All-optical optoacoustic microscopy based on probe beam deflection technique

Optical ultrasound sensors for photoacoustic imaging: a review

Significance

Photoacoustic (PA) imaging is an emerging biomedical imaging modality that can map optical absorption contrast in biological tissues by detecting ultrasound signal. Piezoelectric transducers are commonly used in PA imaging to detect the ultrasound signals. However, piezoelectric transducers suffer from low sensitivity when the dimensions are reduced and are easily influenced by electromagnetic interference. To avoid these limitations, various optical ultrasound sensors have been developed and shown their great potential in PA imaging.

Aim

Our study aims to summarize recent progress in optical ultrasound sensor technologies and their applications in PA imaging.

Approach

The commonly used optical ultrasound sensing techniques and their applications in PA systems are reviewed. The technical advances of different optical ultrasound sensors are summarized.

Results

Optical ultrasound sensors can provide wide bandwidth and improved sensitivity with miniatured size, which enables their applications in PA imaging.

Conclusions

The optical ultrasound sensors are promising transducers in PA imaging to provide higher-resolution images and can be used in new applications with their unique advantages.

Estimation of underwater acoustic direction-of-arrival using the probe beam deflection technique

This paper proposes a method of estimating the underwater acoustic direction-of-arrival using several laser beams impinging on a propagating underwater acoustic wave. The deflection of the laser beam caused by the spatial variation of the optical refractive index, which is further due to the modulation of the acoustic wave, reflects the information of direction-of-arrival and is sensed by the position sensitive detector (PSD). The sensing of the minute displacement on the PSD, in fact, introduces an extra dimension in the depth direction, which is a significant advantage over the conventional piezoelectric sensing regime. The employment of the extra sensing dimension can overcome several shortcomings, represented by spatial aliasing and phase ambiguity, existing in the current direction-of-arrival estimating methods. In addition, the ringing phenomenon of the piezoelectric effect is greatly reduced in the proposed laser-based sensing regime. By the flexibility of placing the laser beams, a prototype of the hydrophone is designed and manufactured, and a series of testing is performed. The results show that, benefiting from the probe beam deflection technique and combining the rough estimate and fine calculation, the resolution of the underwater acoustic direction-of-arrival can be improved to better than 0.016°, which can support and reform many underwater applications such as underwater acoustic communication, underwater detection, and ocean monitoring.

High current short pulse driver using a high power diode laser for optoacoustic biomedical imaging techniques

Optoacoustic biomedical imaging combines the high spatial resolution of the ultrasound imaging with the specificity of the optical absorption spectroscopy techniques. It is being used in various scenarios such as anatomical, functional and molecular imaging. Typically light sources for this imaging technique is based on solid state lasers since they can produce high energy short optical pulses. However, they are bulky, expensive and the imaging speed is limited because their low pulse repetition rate. High power diode lasers (HPDLs) are a promising alternative for imaging small volume absorbers as they are compact, affordable and allow high repetition rates. However, HPDLs provide relative low peak optical power compared to solid state lasers. Therefore, imaging systems based on diode lasers require much longer pulse duration resulting in lower in-depth resolution and optoacoustic conversion efficiency. HPDLs need dedicated fast electronics to generate short optical pulses. In this work, we have designed, built and test a pulsed diode laser driver based on RF power MOSFETs, specifically considering the optimization of the current pulse in order to maximize the optical peak power, achieving current pulses of more than 900 A with a duration of 50 ns. We have studied the operation of a low cost HPDL out of the manufacturers datasheet ratings without noticeable degradation at high current (> 250 A) and short pulse duration (< 60 ns). We have obtained an optical peak power of 750 W and a energy per pulse of 31.2 µJ at 40 ns optical pulse duration. The optoacoustic images obtained in this operation regime shown a clear enhancement respect to the ones obtained in standard operation of the HPDL.

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.

Reconstruction of the gradient field in the cross-section of an acoustic wave and its usefulness in processing acoustic wave fields

This paper proposes a method of reconstructing the gradient field in a cross-section of the acoustic wave using the laser beam deflection tomography, then verifing that the simultaneous acquisitions of the relative acoustic pressure distribution and the gradient field can make the direct employment of Kirchhoff's integral theorem feasible. Specifically, a position-sensitive detector (PSD) is used to sense the deflection of a laser beam impinging on a propagating acoustic wave. The deflection of the laser beam can be divided into two parts; one is in the plane that laser beams go through, and the other is perpendicular to the plane. Combining the tomographic results using the two parts of the deflection, the gradient field of the propagating acoustic wave in a cross-section is obtained, which is an extended version of beam deflection tomography. Based on the gradient of a wavefield along with the relative sound pressure distribution, Kirchhoff's integral theorem can be directly employed to calculate and analyze the wavefield further, which was hardly achieved in the past due to the lack of dense gradient sensing regimes. To verify the usefulness, two experiments are conducted, whose results indicate that the densely and precisely acquired gradient field of an acoustic wave is useful in solving the problem of port and starboard ambiguity, and the problem of accurate near-field prediction can also be well addressed, which in a deeper sense benefit from the direct employment of Kirchhoff's integral theorem in practical applications.

Optical ultrasound sensors for photoacoustic imaging: a narrative review

Optical ultrasound sensors have been increasingly employed in biomedical diagnosis and photoacoustic imaging (PAI) due to high sensitivity and resolution. PAI could visualize the distribution of ultrasound excited by laser pulses in biological tissues. The information of tissues is detected by ultrasound sensors in order to reconstruct structural images. However, traditional ultrasound transducers are made of piezoelectric films that lose sensitivity quadratically with the size reduction. In addition, the influence of electromagnetic interference limits further applications of traditional ultrasound transducers. Therefore, optical ultrasound sensors are developed to overcome these shortcomings. In this review, optical ultrasound sensors are classified into resonant and non-resonant ones in view of physical principles. The principles and basic parameters of sensors are introduced in detail. Moreover, the state of the art of optical ultrasound sensors and applications in PAI are also presented. Furthermore, the merits and drawbacks of sensors based on resonance and non-resonance are discussed in perspectives. We believe this review could provide researchers with a better understanding of the current status of optical ultrasound sensors and biomedical applications.

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.

Perspective on fast-evolving photoacoustic tomography

SIGNIFICANCE

Acoustically detecting the rich optical absorption contrast in biological tissues, photoacoustic tomography (PAT) seamlessly bridges the functional and molecular sensitivity of optical excitation with the deep penetration and high scalability of ultrasound detection. As a result of continuous technological innovations and commercial development, PAT has been playing an increasingly important role in life sciences and patient care, including functional brain imaging, smart drug delivery, early cancer diagnosis, and interventional therapy guidance.

AIM

Built on our 2016 tutorial article that focused on the principles and implementations of PAT, this perspective aims to provide an update on the exciting technical advances in PAT.

APPROACH

This perspective focuses on the recent PAT innovations in volumetric deep-tissue imaging, high-speed wide-field microscopic imaging, high-sensitivity optical ultrasound detection, and machine-learning enhanced image reconstruction and data processing. Representative applications are introduced to demonstrate these enabling technical breakthroughs in biomedical research.

CONCLUSIONS

We conclude the perspective by discussing the future development of PAT technologies.

Towards non-contact photoacoustic imaging [review]

Photoacoustic imaging (PAI) takes advantage of both optical and ultrasound imaging properties to visualize optical absorption with high resolution and contrast. Photoacoustic microscopy (PAM) is usually categorized with all-optical microscopy techniques such as optical coherence tomography or confocal microscopes. Despite offering high sensitivity, novel imaging contrast, and high resolution, PAM is not generally an all-optical imaging method unlike the other microscopy techniques. One of the significant limitations of photoacoustic microscopes arises from their need to be in physical contact with the sample through a coupling media. This physical contact, coupling, or immersion of the sample is undesirable or impractical for many clinical and pre-clinical applications. This also limits the flexibility of photoacoustic techniques to be integrated with other all-optical imaging microscopes for providing complementary imaging contrast. To overcome these limitations, several non-contact photoacoustic signal detection approaches have been proposed. This paper presents a brief overview of current non-contact photoacoustic detection techniques with an emphasis on all-optical detection methods and their associated physical mechanisms.

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.

Infrared spectra of micro-structured samples with microPhotoacoustic spectroscopy and synchrotron radiation

Photoacoustic spectroscopy (PAS) measures the photon absorption spectrum of a sample through detection of the acoustic wave generated by the photothermal effect as one modulates the intensity of the incident radiation at each wavelength. We have recently demonstrated the implementation of PAS in a microscopy configuration with mid-infrared radiation (microPAS). In the present work, we describe the performance of microPAS using synchrotron radiation (SR) in diffraction-limited spectromicroscopy and imaging experiments. Spectra were obtained for polystyrene beads, polypropylene fibres, and single fibres of human hair. SR produced microPAS spectra of much higher intensity as compared with those obtained using conventional mid- and near-infrared sources. For hair samples, the penetration depth of mid-infrared light, even with bright SR, is significantly shorter than the probed sample thickness at very low modulation frequencies resulting in saturated PAS spectra. In contrast, microPAS spectra of polymer beads were in general of much better quality than those obtained with conventional sources. We also demonstrated the capability to collect line profiles and line spectra at diffraction limited spatial resolution. The microPAS spectra of beads appear free from appreciable bandshape distortions arising from the real part of the refractive index of the sample. This observation confirms microPAS as an absorption-only technique and establishes it as a valuable new tool in the microspectroscopic analysis of particulates and of samples with a complex topography.

Optoacoustic mesoscopy for biomedicine

Fuelled by innovation, optical microscopy plays a critical role in the life sciences and medicine, from basic discovery to clinical diagnostics. However, optical microscopy is limited by typical penetration depths of a few hundred micrometres for in vivo interrogations in the visible spectrum. Optoacoustic microscopy complements optical microscopy by imaging the absorption of light, but it is similarly limited by penetration depth. In this Review, we summarize progress in the development and applicability of optoacoustic mesoscopy (OPAM); that is, optoacoustic imaging with acoustic resolution and wide-bandwidth ultrasound detection. OPAM extends the capabilities of optical imaging beyond the depths accessible to optical and optoacoustic microscopy, and thus enables new applications. We explain the operational principles of OPAM, its placement as a bridge between optoacoustic microscopy and optoacoustic macroscopy, and its performance in the label-free visualization of tissue pathophysiology, such as inflammation, oxygenation, vascularization and angiogenesis. We also review emerging applications of OPAM in clinical and biological imaging.

Comparison of Piezoelectric and Optical Projection Imaging for Three-Dimensional In Vivo Photoacoustic Tomography

Ultrasound sensor arrays for photoacoustic tomography (PAT) are investigated that create line projections of the pressure generated in an object by pulsed light illumination. Projections over a range of viewing angles enable the reconstruction of a three-dimensional image. Two line-integrating arrays are compared in this study for the in vivo imaging of vasculature, a piezoelectric array, and a camera-based setup that captures snapshots of the acoustic field emanating from the sample. An array consisting of 64 line-shaped sensors made of piezoelectric polymer film, which was arranged on a half-cylindrical area, was used to acquire spatiotemporal data from a human finger. The optical setup used phase contrast to visualize the acoustic field generated in the leg of a mouse after a selected delay time. Time-domain back projection and frequency-domain back propagation were used for image reconstruction from the piezoelectric and optical data, respectively. The comparison yielded an about threefold higher resolution for the optical setup and an about 13-fold higher sensitivity of the piezoelectric array. Due to the high density of data in the camera images, the optical technique gave images without streak artifacts, which were visible in the piezo array images due to the discrete detector positions. Overall, both detection concepts are suited for almost real-time projection imaging and three-dimensional imaging with a data acquisition time of less than a minute without averaging, which was limited by the repetition rate of the laser.

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.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

2 MHz multi-wavelength pulsed laser for functional photoacoustic microscopy

Fast functional photoacoustic microscopy requires multi-wavelength pulsed laser sources with high pulse repetition rates, short wavelength switching time, and sufficient pulse energies. Here, we report the development of a stimulated-Raman-scattering-based multi-wavelength pulsed laser source for fast functional photoacoustic imaging. The new laser source is pumped with a 532 nm 1 MHz pulsed laser. The 532 nm laser beam is split into two: one pumps a 5 m optical fiber to excite a 558 nm wavelength via stimulated Raman scattering; the other goes through a 50 m optical fiber to delay the 532 nm pulse by 220 ns. The two beams are combined and coupled into an optical fiber for photoacoustic excitation. As a result, the new laser source can generate 2 million pulses per second, switch wavelengths in 220 ns, and provide hundreds of nanojoules pulse energy for each wavelength. Using this laser source, we demonstrate optical-resolution photoacoustic imaging of microvascular structures and oxygen saturation in the mouse ear. The ultrashort wavelength switching time enables oxygen saturation imaging of flowing red blood cells, which is valuable for high-resolution functional imaging.

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.

Non-Contact Photoacoustic Imaging Using a Commercial Heterodyne Interferometer

Most current photoacoustic imaging (PAI) systems employ piezoelectric transducers to receive photoacoustic signals, which requires coupling medium to facilitate photoacoustic wave propagation and are not favored in many applications. Here, we report an all-optical non-contact PAI system based on a commercial heterodyne interferometer working at 1550 nm. The interferometer remotely detects ultrasound-induced surface vibration and does not involve any physical contact with the sample. The theoretically predicated and experimentally measured noise equivalent detection limits of the optical sensor are about 4.5 and 810 Pa over 1.2 MHz bandwidth. Using a raster-scan PAI system equipped with the non-contact design, stereotactic boundaries of an artificial tumor in a pig brain were accurately delineated. The non-contact design also enables the tomographic PAI of biological tissue samples in a non-invasive manner. The preliminary results and analyses reveal that the heterodyne interferometer-based non-contact PAI system holds good potential in biomedical imaging.