All-optical highly sensitive akinetic sensor for ultrasound detection and photoacoustic imaging

Technical note: Application of an optical hydrophone to ionoacoustic range detection in a tissue-mimicking agar phantom

BACKGROUND

Ionoacoustics is a promising approach to reduce the range uncertainty in proton therapy. A miniature-sized optical hydrophone (OH) was used as a measuring device to detect weak ionoacoustic signals with a high signal-to-noise ratio in water. However, further development is necessary to prevent wave distortion because of nearby acoustic impedance discontinuities while detection is conducted on the patient's skin.

PURPOSE

A prototype of the probe head attached to an OH was fabricated and the required dimensions were experimentally investigated using a 100-MeV proton beam from a fixed-field alternating gradient accelerator and k-Wave simulations. The beam range of the proton in a tissue-mimicking phantom was estimated by measuring γ-waves and spherical ionoacoustic waves with resonant frequency (SPIRE).

METHODS

Four sizes of probe heads were fabricated from agar blocks for the OH. Using the prototype, the γ-wave was detected at distal and lateral positions to the Bragg peak on the phantom surface for proton beams delivered at seven positions. For SPIRE, independent measurements were performed at distal on- and off-axis positions. The range positions were estimated by solving the linear equation using the sensitive matrix for the γ-wave and linear fitting of the correlation curve for SPIRE; they were compared with those measured using a film.

RESULTS

The first peak of the γ-wave was undistorted with the 3 × 3 × 3-cm3 probe head used at the on-axis and 3-cm off-axis positions. The range positions estimated by the γ-wave agreed with the film-based range in the depth direction (the maximum deviation was 0.7 mm), although a 0.6-2.1 mm deviation was observed in the lateral direction. For SPIRE, the deviation was <1 mm for the two measurement positions.

CONCLUSIONS

The attachment of a relatively small-sized probe head allowed the OH to measure the beam range on the phantom surface.

Alternative Approach to Design and Optimization of High-Q Ring Resonators for Membrane-Free Acoustic Sensors

Membrane-free acoustic sensors based on new principle and structure are becoming a research hotspot, because of many advantages, e.g., their wide bandwidth and high sensitivity. It is proposed that a membrane-free acoustic sensor employs a semi-buried optical waveguide ring resonator (SOWRR) as a sensing element. Using air as the upper cladding medium, the excited evanescent field in the air cladding medium would be modulated by acoustic wave. On this basis, the acoustic sensing model is established. Taking high Q factor and resonance depth as design criteria, the optimal design parameters are given. The optimal values of the air/SiO2: Ge/SiO2 waveguide resonator length and coupling spacing are obtained as 50 mm and 5.6 μm, respectively. The Q factor of the waveguide resonator of this size is as high as 8.33 × 106. The theoretical simulation indicates that the frequency response ranges from 1 Hz to 1.58 MHz and that the minimum detectable sound pressure is 7.48 µPa using a laser with linewidth of 1 kHz. Because of its advantages of wide bandwidth and high sensitivity, the membrane-free sensor is expected to become one of the most promising candidates for the next-generation acoustic sensor.

Sensitivity-enhanced Fabry-Perot interferometric fiber-optic microphone using hollow cantilever

Transducer components are crucial in optimizing the sensitivity of microphones. Cantilever structure is commonly used as a structural optimization technique. Here, we present a novel Fabry-Perot (F-P) interferometric fiber-optic microphone (FOM) using a hollow cantilever structure. The proposed hollow cantilever aims to reduce the effective mass and spring constant of the cantilever, thereby enhancing the sensitivity of the FOM. Experimental results demonstrate that the proposed structure outperforms the original cantilever design in terms of sensitivity. The sensitivity and minimum detectable acoustic pressure level (MDP) can reach 91.40 mV/Pa and 6.20 µPa/Hz at 1.7 kHz, respectively. Notably, the hollow cantilever provides an optimization framework for highly sensitive FOMs.

Measurement of the Acoustic Relaxation Absorption Spectrum of CO2 Using a Distributed Bragg Reflector Fiber Laser

Reconstruction of the acoustic relaxation absorption curve is a powerful approach to ultrasonic gas sensing, but it requires knowledge of a series of ultrasonic absorptions at various frequencies around the effective relaxation frequency. An ultrasonic transducer is the most widely deployed sensor for ultrasonic wave propagation measurement and works only at a fixed frequency or in a specific environment like water, so a large number of ultrasonic transducers operating at various frequencies are required to recover an acoustic absorption curve with a relative large bandwidth, which cannot suit large-scale practical applications. This paper proposes a wideband ultrasonic sensor using a distributed Bragg reflector (DBR) fiber laser for gas concentration detection through acoustic relaxation absorption curve reconstruction. With a relative wide and flat frequency response, the DBR fiber laser sensor measures and restores a full acoustic relaxation absorption spectrum of CO₂ using a decompression gas chamber between 0.1 and 1 atm to accommodate the main molecular relaxation processes, and interrogates with a non-equilibrium Mach-Zehnder interferometer (NE-MZI) to gain a sound pressure sensitivity of -45.4 dB. The measurement error of the acoustic relaxation absorption spectrum is less than 1.32%.

Absolute Grüneisen parameter measurement in deep tissue based on X-ray-induced acoustic computed tomography

The Grüneisen parameter is a primary parameter of the initial sound pressure signal in the photoacoustic effect, which can provide unique biological information and is related to the temperature change information of an object. The accurate measurement of this parameter is of great significance in biomedical research. Combining X-ray-induced acoustic tomography and conventional X-ray computed tomography, we proposed a method to obtain the absolute Grüneisen parameter. The theory development, numerical simulation, and biomedical application scenarios are discussed. The results reveal that our method not only can determine the Grüneisen parameter but can also obtain the body internal temperature distribution, presenting its potential in the diagnosis of a broad range of diseases.

Non-contact detection of ultrasound with light - Review of recent progress

In this article, we present an overview of recent progress in non-contact remote optical detection of ultrasound in application to nondestructive testing and evaluation of materials. The focus of the review is on the latest advances in optical detection that offer mature and robust field-applicable solutions for diagnostics and imaging of engineered structures. We provide a detailed description of these solutions, including their operation principles, their evolution from the previously known designs to commercial devices, and their contribution to solving the most important problems associated with non-contact optical detection of ultrasound. Several application examples are presented to demonstrate the capabilities of optical detection and provide ideas to a reader on how it can be used in practice. We also discuss the main challenges of modern non-contact detectors which have not yet been addressed, as well as the directions and prospects for their development.

Broadband Air-Coupled Ultrasound Emitter and Receiver Enable Simultaneous Measurement of Thickness and Speed of Sound in Solids

Air-coupled ultrasound sensors have advantages over contact ultrasound sensors when a sample should not become contaminated or influenced by the couplant or the measurement has to be a fast and automated inline process. Thereby, air-coupled transducers must emit high-energy pulses due to the low air-to-solid power transmission ratios (10^-3 to 10^-8). Currently used resonant transducers trade bandwidth-a prerequisite for material parameter analysis-against pulse energy. Here we show that a combination of a non-resonant ultrasound emitter and a non-resonant detector enables the generation and detection of pulses that are both high in amplitude (130 dB) and bandwidth (2 µs pulse width). We further show an initial application: the detection of reflections inside of a carbon fiber reinforced plastic plate with thicknesses between 1.7 mm and 10 mm. As the sensors work contact-free, the time of flight and the period of the in-plate reflections are independent parameters. Hence, a variation of ultrasound velocity is distinguishable from a variation of plate thickness and both properties are determined simultaneously. The sensor combination is likely to find numerous industrial applications necessitating high automation capacity and opens possibilities for air-coupled, single-side ultrasonic inspection.

Ionoacoustic application of an optical hydrophone to detect proton beam range in water

BACKGROUND

Proton range uncertainty has been the main factor limiting the ability of proton therapy to concentrate doses to tumors to their full potential. Ionoacoustic (IA) range verification is an approach to reducing this uncertainty by detecting thermoacoustic waves emitted from an irradiated volume immediately following a pulsed proton beam delivery; however, the signal weakness has been an obstacle to its clinical application. To increase the signal-to-noise ratio (SNR) with the conventional piezoelectric hydrophone (PH), the detector-sensitive volume needs to be large, but it could narrow the range of available beam angles and disturb real-time images obtained during beam delivery.

PURPOSE

To prevent this issue, we investigated a millimeter-sized optical hydrophone (OH) that exploits the laser interferometric principle. For two types of IA waves [γ-wave emitted from the Bragg peak (BP) and a spherical IA wave with resonant frequency (SPIRE) emitted from the gold fiducial marker (GM)], comparisons were made with PH in terms of waveforms, SNR, range detection accuracy, and signal intensity robustness against the small detector misalignment, particularly for SPIRE.

METHODS

A 100-MeV proton beam with a 27 ns pulse width and 4 mm beam size was produced using a fixed-field alternating gradient accelerator and was irradiated to the water phantom. The GM was set on the beam's central axis. Acrylic plates of various thicknesses, up to 12 mm, were set in front of the phantoms to shift the proton range. OH was set distal and lateral to the beam, and the range was estimated using the time-of-flight method for γ-wave and by comparing with the calibration data (SPIRE intensity versus the distance between the GM and BP) derived from an IA wave transport simulation for SPIRE. The BP dose per pulse was 0.5-0.6 Gy. To measure the variation in SPIRE amplitude against the hydrophone misalignment, the hydrophone was shifted by ± 2 mm at a maximum in lateral directions.

RESULTS

Despite its small size, OH could detect γ-wave with a higher SNR than the conventional PH (diameter, 29 mm), and a single measurement was sufficient to detect the beam range with a submillimeter accuracy in water. In the SPIRE measurement, OH was far more robust against the detector misalignment than the focused PH (FPH) used in our previous study [5%/mm (OH) versus 80%/mm (FPH)], and the correlation between the measured SPIRE intensity and the distance between the GM and BP agreed well with the simulation results. However, the OH sensitivity was lower than the FPH sensitivity, and about 5.6-Gy dose was required to decrease the intensity variation among measurements to less than 10%.

CONCLUSION

The miniature OH was found to detect weak IA signals produced by proton beams with a BP dose used in hypofractionated regimens. The OH sensitivity improvement at the MHz regime is worth exploring as the next step.

All-optical ultrasonic detector based on differential interference

We report on an all-optical ultrasonic detecting method based on differential interference. A linearly polarized probe beam is split into two closely separated ones with orthogonal polarization. After interacting with propagating ultrasonic waves in a coupling media, the split beams are recombined into one beam, with its polarization being changed into an elliptical one by the elastic-optical effect. The recombined beam is filtered by an analyzer and detected by a photodetector. The bandwidth and noise-equivalent pressure (NEP) of the acoustic detector are determined to be 107.4 MHz and 2.18 kPa, respectively. We also demonstrate its feasibility for photoacoustic microscopy (PAM) using agar-embedded phantoms.

Ultrasound sensing at thermomechanical limits with optomechanical buckled-dome microcavities

We describe the use of monolithic, buckled-dome cavities as ultrasound sensors. Patterned delamination within a compressively stressed thin film stack produces high-finesse plano-concave optical resonators with sealed and empty cavity regions. The buckled mirror also functions as a flexible membrane, highly responsive to changes in external pressure. Owing to their efficient opto-acousto-mechanical coupling, thermal-displacement-noise limited sensitivity is achieved at low optical interrogation powers and for modest optical (Q ∼ 10³) and mechanical (Q ∼ 10²) quality factors. We predict and verify broadband (up to ∼ 5 MHz), air-coupled ultrasound detection with noise-equivalent pressure (NEP) as low as ∼ 30-100 µPa/Hz^1/2. This corresponds to an ultrasonic force sensitivity ∼ 2 × 10^-13 N/Hz^1/2 and enables the detection of MHz-range signals propagated over distances as large as ∼ 20 cm in air. In water, thermal-noise-limited sensitivity is demonstrated over a wide frequency range (up to ∼ 30 MHz), with NEP as low as ∼ 100-800 µPa/Hz^1/2. These cavities exhibit a nearly omnidirectional response, while being ∼ 3-4 orders of magnitude more sensitive than piezoelectric devices of similar size. Easily realized as large arrays and naturally suited to direct coupling by free-space beams or optical fibers, they offer significant practical advantages over competing optical devices, and thus could be of interest for several emerging applications in medical and industrial ultrasound imaging.

Multi-channel parallel ultrasound detection based on a photothermal tunable fiber optic sensor array

A multi-channel parallel ultrasound detection system based on a photothermal tunable fiber optic sensor array is proposed. The resonant wavelength of the ultrasound sensor has a quadratic relationship with the power of a 980-nm heating laser. The maximum tuning range is larger than 15 nm. Through photothermal tuning, the inconsistent operating wavelengths of the Fabry-Perot (FP) sensor array can be solved, and then a multiplexing capacity of up to 53 can be theoretically realized, which could greatly reduce the time required for data acquisition. Then, a fixed wavelength laser with ultra-narrow linewidth is used to interrogate the sensor array. The interrogation system demonstrates a noise equivalent pressure (NEP) as low as 0.12 kPa, which is 5.5-times lower than the commercial hydrophone. Furthermore, a prototype of a four-channel ultrasound detection system is built to demonstrate the parallel detection capability. Compared with the independent detection, the SNR of parallel detection does not deteriorate, proving that the parallel detection system and the sensor array own very low cross talk characteristics. The parallel detection technique paves a way for real-time photoacoustic/ultrasound imaging.

Research on high-temperature characteristics of a miniature Fabry-Pérot cavity acoustic sensor

The applications of fiber-optic acoustic sensors are expanded to the high-temperature field, but it still faces challenges to realize the wide-band and high-sensitivity acoustic signal detection in high-temperature environments. Here, we propose a miniature membrane-free fiber-optic acoustic sensor based on a rigid Fabry-Pérot (F-P) cavity and construct an acoustic signal detection system. The system can achieve high-sensitivity acoustic detection while maintaining a wide frequency band in temperatures ranging from 20 °C to 200 °C. The prepared F-P cavity based on optical contact technology is the sensitive unit of the sensor, and has a high-quality factor of 8.8×10⁵. Specifically, with the increasing of temperature, the sensitivity gradually increases, and the frequency response range does not change. A maximum sensitivity of 491.2 mV/Pa and a high signal-to-noise ratio of 60.9 dB are achieved at 200 °C. The sensor has an excellent acoustic signal response in the frequency range of 10 Hz-50 kHz with a flatness of ±2 dB. This study is important for the application of the fiber-optic acoustic sensor in high-temperature environments.

Microacoustic Metagratings at Ultra-High Frequencies Fabricated by Two-Photon Lithography

The recently proposed bianisotropic acoustic metagratings offer promising opportunities for passive acoustic wavefront manipulation, which is of particular interest in flat acoustic lenses and ultrasound imaging at ultra-high frequency ultrasound. Despite this fact, acoustic metagratings have never been scaled to MHz frequencies that are common in ultrasound imaging. One of the greatest challenges is the production of complex microscopic structures. Owing to two-photon polymerization, a novel fabrication technique from the view of acoustic metamaterials, it is now possible to precisely manufacture sub-wavelength structures in this frequency range. However, shrinking in size poses another challenge; the increasing thermoviscous effects lead to a drop in efficiency and a frequency downshift of the transmission peak and must therefore be taken into account in the design. In this work three microacoustic metagrating designs refracting a normally incident wave toward -35° at 2 MHz are proposed. In order to develop meta-atoms insensitive to thermoviscous effects shape optimization techniques incorporating the linearized Navier-Stokes equations discretized with finite element method are used. The authors report for the first time microscopic acoustic metamaterials manufactured using two-photon polymerization and, subsequently, experimentally verify their effectiveness using an optical microphone as a detector in a range from 1.8 to 2.2 MHz.

Broadband High-Frequency Ultrasonic Transducer Based Functional Photoacoustic Mesoscopy for Psoriasis Progression

In vivo imaging of skin is commonly used to investigate dynamic processes in the progression and treatment of psoriasis. Photoacoustic mesoscopy is a new non-invasive imaging modality widely used in bio-imaging, and has recently been applied to imaging skin in vivo. However, photoacoustic imaging has shortcomings. Although high-frequency ultrasonic transducers enable high-resolution photoacoustic imaging, the images may be bandwidth-limited. To overcome this limitation, we designed and fabricated a broadband ultrasonic transducer for photoacoustic mesoscopy. The center frequency of the transducer was 32 MHz (88% bandwidth at -6 dB). The transducer was used to visualize mouse and human skin morphology. Colocalization of high- and low-frequency components revealed information about both the skin surface and dermis. To explore dynamic structural changes in mouse back skin during psoriasis progression, we measured blood oxygen saturation and total hemoglobin in a mouse model using multiwavelength imaging without contrast agents. The results indicate that functional photoacoustic mesoscopy using a broadband high-frequency transducer has great potential for clinical imaging of skin disease.

Performance enhancement of polyurethane foam applied to optical fiber microphones

The application of polyurethane foam to optical fiber microphone sensitization is proposed. In this experiment, the Michelson interference system is used, and polyurethane foam is coated on the optical fiber of the signal arm. By changing the optical fiber material of the signal arm and the reference arm, four sets of comparative experiments are designed to test the sensitivity of the optical microphone. Through a scanning electron microscope (SEM) test and a pore size distribution test, the porous structure and non-closed cell structure characteristics, pore size range, etc., of the polyurethane foam were determined. The average sound absorption coefficient of the polyurethane foam is 0.66 through the sound absorption coefficient and sound insulation test. The sound absorption coefficient of each frequency band is above 0.2, the sound insulation is below 30 dB, and the overall sound insulation performance is poor, which can be regarded as an ideal sound absorption material. The sound-absorbing effect of polyurethane foam is better than that of nylon tight-packed materials in the frequency range of 500-2800 Hz, and it has a significant sensitization effect in this frequency band. In the frequency band above 2800 Hz, the sound-absorbing effect of nylon tight-packed optical fiber is better than that of polyurethane foam, and the optimal combination is determined by lateral comparison as signal arm (nylon tight-packed fiber + polyurethane foam) + reference arm (bare fiber). Finally, with different coating thicknesses as variables, the results show that the optical fiber microphone has the best sound collection effect when the coating thickness of polyurethane foam is 1.5 cm.

Advances in Endoscopic Photoacoustic Imaging

Photoacoustic (PA) imaging is able to provide extremely high molecular contrast while maintaining the superior imaging depth of ultrasound (US) imaging. Conventional microscopic PA imaging has limited access to deeper tissue due to strong light scattering and attenuation. Endoscopic PA technology enables direct delivery of excitation light into the interior of a hollow organ or cavity of the body for functional and molecular PA imaging of target tissue. Various endoscopic PA probes have been developed for different applications, including the intravascular imaging of lipids in atherosclerotic plaque and endoscopic imaging of colon cancer. In this paper, the authors review representative probe configurations and corresponding preclinical applications. In addition, the potential challenges and future directions of endoscopic PA imaging are discussed.

Acoustic Performance Study of Fiber-Optic Acoustic Sensors Based on Fabry–Pérot Etalons with Different Q Factors

The ideal development direction of the fiber-optic acoustic sensor (FOAS) is toward broadband, a high sensitivity and a large dynamic range. In order to further promote the acoustic detection potential of the Fabry–Pérot etalon (FPE)-based FOAS, it is of great significance to study the acoustic performance of the FOAS with the quality (Q) factor of FPE as the research objective. This is because the Q factor represents the storage capability and loss characteristic of the FPE. The three FOASs with different Q factors all achieve a broadband response from 20 Hz to 70 kHz with a flatness of ±2 dB, which is consistent with the theory that the frequency response of the FOAS is not affected by the Q factor. Moreover, the sensitivity of the FOAS is proportional to the Q factor. When the Q factor is 1.04×106, the sensitivity of the FOAS is as high as 526.8 mV/Pa. Meanwhile, the minimum detectable sound pressure of 347.33 μPa/Hz1/2  is achieved. Furthermore, with a Q factor of 0.27×106, the maximum detectable sound pressure and dynamic range are 152.32 dB and 107.2 dB, respectively, which is greatly improved compared with two other FOASs. Separately, the FOASs with different Q factors exhibit an excellent acoustic performance in weak sound detection and high sound pressure detection. Therefore, different acoustic detection requirements can be met by selecting the appropriate Q factor, which further broadens the application range and detection potential of FOASs.

An Optical Acoustic Detection System Based on Fabry Pérot Etalon Stability Structure

The optical acoustic detection system based on the Fabry Pérot Etalon (FPE) with high quality–factor (High Q) and stability structure is described and tested. The FPE contains two high–reflectivity Plano–Concave lenses, achieving high fineness and stability. The protective structure of the confocal stabilized FPE is composed of an invar tube, copper sheath, Bakelite sheath and aluminum housing to protect the sensor from the effects of ambient temperature and vibration. The audio signal is injected into the cavity through the sound hole located in the center of the cavity. Acoustic waves induce the vibration of the medium in the cavity, which leads to a simultaneous change in the FPE optical path and a shift of the interference spectrum. The acoustic detection system is built, and the frequency of the laser is locked on the resonant frequency points of the FPE by using phase modulation technology, so as to detect acoustic signals of different frequencies and amplitudes. In addition, the sensitivity of the proposed sensor exceeds 34.49 mV/Pa in the range of 20 Hz–20 kHz. A Signal-to-Noise Ratio (SNR) of 37 dB can be achieved at 20 Hz. Acoustic signal detection technology based on the FPE stability model is used to test the theoretical feasibility of the future high sensitivity Fabry Pérot Interferometric (FPI) acoustic sensors.

A backward-mode optical-resolution photoacoustic microscope for 3D imaging using a planar Fabry-Pérot sensor

Optical-resolution photoacoustic microscopy (OR-PAM) combines high spatial resolution and strong absorption-based contrast in tissue, which has enabled structural and spectroscopic imaging of endogenous chromophores, primarily hemoglobin. Conventional piezoelectric ultrasound transducers are typically placed far away from the photoacoustic source due to their opacity, which reduces acoustic sensitivity. Optical ultrasound sensors are an alternative as their transparency allows them to be positioned close to the sample with minimal source-detector distances. In this work, a backward-mode OR-PAM system based on a planar Fabry-Pérot ultrasound sensor and coaxially aligned excitation and interrogation beams was developed. Two 3D imaging modes, using raster-scanning for enhanced image quality and continuous-scanning for fast imaging, were implemented and tested on a leaf skeleton phantom. In fast imaging mode, a scan-rate of 100,000 A-lines/s was achieved. 3D images of a zebrafish embryo were acquired in vivo in raster-scanning mode. The transparency of the FP sensor in the visible and near-infrared wavelength region makes it suitable for combined functional and molecular imaging applications using OR-PAM and multi-photon fluorescence microscopy.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

Depixelation of coherent fiber bundle imaging by fiber-core-targeted scanning

A novel fast proximal scanning method, to the best of our knowledge, termed fiber-core-targeted scanning (FCTS), is proposed for illuminating individual fiber cores sequentially to remove the pixelation effect in fiber bundle (FB) imaging. FCTS is based on a galvanometer scanning system. Through a dynamic control of the scan trajectory and speed using the prior knowledge of fiber core positions, FCTS experimentally verifies a precise sequential delivery of laser pulses into fiber cores at a maximal speed of 45,000 cores per second. By applying FCTS on a FB-based photoacoustic forward-imaging probe, the results demonstrate that FCTS eliminates the pixelation effect and improves the imaging quality.

Progress of clinical translation of handheld and semi-handheld photoacoustic imaging

Photoacoustic imaging (PAI), featuring rich contrast, high spatial/temporal resolution and deep penetration, is one of the fastest-growing biomedical imaging technology over the last decade. To date, numbers of handheld and semi-handheld photoacoustic imaging devices have been reported with corresponding potential clinical applications. Here, we summarize emerged handheld and semi-handheld systems in terms of photoacoustic computed tomography (PACT), optoacoustic mesoscopy (OAMes), and photoacoustic microscopy (PAM). We will discuss each modality in three aspects: laser delivery, scanning protocol, and acoustic detection. Besides new technical developments, we also review the associated clinical studies, and the advantages/disadvantages of these new techniques. In the end, we propose the challenges and perspectives of miniaturized PAI in the future.

Micro-fiber-optic acoustic sensor based on high-Q resonance effect using Fabry-Pérot etalon

A micro-fiber-optic acoustic sensor based on the high-quality-factor (high-Q) resonance effect that uses a Fabry-Pérot etalon (FPE) is presented in this study. The device has been demonstrated experimentally to be a high-sensitivity acoustic sensor with a large dynamic range over a wide frequency band. Optical contact technology was used to improve the robustness of the FPE, which consists of two parallel lenses with high reflectivity exceeding 99%. An acoustic signal detection system based on phase modulation spectrum technology was also constructed. A stable and high-Q value of 10⁶ was measured for the FPE. As a result, high sensitivity of 177.6 mV/Pa was achieved. Because of the change in the refractive index of the air when it is modulated by the acoustic waves, a frequency response of 20 Hz-70 kHz with flatness of ±2 dB was obtained and a large dynamic range of 115.3 dB was measured simultaneously. The excellent performance of the device will be beneficial for optical acoustic sensing.

REAP: revealing drug tolerant persister cells in cancer using contrast enhanced optical coherence and photoacoustic tomography

Despite chemotherapy, residual tumors often rely on so-called drug tolerant persister (DTP) cells, which evade treatment to give rise to therapy-resistant relapse and refractory disease. Detection of residual tumor cells proves to be challenging because of the rarity and heterogeneity of DTP cells. In the framework of a H2020 project, REAP will gather researchers and engineers from six countries, who will combine their expertise in biology, chemistry, oncology, material sciences, photonics, and electrical and biomedical engineering in the hope of revealing DTPs in cancer using contrast enhanced multimodal optical imaging. Laser sources for photoacoustic microscopy, photoacoustic tomography, and optical coherence tomography will be developed to enable the design of a two-photon laser scanning optical coherence photoacoustic microscopy system and an optical coherence photoacoustic tomography system. Furthermore, novel photoacoustic detectors using micro-ring resonator will be designed and fabricated, granting improved sensitivity and easier integration of multiple optical imaging modalities into a single system. Innovative algorithms will be developed to reconstruct and analyze the images quickly and automatically. With successful implementation of this four-year project, we can not only gain insight into the mechanisms governing DTPs, but also significantly advance the technology readiness level of contrast agents, lasers, sensors, and image analysis software through joint efforts.

Photoacoustic Imaging and Sensing: A New Way to See the Eye

Purpose: Photoacoustics (optoacoustics) is a hybrid technology utilizing light excitation of acoustic responses in targets of interest. It has found numerous applications in biomedicine, including eye research, because of its ability to report both morphological and functional data about the interrogated tissue. This presentation will give an overview of current applications. Methods: Wavelength-dependent absorption of light in a tissue chromophore causes local heating, leading to a thermoelastic expansion-contraction cycle. If nanosecond pulses of light are used to excite this process, the resulting pressure wave is an ultrasound signal propagating through the tissue and detectable at the tissue surface. This is highly advantageous because of the known properties of ultrasound propagation in tissue and the ability to use standard, medical ultrasound equipment for detection. The time of arrival and amplitude of ultrasound signals provide information about the location and nature of the absorber. Results: Due to the wavelength dependence of the photoacoustic response, functional and physiological applications are possible. For example, retinal oximetry can be determined from the different optical absorption properties of oxy- and deoxyhemoglobin. Multispectral imaging of the posterior segment can identify pigments such as melanin or lipofuscin or the nature of foreign bodies. The technique can be combined with other imaging modalities such as ultrasound and optical coherence tomography to produce high-resolution images of retinal structures along with functional information. Conclusion: Photoacoustic technology is a powerful noninvasive tool for ocular research and to study ocular morphology, fundamental physiological parameters, cellular responses, and molecular expression.

Thickness measurement via local ultrasonic resonance spectroscopy

Local ultrasonic resonance spectroscopy (LURS) is a new approach to material inspection, where the specimen is locally excited by a short mechanical impulse while its local mechanical response is recorded at a position nearby. The local material and geometrical properties can be extracted from the frequency spectrum of the response and visualized by performing a scan over the inspected area. In our experiment, the plate thickness and the reliefs of both plate surfaces (plate curvature) were obtained from thickness resonance and time of arrival analysis without physical contact to the specimen. Ultrasound was generated on the specimen surface by a laser pulse. Local mechanical response of a carbon fiber-reinforced polymer plate with a thickness ranging from 0.6 mm to 4.3 mm was recorded with a broadband optical microphone in through-transmission setup. The precision of this arrangement greatly exceeded the precision of conventional methods limited by the ultrasound wavelength. For thicknesses in the range around 1 mm, standard deviations of up to several µm were achieved. An influence of the through-plate ultrasound velocity on the measured relief of the plate surface nearest to the optical microphone was eliminated by a joint evaluation of thickness resonance and time of arrival. Furthermore, we demonstrated that internal delaminations have an influence on the spectrum of the local mechanical response and can therefore be detected by LURS.

Approaches to quantify optical coherence tomography angiography metrics

Optical coherence tomography (OCT) has revolutionized the field of ophthalmology in the last three decades. As an OCT extension, OCT angiography (OCTA) utilizes a fast OCT system to detect motion contrast in ocular tissue and provides a three-dimensional representation of the ocular vasculature in a non-invasive, dye-free manner. The first OCT machine equipped with OCTA function was approved by U.S. Food and Drug Administration in 2016 and now it is widely applied in clinics. To date, numerous methods have been developed to aid OCTA interpretation and quantification. In this review, we focused on the workflow of OCTA-based interpretation, beginning from the generation of the OCTA images using signal decorrelation, which we divided into intensity-based, phase-based and phasor-based methods. We further discussed methods used to address image artifacts that are commonly observed in clinical settings, to the algorithms for image enhancement, binarization, and OCTA metrics extraction. We believe a better grasp of these technical aspects of OCTA will enhance the understanding of the technology and its potential application in disease diagnosis and management. Moreover, future studies will also explore the use of ocular OCTA as a window to link ocular vasculature to the function of other organs such as the kidney and brain.

Overview of Ultrasound Detection Technologies for Photoacoustic Imaging

Ultrasound detection is one of the major components of photoacoustic imaging systems. Advancement in ultrasound transducer technology has a significant impact on the translation of photoacoustic imaging to the clinic. Here, we present an overview on various ultrasound transducer technologies including conventional piezoelectric and micromachined transducers, as well as optical ultrasound detection technology. We explain the core components of each technology, their working principle, and describe their manufacturing process. We then quantitatively compare their performance when they are used in the receive mode of a photoacoustic imaging system.

Photothermally tunable Fabry-Pérot fiber interferometer for photoacoustic mesoscopy

An optical fiber based Fabry-Pérot interferometer whose resonant wavelength can be dynamically tuned was designed and realized for photoacoustic mesoscopy. The optical path length (OPL) of the Fabry-Pérot cavity can be modulated by a photothermal heating process, which was achieved by adjusting the power of a 650 nm heating laser. The optical heating process can effectively change the thickness and refractive index of the polymer spacer of the sensor cavity. The robustness of the sensor can be greatly improved by proper packaging. The interferometer was interrogated by a relatively cheap wavelength-fixed 1550 nm laser for broadband and sensitive ultrasound detection, eliminating the requirement for an expensive tunable interrogation laser. The sensing module was then integrated into a photoacoustic mesoscopic imaging system. Two phantom imaging experiments and an ex vivo imaging experiment demonstrated the capability of such a miniature sensor. The interferometer has an acoustic detection bandwidth of up to 30 MHz and a noise equivalent pressure of 40 mPa/Hz^1/2 (i.e., 220 Pa over the full detection bandwidth). The new tuning mechanism and the batch-production compatibility of the sensor holds promises for commercialization and parallelized detection.

Functional optical coherence tomography and photoacoustic microscopy imaging for zebrafish larvae

We present a dual modality functional optical coherence tomography and photoacoustic microscopy (OCT-PAM) system. The photoacoustic modality employs an akinetic optical sensor with a large imaging window. This imaging window enables direct reflection mode operation, and a seamless integration of optical coherence tomography (OCT) as a second imaging modality. Functional extensions to the OCT-PAM system include Doppler OCT (DOCT) and spectroscopic PAM (sPAM). This functional and non-invasive imaging system is applied to image zebrafish larvae, demonstrating its capability to extract both morphological and hemodynamic parameters in vivo in small animals, which are essential and critical in preclinical imaging for physiological, pathophysiological and drug response studies.

Membrane-free acoustic sensing based on an optical fiber Mach-Zehnder interferometer

Traditional optical fiber acoustic sensors are mostly based on mechanical diaphragms and use indirect coupling between the acoustic and optical signals. The detectable frequency range and sound pressure range of such a sensor have limitations because they are influenced by the membrane or a mechanically deformable material. In this paper, a Mach-Zehnder interferometer-based membrane-free acoustic sensing method is developed. The sensing principle relies on direct detection of sound-pressure-induced changes of the refractive index in the open cavity. This enables an inherently flat frequency response over a broad bandwidth. Simulation and experiment were carried out to verify and demonstrate the idea. The results show that the membrane-free acoustic sensor has a flat frequency response from 500 Hz to 20 kHz.

Multimodal intravascular imaging technology for characterization of atherosclerosis

Early detection of vulnerable plaques is the critical step in the prevention of acute coronary events. Morphology, composition, and mechanical property of a coronary artery have been demonstrated to be the key characteristics for the identification of vulnerable plaques. Several intravascular multimodal imaging technologies providing co-registered simultaneous images have been developed and applied in clinical studies to improve the characterization of atherosclerosis. In this paper, the authors review the present system and probe designs of representative intravascular multimodal techniques. In addition, the scientific innovations, potential limitations, and future directions of these technologies are also discussed.

Simultaneous multi-channel ultrasound detection via phase modulated pulse interferometry

In optical detection of ultrasound, resonators with high Q-factors are often used to maximize sensitivity. However, in order to perform parallel interrogation, conventional interferometric techniques require an overlap between the spectra of all the resonators, which is difficult to achieve with high Q-factor resonators. In this paper, a new method is developed for parallel interrogation of optical resonators with non-overlapping spectra. The method is based on a phase-modulation scheme for pulse interferometry (PM-PI) and requires only a single photodetector and sampling channel per ultrasound detector. Using PM-PI, parallel ultrasound detection is demonstrated with four high Q-factor resonators.

PMN-PT/Epoxy 1-3 composite based ultrasonic transducer for dual-modality photoacoustic and ultrasound endoscopy

Endoscopic dual-modality photoacoustic (PA) and ultrasound (US) imaging has the capability of providing morphology and molecular information simultaneously. An ultrasonic transducer was applied for detecting PA signals and performing US imaging which determines the sensitivity and performance of a dual-modality PA/US system. In our study, a miniature single element 32-MHz lead magnesium niobate-lead titanate (PMN-PT) epoxy 1-3 composite based ultrasonic transducer was developed. A miniature endoscopic probe based on this transducer has been fabricated. Using the dual modality PA/US system with a PMN-PT/epoxy 1-3 composite based ultrasonic transducer, phantom and in vivo animal studies have been conducted to evaluate the performance. The preliminary results show enhanced bandwidths of the new ultrasonic transducer and improved signal-to-noise ratio of PA and US images of rat colorectal wall compared with PMN-PT and lead zirconate titanate (PZT) composite based ultrasonic transducers.

Ultrasound sensing based on an in-fiber dual-cavity Fabry-Perot interferometer

Highly sensitive and broadband ultrasound (tens of megahertz) detectors can extend the capabilities of photo-acoustic imaging in biomedical application and nondestructive test inspection. Here ultra-compact fiber-based multi-mode dual-cavity Fabry-Perot interferometer (DC-FPI) ultrasound sensors are proposed by splicing three sections of cleaved standard single-mode fibers with the fiber off-core cross section in the middle. The contrasts, reflectivity, and linewidths of the DC-FPIs have been comprehensively characterized. The broadband frequency responses, ranging from 5 kHz to 45.4 MHz, are demonstrated by using the high harmonics of a piezoelectric transducer centered at 3.7 MHz. In addition, the influences of different resonant modes in the DC-FPI on the ultrasound frequency response have been demonstrated. The high-frequency and broadband response of such a simple and cost-effective ultrasound device offers new opportunities to the industrial ultrasound-based and advanced biomedical applications.

Intensity Modulated Photothermal Measurements of NO2 with a Compact Fiber-Coupled Fabry-Pérot Interferometer

Sensors for the reliable measurement of nitrogen dioxide concentrations are of high interest due the adverse health effects of this pollutant. This work employs photothermal spectroscopy to measure nitrogen dioxide concentrations at the parts per billion level. Absorption induced temperature changes are detected by means of a fiber-coupled Fabry–Pérot interferometer. The small size of the interferometer enables small detection volumes, paving the way for miniaturized sensing concepts as well as fast response times, demonstrated down to 3 s. A normalized noise equivalent absorption of 7.5×10−8 cm⁻¹W/Hz is achieved. Additionally, due to the rigid structure of the interferometer, the sensitivity to mechanical vibrations is shown to be minor.

Three-wavelength passive demodulation technique for the interrogation of EFPI sensors with arbitrary cavity length

A three-wavelength passive demodulation technique to interrogate extrinsic Fabry-Perot interferometric (EFPI) sensors with arbitrary cavity length has been developed. The DC component is obtained online, then the applied dynamic signal is recovered by using three new signals without the DC component. The performance of the technique is demonstrated by simulations and experiments. The demodulation technique can extract dynamic signals, regardless of whether the phase modulation is larger than 2π. Theoretically, EFPI sensors with arbitrary cavity length can be demodulated by the demodulation technique, and EFPI sensors with cavity lengths in the 22.96-1002.3 μm range are detected successfully by the same demodulator in experiments. The technique is robust with respect to the bending loss of the leading fiber. The demodulation technique provides a robust and accurate solution to measure dynamic signals for EFPI sensors. It has the properties of high frequency, a large dynamic range, and high sensitivity. The paper demonstrates this technique's potential for measuring dynamic signals.

In-Fiber Collimator-Based Fabry-Perot Interferometer with Enhanced Vibration Sensitivity

A simple vibration sensor is proposed and demonstrated based on an optical fiber Fabry-Perot interferometer (FPI) with an in-fiber collimator. The device was fabricated by splicing a quarter-pitch graded index fiber (GIF) with a section of a hollow-core fiber (HCF) interposed between single mode fibers (SMFs). The static displacement sensitivity of the FPI with an in-fiber collimator was 5.17 × 10⁻⁴ μm⁻¹, whereas the maximum static displacement sensitivity of the device without collimator was 1.73 × 10⁻⁴ μm⁻¹. Moreover, the vibration sensitivity of the FPI with the collimator was 60.22 mV/g at 100 Hz, which was significantly higher than the sensitivity of the FPI without collimator (11.09 mV/g at 100 Hz). The proposed FPI with an in-fiber collimator also exhibited a vibration sensitivity nearly one order of magnitude higher than the device without the collimator at frequencies ranging from 40 to 200 Hz. This low-cost FPI sensor is highly-sensitive, robust and easy to fabricate. It could potentially be used for vibration monitoring in remote and harsh environments.

Precision ultrasound sensing on a chip

Ultrasound sensors have wide applications across science and technology. However, improved sensitivity is required for both miniaturisation and increased spatial resolution. Here, we introduce cavity optomechanical ultrasound sensing, where dual optical and mechanical resonances enhance the ultrasound signal. We achieve noise equivalent pressures of 8-300 μPa Hz-1/2 at kilohertz to megahertz frequencies in a microscale silicon-chip-based sensor with >120 dB dynamic range. The sensitivity far exceeds similar sensors that use an optical resonance alone and, normalised to the sensing area, surpasses previous air-coupled ultrasound sensors by several orders of magnitude. The noise floor is dominated by collisions from molecules in the gas within which the acoustic wave propagates. This approach to acoustic sensing could find applications ranging from biomedical diagnostics, to autonomous navigation, trace gas sensing, and scientific exploration of the metabolism-induced-vibrations of single cells.

Ultrasound detection via low-noise pulse interferometry using a free-space Fabry-Pérot

Coherence-restored pulse interferometry (CRPI) is a recently developed method for optical detection of ultrasound that achieves shot-noise-limited sensitivity and high dynamic range. In principle, the wideband source employed in CRPI may enable the interrogation of multiple detectors by using wavelength multiplexing. However, the noise-reduction scheme in CRPI has not been shown to be compatible with wideband operation. In this work, we introduce a new scheme for CRPI that relies on a free-space Fabry-Pérot filter for noise reduction and a pulse stretcher for reducing nonlinear effects. Using our scheme, we demonstrate that shot-noise-limited detection may be achieved for a spectral band of 80 nm and powers of up to 5 mW.

Looking at sound: optoacoustics with all-optical ultrasound detection

Originally developed for diagnostic ultrasound imaging, piezoelectric transducers are the most widespread technology employed in optoacoustic (photoacoustic) signal detection. However, the detection requirements of optoacoustic sensing and imaging differ from those of conventional ultrasonography and lead to specifications not sufficiently addressed by piezoelectric detectors. Consequently, interest has shifted to utilizing entirely optical methods for measuring optoacoustic waves. All-optical sound detectors yield a higher signal-to-noise ratio per unit area than piezoelectric detectors and feature wide detection bandwidths that may be more appropriate for optoacoustic applications, enabling several biomedical or industrial applications. Additionally, optical sensing of sound is less sensitive to electromagnetic noise, making it appropriate for a greater spectrum of environments. In this review, we categorize different methods of optical ultrasound detection and discuss key technology trends geared towards the development of all-optical optoacoustic systems. We also review application areas that are enabled by all-optical sound detectors, including interventional imaging, non-contact measurements, magnetoacoustics, and non-destructive testing.

Ultrasound Detection Using Acoustic Apertures

Ultrasound detection is commonly performed by piezoelectric transducers that are optimized for a specific application. Since the piezoelectric technology is not configurable, transducers designed for one application may not be compatible with other applications. In addition, some designs of ultrasound transducers may be difficult to implement owing to production constraints. In this paper, we propose a simple, low-cost method to reconfigure the geometry of ultrasound transducers. The technique is based on using apertures in thin sheets of acoustic blockers. We experimentally demonstrate this method for an ultrasound transducer with a central frequency of 1 MHz and show that it can emulate detectors of various sizes. An added advantage of this technique is its capability to achieve semi-isotropic detection sensitivity due to diffraction when the aperture size is comparable to the acoustic wavelength even when the angular sensitivity of the transducer is inherently limited.

Dual modality reflection mode optical coherence and photoacoustic microscopy using an akinetic sensor

This Letter presents a novel dual modality reflection mode optical coherence and photoacoustic microscopy (OC-PAM) system. The optical coherence microscopy modality features a broadband source to accomplish 5 μm axial resolution. The photoacoustic microscopy modality uses a rigid akinetic Fabry-Perot etalon encapsulated in an optically transparent medium, which forms a 2  mm×11  mm translucent imaging window, permitting reflection mode dual modality imaging. After characterization, the OC-PAM system was applied to image zebrafish larvae in vivo, demonstrating its capability in biomedical imaging with complementary optical scattering and absorption contrasts by revealing morphology in the fish larvae.

Air-coupled ultrasound detection using capillary-based optical ring resonators

We experimentally demonstrate and theoretically analyze high Q-factor (~107) capillary-based optical ring resonators for non-contact detection of air-coupled ultrasound. Noise equivalent pressures in air as low as 215 mPa/√Hz and 41 mPa/√Hz at 50 kHz and 800 kHz in air, respectively, are achieved. Furthermore, non-contact detection of air-coupled photoacoustic pulses optically generated from a 200 nm thick Chromium film is demonstrated. The interaction of an acoustic pulse and the mechanical mode of the ring resonator is also studied. Significant improvement in detection bandwidth is demonstrated by encapsulating the ring resonator in a damping medium. Our work will enable compact and sensitive ultrasound detection in many applications, such as air-coupled non-destructive ultrasound testing, photoacoustic imaging, and remote sensing. It will also provide a model system for fundamental study of the mechanical modes in the ring resonator.