Review of cost reduction methods in photoacoustic computed tomography

Development of non-contact foreign body imaging base on photoacoustic signal intensity measurement

BACKGROUND

It is challenging to visually identify tiny and concealed foreign objects within the body due to their small size and subcutaneous location while they can cause infections.

METHODS

A non-contact photoacoustic system based on Rosencwaig-Gersho photoacoustic theory and dual modulator method is developed for detecting foreign objects in meat.

RESULT

The experiments conducted validate the successful development of this measurement technique with 10 μm spatial resolution and its corresponding mathematical model, demonstrating an 11% Mean Absolute Percentage Error (MAPE) in comparison to the experimental results. Dual modulator successfully regulates laser energy at MPE limit.

CONCLUSION

The utilization of non-contact photoacoustic signal intensity measurements enables the identification of foreign objects within the body. Further, the application of mathematical modelling can validate the measurement outcomes. These findings serve as a foundation for creating an affordable and straightforward foreign body detector.

Simultaneous photoacoustic and ultrasound imaging: A review

Photoacoustic imaging (PAI) is an emerging biomedical imaging technique that combines the advantages of optical and ultrasound imaging, enabling the generation of images with both optical resolution and acoustic penetration depth. By leveraging similar signal acquisition and processing methods, the integration of photoacoustic and ultrasound imaging has introduced a novel hybrid imaging modality suitable for clinical applications. Photoacoustic-ultrasound imaging allows for non-invasive, high-resolution, and deep-penetrating imaging, providing a wealth of image information. In recent years, with the deepening research and the expanding biomedical application scenarios of photoacoustic-ultrasound bimodal systems, the immense potential of photoacoustic-ultrasound bimodal imaging in basic research and clinical applications has been demonstrated, with some research achievements already commercialized. In this review, we introduce the principles, technical advantages, and biomedical applications of photoacoustic-ultrasound bimodal imaging techniques, specifically focusing on tomographic, microscopic, and endoscopic imaging modalities. Furthermore, we discuss the future directions of photoacoustic-ultrasound bimodal imaging technology.

Photoacoustic imaging for cutaneous melanoma assessment: a comprehensive review

Significance

Cutaneous melanoma (CM) has a high morbidity and mortality rate, but it can be cured if the primary lesion is detected and treated at an early stage. Imaging techniques such as photoacoustic (PA) imaging (PAI) have been studied and implemented to aid in the detection and diagnosis of CM.

Aim

Provide an overview of different PAI systems and applications for the study of CM, including the determination of tumor depth/thickness, cancer-related angiogenesis, metastases to lymph nodes, circulating tumor cells (CTCs), virtual histology, and studies using exogenous contrast agents.

Approach

A systematic review and classification of different PAI configurations was conducted based on their specific applications for melanoma detection. This review encompasses animal and preclinical studies, offering insights into the future potential of PAI in melanoma diagnosis in the clinic.

Results

PAI holds great clinical potential as a noninvasive technique for melanoma detection and disease management. PA microscopy has predominantly been used to image and study angiogenesis surrounding tumors and provide information on tumor characteristics. Additionally, PA tomography, with its increased penetration depth, has demonstrated its ability to assess melanoma thickness. Both modalities have shown promise in detecting metastases to lymph nodes and CTCs, and an all-optical implementation has been developed to perform virtual histology analyses. Animal and human studies have successfully shown the capability of PAI to detect, visualize, classify, and stage CM.

Conclusions

PAI is a promising technique for assessing the status of the skin without a surgical procedure. The capability of the modality to image microvasculature, visualize tumor boundaries, detect metastases in lymph nodes, perform fast and label-free histology, and identify CTCs could aid in the early diagnosis and classification of CM, including determination of metastatic status. In addition, it could be useful for monitoring treatment efficacy noninvasively.

Improvement of LED-based photoacoustic imaging using lag-coherence factor (LCF) beamforming

BACKGROUND

Owing to its portability, affordability, and energy-efficiency, LED-based photoacoustic (PA) imaging is increasingly becoming popular when compared to its laser-based alternative, mainly for superficial vascular imaging applications. However, this technique suffers from low SNR and thereby limited imaging depth. As a result, visual image quality of LED-based PA imaging is not optimal, especially in sub-surface vascular imaging applications.

PURPOSE

Combination of linear ultrasound (US) probes and LED arrays are the most common implementation in LED-based PA imaging, which is currently being explored for different clinical imaging applications. Traditional delay-and-sum (DAS) is the most common beamforming algorithm in linear array-based PA detection. Side-lobes and reconstruction-related artifacts make the DAS performance unsatisfactory and poor for a clinical-implementation. In this work, we explored a new weighting-based image processing technique for LED-based PAs to yield improved image quality when compared to the traditional methods.

METHODS

We are proposing a lag-coherence factor (LCF), which is fundamentally based on the combination of the spatial auto-correlation of the detected PA signals. In LCF, the numerator contains lag-delay-multiply-and-sum (DMAS) beamformer instead of a conventional DAS beamformer. A spatial auto-correlation operation is performed between the detected US array signals before using DMAS beamformer. We evaluated the new method on both tissue-mimicking phantom (2D) and human volunteer imaging (3D) data acquired using a commercial LED-based PA imaging system.

RESULTS

Our novel correlation-based weighting technique showed LED-based PA image quality improvement when it is combined with conventional DAS beamformer. Both phantom and human volunteer imaging results gave a direct confirmation that by introducing LCF, image quality was improved and this method could reduce side-lobes and artifacts when compared to the DAS and coherence-factor (CF) approaches. Signal-to-noise ratio, generalized contrast-to-noise ratio, contrast ratio and spatial resolution were evaluated and compared with conventional beamformers to assess the reconstruction performance in a quantitative way. Results show that our approach offered image quality enhancement with an average signal-to-noise ratio and spatial resolution improvement of around 20% and 25% respectively, when compared with conventional CF based DAS algorithm.

CONCLUSIONS

Our results demonstrate that the proposed LCF based algorithm performs better than the conventional DAS and CF algorithms by improving signal-to-noise ratio and spatial resolution. Therefore, our new weighting technique could be a promising tool to improve the performance of LED-based PA imaging and thus accelerate its clinical translation.

Ultrafast longitudinal imaging of haemodynamics via single-shot volumetric photoacoustic tomography with a single-element detector

Techniques for imaging haemodynamics use ionizing radiation or contrast agents or are limited by imaging depth (within approximately 1 mm), complex and expensive data-acquisition systems, or low imaging speeds, system complexity or cost. Here we show that ultrafast volumetric photoacoustic imaging of haemodynamics in the human body at up to 1 kHz can be achieved using a single laser pulse and a single element functioning as 6,400 virtual detectors. The technique, which does not require recalibration for different objects or during long-term operation, enables the longitudinal volumetric imaging of haemodynamics in vasculature a few millimetres below the skin's surface. We demonstrate this technique in vessels in the feet of healthy human volunteers by capturing haemodynamic changes in response to vascular occlusion. Single-shot volumetric photoacoustic imaging using a single-element detector may facilitate the early detection and monitoring of peripheral vascular diseases and may be advantageous for use in biometrics and point-of-care testing.

Effect of pulsed laser parameters on photoacoustic flow cytometry efficiency in vitro and in vivo

Photoacoustic flow cytometry is one of the most effective approaches to detect "alien" objects in the bloodstream, including circulating tumor cells, blood clots, parasites, and emboli. However, the possibility of detecting high-amplitude signals from these objects against the background of blood depends on the parameters of the laser pulse. So, the dependencies of photoacoustic signals amplitude and number on laser pulse energy (5-150 μJ), pulse length (1, 2, 5 ns), and pulse repetition rate (2, 5, 10 kHz) for the melanoma cells were investigated. First, the PA responses of a melanoma cell suspension in vitro were measured to directly assess the efficiency of converting laser light into an acoustic signal. After it, the same dependence with the developed murine model based on constant rate melanoma cell injection into the animal blood flow was tested. Both in vivo and in vitro experiments show that signal generation efficiency increases with laser pulse energy above 15 μJ. Shorter pulses, especially 1 ns, provide more efficient signal generation as well as higher pulse rates. A higher pulse rate also provides more efficient signal generation, but also leads to overheating of the skin. The results show the limits where the photoacoustic flow cytometry system can be effectively used for the detection of circulating tumor cells in undiluted blood both for in vitro experiments and for in vivo murine models.

Adaptive coherent weighted averaging algorithm for enhancement of photoacoustic tomography images of brain

One common method to improve the low signal-to-noise ratio of the photoacoustic (PA) signal generated from weak absorbers or absorbers located in deep tissue is to acquire signal multiple times from the same region and perform averaging. However, pulse-to-pulse laser fluctuations together with differences in the beam profile of the pulses create undeterministic multiple scattering processes in the tissue. This phenomenon consequently induces a spatiotemporal displacement in the PA signal samples which in turn deteriorates the effectiveness of signal averaging. Here, we present an adaptive coherent weighted averaging algorithm to adjust the locations and values of PA signal samples for more efficient signal averaging. The proposed method is evaluated in a linear array-based PA imaging setup of ex vivo sheep brain.

Ultra-widefield and high-speed spiral laser scanning OR-PAM: System development and characterization

Photoacoustic microscopy (PAM) is a high-resolution imaging modality that has been mainly implemented with small field of view applications. Here, we developed a fast PAM system that utilizes a unique spiral laser scanning mechanism and a wide acoustic detection unit. The developed system can image an area of 12.5 cm² in 6.4 s. The system has been characterized using highly detailed phantoms. Finally, the imaging capabilities of the system were further demonstrated by imaging a sheep brain ex vivo and a rat brain in vivo.

Single-shot 3D photoacoustic tomography using a single-element detector for ultrafast imaging of hemodynamics

Imaging hemodynamics is crucial for the diagnosis, treatment, and prevention of vascular diseases. However, current imaging techniques are limited due to the use of ionizing radiation or contrast agents, short penetration depth, or complex and expensive data acquisition systems. Photoacoustic tomography shows promise as a solution to these issues. However, existing photoacoustic tomography methods collect signals either sequentially or through numerous detector elements, leading to either low imaging speed or high system complexity and cost. To address these issues, here we introduce a method to capture a 3D photoacoustic image of vasculature using a single laser pulse and a single-element detector that functions as 6,400 virtual ones. Our method enables ultrafast volumetric imaging of hemodynamics in the human body at up to 1 kHz and requires only a single calibration for different objects and for long-term operations. We demonstrate 3D imaging of hemodynamics at depth in humans and small animals, capturing the variability in blood flow speeds. This concept can inspire other imaging technologies and find applications such as home-care monitoring, biometrics, point-of-care testing, and wearable monitoring.

Sound out the impaired perfusion: Photoacoustic imaging in preclinical ischemic stroke

Acoustically detecting the optical absorption contrast, photoacoustic imaging (PAI) is a highly versatile imaging modality that can provide anatomical, functional, molecular, and metabolic information of biological tissues. PAI is highly scalable and can probe the same biological process at various length scales ranging from single cells (microscopic) to the whole organ (macroscopic). Using hemoglobin as the endogenous contrast, PAI is capable of label-free imaging of blood vessels in the brain and mapping hemodynamic functions such as blood oxygenation and blood flow. These imaging merits make PAI a great tool for studying ischemic stroke, particularly for probing into hemodynamic changes and impaired cerebral blood perfusion as a consequence of stroke. In this narrative review, we aim to summarize the scientific progresses in the past decade by using PAI to monitor cerebral blood vessel impairment and restoration after ischemic stroke, mostly in the preclinical setting. We also outline and discuss the major technological barriers and challenges that need to be overcome so that PAI can play a more significant role in preclinical stroke research, and more importantly, accelerate its translation to be a useful clinical diagnosis and management tool for human strokes.

Posterior photoacoustic/ultrasound imaging of the periodontal pocket with a compact intraoral transducer

Periodontitis is a public issue and imaging periodontal pocket is important to evaluate periodontitis. Regular linear transducers have limitations in imaging the posterior teeth due to their geometry restrictions. Here we characterized a transducer that can image the posterior teeth including assessment of periodontal pockets via a combination of photoacoustic and ultrasound imaging. Unlike conventional transducer design, this device has a toothbrush-shaped form factor with a side-view transducer to image molars (total size: 1 ×1.9 cm). A laser diode was integrated as the light source to reduce the cost and size and facilitates clinical transition. The in vivo imaging of a molar of a periodontal patient demonstrated that the transducer could image in the posterior area of gum in vivo; the value determined by imaging was within 7 % of the value measured clinically.

Improved photoacoustic images via wavefront shaping modulation based on the scattering structure

Multispectral optoacoustic tomography (MSOT) has become the dominant technical solution for photoacoustic imaging (PAI). However, the laser source of fiber output in the current MSOT method is typically a TEM₀₀ Gaussian beam, which is prone to artifacts and incomplete due to the uneven distribution of the irradiated light intensity. Here, we propose a novel method to improve the quality of photoacoustic image reconstruction by modulating the wavefront shaping of the incident laser beam based on the designed scattering structure. In the experiment, we add the designed scattering structure to the current hemispherical photoacoustic transducer array device. Through experiments and simulations, we investigate and compare the effects of different scattering structures on laser intensity modulation. The results show that an ED1-C20 diffusion structure with a scattering angle of 20 degrees has the most effective modulation of the beam intensity distribution. And we choose gold nanoparticles of 50-100 nanometers (nm) diameters and index finger capillary vessels respectively as the medium of PAI. We obtain the highest ratio of PAI area increases of gold nanoparticles and index finger to devices compare without scattering structure is 29.69% and 634.94%, respectively. Experimental results demonstrate that our method is significantly higher quality than traditional methods, which has great potential for theoretical application in medical PAI.

Time-domain ultrasound as prior information for frequency-domain compressive ultrasound for intravascular cell detection: A 2-cell numerical model

This study proposes a new method for the detection of a weak scatterer among strong scatterers using prior-information ultrasound (US) imaging. A perfect application of this approach is in vivo cell detection in the bloodstream, where red blood cells (RBCs) serve as identifiable strong scatterers. In vivo cell detection can help diagnose cancer at its earliest stages, increasing the chances of survival for patients. This work combines time-domain US with frequency-domain compressive US imaging to detect a 20-μ MCF-7 circulating tumor cell (CTC) among a number of RBCs within a simulated venule inside the mouth. The 2D image reconstructed from the time-domain US is employed to simulate the reflected and scattered pressure field from the RBCs, which is then measured at the location of the receivers. The RBCs are tagged one time by a human operator and another time, automatically, by template-based computer vision. Next, the resulting signal from the RBCs is subtracted from the measured total signal in frequency domain to generate the scattered-field data, coming from the CTC alone. Feeding that signal and the background pressure field into a norm-one-based compressive sensing code enables detecting the CTC at various locations. As errors could arise in determining the location of the RBCs and their acoustic properties in the real world, small errors (up to 10% in the former and 5% in the latter) are purposefully introduced to the model, to which the proposed method is shown to be resilient. Localization errors are smaller than 12 μ when a human tags the RBCs and smaller than 25 μ when computer vision is applied. Despite its limitations, this study, for the first time, reports the results of combining two US modalities aimed at cell detection and introduces a unique and useful application for ultrahigh-frequency US imaging. It should be noted that this method can be used in detecting weak scatterers with ultrasound waves in other applications as well.

Tutorial on Development of 3D Vasculature Digital Phantoms for Evaluation of Photoacoustic Image Reconstruction Algorithms

A synthetic phantom model is typically utilized to evaluate the initial performance of a photoacoustic image reconstruction algorithm. The characteristics of the phantom model (structural, optical, and acoustic) are required to be very similar to those of the biological tissue. Typically, generic two-dimensional shapes are used as imaging targets to calibrate reconstruction algorithms. However, these structures are not representative of complex biological tissue, and therefore the artifacts that exist in reconstructed images of biological tissue vasculature are ignored. Real data from 3D MRI/CT volumes can be extrapolated to create high-quality phantom models; however, these sometimes involve complicated pre-processing and mostly are challenging, due to the inaccessibility of these datasets or the requirement for approval to utilize the data. Therefore, it is necessary to develop a 3D tissue-mimicking phantom model consisting of different compartments with characteristics that can be easily modified. In this tutorial, we present an optimized development process of a generic 3D complex digital vasculature phantom model in Blender. The proposed workflow is such that an accurate and easily editable digital phantom can be developed. Other workflows for creating the same phantom will take much longer to set up and require more time to edit. We have made a few examples of editable 3D phantom models, which are publicly available to test and modify.

Interleave-sampled photoacoustic imaging: a doubled and equivalent sampling rate for high-frequency imaging

High-frequency photoacoustic (PA) imaging (>20 MHz) requires data acquisition (DAQ) with a commensurately high sampling rate, which leads to hardware challenges and increased costs. We report here a new, to the best of our knowledge, method-interleave-sampled PA imaging-that enables high-frequency imaging with a relatively low sampling rate, e.g., a 41.67-MHz sampling rate with a 30-MHz transducer. This method harnesses two acquisitions at a low sampling rate to effectively double the sampling rate which consequently reduces the frame rate by a factor of two. It modulates the delay of the light pulses and can thus be applied to any PA DAQ system. We perform both phantom and in vivo studies with a 30-MHz transducer. The results demonstrate that interleaved sampling at 41.67 MHz can capture high frequency information above 30 MHz but a conventional 41.67-MHz sampling rate cannot. The axial and lateral resolution are as high as 63 µm and 91 µm via interleaved sampling which are much higher than those of conventional 41.67-MHz sampling (130 µm and 136 µm).

Programmable Acoustic Delay-Line Enabled Low-Cost Photoacoustic Tomography System

Photoacoustic tomography (PAT) is an emerging technology for biomedical imaging that combines the superiorities of high optical contrast and acoustic penetration. In the PAT system, more PA signals are preferred to be detected from full field of view to reconstruct the PA images with higher fidelity. However, the requirement for more PA signals' detection leads to more time consumption for single-channel scanning-based PAT system or higher cost of data acquisition (DAQ) module for an array-based PAT system. To address this issue, we proposed a programmable acoustic delay-line (PADL) module to reduce DAQ cost and accelerate imaging speed for PAT system. The module is based on bidirectional conversion between acoustic signals and electrical signals, including ultrasound transmission in between to provide sufficient time delay. The acoustic delay-line module achieves tens or hundreds of microseconds' delay for each channel and is controlled by a programmable control unit. In this work, it achieves to merge four inputs of PA signals into one output signal, which can be recovered into original four PA signals in the digital domain after DAQ. The imaging experiments of pencil leads embedded in agar phantom are conducted by the PAT system equipped with the proposed PADL module, which demonstrated its feasibility to reduce the cost of the PAT system. An in vivo study of human finger PAT imaging with delay-line module verified its feasibility for biomedical imaging applications.

Photoacoustic Imaging in Biomedicine and Life Sciences

Photo-acoustic imaging, also known as opto-acoustic imaging, has become a widely popular modality for biomedical applications. This hybrid technique possesses the advantages of high optical contrast and high ultrasonic resolution. Due to the distinct optical absorption properties of tissue compartments and main chromophores, photo-acoustics is able to non-invasively observe structural and functional variations within biological tissues including oxygenation and deoxygenation, blood vessels and spatial melanin distribution. The detection of acoustic waves produced by a pulsed laser source yields a high scaling range, from organ level photo-acoustic tomography to sub-cellular or even molecular imaging. This review discusses significant novel technical solutions utilising photo-acoustics and their applications in the fields of biomedicine and life sciences.

Performance comparison of high-speed photoacoustic microscopy: opto-ultrasound combiner versus ring-shaped ultrasound transducer

Photoacoustic microscopy (PAM) embedded with a 532 nm pulse laser is widely used to visualize the microvascular structures in both small animals and humans in vivo. An opto-ultrasound combiner (OUC) is often utilized in high-speed PAM to confocally align the optical and acoustic beams to improve the system's sensitivity. However, acoustic impedance mismatch in the OUC results in little improvement in the sensitivity. Alternatively, a ring-shaped ultrasound transducer (RUT) can also accomplish the confocal configuration. Here, we compare the performance of OUC and RUT modules through ultrasound pulse-echo tests and PA imaging experiments. The signal-to-noise ratios (SNRs) of the RUT-based system were 15 dB, 12 dB, and 7 dB higher when compared to the OUC-based system for ultrasound pulse-echo test, PA phantom imaging test, and PA in-vivo imaging test, respectively. In addition, the RUT-based system could image the microvascular structures of small parts of a mouse body in a few seconds with minimal loss in SNR. Thus, with increased sensitivity, improved image details, and fast image acquisition, we believe the RUT-based systems could play a significant role in the design of future fast-PAM systems.

Transfontanelle photoacoustic imaging: ultrasound transducer selection analysis

Transfontanelle ultrasound imaging (TFUI) is the conventional approach for diagnosing brain injury in neonates. Despite being the first stage imaging modality, TFUI lacks accuracy in determining the injury at an early stage due to degraded sensitivity and specificity. Therefore, a modality like photoacoustic imaging that combines the advantages of both acoustic and optical imaging can overcome the existing TFUI limitations. Even though a variety of transducers have been used in TFUI, it is essential to identify the transducer specification that is optimal for transfontanelle imaging using the photoacoustic technique. In this study, we evaluated the performance of 6 commercially available ultrasound transducer arrays to identify the optimal characteristics for transfontanelle photoacoustic imaging. We focused on commercially available linear and phased array transducer probes with center frequencies ranging from 2.5MHz to 8.5MHz which covers the entire spectrum of the transducer arrays used for brain imaging. The probes were tested on both in vitro and ex vivo brain tissue, and their performance in terms of transducer resolution, size, penetration depth, sensitivity, signal to noise ratio, signal amplification and reconstructed image quality were evaluated. The analysis of selected transducers in these areas allowed us to determine the optimal transducer for transfontanelle imaging, based on vasculature depth and blood density in tissue using ex vivo sheep brain. The outcome of this evaluation identified the two most suitable ultrasound transducer probes for transfontanelle photoacoustic imaging.

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.

Development of a Compact Photoacoustic Tomography Imaging System with Dual Single-Element Transducers for Image Enhancement

OBJECTIVE

This paper proposes a new photoacoustic computed tomography (PACT) imaging system employing dual ultrasonic transducers with different frequencies. When imaging complex biological tissues, photoacoustic (PA) signals with multiple frequencies are produced simultaneously; however, due to the limited bandwidth of a single-frequency transducer, the received PA signals with specific frequencies may be missing, leading to a low imaging quality.

METHODS

In contrast to our previous work, the proposed system has a compact volume as well as specific selection of the detection center frequency of the transducer, which can provide a comprehensive range for the detection of PA signals. In this study, a series of numerical simulation and phantom experiments were performed to validate the efficacy of the developed PACT system.

RESULTS

The images generated by our system combined the advantages of both high resolution and ideal brightness/contrast.

CONCLUSION

The interchangeability of transducers with different frequencies provides potential for clinical deployment under the circumstance where a single frequency transducer cannot perform well.

Recent Progress on Molecular Photoacoustic Imaging with Carbon-Based Nanocomposites

For biomedical imaging, the interest in noninvasive imaging methods is ever increasing. Among many modalities, photoacoustic imaging (PAI), which is a combination of optical and ultrasound imaging techniques, has received attention because of its unique advantages such as high spatial resolution, deep penetration, and safety. Incorporation of exogenous imaging agents further amplifies the effective value of PAI, since they can deliver other specified functions in addition to imaging. For these agents, carbon-based materials can show a large specific surface area and interesting optoelectronic properties, which increase their effectiveness and have proved their potential in providing a theragnostic platform (diagnosis + therapy) that is essential for clinical use. In this review, we introduce the current state of the PAI modality, address recent progress on PAI imaging that takes advantage of carbon-based agents, and offer a future perspective on advanced PAI systems using carbon-based agents.

A Review of Transparent Sensors for Photoacoustic Imaging Applications

Photoacoustic imaging is a new type of noninvasive, nonradiation imaging modality that combines the deep penetration of ultrasonic imaging and high specificity of optical imaging. Photoacoustic imaging systems employing conventional ultrasonic sensors impose certain constraints such as obstructions in the optical path, bulky sensor size, complex system configurations, difficult optical and acoustic alignment, and degradation of signal-to-noise ratio. To overcome these drawbacks, an ultrasonic sensor in the optically transparent form has been introduced, as it enables direct delivery of excitation light through the sensors. In recent years, various types of optically transparent ultrasonic sensors have been developed for photoacoustic imaging applications, including optics-based ultrasonic sensors, piezoelectric-based ultrasonic sensors, and microelectromechanical system-based capacitive micromachined ultrasonic transducers. In this paper, the authors review representative transparent sensors for photoacoustic imaging applications. In addition, the potential challenges and future directions of the development of transparent sensors are discussed.

Photoacoustic computed tomography for functional human brain imaging [Invited]

The successes of magnetic resonance imaging and modern optical imaging of human brain function have stimulated the development of complementary modalities that offer molecular specificity, fine spatiotemporal resolution, and sufficient penetration simultaneously. By virtue of its rich optical contrast, acoustic resolution, and imaging depth far beyond the optical transport mean free path (∼1 mm in biological tissues), photoacoustic computed tomography (PACT) offers a promising complementary modality. In this article, PACT for functional human brain imaging is reviewed in its hardware, reconstruction algorithms, in vivo demonstration, and potential roadmap.

Technical validation studies of a dual-wavelength LED-based photoacoustic and ultrasound imaging system

Recent advances in high power, pulsed, light emitting diodes (LEDs) have shown potential as fast, robust and relatively inexpensive excitation sources for photoacoustic imaging (PAI), yet systematic characterization of performance for biomedical imaging is still lacking. We report here technical and biological validation studies of a commercial dual-wavelength LED-based PAI and ultrasound system. Phantoms and small animals were used to assess temporal precision. In phantom studies, we found high temporal stability of the LED-based PAI system, with no significant drift in performance observed during 6 h of operation or over 30 days of repeated measurements. In vivo dual-wavelength imaging was able to map the dynamics of changes in blood oxygenation during oxygen-enhanced imaging and reveal the kinetics of indocyanine green contrast agent inflow after intravenous administration (Tmax∼6 min). Taken together, these studies indicate that LED-based excitation could be promising for future application in functional and molecular PAI.

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.

Advances in multimodal imaging in ophthalmology

Multimodality ophthalmic imaging systems aim to enhance the contrast, resolution, and functionality of existing technologies to improve disease diagnostics and therapeutic guidance. These systems include advanced acquisition and post-processing methods using optical coherence tomography (OCT), combined scanning laser ophthalmoscopy and OCT systems, adaptive optics, surgical guidance, and photoacoustic technologies. Here, we provide an overview of these ophthalmic imaging systems and their clinical and basic science applications.

Extending Imaging Depth in PLD-Based Photoacoustic Imaging: Moving Beyond Averaging

Pulsed laser diodes (PLDs) promise to be an attractive alternative to solid-state laser sources in photoacoustic tomography (PAT) due to their portability, high-pulse repetition frequency (PRF), and cost effectiveness. However, due to their lower energy per pulse, which, in turn, results in lower fluence required per photoacoustic signal generation, PLD-based photoacoustic systems generally have maximum imaging depth that is lower in comparison to solid-state lasers. Averaging of multiple frames is usually employed as a common practice in high PRF PLD systems to improve the signal-to-noise ratio of the PAT images. In this work, we demonstrate that by combining the recently described approach of subpitch translation on the receive-side ultrasound transducer alongside averaging of multiple frames, it is feasible to increase the depth sensitivity in a PLD-based PAT imaging system. Here, experiments on phantom containing diluted India ink targets were performed at two different laser energy level settings, that is, 21 and [Formula: see text]. Results obtained showed that the imaging depth improves by ~38.5% from 9.1 to 12.6 mm for 21- [Formula: see text] energy level setting and by ~33.3% from 10.8 to 14.4 mm for 27- [Formula: see text] energy level setting by using λ /4-pitch translation and average of 128 frames in comparison to λ -pitch data acquired with the average of 128 frames. However, the achievable frame rate is reduced by a factor of 2 and 4 for λ /2 and λ /4 subpitch translation, respectively.

Technical considerations in the Verasonics research ultrasound platform for developing a photoacoustic imaging system

Photoacoustic imaging (PAI) is an emerging functional and molecular imaging technology that has attracted much attention in the past decade. Recently, many researchers have used the vantage system from Verasonics for simultaneous ultrasound (US) and photoacoustic (PA) imaging. This was the motivation to write on the details of US/PA imaging system implementation and characterization using Verasonics platform. We have discussed the experimental considerations for linear array based PAI due to its popularity, simple setup, and high potential for clinical translatability. Specifically, we describe the strategies of US/PA imaging system setup, signal generation, amplification, data processing and study the system performance.

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.

In vivo magnetic nanoparticle hyperthermia: a review on preclinical studies, low-field nano-heaters, noninvasive thermometry and computer simulations for treatment planning

Magnetic nanoparticle hyperthermia (MNH) is a promising nanotechnology-based cancer thermal therapy that has been approved for clinical use, together with radiation therapy, for treating brain tumors. Almost ten years after approval, few new clinical applications had appeared, perhaps because it cannot benefit from the gold standard noninvasive MRI thermometry technique, since static magnetic fields inhibit heat generation. This might limit its clinical use, in particular as a single therapeutic modality. In this article, we review the in vivo MNH preclinical studies, discussing results of the last two decades with emphasis on safety as a clinical criteria, the need for low-field nano-heaters and noninvasive thermal dosimetry, and the state of the art of computational modeling for treatment planning using MNH. Limitations to more effective clinical use are discussed, together with suggestions for future directions, such as the development of ultrasound-based, computed tomography-based or magnetic nanoparticle-based thermometry to achieve greater impact on clinical translation of MNH.

Melanoma Biomarkers and Their Potential Application for In Vivo Diagnostic Imaging Modalities

Melanoma is the deadliest form of skin cancer and remains a diagnostic challenge in the dermatology clinic. Several non-invasive imaging techniques have been developed to identify melanoma. The signal source in each of these modalities is based on the alteration of physical characteristics of the tissue from healthy/benign to melanoma. However, as these characteristics are not always sufficiently specific, the current imaging techniques are not adequate for use in the clinical setting. A more robust way of melanoma diagnosis is to "stain" or selectively target the suspect tissue with a melanoma biomarker attached to a contrast enhancer of one imaging modality. Here, we categorize and review known melanoma diagnostic biomarkers with the goal of guiding skin imaging experts to design an appropriate diagnostic tool for differentiating between melanoma and benign lesions with a high specificity and sensitivity.

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).

Portable and Affordable Light Source-Based Photoacoustic Tomography

Photoacoustic imaging is a hybrid imaging modality that offers the advantages of optical (spectroscopic contrast) and ultrasound imaging (scalable spatial resolution and imaging depth). This promising modality has shown excellent potential in a wide range of preclinical and clinical imaging and sensing applications. Even though photoacoustic imaging technology has matured in research settings, its clinical translation is not happening at the expected pace. One of the main reasons for this is the requirement of bulky and expensive pulsed lasers for excitation. To accelerate the clinical translation of photoacoustic imaging and explore its potential in resource-limited settings, it is of paramount importance to develop portable and affordable light sources that can be used as the excitation light source. In this review, we focus on the following aspects: (1) the basic theory of photoacoustic imaging; (2) inexpensive light sources and different implementations; and (3) important preclinical and clinical applications, demonstrated using affordable light source-based photoacoustics. The main focus will be on laser diodes and light-emitting diodes as they have demonstrated promise in photoacoustic tomography-the key technological developments in these areas will be thoroughly reviewed. We believe that this review will be a useful opus for both the beginners and experts in the field of biomedical photoacoustic imaging.

3D finger vein biometric authentication with photoacoustic tomography

Biometric authentication is the recognition of human identity via unique anatomical features. The development of novel methods parallels widespread application by consumer devices, law enforcement, and access control. In particular, methods based on finger veins, as compared to face and fingerprints, obviate privacy concerns and degradation due to wear, age, and obscuration. However, they are two-dimensional (2D) and are fundamentally limited by conventional imaging and tissue-light scattering. In this work, for the first time, to the best of our knowledge, we demonstrate a method of three-dimensional (3D) finger vein biometric authentication based on photoacoustic tomography. Using a compact photoacoustic tomography setup and a novel recognition algorithm, the advantages of 3D are demonstrated via biometric authentication of index finger vessels with false acceptance, false rejection, and equal error rates <1.23%, <9.27%, and <0.13%, respectively, when comparing one finger, a false acceptance rateimprovement>10× when comparing multiple fingers, and <0.7% when rotating fingers ±30.

A New Possibility for Fermentation Monitoring by Electrical Driven Sensing of Ultraviolet Light and Glucose

Industrial fermentation generates products through microbial growth associated with the consumption of substrates. The efficiency of industrial production of high commercial value microbial products such as ethanol from glucose (GLU) is dependent on bacterial contamination. Controlling the sugar conversion into products as well as the sterility of the fermentation process are objectives to be considered here by studying GLU and ultraviolet light (UV) sensors. In this work, we present two different approaches of SnO₂ nanowires grown by the Vapor–Liquid–Solid (VLS) method. In the GLU sensor, we use SnO₂ nanowires as active electrodes, while for the UV sensor, a nanowire film was built for detection. The results showed a wide range of GLU sensing and as well as a significant influence of UV in the electrical signal. The effect of a wide range of GLU concentrations on the responsiveness of the sensor through current–voltage based on SnO₂ nanowire films under different concentration conditions ranging was verified from 1 to 1000 mmol. UV sensors show a typical amperometric response of SnO₂ nanowires under the excitation of UV and GLU in ten cycles of 300 s with 1.0 V observing a stable and reliable amperometric response. GLU and UV sensors proved to have a promising potential for detection and to control the conversion of a substrate into a product by GLU control and decontamination by UV control in industrial fermentation systems.

Investigation of the Effect of the Skull in Transcranial Photoacoustic Imaging: A Preliminary Ex Vivo Study

Although transcranial photoacoustic imaging (TCPAI) has been used in small animal brain imaging, in animals with thicker skull bones or in humans both light illumination and ultrasound propagation paths are affected. Hence, the PA image is largely degraded and in some cases completely distorted. This study aims to investigate and determine the maximum thickness of the skull through which photoacoustic imaging is feasible in terms of retaining the imaging target structure without incorporating any post processing. We identify the effect of the skull on both the illumination path and acoustic propagation path separately and combined. In the experimental phase, the distorting effect of ex vivo sheep skull bones with thicknesses in the range of 0.7~1.3 mm are explored. We believe that the findings in this study facilitate the clinical translation of TCPAI.

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.

Dictionary learning technique enhances signal in LED-based photoacoustic imaging

There has been growing interest in low-cost light sources such as light-emitting diodes (LEDs) as an excitation source in photoacoustic imaging. However, LED-based photoacoustic imaging is limited by low signal due to low energy per pulse-the signal is easily buried in noise leading to low quality images. Here, we describe a signal de-noising approach for LED-based photoacoustic signals based on dictionary learning with an alternating direction method of multipliers. This signal enhancement method is then followed by a simple reconstruction approach delay and sum. This approach leads to sparse representation of the main components of the signal. The main improvements of this approach are a 38% higher contrast ratio and a 43% higher axial resolution versus the averaging method but with only 4% of the frames and consequently 49.5% less computational time. This makes it an appropriate option for real-time LED-based photoacoustic imaging.

Tomographic imaging with an ultrasound and LED-based photoacoustic system

Pulsed lasers in photoacoustic tomography systems are expensive, which limit their use to a few clinics and small animal labs. We present a method to realize tomographic ultrasound and photoacoustic imaging using a commercial LED-based photoacoustic and ultrasound system. We present two illumination configurations using LED array units and an optimal number of angular views for tomographic reconstruction. The proposed method can be a cost-effective solution for applications demanding tomographic imaging and can be easily integrated into conventional linear array-based ultrasound systems. We present a potential application for finger joint imaging in vivo, which can be used for point-of-care rheumatoid arthritis diagnosis and monitoring.

Shedding light on pediatric diseases: multispectral optoacoustic tomography at the doorway to clinical applications

Optoacoustic imaging (OAI), or photoacoustic imaging (PAI), has fundamentally influenced basic science by providing high-resolution visualization of biological mechanisms. With the introduction of multispectral optoacoustic tomography (MSOT), these technologies have now moved closer to clinical applications. MSOT utilizes short-pulsed near-infrared laser light to induce thermoelastic expansion in targeted tissues. This results in acoustic pressure waves, which are used to resolve specific endo- and exogenous chromophores. Especially in the pediatric population, this non-invasive imaging approach might hold fundamental advantages compared to conventional cross-sectional imaging modalities. As this technology allows the visualization of quantitative molecular tissue composition at high spatial resolution non-invasively in sufficient penetration depth, it paves the way to personalized medicine in pediatric diseases.

The Acquisition Rate and Soundness of a Low-Cost Data Acquisition System (LC-DAQ) for High Frequency Applications

This article presents a novel and reliable low-cost data acquisition solution for high frequency and real-time applications in vehicular dynamics. Data acquisition systems for highly dynamic systems based on low-cost platforms face different challenges such as a constrained data retrieval rate. Basic data reading functions in these platforms are inefficient and, when used, they limit electronics acquisition rate capabilities. This paper explains a new low-cost, modular and open platform to read different types of sensors at high speed rates. Conventional reading functions are avoided to speed up acquisition rate, but this negatively affects data reliability of the system. To solve this and exploit higher data managing rates, a number of custom secure layers are implemented to secure a reliable acquisition. This paper describes the new low-cost electronics developed for high rate acquisition applications and inspects its performance and robustness against the introduction of an increasing number of sensors connected to the board. In most cases, acquisition rates of the system are duplicated using this new solution.

Development of a Stationary 3D Photoacoustic Imaging System Using Sparse Single-Element Transducers: Phantom Study

Photoacoustic imaging (PAI) is an emerging label-free and non-invasive modality for imaging biological tissues. PAI has been implemented in different configurations, one of which is photoacoustic computed tomography (PACT) with a potential wide range of applications, including brain and breast imaging. Hemispherical Array PACT (HA-PACT) is a variation of PACT that has solved the limited detection-view problem. Here, we designed an HA-PACT system consisting of 50 single element transducers. For implementation, we initially performed a simulation study, with parameters close to those in practice, to determine the relationship between the number of transducers and the quality of the reconstructed image. We then used the greatest number of transducers possible on the hemisphere and imaged copper wire phantoms coated with a light absorbing material to evaluate the performance of the system. Several practical issues such as light illumination, arrangement of the transducers, and an image reconstruction algorithm have been comprehensively studied.