Field conjugate acoustic lenses for ultrasound hyperthermia

Holographic generation of arbitrary ultrasonic fields by simultaneous modulation of amplitude and phase

Acoustic holograms have been used widely to generate desired acoustic fields. Following the rapid development of 3D printing technology, the use of holographic lenses has become an efficient method to produce acoustic fields with high resolution and low cost. In this paper, we demonstrate a technique to modulate the amplitude and phase of ultrasonic waves simultaneously using a holographic method with high transmission efficiency and high accuracy. On this basis, we generate an Airy beam with high propagation invariance. We then discuss the advantages and disadvantages of the proposed method when compared with the conventional acoustic holographic method. Finally, we design a sinusoidal curve with a phase gradient and a constant pressure amplitude and realize transport of a particle on a water surface along a curve.

Experiments and simulations demonstrating the rapid ultrasonic rewarming of frozen tissue cryovials

The development of methods to safely rewarm large cryopreserved biological samples remains a barrier to the widespread adoption of cryopreservation. Here, experiments and simulations were performed to demonstrate that ultrasound can increase rewarming rates relative to thermal conduction alone. An ultrasonic rewarming setup based on a custom 444 kHz tubular piezoelectric transducer was designed, characterized, and tested with 2 ml cryovials filled with frozen ground beef. Rewarming rates were characterized in the -20 °C to 5 °C range. Thermal conduction-based rewarming was compared to thermal conduction plus ultrasonic rewarming, demonstrating a tenfold increase in rewarming rate when ultrasound was applied. The maximum recorded rewarming rate with ultrasound was 57° C/min, approximately 2.5 times faster than with thermal conduction alone. Coupled acoustic and thermal simulations were developed and showed good agreement with the heating rates demonstrated experimentally and were also used to demonstrate spatial heating distributions with small (<3° C) temperature differentials throughout the sample when the sample was below 0° C. The experiments and simulations demonstrate the potential for ultrasonic cryovial rewarming with a possible application to large volume rewarming, as faster rewarming rates may improve the viability of cryopreserved tissues and reduce the time needed for cells to regain normal function.

Spatio-temporal ultrasound beam modulation to sequentially achieve multiple foci with a single planar monofocal lens

Ultrasound focusing is a hot topic due to its multiple applications in many fields, including biomedical imaging, thermal ablation of cancerous tissues, and non destructive testing in industrial environments. In such applications, the ability to control the focal distance of the ultrasound device in real-time is a key advantage over conventional devices with fixed focal parameters. Here, we present a method to achieve multiple time-modulated ultrasound foci using a single planar monofocal Fresnel Zone Plate. The method takes advantage of the focal distance linear dependence on the operating frequency of this kind of lenses to design a sequence of contiguous modulated rectangular pulses that achieve different focal distances and intensities as a function of time. Both numerical simulations and experimental results are presented, demonstrating the feasibility and potential of this technique.

Rapid quantitative imaging of high intensity ultrasonic pressure fields

High intensity focused ultrasound (FUS) is a noninvasive technique for treatment of tissues that can lie deep within the body. There is a need for methods to rapidly and quantitatively map FUS pressure beams for quality assurance and accelerate development of FUS systems and techniques. However, conventional ultrasound pressure beam mapping instruments, including hydrophones and optical techniques, are slow, not portable, and expensive, and most cannot map beams at actual therapeutic pressure levels. Here, a rapid projection imaging method to quantitatively map FUS pressure beams based on continuous-wave background-oriented schlieren (CW-BOS) imaging is reported. The method requires only a water tank, a background pattern, and a camera and uses a multi-layer deep neural network to reconstruct two-dimensional root-mean-square (RMS) projected pressure maps that resolve the ultrasound propagation dimension and one lateral dimension. In this work, the method was applied to collect beam maps over a 3 × 1 cm2 field-of-view with 0.425 mm resolution for focal pressures up to 9 MPa. Results at two frequencies and comparisons to hydrophone measurements show that CW-BOS imaging produces high-resolution quantitative RMS projected FUS pressure maps in under 10 s, the technique is linear and robust to beam rotations and translations, and it can map aberrated beams.

Bifocal Ultrasound Focusing Using Bi-Fresnel Zone Plate Lenses

In this work, we present a bifocal Fresnel zone plate (BiFZP) capable of generating focusing profiles with two different foci. The performance of the BiFZP is demonstrated in the ultrasound domain, with a very good agreement between the experimental measurements and the finite element method (FEM) simulations. This lens becomes an appealing alternative to other dual-focusing lenses, in which the foci location can only be set at a limited range of positions, such as M-bonacci zone plates. Moreover, the variation of the operating frequency has also been analyzed, providing an additional dynamic control parameter in this type of lenses.

Metamaterial bricks and quantization of meta-surfaces

Controlling acoustic fields is crucial in diverse applications such as loudspeaker design, ultrasound imaging and therapy or acoustic particle manipulation. The current approaches use fixed lenses or expensive phased arrays. Here, using a process of analogue-to-digital conversion and wavelet decomposition, we develop the notion of quantal meta-surfaces. The quanta here are small, pre-manufactured three-dimensional units-which we call metamaterial bricks-each encoding a specific phase delay. These bricks can be assembled into meta-surfaces to generate any diffraction-limited acoustic field. We apply this methodology to show experimental examples of acoustic focusing, steering and, after stacking single meta-surfaces into layers, the more complex field of an acoustic tractor beam. We demonstrate experimentally single-sided air-borne acoustic levitation using meta-layers at various bit-rates: from a 4-bit uniform to 3-bit non-uniform quantization in phase. This powerful methodology dramatically simplifies the design of acoustic devices and provides a key-step towards realizing spatial sound modulators.

Ultrasonic neuromodulation

Ultrasonic waves can be non-invasively steered and focused into mm-scale regions across the human body and brain, and their application in generating controlled artificial modulation of neuronal activity could therefore potentially have profound implications for neural science and engineering. Ultrasonic neuro-modulation phenomena were experimentally observed and studied for nearly a century, with recent discoveries on direct neural excitation and suppression sparking a new wave of investigations in models ranging from rodents to humans. In this paper we review the physics, engineering and scientific aspects of ultrasonic fields, their control in both space and time, and their effect on neuronal activity, including a survey of both the field's foundational history and of recent findings. We describe key constraints encountered in this field, as well as key engineering systems developed to surmount them. In closing, the state of the art is discussed, with an emphasis on emerging research and clinical directions.

Towards multifocal ultrasonic neural stimulation: pattern generation algorithms

Focused ultrasound (FUS) waves directed onto neural structures have been shown to dynamically modulate neural activity and excitability, opening up a range of possible systems and applications where the non-invasiveness, safety, mm-range resolution and other characteristics of FUS are advantageous. As in other neuro-stimulation and modulation modalities, the highly distributed and parallel nature of neural systems and neural information processing call for the development of appropriately patterned stimulation strategies which could simultaneously address multiple sites in flexible patterns. Here, we study the generation of sparse multi-focal ultrasonic distributions using phase-only modulation in ultrasonic phased arrays. We analyse the relative performance of an existing algorithm for generating multifocal ultrasonic distributions and new algorithms that we adapt from the field of optical digital holography, and find that generally the weighted Gerchberg-Saxton algorithm leads to overall superior efficiency and uniformity in the focal spots, without significantly increasing the computational burden. By combining phased-array FUS and magnetic-resonance thermometry we experimentally demonstrate the simultaneous generation of tightly focused multifocal distributions in a tissue phantom, a first step towards patterned FUS neuro-modulation systems and devices.

Time-reversal acoustic focusing system as a virtual random phased array

This paper compares the performance of two different systems for dynamic focusing of ultrasonic waves: conventional 2-D phased arrays (PA) and a focusing system based on the principles of time-reversed acoustics (TRA). Focused ultrasound fields obtained in the experiments with the TRA focusing system (TRA FS), which employs a liquid-filled reverberator with 4 piezotransducers attached to its wall, are compared with the focused fields obtained by mathematical simulation of PAs comprised from several tens to several hundreds of elements distributed randomly on the array surface. The experimental and simulated focusing systems had the same aperture and operated at a frequency centered about 600 kHz. Experimental results demonstrated that the TRA FS with a small number of channels can produce complex focused patterns and can steer them with efficiency comparable to that of a PA with hundreds of elements. It is shown that the TRA FS can be realized using an extremely simple means, such as a reverberator made of a water-filled plastic bottle with just a few piezotransducers attached to its walls.

[Sound-field modification with acoustic lenses for high-intensity focused ultrasound therapy]

High-intensity focused ultrasound (HIFU) therapy is a minimally invasive method for precise thermal tissue destruction. Most applications make use of spherical, sharply focused fixed-focus transducers. These allow only small ablation rates, thus the treatment of large tumours is a very time-consuming process. The present paper describes the design of ancillary lenses, which combined with a spherical focused ultrasound transducer can be employed to reduce treatment time. The ancillary lenses shift the phase of the focused ultrasound waves, so that instead of one ellipsoidal focus multiple foci are generated. Thus, a single sonication can achieve a larger ablated area. To estimate the therapeutic benefits of the designed lenses, the lesion development during sonication was numerically simulated. Two "optimal" lenses were made of polystyrene and acoustically analysed. Using the designed lenses the theoretical lesion rate was increased by a factor of 3.7-5. There was a good correlation between simulated and measured ultrasound pressure field. The lenses reduced the system efficiency by only 13%. Thus, ancillary lenses present a technically simple and cost-effective method to accelerate ablative ultrasound therapy.

Theoretical evaluation of moderately focused spherical transducers and multi-focus acoustic lens/transducer systems for ultrasound thermal therapy

Impractically long treatment times are required for highly focused spherical transducers to destroy large tumours because thermal lesions generated by these transducers are small and a large number of such lesions are required. Moderately focused spherical transducers and multi-focus acoustic lens/transducer systems can generate larger thermal lesions compared to those produced by highly focused spherical transducers, and therefore shorter treatment times can be expected. The decrease in total treatment time by the use of moderately focused spherical transducers and acoustic lens/transducer systems was quantified in this study. A 3D ultrasound thermal model was developed to predict thermal lesion volumes generated by ultrasound transducers. A target model was constructed in order to compare various transducer designs under identical treatment conditions and with identical treatment goals. A design method was developed to determine the thickness of lens elements for production of specified multi-focus fields. Then, a highly focused and a moderately focused spherical transducer, and an acoustic lens/transducer system were compared in terms of total time required to treat a tumour. These transducers had identical apertures and operating frequencies. The radius of curvature of the moderately focused spherical transducer was chosen such that the length of thermal lesions it generated over 10 s single exposures was slightly greater than that of the target. The lens/transducer system was designed to produce a 9 focus field. The simulation results show that for the treatment of a 2 x 2 x 2 cm3 tumour at a depth of 5 cm in the body, the highly focused spherical transducer, the moderately focused spherical transducer and the lens/transducer system required 150, 42 and 30 min, respectively.

A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3x3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.

Optimizing ultrasound focus distributions for hyperthermia

A method is described for generating ultrasound focus patterns for ultrasound hyperthermia treatment planning for steady state and transient hyperthermia. The solution for placement and intensity of ultrasound focus points is based on two types of temperature constraints: 1) equality constraints on the tumor boundary (temperature is held at maximum safe level) and 2) inequality constraints in the tumor interior (in a therapeutic range of temperatures). The method employs a simplex algorithm to solve a series of linear equations which approximate the heating distribution in tissue. Examples are given for field conjugate acoustic lens applicators capable of generating multiple foci simultaneously.