two subarrays. Layout and weights for the subarray having larger aperture are designed by GA. Layout and weights for the second subarray are determined according to results of the aforementioned design. Dual apodization is applied to the outputs of the subarrays to obtain the output. Results show that there is an improvement in peak side-lobe level, beam width, and current taper ratio for particular test problems in the literature.

Ultrasonic Reverberation Clutter Suppression Using Multiphase Apodization With Cross Correlation.

Despite numerous recent advances in medical ultrasound imaging, reverberation clutter from near-field anatomical structures, such as the abdominal wall, ribs, and tissue layers, is one of the major sources of ultrasound image quality degradation. Reverberation clutter signals are undesirable echoes, which arise as a result of multiple reflections of acoustic waves between the boundaries of these structures, and cause fill-in to lower image contrast. In order to mitigate the undesirable reverberation clutter effects, we present, in this paper, a new beamforming technique called multiphase apodization with cross correlation (MPAX), which is an improved version of our previous technique, dual apodization with cross correlation (DAX). While DAX uses a single pair of complementary amplitude apodizations, MPAX utilizes multiple pairs of complementary sinusoidal phase apodizations to intentionally introduce grating lobes from which an improved weighting matrix can be produced to effectively suppress reverberation clutter. Our experimental sponge phantom and preliminary in vivo results from human subjects presented in this paper suggest that MPAX is a highly effective technique in suppressing reverberation clutter and has great potential for producing high contrast ultrasound images for more accurate diagnosis in clinics.

Harmonic Imaging with Fresnel Beamforming in the Presence of Phase Aberration

Fresnel beamforming is a beamforming method with a delay profile similar in shape to a physical Fresnel lens. The advantage of Fresnel beamforming is the reduced channel count, which consists of four to eight transmit and two analog-to-digital receive channels. Fresnel beamforming was found to perform comparably to conventional delay-and-sum beamforming. However, the performance of Fresnel beamforming is highly dependent on focal errors. These focal errors result in high side-lobe levels and further reduce the performance of Fresnel beamforming in the presence of phase aberration. With the advantages of lower side-lobe levels and suppression of aberration effects, harmonic imaging offers an effective solution to the limitations of Fresnel beamforming. We describe the implementation of tissue harmonic imaging and pulse inversion harmonic imaging in Fresnel beamforming, followed by dual apodization with cross-correlation, to improve image quality. Compared with conventional delay-and-sum beamforming, experimental results indicated contrast-to-noise ratio improvements of 10%, 49% and 264% for Fresnel beamforming using tissue harmonic imaging in the cases of no aberrator, 5-mm pork aberrator and 12-mm pork aberrator, respectively. These improvements were 22%, 57% and 352% for Fresnel beamforming using pulse inversion harmonic imaging. Moreover, dual apodization with cross-correlation was found to further improve the contrast-to-noise ratios in all cases. Harmonic imaging was also found to narrow the lateral beamwidth and shorten the axial pulse length by at least 25% and 21%, respectively, for Fresnel beamforming at different aberration levels. These results suggest the effectiveness of harmonic imaging in improving image quality for Fresnel beamforming, especially in the presence of phase aberration. Even though this combination of Fresnel beamforming and harmonic imaging does not outperform delay-and-sum beamforming combined with harmonic imaging, it provides the benefits of reduced channel count and potentially reduced cost and size of ultrasound systems.

Effects of dual apodization with cross- correlation on tissue harmonic and pulse inversion harmonic imaging in the presence of phase aberration

Dual apodization with cross-correlation (DAX) is a relatively new beamforming technique which can suppress side lobes and clutter to enhance ultrasound image contrast. However, previous studies have shown that with increasing aberrator strength, contrast enhancements with DAX diminish and DAX becomes more prone to image artifacts. In this paper, we propose integrating DAX with tissue harmonic imaging (THI) or pulse inversion harmonic imaging (PIHI) to overcome their shortcomings and achieve higher image contrast. Compared with conventional imaging, our experimental results showed that DAX with THI allows for synergistic enhancements of image contrast with improvements of more than 231% for a 5-mm pork aberrator and 703% for a 12-mm pork aberrator. With PIHI, improvements of 238% and 890% were observed for the two pork tissue samples. Our results suggest that the complementary contrast enhancement mechanism employed by the proposed method may be useful in improving imaging of technically difficult patients in clinics.

Performance improvement of fresnel beamforming using dual apodization with cross-correlation

Fresnel beamforming is a beamforming method that has a delay profile with a shape similar to a physical Fresnel lens. With 4 to 8 transmit channels, 2 receive channels, and a network of single-pole/single-throw switches, Fresnel beamforming can reduce the size, cost, and complexity of a beamformer. The performance of Fresnel beamforming is highly dependent on focal errors resulting from phase wraparound and quantization of its delay profile. Previously, we demonstrated that the performance of Fresnel beamforming relative to delayand- sum (DAS) beamforming is comparable for linear arrays at f-number = 2 and 50% bandwidth. However, focal errors for Fresnel beamforming are larger because of larger path length differences between elements, as in the case of curvilinear arrays compared with linear arrays. In this paper, we present the concept and performance evaluation of Fresnel beamforming combined with a novel clutter suppression method called dual apodization with cross-correlation (DAX) for curvilinear arrays. The contrast-to-noise ratios (CNRs) of Fresnel beamforming followed by DAX are highest at f-number = 3. At f-number = 3, the experimental results show that using DAX, the CNR for Fresnel beamforming improves from 3.7 to 10.6, compared with a CNR of 5.2 for DAS beamforming. Spatial resolution is shown to be unaffected by DAX. At f-number = 3, the lateral beamwidth and axial pulse length for Fresnel beamforming with DAX are 1.44 and 1.00 mm larger than those for DAS beamforming (about 14% and 21% larger), respectively. These experimental results are in good agreement with simulation results.

Synergistic enhancements of ultrasound image contrast with a combination of phase aberration correction and dual apodization with cross-correlation

Dual apodization with cross-correlation (DAX) is a novel adaptive beamforming technique which utilizes two distinct apodization functions in suppressing side lobes and clutter. Previous studies have shown that the performance of DAX in minimizing the effects of phase aberration diminishes with increasing aberrator strength. To achieve greater improvement in image contrast, we propose, in this paper, to combine DAX with a phase aberration correction algorithm based on nearest-neighbor cross-correlation (NNCC). Our simulation and experimental results presented in this work showed that the proposed method allows for synergistic enhancements of image contrast and achieves greater improvement in image quality than using DAX alone or phase aberration correction alone in the presence of weak and strong aberrators. Compared with standard delay-and-sum (DAS) beamforming, using the proposed method on simulated data with weak and strong aberrations increased the contrast-to-noise ratio (CNR) values from 4.10 to 10.96 and from 1.69 to 9.80, respectively. Experimental results were obtained using pork tissues of 4 and 10 mm thickness and a tissue-mimicking phantom. The CNR values increased from 3.74 to 9.72 for the 4-mm pork aberrator and from 1.27 to 8.17 for the 10-mm pork aberrator.

Design and Demonstration of Apodized Comb Filters on SOI

We report on the design, modeling, and characterization of apodized comb filters in the silicon-on-insulator platform. These devices are based on sampled gratings, wherein the comb response is generated by a convolution of a regular grating by a predetermined periodic sampling function. We show that dual and complementary apodization of the etch depth in the marks and spaces ensures a near-zero dc index change over the length of the device and avoids the unwanted sidelobes obtained in a single apodized device. Comb filters with a 40-dB side-mode suppression ratio, a 92-GHz clear bandwidth, 200-GHz channel spacing, and a 120-dB/dec rolloff have been theoretically proposed. An experimental demonstration of the fabricated devices is also presented, and the performance is compared with the numerical simulations.

Evaluating the robustness of dual apodization with cross-correlation.

We have recently presented a new method to suppress side lobes and clutter in ultrasound imaging called dual apodization with cross-correlation (DAX). However, due to the random nature of speckle, artifactual black spots may arise with DAX-processed images. In this paper, we present one possible solution, called dynamic DAX, to reduce these black spots. We also evaluate the robustness of dynamic DAX in the presence of phase aberration and noise. Simulation results using a 5 MHz, 128-element linear array are presented using dynamic DAX with aberrator strengths ranging from 25 ns root-mean-square (RMS) to 45 ns RMS and correlation lengths of 3 mm and 5 mm. When simulating a 3 mm diameter anechoic cyst, at least 100% improvement in the contrast-to-noise ratio (CNR) compared with standard beamforming is seen using dynamic DAX, except in the most severe case. Layers of pig skin, fat, and muscle were used as experimental aberrators. Simulation and experimental results are also presented using dynamic DAX in the presence of noise. With a system signal-to-noise ratio (SNR) of at least 15 dB, we have a CNR improvement of more than 100% compared with standard beamforming. This work shows that dynamic DAX is able to improve the contrast-to-noise ratio reliably in the presence of phase aberration and noise.

Sidelobe suppression in ultrasound imaging using dual apodization with cross-correlation

This paper introduces a novel sidelobe and clutter suppression method in ultrasound imaging called dual apodization with cross-correlation or DAX. DAX dramatically improves the contrast-to-noise ratio (CNR) allowing for easier visualization of anechoic cysts and blood vessels. This technique uses dual apodization or weighting strategies that are effective in removing or minimizing clutter and efficient in terms of computational load and hardware/software needs. This dual apodization allows us to determine the amount of mainlobe versus clutter contribution in a signal by cross-correlating RF data acquired from 2 apodization functions. Simulation results using a 128 element 5 MHz linear array show an improvement in CNR of 139% compared with standard beamformed data with uniform apodization in a 3 mm diameter anechoic cylindrical cyst. Experimental CNR using a tissue-mimicking phantom with the same sized cyst shows an improvement of 123% in a DAX processed image.

Dual apodization technique for improved contrast

We propose a method to use dual apertures or dual apodization functions to reduce side lobes and clutter for ultrasound imaging. Using a common transmit aperture but different receive apodizations or apertures, we create two point spread functions with very similar main lobes and different sidelobe and clutter signals. Main lobe signals can be distinguished from clutter signals using normalized cross‐correlation of the raw radio‐frequency data. The normalized cross‐correlation coefficient is used as a pixel‐by‐pixel weighting to pass main‐lobe signals and suppress side lobe and clutter signals. Main lobe signals will have a high cross‐correlation coefficient near 1 and clutter signals will have coefficients between 0 and ‐1. Point target simulations show a narrowing of the main beam compared to conventional beamforming at beamwidths ‐20 dB and lower. Using a 5 MHz 128‐element linear array, improvements of contrast‐to‐noise ratio (CNR) of an anechoic cyst compared to delay and sum beamforming exceed 130% in both simulations and experiments. We also evaluated this method for robustness in the presence of phase aberration. Aberrators ranging from 25‐45 ns rms with correlation lengths of 3‐5 mm were used. Here, improvements in CNR also exceed 100% in many cases.