A reconfigurable all-optical ultrasound transducer array for 3D endoscopic imaging

Online evolution of a phased array for ultrasonic imaging by a novel adaptive data acquisition method

Ultrasonic imaging, using ultrasonic phased arrays, has an enormous impact in science, medicine and society and is a widely used modality in many application fields. The maximum amount of information which can be captured by an array is provided by the data acquisition method capturing the complete data set of signals from all possible combinations of ultrasonic generation and detection elements of a dense array. However, capturing this complete data set requires long data acquisition time, large number of array elements and transmit channels and produces a large volume of data. All these reasons make such data acquisition unfeasible due to the existing phased array technology or non-applicable to cases requiring fast measurement time. This paper introduces the concept of an adaptive data acquisition process, the Selective Matrix Capture (SMC), which can adapt, dynamically, to specific imaging requirements for efficient ultrasonic imaging. SMC is realised experimentally using Laser Induced Phased Arrays (LIPAs), that use lasers to generate and detect ultrasound. The flexibility and reconfigurability of LIPAs enable the evolution of the array configuration, on-the-fly. The SMC methodology consists of two stages: a stage for detecting and localising regions of interest, by means of iteratively synthesising a sparse array, and a second stage for array optimisation to the region of interest. The delay-and-sum is used as the imaging algorithm and the experimental results are compared to images produced using the complete generation-detection data set. It is shown that SMC, without a priori knowledge of the test sample, is able to achieve comparable results, while preforming ∼ 10 times faster data acquisition and achieving ∼ 10 times reduction in data size.

A Comprehensive Review on Photoacoustic-Based Devices for Biomedical Applications

The photoacoustic effect is an emerging technology that has sparked significant interest in the research field since an acoustic wave can be produced simply by the incidence of light on a material or tissue. This phenomenon has been extensively investigated, not only to perform photoacoustic imaging but also to develop highly miniaturized ultrasound probes that can provide biologically meaningful information. Therefore, this review aims to outline the materials and their fabrication process that can be employed as photoacoustic targets, both biological and non-biological, and report the main components' features to achieve a certain performance. When designing a device, it is of utmost importance to model it at an early stage for a deeper understanding and to ease the optimization process. As such, throughout this article, the different methods already implemented to model the photoacoustic effect are introduced, as well as the advantages and drawbacks inherent in each approach. However, some remaining challenges are still faced when developing such a system regarding its fabrication, modeling, and characterization, which are also discussed.

CuInS2 Quantum Dot and Polydimethylsiloxane Nanocomposites for All-Optical Ultrasound and Photoacoustic Imaging

Dual-modality imaging employing complementary modalities, such as all-optical ultrasound and photoacoustic imaging, is emerging as a well-suited technique for guiding minimally invasive surgical procedures. Quantum dots are a promising material for use in these dual-modality imaging devices as they can provide wavelength-selective optical absorption. The first quantum dot nanocomposite engineered for co-registered laser-generated ultrasound and photoacoustic imaging is presented. The nanocomposites developed, comprising CuInS2 quantum dots and medical-grade polydimethylsiloxane (CIS-PDMS), are applied onto the distal ends of miniature optical fibers. The films exhibit wavelength-selective optical properties, with high optical absorption (> 90%) at 532 nm for ultrasound generation, and low optical absorption (< 5%) at near-infrared wavelengths greater than 700 nm. Under pulsed laser irradiation, the CIS-PDMS films generate ultrasound with pressures exceeding 3.5 MPa, with a corresponding bandwidth of 18 MHz. An ultrasound transducer is fabricated by pairing the coated optical fiber with a Fabry-Pérot (FP) fiber optic sensor. The wavelength-selective nature of the film is exploited to enable co-registered all-optical ultrasound and photoacoustic imaging of an ink-filled tube phantom. This work demonstrates the potential for quantum dots as wavelength-selective absorbers for all-optical ultrasound generation.

Freehand and video-rate all-optical ultrasound imaging

All-optical ultrasound (AOUS) imaging, which uses light to both generate and detect ultrasound, is an emerging alternative to conventional electronic ultrasound imaging. To date, AOUS imaging has been performed using paradigms that either resulted in long acquisition times or employed bench-top imaging systems that were impractical for clinical use. In this work, we present a novel AOUS imaging paradigm where scanning optics are used to rapidly synthesise an imaging aperture. This paradigm enabled the first AOUS system with a flexible, handheld imaging probe, which represents a critical step towards clinical translation. This probe, which provides video-rate imaging and a real-time display, is demonstrated with phantoms and in vivo human tissue.

Broadband All-Optical Plane-Wave Ultrasound Imaging System Based on a Fabry-Perot Scanner

A broadband all-optical plane-wave ultrasound imaging system for high-resolution 3-D imaging of biological tissues is presented. The system is based on a planar Fabry-Perot (FP) scanner for ultrasound detection and the photoacoustic generation of ultrasound in a carbon-nanotube-polydimethylsiloxane (CNT-PDMS) composite film. The FP sensor head was coated with the CNT-PDMS film which acts as an ultrasound transmitting layer for pulse-echo imaging. Exciting the CNT-PDMS coating with nanosecond laser pulses generated monopolar plane-wave ultrasound pulses with MPa-range peak pressures and a -6-dB bandwidth of 22 MHz, which were transmitted into the target. The resulting scattered acoustic field was detected across a 15 mm ×15 mm scan area with a step size of 100 [Formula: see text] and an optically defined element size of [Formula: see text]. The -3-dB bandwidth of the sensor was 30 MHz. A 3-D image of the scatterer distribution was then recovered using a k -space reconstruction algorithm. To obtain a measure of spatial resolution, the instrument line-spread function (LSF) was measured as a function of position. At the center of the scan area, the depth-dependent lateral LSF ranged from 46 to 65 [Formula: see text] for depths between 1 and 12 mm. The vertical LSF was independent of position and measured to be [Formula: see text] over the entire field of view. To demonstrate the ability of the system to provide high-resolution 3-D images, phantoms with well-defined scattering structures of arbitrary geometry were imaged. To demonstrate its suitability for imaging biological tissues, phantoms with similar impedance mismatches, sound speed and scattering properties to those present in the tissue, and ex vivo tissue samples were imaged. Compared with conventional piezoelectric-based ultrasound scanners, this approach offers the potential for improved image quality and higher resolution for superficial tissue imaging. Since the FP scanner is capable of high-resolution 3-D photoacoustic imaging of in vivo biological tissues, the system could ultimately be developed into an instrument for dual-mode all-optical ultrasound and photoacoustic imaging.

Photoacoustic imaging with fiber optic technology: A review

Photoacoustic imaging (PAI) has achieved remarkable growth in the past few decades since it takes advantage of both optical and ultrasound (US) imaging. In order to better promote the wide clinical applications of PAI, many miniaturized and portable PAI systems have recently been proposed. Most of these systems utilize fiber optic technologies. Here, we overview the fiber optic technologies used in PAI. This paper discusses three different fiber optic technologies: fiber optic light transmission, fiber optic US transmission, and fiber optic US detection. These fiber optic technologies are analyzed in different PAI modalities including photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and minimally invasive photoacoustic imaging (MIPAI).

Candle Soot Carbon Nanoparticles in Photoacoustics: Advantages and Challenges for Laser Ultrasound Transmitters

This manuscript provides a review of candle-soot nanoparticle (CSNP) composite laser ultrasound transmitters (LUT), and compares and contrasts this technology to other carboncomposite designs. Among many carbon-based composite LUTs, a CSNP composite has shown its advantages of maximum energy conversion and fabrication simplicity for developing highly efficient ultrasound transmitters. This review focuses on the advantages and challenges of the CSNP-composite transmitter in the aspects of nanostructure design, fabrication procedure, and promising applications. Included are a brief description of the basic principles of the laser ultrasound transmitter, a review of general properties of CSNPs, as well as details on the fabrication method, photoacoustic performance, and design factors. A comparison of the CSNP-nanocomposite to other carbon-nanocomposites is provided. Lastly, representative applications of carbon-nanocomposite transmitters and future perspectives on CSNP-composite transmitters are presented.

Side-Polished Fiber-Optic Line Sensor for High-Frequency Broadband Ultrasound Detection

We demonstrate a side-polished fiber-optic ultrasound sensor (SPFS) with a broad frequency bandwidth (dc⁻46 MHz at 6-dB reduction) and a wide amplitude detection range from several kPa to 4.8 MPa. It also exhibits a high acoustic sensitivity of 426 mV/MPa with a signal-to-noise ratio of 35 dB and a noise-equivalent pressure of 6.6 kPa (over 1⁻50 MHz bandwidth) measured at 7-MHz frequency. The SPFS does not require multi-layer-coated structures that are used in other high-sensitivity optical detectors. Without any coating, this uses a microscale-roughened structure for evanescent-field interaction with an external medium acoustically modulated. Such unique structure allows significantly high sensitivity despite having a small detection area of only 0.016 mm² as a narrow line sensor with a width of 8 μm. The SPFS performance is characterized in terms of acoustic frequency, amplitude responses, and sensitivities that are compared with those of a 1-mm diameter piezoelectric hydrophone used as a reference.

Video-rate all-optical ultrasound imaging

All-optical ultrasound imaging, where ultrasound is generated and detected using light, has recently been demonstrated as a viable modality that is inherently insensitive to electromagnetic interference and exhibits wide bandwidths. High-quality 2D and 3D all-optical ultrasound images of tissues have previously been presented; however, to date, long acquisition times (ranging from minutes to hours) have hindered clinical application. Here, we present the first all-optical ultrasound imaging system capable of video-rate, real-time two-dimensional imaging of biological tissue. This was achieved using a spatially extended nano-composite optical ultrasound generator, a highly sensitive fibre-optic acoustic receiver, and eccentric illumination resulting in an acoustic source exhibiting optimal directivity. This source was scanned across a one-dimensional source aperture using a fast galvo mirror, thus enabling the dynamic synthesis of source arrays comprising spatially overlapping sources at non-uniform source separation distances. The resulting system achieved a sustained frame rate of 15 Hz, a dynamic range of 30 dB, a penetration depth of at least 6 mm, a resolution of 75 µm (axial) by 100 µm (lateral), and enabled the dynamics of a pulsating ex vivo carotid artery to be captured.

Optical Ultrasound Generation and Detection for Intravascular Imaging: A Review

Combined ultrasound and photoacoustic imaging has attracted significant interests for intravascular imaging such as atheromatous plaque detection, with ultrasound imaging providing spatial location and morphology and photoacoustic imaging highlighting molecular composition of the plaque. Conventional ultrasound imaging systems utilize piezoelectric ultrasound transducers, which suffer from limited frequency bandwidths and reduced sensitivity with miniature transducer elements. Recent advances on optical methods for both ultrasound generation and detection have shown great promise, as they provide efficient and ultrabroadband ultrasound generation and sensitive and ultrabroadband ultrasound detection. As such, all-optical ultrasound imaging has a great potential to become a next generation ultrasound imaging method. In this paper, we review recent developments on optical ultrasound transmitters, detectors, and all-optical ultrasound imaging systems, with a particular focus on fiber-based probes for intravascular imaging. We further discuss our thoughts on future directions on developing combined all-optical photoacoustic and ultrasound imaging systems for intravascular imaging.

Label-free photoacoustic microscopy for in-vivo tendon imaging using a fiber-based pulse laser

Tendons are tough, flexible, and ubiquitous tissues that connect muscle to bone. Tendon injuries are a common musculoskeletal injury, which affect 7% of all patients and are involved in up to 50% of sports-related injuries in the United States. Various imaging modalities are used to evaluate tendons, and both magnetic resonance imaging and sonography are used clinically to evaluate tendons with non-invasive and non-ionizing radiation. However, these modalities cannot provide 3-dimensional (3D) structural images and are limited by angle dependency. In addition, anisotropy is an artifact that is unique to the musculoskeletal system. Thus, great care should be taken during tendon imaging. The present study evaluated a functional photoacoustic microscopy system for in-vivo tendon imaging without labeling. Tendons have a higher density of type 1 collagen in a cross-linked triple-helical formation (65–80% dry-weight collagen and 1–2% elastin in a proteoglycan-water matrix) than other tissues, which provides clear endogenous absorption contrast in the near-infrared spectrum. Therefore, photoacoustic imaging with a high sensitivity to absorption contrast is a powerful tool for label-free imaging of tendons. A pulsed near-infrared fiber-based laser with a centered wavelength of 780 nm was used for the imaging, and this system successfully provided a 3D image of mouse tendons with a wide field of view (5 × 5 mm²).