Ultrasonic Lens Research Articles

Tunable Hybrid Phononic Crystal Lens Using Thermo-Acoustic Polymers.

Solid phononic crystal (PnC) lenses were made active on infiltration with thermosensitive polymers to produce a thermoactuated hybrid solid lens with variable focusing. Acoustic lenses, both solid state and PnCbased, are passive elements with a fixed focal length. Their focal characteristics are functions of the lens structure or the arrangement of the PnC unit cell. Dispersion effects, liquid-filled membranes, and phase delay in a multi-element emitter have been used for variable focusing. The high thermal, electric, and electromagnetic sensitivity of the elastic properties of poly(vinyl alcohol) (PVA) poly(N-isopropylacrylamide) (PNIPAm)-based hydrogels enable them to operate as tunable solids. However, these solids do not have strong enough contrast with water or well-controlled shape parameters to function as standalone lenses. Here, a tunable hybrid solid ultrasonic lens is realized by combining a PnC lens with PVA-PNIPAm thermoacoustic hydrogel to modify the transmission and dispersion properties of transient acoustic waves. Variable focusing is demonstrated from 40 to 50 mm using the anomalous thermosensitivity of the elasticity and speed of sound of the hydrogel.

A focused lens for measuring object response to modulated radiation pressure

An ultrasonic lens was designed and tested for use in modulated radiation pressure experiments for the purpose of exciting and identifying the low frequency modes of objects in water. The lens was designed to produce a focused beam, and care was taken to minimize the spherical aberration introduced by the lens. This was done by carefully specifying the shape of the lens surface that was then machined using a computer-controlled lathe. The lens was placed in contact with an electrically excited piezoceramic plate. The quality of the focused sound field was tested and found to display properties predicted by simple models of the focal plane wave-field. The lens was then used to drive low-frequency modes of elastic objects in water using modulated ultrasonic radiation pressure in a way previously summarized involving a commercially made focused transducer [T. D. Daniel et al., J. Acoust. Soc. Am. 140, 3123 (2016)]. [Work supported by ONR.]

The technology of laser cataract extraction in diabetic patients

In 1994, academician S.N. Fyodorov directed the development of laser cataract extraction (LCE) technique, which does not require manual or ultrasonic lens fragmentation. To determine the effectiveness and safety of Russian LCE technology in patients with diabetes mellitus using comparative analysis, and to develop practical guidelines. The study analyzed the results of 154 operations in patients with complicated cataract and diabetes mellitus (DM). 80 LCE operations and 74 ultrasonic cataract phacoemulsification (PhEC) surgeries including implantation of intraocular lens were performed. The duration of DM was 12.5±7.1 years in the LCE group and 11.9±7.3 years in the PhEC group. The energy of neodymium Nd:YAG laser endo-dissector with original wavelength of 1.44 μm together with low-intensity He-Ne laser radiation of 0.63 μm were delivered simultaneously to the eye cavity. Corneal endothelial cells loss was significantly lower (by 1.8 times) after LCE, in the early postop period, in 1 year, and 2 years after the surgery; the incidence of intraocular pressure increase and ciliary body edema were lower as well. Moreover, there is no negative influence on the posterior segment of the eye because the radiation of laser endo-dissector during the duty cycle of cataract removal does not extend beyond the lens capsule, and does not affect other tissues of the eye. In comparison to ultrasound cataract surgery, LCE is a more safe and effective method of removing complicated cataracts in diabetic patients with any degree of lens density.

Planar Ultrasonic Lenses Formed by Concentric Circular Sandwiched‐Ring Arrays

AbstractAn underwater ultrasonic planar lens is numerically and experimentally developed with a small thickness and maintains a highly efficient transmission. The lens is formed by an array of concentric circular resin rings sandwiched between two identical silicone rings. Since the velocity of the water is closely in between that of the resin and silicone, a 2π phase variation can be achieved by changing the thickness ratios of the resin to the silicone ring within a thin sandwiched structure. Based on this principle, both single‐point and multipoint focusing lenses with high transmission in the axial direction are realized, which are believed to have potential applications in underwater imaging and detection.

Polyadic Cantor Fractal Ultrasonic Lenses: Design and Characterization

Traditional acoustic lenses modulate the ultrasonic beam due to their curved surfaces and the refractive material of which they are made. In this work, a different type of acoustic lens, based on Polyadic Cantor Fractals (PCF), is presented and thoroughly analyzed. These new Polyadic Cantor Fractal Lenses (PCFLs) are completely flat and easy to build, and they present interesting modulation capabilities over the acoustic profile. The dependence of the focusing profile on the PCFL design parameters is fully characterized, and it is shown that certain design parameters provide a dynamic control, which is critical in many medical applications such as thermal ablation of tumors.

An Ultrasonic Lens Design Based on Prefractal Structures

The improvement in focusing capabilities of a set of annular scatterers arranged in a fractal geometry is theoretically quantified in this work by means of the finite element method (FEM). Two different arrangements of rigid rings in water are used in the analysis. Thus, both a Fresnel ultrasonic lens and an arrangement of rigid rings based on Cantor prefractals are analyzed. Results show that the focusing capacity of the modified fractal lens is better than the Fresnel lens. This new lens is believed to have potential applications for ultrasonic imaging and medical ultrasound fields.

The Research of a Variable-Focus Ultrasonic Liquid Lens

In this paper, a variable-focus ultrasonic liquid lens controlled by the external voltage is demonstrated. That is, the liquid lens can be reshaped with the external voltage, which will cause the focal length to change. The relationship between liquid lens’ shape and the control voltage was obtained. The influence factors of the response time were researched. And the distribution of the ultrasonic field after the lens under different external control voltages was simulated.

Ultrasonic optical lens array with variable focal length and pitch

An optical lens array whose focal length and lens pitch can be varied by acoustic radiation force was investigated. It has no mechanical moving parts and a simple structure that consists of a glass plate, four ultrasound transducers, and a viscoelastic film. A lens array can be formed by generating acoustic radiation force through flexural vibrations of the glass plate. Its focal length can be controlled by varying the voltage applied to the transducers and the lens pitch can be altered by changing the driving frequency.

Broadband aperiodic air coupled ultrasonic lens

This paper demonstrates the possibility of subwavelength, defined as less than the incident wavelength, broadband focusing in an aperiodic air coupled ultrasonic lens. A near field probe is used to detect well defined resonances from 75 to 125 kHz. The spatial resolution at each of the resonant frequencies is determined and demonstrated to be smaller than the wavelength of the ultrasonic waves. The strongest resonance is observed at 82.9 kHz with a focal spot size of 3.12 mm. The subwavelength spatial resolution of the lens structures at the resonances is attributed to the near field scattering of the acoustic waves.

3Pa3-1 Ultrasonic Optical Lens Array with Variable Focal Length and Pitch(Poster Session)

3Pa3-1 Ultrasonic Optical Lens Array with Variable Focal Length and Pitch(Poster Session)

HIGH INTENSITY FOCUSED ULTRASOUND TRANSDUCER WITH ACOUSTIC LENS

A focused ultrasound transducer includes a first ultrasonic emitter and at least one metallic ultrasonic lens acoustically coupled thereto. The emitter generates ultrasonic energy that propagates along a beam path projecting therefrom. The at least one metallic ultrasonic lens is positioned at least partially in the beam path so that it can direct (e.g., focus, defocus, and/or collimate) in at least one direction (or along at least one plane) at least some of the ultrasonic energy propagating from the emitter. The metallic lens may be formed by extrusion, by molding (e.g., diecast molding or thermoforming), or by sintering (e.g., powder metallurgy). The metallic lens also advantageously functions as a heat sink, improving thermal performance of the ultrasound transducer.

Focusing of longitudinal ultrasonic waves in air with an aperiodic flat lens

Modeling and experimental results of an ultrasonic aperiodic flat lens for use in air are presented. Predictive modeling of the lens is performed using a hybrid genetic-greedy algorithm constrained to a linear structure. The optimized design parameters are used to fabricate a lens. A method combining a fiber-disk arrangement and scanning laser vibrometer measurements is developed to characterize the acoustic field distribution generated by the lens. The focal spot size is determined to be 0.88 of the incident wavelength of 80-90 kHz at a distance of 2.5 mm from the lens. Theoretically computed field distributions, optimized frequency of operation, and spatial resolution focal length are compared with experimental measurements. The differences between experimental measurements and the theoretical computations are analyzed. The theoretical calculation of the focal spot diameter is 1.7 mm which is 48% of the experimental measurement at a frequency of 80-90 kHz. This work illustrates the capabilities of a hybrid algorithm approach to design of flat acoustic lenses to operate in air with a resolution of greater than the incident wavelength and the challenges of characterizing acoustic field distribution in air.

Real-time full-field photoacoustic imaging using an ultrasonic camera

A photoacoustic imaging system that incorporates a commercial ultrasonic camera for real-time imaging of two-dimensional (2-D) projection planes in tissue at video rate (30 Hz) is presented. The system uses a Q-switched frequency-doubled Nd:YAG pulsed laser for photoacoustic generation. The ultrasonic camera consists of a 2-D 12 x 12 mm CCD chip with 120 x 120 piezoelectric sensing elements used for detecting the photoacoustic pressure distribution radiated from the target. An ultrasonic lens system is placed in front of the chip to collect the incoming photoacoustic waves, providing the ability for focusing and imaging at different depths. Compared with other existing photoacoustic imaging techniques, the camera-based system is attractive because it is relatively inexpensive and compact, and it can be tailored for real-time clinical imaging applications. Experimental results detailing the real-time photoacoustic imaging of rubber strings and buried absorbing targets in chicken breast tissue are presented, and the spatial resolution of the system is quantified.

High resolution fast digital photoacoustic CT for breast cancer diagnosis

Photoacoustic imaging combines advantages of both ultrasound imaging and optical imaging to provide high ultrasonic resolution and high optical contrast in noninvasive medical images. A photoacoustic computer tomography （PCT） system with a focused digital phased array transducer is proposed. In particular each element of the transducer array has a thin cylindrical ultrasonic lens to select 2D image plane and suppress the out-of-plane signals to realize photoacoustic computed tomography. A fast full-view photoacoustic imaging system for visualizing tissue structures using a linear transducer array with combined scan, whies; detects photoacoustic signals by combined electronic and mechanical scanning. An improved limited-field filtered back projection algorithm with directivity factors was applied to reconstruct the optical absorption distribution and further improve the lateral resolution. In the experiment, the spatial resolution of this imaging system is determined as 0.2mm, and the slice resolution of Z axis is 1.5mm. Mechanical step scanning was performed, leading to a measurement time of 6.4 seconds per position, and a total scan duration of 150 seconds to obtain the two-dimensional photoacoustic image. High spatial resolution of 2D photoacoustic vascular image and quasi-3D photoacoustic CT for breast cancer imaging are achicned. The experimental results demonstrate that this method and imaging system can realize the high resolution photoacoustic tomography wtich can be applied to noninvasive detection of early breast cancer.

Pulse laser induced wave propagation in graded media and focusing devices

Near-infrared-laser pulses having durations of 100 fs are used to excite elastic waves thermoelastically propagating in a sub-THz frequency range. The elastic waves interact with inhomogeneities and carry information to the surface. The arrivals of the elastic pulses at the surface lead to transient changes of the optical reflectance which are monitored with short laser pulses which have a defined and controlled time delay relative to the initial pulses. Two activities of the research group are presented: The reflection and transmission behavior of acoustic waves propagating in graded media shows a frequency dependent nature and can therefore be used for filtering purpose. Time-boundary value problems are solved for various gradients with a finite-difference method. Results of the numerical simulation are presented and compared with laser-acoustic measurements. A 'classical' photoacoustic set-up provides an in-depth resolution of about 5 nm whereas the lateral resolution is in the order of 5 to 10 microns. To enhance the lateral resolution of the pump-probe technique, the elastic wave propagation along structures with arbitrary tip-like geometries consisting of orthotropic material is analyzed. With such structures representing ultrasonic lenses, the elastic energy is focused to a spot size given by the sharpness of the tip thereby leading to a higher lateral resolution.

Negative refraction and focusing of ultrasound in two-dimensional phononic crystals

We present experimental demonstrations of negative refraction and focusing of ultrasonic waves in two-dimensional phononic crystals made of stainless steel rods assembled in a triangular lattice and immersed in a liquid. Negative refraction is achieved for the range of frequencies in the second band, where the directions of the wave vector and group velocity are antiparallel to each other due to circular equifrequency contours. Negative refraction is unambiguously observed using a prism-shaped crystal. By exploiting the circular equifrequency contours in the second band, focusing of the ultrasonic field emitted by a pointlike source was demonstrated using a flat phononic crystal filled with and immersed in water. During these experiments, the importance of imaging in the regime of all angle negative refraction (AANR) was established for obtaining high-quality images. The regime of AANR was achieved in a similar flat crystal, in which the liquid inside the crystal (methanol) was different from the outside medium (water). This design resulted in matching circular equifrequency contours at the frequency of $0.55\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, implying that a flat ultrasonic lens with an effective refractive index of $\ensuremath{-}1$ was realized. By imaging a subwavelength line source with this crystal, a resolution of $0.55\ensuremath{\lambda}$ was observed, which is just above the diffraction limit.

2P7b-5 Comparison of sound pressure distribution analysis with scale up experiment for small ultrasonic acoustic lens probe(Poster Session)

2P7b-5 Comparison of sound pressure distribution analysis with scale up experiment for small ultrasonic acoustic lens probe(Poster Session)

High-temperature and broadband immersion ultrasonic probes

Immersion ultrasonic probes for measurements and imaging at high temperature are presented. The probes consist of sol-gel-sprayed thick films as piezoelectric ultrasonic transducers (UTs) directly deposited onto steel buffer rods. They operate in pulse-echo mode at temperatures up to 500degC. The operating ultrasonic frequency is between 5 MHz and 20 MHz, controlled by the film thickness. The ultrasonic thickness measurement of a steel plate with the probe fully immersed in molten zinc at 450degC was demonstrated using ultrasonic plane waves. For imaging purposes, the probing end of the steel buffer rod was machined into a semispherical concave shape to form an ultrasonic lens and achieve high spatial resolution with focused ultrasound in liquids. Ultrasonic surface and subsurface imaging using a mechanical raster scan of the focused probe in silicone oil at 200degC was also carried out. The importance of the signal-to-noise ratio (SNR) in the pulse-echo measurement is discussed

Improved in vivo photoacoustic microscopy based on a virtual-detector concept

Recently an in vivo high-resolution backward-mode photoacoustic microscope was developed that shows potential for applications in dermatology and related cancer research. However, the limited depth of focus of the large-numerical-aperture (NA) ultrasonic lens employed in this system causes the image quality to deteriorate significantly in the out-of-focus region. To solve this problem, we devised and explored, for the first time to our knowledge, a virtual-detector-based synthetic-aperture focusing technique, combined with coherence weighting, for photoacoustic microscopy with such a large-NA transducer. Images of phantoms show that the proposed technique improves the -6 dB lateral resolution from 49-379 to 46-53 microm and increases the signal-to-noise ratio by up to 29 dB, depending on the distance from the ultrasonic focal point. In vivo experiments show that the technique also provides a clearer representation of the vascular distribution in the rat's scalp.

Ultrasonic imaging in molten magnesium

Processing magnesium (Mg) and its alloys in the molten state is often necessary forthe refining and recycling, and for the casting to achieve the net shape forming.The containers to hold the molten Mg are commonly made of steel. There is aconcern that the steel wall of the container will corrode from the inside, whichmight induce cracks resulting in dangerous and expensive spillage. In this note,development of ultrasonic techniques for imaging in molten Mg using clad steelbuffer rods operated at 10 MHz is presented. The probing end of the buffer rod,having an ultrasonic lens, was immersed into molten Mg, while the other end,with an ultrasonic transducer, was air cooled to room temperature. An ultrasonicimage of a character ‘N’, engraved on a stainless steel plate, which simulates asurface defect on a plate, immersed in molten Mg has been successfully observed at690 °C using the focused probe.