Acoustic Lens System Research Articles

Enhancing azimuth estimation in low-resolution underwater acoustic lens systems for advanced communication and positioning

Here we aimed to enhance azimuth estimation to accurately locate a communication partner’s position by utilizing a low-resolution underwater acoustic lens system for communication. We proposed a method to accurately estimate the direction-of-arrival (DoA) by finding the center of gravity of the focal distribution using two adjacent elements of the transducer array. Ranging was to be based on time-of-flight (ToF). The method’s effectiveness was evaluated by two-dimensional finite difference time domain simulation. The lens’s focused sound field was calculated and the array was designed based on the result. DoA and ToF were estimated from the received signals and evaluated by comparing them with the true values under conditions involving 1-, 2-, and 3- users. The results confirmed DoA estimation with an error of almost 4°, even when using a lens system with an azimuth resolution of 10°. As the number of users increased, the maximum error value increased, though slightly.

Design of a wide-angle compound eye acoustic lens system suitable for multi-user underwater acoustic communication

Mobile underwater acoustic (UWA) communication is expected to enable advanced, efficient, and reliable underwater exploration. However, crosstalk among terminals occurs when multiple vehicles emit signals at the same time. In this paper, we propose a UWA lens that has the potential to achieve space-division multiple access by avoiding crosstalk between signals that come from a different angle. Specifically, we used wave theory to design a wide-angle compound eye lens and a hydrophone array geometrically. The two-dimensional finite-difference time-domain method was used to calculate wave propagation. Multi-user communication using a single-carrier scheme (two sources emit the signal simultaneously) was simulated with the designed lens system. The communication method with the lens system showed superior performance to that without the lens system. Additionally, the performance was improved by inserting partitions between lens segments.

Shape and topology optimization of acoustic lens system using phase field method

A layout optimization method for a two-dimensional acoustic lens system used in underwater imaging is presented. To this end, a shape and topology optimization is formulated for the design problem of a lens system for the first time. The layout of a lens system to be optimized includes the number of lenses, shape of lens surfaces, distances between lenses, and lens materials. A phase field function is employed to implicitly parameterize the boundaries of the lenses, which move according to design sensitivities during optimization. Multiple lenses with different materials are optimized using a single phase field function. Because the ratio of the acoustic wavelength with respect to lens dimensions is large, diffraction effects should be taken into account. Accordingly, the performance of a lens system should be analyzed using wave acoustics and not the ray tracing method. The optimization problem is formulated to remove the aberrations of coma and field curvature. The validity of the proposed optimization method is demonstrated by solving benchmark design problems including a lens system with a large field of view.

Characterization of CMOS MEMS Capacitive Ultrasonic Sensors for Fast Photoacoustic Imaging

This work proposes to develop a CMOS (complementary metal oxide semiconductor) capacitive ultrasonic sensor chip that can be jointly operated with an acoustic lens system to achieve fast 3-D photoacoustic imaging. Compared to conventional piezoelectric technology, capacitive transducers have a wider bandwidth and can be miniaturized and fabricated in a dense array to provide better imaging quality. Different from our previous approach that requires a long acquisition time to measure the complete waveform of each sensor for 3-D image reconstruction, the new scheme only has to detect the peak amplitude of a waveform. This paper reports the characterization of the fabricated sensors, which shows the sensors have a resonant frequency near 3.5 MHz and the peak detection errors are typically within ±10% for test signals from 1 to 10 MHz with the amplitude from 50 to 400 mV.

PE-CMOS based C-scan ultrasound for foreign object detection in soft tissue

In this paper, we introduce a C-scan ultrasound prototype and three imaging modalities for the detection of foreign objects inserted in porcine soft tissue. The object materials include bamboo, plastics, glass and aluminum alloys. The images of foreign objects were acquired using the C-scan ultrasound, a portable B-scan ultrasound, film-based radiography, and computerized radiography. The C-scan ultrasound consists of a plane wave transducer, a compound acoustic lens system, and a newly developed ultrasound sensor array based on the complementary metal-oxide semiconductor coated with piezoelectric material (PE-CMOS). The contrast-to-noise ratio (CNR) of the images were analyzed to quantitatively evaluate the detectability using different imaging modalities. The experimental results indicate that the C-scan prototype has better CNR values in 4 out of 7 objects than other modalities. Specifically, the C-scan prototype provides more detail information of the soft tissues without the speckle artifacts that are commonly seen with conventional B-scan ultrasound, and has the same orientation as the standard radiographs but without ionizing radiation.

Laboratory‐scale study of field of view and the seismic interpretation of fracture specific stiffness

ABSTRACTThe effects of the scale of measurement, i.e., the field of view, on the interpretation of fracture properties from seismic wave propagation was investigated using an acoustic lens system to produce a pseudo‐collimated wavefront. The incident wavefront had a controllable beam diameter that set the field of view at 15 mm, 30 mm and 60 mm. On a smaller scale, traditional acoustic scans were used to probe the fracture in 2 mm increments. This laboratory approach was applied to two limestone samples, each containing a single induced fracture and compared to an acrylic control sample. From the analysis of the average coherent sum of the signals measured on each scale, we observed that the scale of the field of view affected the interpretation of the fracture specific stiffness. Many small‐scale measurements of the seismic response of a fracture, when summed, did not predict the large‐scale response of the fracture. The change from a frequency‐independent to frequency‐dependent fracture stiffness occurs when the scale of the field of view exceeds the spatial correlation length associated with fracture geometry. A frequency‐independent fracture specific stiffness is not sufficient to classify a fracture as homogeneous. A nonuniform spatial distribution of fracture specific stiffness and overlapping geometric scales in a fracture cause a scale‐dependent seismic response, which requires measurements at different field of views to fully characterize the fracture.

An investigation of the use of transmission ultrasound to measure acoustic attenuation changes in thermal therapy

The potential of using a commercial ultrasound transmission imaging system to quantitatively monitor tissue attenuation changes after thermal therapy was investigated. The ultrasound transmission imaging system used, the AcoustoCam (Imperium Inc., MD) allows ultrasonic images to be captured using principles similar to that of a CCD-type camera that collects light. Ultrasound energy is focused onto a piezoelectric array by an acoustic lens system, creating a gray scale acoustic image. In this work, the pixel values from the acoustic images were assigned acoustic attenuation values by imaging polyacrylamide phantoms of varying known attenuation. After the calibration procedure, data from heated polyacrylamide/bovine serum albumin (BSA) based tissue-mimicking (TM) phantoms and porcine livers were acquired. Samples were heated in water at temperatures of 35, 45, 55, 65, and 75 degrees C for 1 h. Regions of interest were chosen in the images and acoustic attenuation values before and after heating were compared. An increase in ultrasound attenuation was found in phantoms containing BSA and in porcine liver. In the presence of BSA, attenuation in the TM phantom increased by a factor of 1.5, while without BSA no significant changes were observed. The attenuation of the porcine liver increased by up to a factor of 2.4, consistent with previously reported studies. The study demonstrates the feasibility of using a quantitative ultrasound transmission imaging system for monitoring thermal therapy.

Finite Difference Time Domain Analysis of Underwater Acoustic Lens System for Ambient Noise Imaging

Much attention has been paid to the new idea of detecting objects using ocean ambient noise. This concept is called ambient noise imaging (ANI). In this study, sound fields focused by an acoustic lens system constructed with a single biconcave lens were analyzed using the finite difference time domain (FDTD) method for realizing an ANI system. The size of the lens aperture that would have sufficient resolution–for example, the beam width is 1° at 60 kHz–was roughly determined by comparing the image points and -3 dB areas of sound pressure fields generated by lenses with various apertures. Then, in another FDTD analysis, we successfully used a lens with a determined aperture to detect rigid target objects in an acoustic noise field generated by a large number of point sources.

C-3 Wave Theory Analysis of Acoustic Lens System for Real Time and High Resolution Imaging Sonar System

C-3 Wave Theory Analysis of Acoustic Lens System for Real Time and High Resolution Imaging Sonar System

Real-time three-dimensional optoacoustic imaging using an acoustic lens system

In medical optoacoustics (photoacoustics), absorbing structures, such as blood vessels, hidden inside scattering media are illuminated with short laser pulses resulting in the generation of thermoelastic pressure transients. This initial three-dimensional (3D) acoustic pressure distribution, which exactly resembles the absorption distribution, was imaged into a water container with a 4f acoustic lens system. An optical dark-field stereo imaging system using a 30ns flash illumination light was used to capture a snapshot of the pressure-induced refraction index changes in the water container at a predetermined time after the original laser pulse. The imaging system works at 20Hz frame rate and was designed toward a theoretical resolution of 50μm. The proposed method directly provides 3D images of absorbing structures without the need of computational reconstruction algorithms.

Development of an Acoustic Lens for an Imaging Sonar for Autonomous Underwater Vehicle “Urashima” and Experimentation in a Water Tank

Underwater imaging sonar has been developed for obstacle avoidance and navigation of the autonomous underwater vehicle (AUV) “URASHIMA”. The acoustic image, the distance and the azimuth of the target are obtained using an acoustic lens. Tone burst acoustic waves are transmitted and scattered at the target, and a direct image of the target can be created on a two-dimensional receiving array. The merits of the acoustic lens system are the possibility to omit the extensive calculation required for signal processing of received signals at each element to form two-dimensional beams, and reduction of equipment size. In this report, the outline of the imaging sonar with a single spherical lens and the experimental results for the acoustic lens in a water tank are presented. Then, theoretical calculation of the sound field through the lens using the parabolic equation method is introduced as a useful tool for improvement of lens performance to solve problems in resolution and aberration.

Acoustic lens system

Acoustic lens system

Acoustic lens system

Acoustic lens system

Acoustic lens system

An acoustic lens system is constructed so that at least one surface of acoustic lenses constituting the acoustic lens system is an aspherical surface, which has such a shape that curvature moderates progressively in separating from the axis of the acoustic lens system, and an acoustic beam stop is provided therein. As a result, aberrations can be favorably corrected even when the angle of view and the numerical aperture are increased and this brings about the acoustic lens system suitable for an objective lens of an acoustic system for securing an image of an object having a two-dimensional size in particular.