Handheld array-based photoacoustic probe for guiding needle biopsy of sentinel lymph nodes

Handheld interventional ultrasound/photoacoustic puncture needle navigation based on deep learning segmentation

Interventional ultrasound (US) has challenges in accurate localization of the puncture needle due to intrinsic acoustic interferences, which lead to blurred, indistinct, and even invisible needles in handheld linear array transducer-based US navigation, especially the incorrect needle tip positioning. Photoacoustic (PA) imaging can provide complementary image contrast, without additional data acquisition. Herein, we proposed an internal illumination to solely light up the needle tip in PA imaging. Then deep-learning-based feature segmentation alleviates acoustic interferences, enhancing the needle shaft-tip visibility. Further, needle shaft-tip compensation aligned the needle shaft in US image and the needle tip in the PA image. The experiments on phantom, ex vivo chicken breast, preclinical radiofrequency ablation and in vivo biopsy of sentinel lymph nodes were piloted. The target registration error can reach the submillimeter level, achieving precise puncture needle tracking ability with in-plane US/PA navigation.

Niche preclinical and clinical applications of photoacoustic imaging with endogenous contrast

In the past decade, photoacoustic (PA) imaging has attracted a great deal of popularity as an emergent diagnostic technology owing to its successful demonstration in both preclinical and clinical arenas by various academic and industrial research groups. Such steady growth of PA imaging can mainly be attributed to its salient features, including being non-ionizing, cost-effective, easily deployable, and having sufficient axial, lateral, and temporal resolutions for resolving various tissue characteristics and assessing the therapeutic efficacy. In addition, PA imaging can easily be integrated with the ultrasound imaging systems, the combination of which confers the ability to co-register and cross-reference various features in the structural, functional, and molecular imaging regimes. PA imaging relies on either an endogenous source of contrast (e.g., hemoglobin) or those of an exogenous nature such as nano-sized tunable optical absorbers or dyes that may boost imaging contrast beyond that provided by the endogenous sources. In this review, we discuss the applications of PA imaging with endogenous contrast as they pertain to clinically relevant niches, including tissue characterization, cancer diagnostics/therapies (termed as theranostics), cardiovascular applications, and surgical applications. We believe that PA imaging's role as a facile indicator of several disease-relevant states will continue to expand and evolve as it is adopted by an increasing number of research laboratories and clinics worldwide.

Sentinel lymph node mapping in patients with breast cancer using a photoacoustic/ultrasound dual-modality imaging system with carbon nanoparticles as the contrast agent: a pilot study

Assessing the metastatic status of axillary lymph nodes is a common clinical practice in the staging of early breast cancers. Yet sentinel lymph nodes (SLNs) are the regional lymph nodes believed to be the first stop along the lymphatic drainage path of the metastasizing cancer cells. Compared to axillary lymph node dissection, sentinel lymph node biopsy (SLNB) helps reduce morbidity and side effects. Current SLNB methods, however, still have suboptimum properties, such as restrictions due to nuclide accessibility and a relatively low therapeutic efficacy when only a single contrast agent is used. To overcome these limitations, researchers have been motivated to develop a non-radioactive SLN mapping method to replace or supplement radionuclide mapping. We proposed and demonstrated a clinical procedure using a dual-modality photoacoustic (PA)/ultrasound (US) imaging system to locate the SLNs to offer surgical guidance. In our work, the high contrast of PA imaging and its specificity to SLNs were based on the accumulation of carbon nanoparticles (CNPs) in the SLNs. A machine-learning model was also trained and validated to distinguish stained SLNs based on single-wavelength PA images. In the pilot study, we imaged 11 patients in vivo, and the specimens from 13 patients were studied ex vivo. PA/US imaging identified stained SLNs in vivo without a single false positive (23 SLNs), yielding 100% specificity and 52.6% sensitivity based on the current PA imaging system. Our machine-learning model can automatically detect SLNs in real time. In the new procedure, single-wavelength PA/US imaging uses CNPs as the contrast agent. The new system can, with that contrast agent, noninvasively image SLNs with high specificity in real time based on the unique features of the SLNs in the PA images. Ultimately, we aim to use our systems and approach to substitute or supplement nuclide tracers for a non-radioactive, less invasive SLN mapping method in SLNB for the axillary staging of breast cancer.

Recent Advances in Contrast-Enhanced Photoacoustic Imaging: Overcoming the Physical and Practical Challenges

For decades now, photoacoustic imaging (PAI) has been investigated to realize its potential as a niche biomedical imaging modality. Despite its highly desirable optical contrast and ultrasonic spatiotemporal resolution, PAI is challenged by such physical limitations as a low signal-to-noise ratio (SNR), diminished image contrast due to strong optical attenuation, and a lower-bound on spatial resolution in deep tissue. In addition, contrast-enhanced PAI has faced practical limitations such as insufficient cell-specific targeting due to low delivery efficiency and difficulties in developing clinically translatable agents. Identifying these limitations is essential to the continuing expansion of the field, and substantial advances in developing contrast-enhancing agents, complemented by high-performance image acquisition systems, have synergistically dealt with the challenges of conventional PAI. This review covers the past four years of research on pushing the physical and practical challenges of PAI in terms of SNR/contrast, spatial resolution, targeted delivery, and clinical application. Promising strategies for dealing with each challenge are reviewed in detail, and future research directions for next generation contrast-enhanced PAI are discussed.

The emerging role of photoacoustic imaging in clinical oncology

Clinical oncology can benefit substantially from imaging technologies that reveal physiological characteristics with multiscale observations. Complementing conventional imaging modalities, photoacoustic imaging (PAI) offers rapid imaging (for example, cross-sectional imaging in real time or whole-breast scanning in 10-15 s), scalably high levels of spatial resolution, safe operation and adaptable configurations. Most importantly, this novel imaging modality provides informative optical contrast that reveals details on anatomical, functional, molecular and histological features. In this Review, we describe the current state of development of PAI and the emerging roles of this technology in cancer screening, diagnosis and therapy. We comment on the performance of cutting-edge photoacoustic platforms, and discuss their clinical applications and utility in various clinical studies. Notably, the clinical translation of PAI is accelerating in the areas of macroscopic and mesoscopic imaging for patients with breast or skin cancers, as well as in microscopic imaging for histopathology. We also highlight the potential of future developments in technological capabilities and their clinical implications, which we anticipate will lead to PAI becoming a desirable and widely used imaging modality in oncological research and practice.

Copper-Catalyzed Covalent Dimerization of Near-Infrared Fluorescent Cyanine Dyes: Synergistic Enhancement of Photoacoustic Signals for Molecular Imaging of Tumors

Photoacoustic (PA) imaging relies on the absorption of light by chromophores to generate acoustic waves used to delineate tissue structures and physiology. Here, we demonstrate that Cu(II) efficiently catalyzes the dimerization of diverse near-infrared (NIR) cyanine molecules, including a peptide conjugate. NMR spectroscopy revealed a C-C covalent bond along the heptamethine chains, creating stable molecules under conditions such as a wide range of solvents and pH mediums. Dimerization achieved >90% fluorescence quenching, enhanced photostability, and increased PA signals by a factor of about 4 at equimolar concentrations compared to the monomers. In vivo study with a mouse cancer model revealed that dimerization enhanced tumor retention and PA signal, allowing cancer detection at doses where the monomers are less effective. While the dye dimers highlighted peritumoral blood vessels, the PA signal for dimeric tumor-targeting dye-peptide conjugate, LS301, was diffuse throughout the entire tumor mass. A combination of the ease of synthesis, diversity of molecules that are amenable to Cu(II)-catalyzed dimerization, and the high acoustic wave amplification by these stable dimeric small molecules ushers a new strategy to develop clinically translatable PA molecular amplifiers for the emerging field of molecular photoacoustic imaging.

Superiorized Photo-Acoustic Non-NEgative Reconstruction (SPANNER) for Clinical Photoacoustic Imaging

Photoacoustic (PA) imaging can revolutionize medical ultrasound by augmenting it with molecular information. However, clinical translation of PA imaging remains a challenge due to the limited viewing angles and imaging depth. Described here is a new robust algorithm called Superiorized Photo-Acoustic Non-NEgative Reconstruction (SPANNER), designed to reconstruct PA images in real-time and to address the artifacts associated with limited viewing angles and imaging depth. The method utilizes precise forward modeling of the PA propagation and reception of signals while accounting for the effects of acoustic absorption, element size, shape, and sensitivity, as well as the transducer's impulse response and directivity pattern. A fast superiorized conjugate gradient algorithm is used for inversion. SPANNER is compared to three reconstruction algorithms: delay-and-sum (DAS), universal back-projection (UBP), and model-based reconstruction (MBR). All four algorithms are applied to both simulations and experimental data acquired from tissue-mimicking phantoms, ex vivo tissue samples, and in vivo imaging of the prostates in patients. Simulations and phantom experiments highlight the ability of SPANNER to improve contrast to background ratio by up to 20 dB compared to all other algorithms, as well as a 3-fold increase in axial resolution compared to DAS and UBP. Applying SPANNER on contrast-enhanced PA images acquired from prostate cancer patients yielded a statistically significant difference before and after contrast agent administration, while the other three image reconstruction methods did not, thus highlighting SPANNER's performance in differentiating intrinsic from extrinsic PA signals and its ability to quantify PA signals from the contrast agent more accurately.

Laser-Generated Leaky Acoustic Wave Imaging for Interventional Guidewire Guidance

Ultrasound (US) is widely used to visualize both tissue and the positions of surgical instruments in real time during surgery. Previously we proposed a new method to exploit US imaging and laser-generated leaky acoustic waves (LAWs) for needle visualization. Although successful, that method only detects the position of a needle tip, with the location of the entire needle deduced from knowing that the needle is straight. The purpose of the current study was to develop a beamforming-based method for the direct visualization of objects. The approach can be applied to objects with arbitrary shapes, such as the guidewires that are commonly used in interventional guidance. With this method, illumination by a short laser pulse generates photoacoustic waves at the top of the guidewire that propagate down its metal surface. These waves then leak into the surrounding tissue, which can be detected by a US array transducer. The time of flight consists of two parts: 1) the propagation time of the guided waves on the guidewire and 2) the propagation time of the US that leaks into the tissue. In principle, an image of the guidewire can be formed based on array beamforming by taking the propagation time on the metal into consideration. Furthermore, we introduced directional filtering and a matched filter to compress the dispersion signal associated with long propagation times. The results showed that guidewires could be detected at depths of at least 70 mm. The maximum detectable angle was 56.3°. LAW imaging with a 1268-mm-long guidewire was also demonstrated. The proposed method has considerable potential in new clinical applications.

Deep learning for biomedical photoacoustic imaging: A review

Photoacoustic imaging (PAI) is a promising emerging imaging modality that enables spatially resolved imaging of optical tissue properties up to several centimeters deep in tissue, creating the potential for numerous exciting clinical applications. However, extraction of relevant tissue parameters from the raw data requires the solving of inverse image reconstruction problems, which have proven extremely difficult to solve. The application of deep learning methods has recently exploded in popularity, leading to impressive successes in the context of medical imaging and also finding first use in the field of PAI. Deep learning methods possess unique advantages that can facilitate the clinical translation of PAI, such as extremely fast computation times and the fact that they can be adapted to any given problem. In this review, we examine the current state of the art regarding deep learning in PAI and identify potential directions of research that will help to reach the goal of clinical applicability.

Functional photoacoustic/ultrasound imaging for the assessment of breast intraductal lesions: preliminary clinical findings

This study aimed to identify features of breast intraductal lesions in photoacoustic/ultrasound (PA/US) imaging and compare PA/US with color Doppler flow/ultrasound (CDFI/US) in the evaluation of breast intraductal lesions. In the nine patients with 10 breast intraductal lesions and 8 patients with 8 benign lesions, total vessel scores evaluated from PA/US are significantly greater than those from CDFI/US (p=0.005). PA internal vessel scores and oxygen saturation (SO2) score are significantly increased in breast intraductal lesions than in benign lesions (p=0.016, p=0.006). With a cutoff PA score (sum of PA internal vessel score and SO2 score) of 2.5, we obtained a sensitivity of 90% and a specificity of 87.5% in differentiation of two groups. PA/US upgraded 40% of breast intraductal lesions, and downgraded 50% of benign lesions from the Breast Imaging Reporting and Data System grading results based on CDFI/US. PA/US functional imaging has the potential in differentiating breast intraductal lesions.

A colorful approach towards developing new nano-based imaging contrast agents for improved cancer detection

Providing physicians with new imaging agents to help detect cancer with better sensitivity and specificity has the potential to significantly improve patient outcomes. Development of new imaging agents could offer improved early cancer detection during routine screening or help surgeons identify tumor margins for surgical resection. In this study, we evaluate the optical properties of a colorful class of dyes and pigments that humans routinely encounter. The pigments are often used in tattoo inks and the dyes are FDA approved for the coloring of foods, drugs, and cosmetics. We characterized their absorption, fluorescence and Raman scattering properties in the hopes of identifying a new panel of dyes that offer exceptional imaging contrast. We found that some of these coloring agents, coined as "optical inks", exhibit a multitude of useful optical properties, outperforming some of the clinically approved imaging dyes on the market. The best performing optical inks (Green 8 and Orange 16) were further incorporated into liposomal nanoparticles to assess their tumor targeting and optical imaging potential. Mouse xenograft models of colorectal, cervical and lymphoma tumors were used to evaluate the newly developed nano-based imaging contrast agents. After intravenous injection, fluorescence imaging revealed significant localization of the new "optical ink" liposomal nanoparticles in all three tumor models as opposed to their neighboring healthy tissues (p < 0.05). If further developed, these coloring agents could play important roles in the clinical setting. A more sensitive imaging contrast agent could enable earlier cancer detection or help guide surgical resection of tumors, both of which have been shown to significantly improve patient survival.

Photoacoustic image improvement based on a combination of sparse coding and filtering

SIGNIFICANCE

Photoacoustic imaging (PAI) has been greatly developed in a broad range of diagnostic applications. The efficiency of light to sound conversion in PAI is limited by the ubiquitous noise arising from the tissue background, leading to a low signal-to-noise ratio (SNR), and thus a poor quality of images. Frame averaging has been widely used to reduce the noise; however, it compromises the temporal resolution of PAI.

AIM

We propose an approach for photoacoustic (PA) signal denoising based on a combination of low-pass filtering and sparse coding (LPFSC).

APPROACH

LPFSC method is based on the fact that PA signal can be modeled as the sum of low frequency and sparse components, which allows for the reduction of noise levels using a hybrid alternating direction method of multipliers in an optimization process.

RESULTS

LPFSC method was evaluated using in-silico and experimental phantoms. The results show a 26% improvement in the peak SNR of PA signal compared to the averaging method for in-silico data. On average, LPFSC method offers a 63% improvement in the image contrast-to-noise ratio and a 33% improvement in the structural similarity index compared to the averaging method for objects located at three different depths, ranging from 10 to 20 mm, in a porcine tissue phantom.

CONCLUSIONS

The proposed method is an effective tool for PA signal denoising, whereas it ultimately improves the quality of reconstructed images, especially at higher depths, without limiting the image acquisition speed.

Photoacoustic imaging for surgical guidance: Principles, applications, and outlook

Minimally invasive surgeries often require complicated maneuvers and delicate hand-eye coordination and ideally would incorporate "x-ray vision" to see beyond tool tips and underneath tissues prior to making incisions. Photoacoustic imaging has the potential to offer this feature but not with ionizing x-rays. Instead, optical fibers and acoustic receivers enable photoacoustic sensing of major structures-such as blood vessels and nerves-that are otherwise hidden from view. This imaging process is initiated by transmitting laser pulses that illuminate regions of interest, causing thermal expansion and the generation of sound waves that are detectable with conventional ultrasound transducers. The recorded signals are then converted to images through the beamforming process. Photoacoustic imaging may be implemented to both target and avoid blood-rich surgical contents (and in some cases simultaneously or independently visualize optical fiber tips or metallic surgical tool tips) in order to prevent accidental injury and assist device operators during minimally invasive surgeries and interventional procedures. Novel light delivery systems, counterintuitive findings, and robotic integration methods introduced by the Photoacoustic & Ultrasonic Systems Engineering Lab are summarized in this invited Perspective, setting the foundation and rationale for the subsequent discussion of the author's views on possible future directions for this exciting frontier known as photoacoustic-guided surgery.

In Vivo Dual-Modal Photoacoustic and Ultrasound Imaging of Sentinel Lymph Nodes Using a Solid-State Dye Laser System

Photoacoustic imaging (PAI) is being actively investigated as a non-invasive and non-radioactive imaging technique for sentinel lymph node (SLN) biopsy. By taking advantage of optical and ultrasound imaging, PAI probes SLNs non-invasively with methylene blue (MB) in both live animals and breast cancer patients. However, these PAI systems have limitations for widespread use in clinics and commercial marketplaces because the lasers used by the PAI systems, e.g., tunable liquid dye laser systems and optical parametric oscillator (OPO) lasers, are bulky in size, not economical, and use risky flammable and toxic liquid dyes. To overcome these limitations, we are proposing a novel dual-modal photoacoustic and ultrasound imaging system based on a solid-state dye laser (SD-PAUSI), which is compact, convenient, and carries far less risk of flammability and toxicity. Using a solid-state dye handpiece that generates 650-nm wavelength, we successfully imaged the MB tube positioned deeply (~3.9 cm) in chicken breast tissue. The SLNs were also photoacoustically detected in the in vivo rats beneath a 2.2-cm-thick layer of chicken breast, which is deeper than the typical depth of SLNs in humans (1.2 ± 0.5 cm). Furthermore, we showed the multispectral capability of the PAI by switching the dye handpiece, in which the MB-dyed SLN was selectively highlighted from the surrounding vasculature. These results demonstrated the great potential of the SD-PAUSI as an easy but effective modality for SLN detection.

Towards Clinical Translation of LED-Based Photoacoustic Imaging: A Review

Photoacoustic imaging, with the capability to provide simultaneous structural, functional, and molecular information, is one of the fastest growing biomedical imaging modalities of recent times. As a hybrid modality, it not only provides greater penetration depth than the purely optical imaging techniques, but also provides optical contrast of molecular components in the living tissue. Conventionally, photoacoustic imaging systems utilize bulky and expensive class IV lasers, which is one of the key factors hindering the clinical translation of this promising modality. Use of LEDs which are portable and affordable offers a unique opportunity to accelerate the clinical translation of photoacoustics. In this paper, we first review the development history of LED as an illumination source in biomedical photoacoustic imaging. Key developments in this area, from point-source measurements to development of high-power LED arrays, are briefly discussed. Finally, we thoroughly review multiple phantom, ex-vivo, animal in-vivo, human in-vivo, and clinical pilot studies and demonstrate the unprecedented preclinical and clinical potential of LED-based photoacoustic imaging.

An economic photoacoustic imaging platform using automatic laser synchronization and inverse beamforming

We present a proof-of-concept of an automatic integration of photoacoustic (PA) imaging on clinical ultrasound (US) imaging platforms. Here we tackle two critical challenges: the laser synchronization and the inaccessibility to the beamformer core embedded in commercial US imaging platform. In particular, the line trigger frequency (LTF) estimation and the asynchronous synthetic aperture inverse beamforming (ASAIB) were developed and evaluated in both k-Wave simulation and phantom experiment. The proposed method is an economical solution to enable PA imaging on a greater number of US equipment to further thrive the PA imaging research community.

Photoacoustic Imaging for Management of Breast Cancer: A Literature Review and Future Perspectives

In this review article, a detailed chronological account of the research related to photoacoustic imaging for the management of breast cancer is presented. Performing a detailed analysis of the breast cancer detection related photoacoustic imaging studies undertaken by different research groups, this review attempts to present the clinical evidence in support of using photoacoustic imaging for breast cancer detection. Based on the experimental evidence obtained from the clinical studies conducted so far, the performance of photoacoustic imaging is compared with that of conventional breast imaging modalities. While we find that there is enough experimental evidence to support the use of photoacoustic imaging for breast cancer detection, additional clinical studies are required to be performed to evaluate the diagnostic potential of photoacoustic imaging for identifying different types of breast cancer. To establish the utility of photoacoustic imaging for breast cancer screening, clinical studies with high-risk asymptomatic patients need to be done.

Development of a Handheld Volumetric Photoacoustic Imaging System With a Central-Holed 2D Matrix Aperture

OBJECTIVE

Photoacoustics has been evolving rapidly in the recent several decades. With the aim of clinical translation, a photoacoustic imaging platform with a portable system size, a miniaturized imaging probe, and convenient handheld capability is of great demand.

METHODS

In this work, by adopting an ultrathin central-holed matrix array and a compact coaxial photoacoustic design, a water-free handheld photoacoustic imager is developed (weight: 44 g). Optical Monte Carlo simulation and acoustic k-wave simulation are performed to confirm a relatively homogenous photoacoustic sensitivity distribution within a wide field of view (larger than 10 × 10 × 10 mm³).

RESULTS

Imaging resolution characterized by imaging two hairs is estimated to be about 0.80 mm in the lateral direction and 0.73 mm in the axial direction. To demonstrate the handheld capability in real time, handheld imaging guided needle biopsy in the rat's abdomen is performed at a rate of about 10 Hz (CNR ∼ 14.3 dB). Furthermore, handheld imaging is also demonstrated on visualizing vasculature (mainly cephalic vein) in the human arm (CNR > 4.1 dB).

CONCLUSION

Results demonstrate that the volumetric photoacoustic imaging system using a central-holed 2D matrix array is an attractive choice for achieving convenient handheld operation in real time.

SIGNIFICANCE

Such a compact handheld design may promote the clinical translation of photoacoustic technique.

Three-dimensional interventional photoacoustic imaging for biopsy needle guidance with a linear array transducer

Needle placement is important for many clinical interventions, such as tissue biopsy, regional anesthesia and drug delivery. It is essential to visualize the spatial position of the needle and the target tissue during the interventions using appropriate imaging techniques. Based on the contrast of optical absorption, photoacoustic imaging is well suited for the guidance of interventional procedures. However, conventional photoacoustic imaging typically provides two-dimensional (2D) slices of the region of interest and could only visualize the needle and the target when they are within the imaging plane of the probe at the same time. This requires great alignment skill and effort. To ease this problem, we developed a 3D interventional photoacoustic imaging technique by fast scanning a linear array ultrasound probe and stitching acquired image slices. in vivo sentinel lymph node biopsy experiment shows that the technique could precisely locate a needle and a sentinel lymph node in a tissue volume while a perfusion experiment demonstrates that the technique could visualize the 3D distribution of injected methylene blue dye underneath the skin at high temporal and spatial resolution. The proposed technique provides a practical way for photoacoustic image-guided interventions.

In Vivo Photoacoustic Sentinel Lymph Node Imaging Using Clinically-Approved Carbon Nanoparticles

OBJECTIVE

Breast cancer is the most common type of invasive cancer and one of the leading causes of cancer death in women worldwide. Correct staging of breast cancer is critical to the survival rate of the patients. Sentinel lymph node (SLN) biopsy (SLNB), currently the gold standard technique for breast cancer staging, requires preoperative and intraoperative image guidance for noninvasive SLN identification and minimal surgical invasion. However, existing image guidance techniques suffer from a variety of limitations, such as ionizing radiation, high cost, and poor imaging depth. To address the clinical challenges, new methodology has to be developed.

METHODS

We developed a photoacoustic (PA) imaging procedure for noninvasive and nonradioactive SLN identification and biopsy guidance enhanced with a clinically-approved lymphatic tracer, i.e., carbon nanoparticles (CNPs) suspension injection.

RESULTS

In vivo experiments show that the proposed procedure could sensitively identify the SLN and provide high-contrast image guidance for fine-needle aspiration simulation. In addition, we demonstrated that CNPs have significantly better performance than other commonly-used contrast agents, such as methylene blue and indocyanine green.

CONCLUSION

PA imaging technique using clinically-approved CNPs as the contrast agent is capable for noninvasive and nonradioactive SLN identification and high-contrast biopsy guidance, and should be considered as a new tool for assisting SLNB in breast cancer staging.

SIGNIFICANCE

The proposed CNPs-enhanced PA imaging technique provides a practical way for SLN identification and biopsy guidance for breast cancer patients and paves the way for clinical translation of PA SLN imaging.

Handheld Photoacoustic Imager for Theranostics in 3D

A handheld approach to 3D photoacoustic imaging is essential in clinical applications. To this end, we develop a 3D handheld photoacoustic imager for dynamic (temporally and spatially) volumetric visualization. In this 3D imager, the optically transmitting part and the acoustically receiving part are integrated into a single handheld probe with a compact size about 160 mm ×64 mm ×40 mm. Besides, a dedicated imaging reconstruction algorithm for the heterogeneous medium is developed based on the phase-shift migration method in the frequency domain, which deals well with the stratified condition in the designed system. Dynamic 3D imaging supporting flexible handheld operation is demonstrated with needle biopsy and in vitro temperature measurement for photothermal therapy. The development of such a 3D handheld photoacoustic system paves the way for compact and handheld-operating implementations, and its further clinical exploration is promising.

Photoacoustic clinical imaging

Photoacoustic is an emerging biomedical imaging modality, which allows imaging optical absorbers in the tissue by acoustic detectors (light in - sound out). Such a technique has an immense potential for clinical translation since it allows high resolution, sufficient imaging depth, with diverse endogenous and exogenous contrast, and is free from ionizing radiation. In recent years, tremendous developments in both the instrumentation and imaging agents have been achieved. These opened avenues for clinical imaging of various sites allowed applications such as brain functional imaging, breast cancer screening, diagnosis of psoriasis and skin lesions, biopsy and surgery guidance, the guidance of tumor therapies at the reproductive and urological systems, as well as imaging tumor metastases at the sentinel lymph nodes. Here we survey the various clinical and pre-clinical literature and discuss the potential applications and hurdles that still need to be overcome.

Real-Time Photoacoustic Thermometry Combined With Clinical Ultrasound Imaging and High-Intensity Focused Ultrasound

High-intensity focused ultrasound (HIFU) treatment is a promising non-invasive method for killing or destroying the diseased tissues by locally delivering thermal and mechanical energy without damaging surrounding normal tissues. In HIFU, measuring the temperature at the site of delivery is important for improving therapeutic efficacy, controlling safety, and appropriately planning a treatment. Several researchers have proposed photoacoustic thermometry for monitoring HIFU treatment, but they had many limitations, including the inability to image while the HIFU is on, inability to provide two-dimensional monitoring, and the inability to be used clinically. In this paper, we propose a novel integrated real-time photoacoustic thermometry system for HIFU treatment monitoring. The system provides ultrasound B-mode imaging, photoacoustic structural imaging, and photoacoustic thermometry during HIFU treatment in real-time for both in vitro and in vivo environments, without any interference from the strong therapeutic HIFU waves. We have successfully tested the real-time photoacoustic thermometry by investigating the relationship between the photoacoustic amplitude and the measured temperature with in vitro phantoms and in vivo tumor-bearing mice. The results show the feasibility of a real-time photoacoustic thermometry system for safe and effective monitoring of HIFU treatment.

Breast sentinel lymph node biopsy with imaging towards minimally invasive surgery

Breast sentinel lymph nodes are still commonly assessed through complete lymph node dissections, which is a time-consuming and radical approach because the nodes are difficult to identify. To prevent false diagnosis and achieve accurate results, minimally invasive, image-guided procedures are applied and constantly improved. The purpose of this paper is to present the currently used imaging modalities ultrasound, fluorescence, single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI) and hybrid imaging methods and comparing their effectiveness for breast sentinel lymph node biopsy. A definition for an ideal imaging system combining efficient minimally invasive techniques with workflow considerations is also discussed. As a conclusion, upcoming imaging methods and their future outlook with areas of advancement are presented.

Real-time 3D Photoacoustic Visualization System with a Wide Field of View for Imaging Human Limbs

Background: A breast-specific photoacoustic imaging (PAI) system prototype equipped with a hemispherical detector array (HDA) has been reported as a promising system configuration for providing high morphological reproducibility for vascular structures in living bodies. Methods: To image the vasculature of human limbs, a newly designed PAI system prototype (PAI-05) with an HDA with a higher density sensor arrangement was developed. The basic device configuration mimicked that of a previously reported breast-specific PAI system. A new imaging table and a holding tray for imaging a subject's limb were adopted. Results: The device's performance was verified using a phantom. Contrast of 8.5 was obtained at a depth of 2 cm, and the viewing angle reached up to 70 degrees, showing sufficient performance for limb imaging. An arbitrary wavelength was set, and a reasonable PA signal intensity dependent on the wavelength was obtained. To prove the concept of imaging human limbs, various parts of the subject were scanned. High-quality still images of a living human with a wider size than that previously reported were obtained by scanning within the horizontal plane and averaging the images. The maximum field of view (FOV) was 270 mm × 180 mm. Even in movie mode, one-shot 3D volumetric data were obtained in an FOV range of 20 mm in diameter, which is larger than values in previous reports. By continuously acquiring these images, we were able to produce motion pictures. Conclusion: We developed a PAI prototype system equipped with an HDA suitable for imaging limbs. As a result, the subject could be scanned over a wide range while in a more comfortable position, and high-quality still images and motion pictures could be obtained.

Real-time 3D Photoacoustic Visualization System with a Wide Field of View for Imaging Human Limbs

Background: A breast-specific photoacoustic imaging (PAI) system prototype equipped with a hemispherical detector array (HDA) has been reported as a promising system configuration for providing high morphological reproducibility for vascular structures in living bodies. Methods: To image the vasculature of human limbs, a newly designed PAI system prototype (PAI-05) with an HDA with a higher density sensor arrangement was developed. The basic device configuration mimicked that of a previously reported breast-specific PAI system. A new imaging table and a holding tray for imaging a subject's limb were adopted. Results: The device's performance was verified using a phantom. Contrast of 8.5 was obtained at a depth of 2 cm, and the viewing angle reached up to 70 degrees, showing sufficient performance for limb imaging. An arbitrary wavelength was set, and a reasonable PA signal intensity dependent on the wavelength was obtained. To prove the concept of imaging human limbs, various parts of the subject were scanned. High-quality still images of a living human with a wider size than that previously reported were obtained by scanning within the horizontal plane and averaging the images. The maximum field of view (FOV) was 270 mm × 180 mm. Even in movie mode, one-shot 3D volumetric data were obtained in an FOV range of 20 mm in diameter, which is larger than values in previous reports. By continuously acquiring these images, we were able to produce motion pictures. Conclusion: We developed a PAI prototype system equipped with an HDA suitable for imaging limbs. As a result, the subject could be scanned over a wide range while in a more comfortable position, and high-quality still images and motion pictures could be obtained.

Design and Fabrication of a Miniaturized Convex Array for Combined Ultrasound and Photoacoustic Imaging of the Prostate

Although transrectal ultrasound (TRUS) imaging is widely used for screening and diagnosing prostate cancer, it is often not found on TRUS images, depending on its stage, size, and location. In addition, due to the weak echo signal and the low contrast of TRUS images, it is difficult to diagnose early-stage prostate cancers and distinguish malignant tumors from benign prostatic hyperplasia. For this reason, TRUS image-guided biopsy is mandatory to confirm the malignancy of the suspicious tumor, but the diagnostic accuracy of initial biopsy is only 20%-30%, so that the patients inevitably undergo repeated biopsies. TRUS-photoacoustic (TRUS-PA) imaging is one way to resolve those problems. However, the development of a TRUS-PA probe, in which an ultrasound array transducer and optical fibers are integrated, is demanding because the overall size of the probe should be as small as possible for the convenience of the patients, while providing the desired performances. Here, we report a recently developed TRUS-PA probe. The core element of the TRUS-PA is a miniaturized 128-element, 7-MHz convex array transducer of which size in the lateral and elevational directions is 11.4 and 5 mm, respectively. A new concept of a flexible printed circuit board was also developed to limit the size of the TRUS-PA probe to less than 15 mm. From the performance evaluation, it was found that the developed array with a field-of-view of 134° has a center frequency of 6.75 MHz, a -6-dB fractional bandwidth of 66%, and a crosstalk of less than -45 dB. In the tissue-mimicking phantom test and ex vivo experiments, the miniaturized convex array proved to be capable of providing combined US and PA images with acceptable imaging quality in spite of its small size.

Visualising peripheral arterioles and venules through high-resolution and large-area photoacoustic imaging

Photoacoustic (PA) imaging (PAI) has been shown to be a promising tool for non-invasive blood vessel imaging. A PAI system comprising a hemispherical detector array (HDA) has been reported previously as a method providing high morphological reproducibility. However, further improvements in diagnostic capability will require improving the image quality of PAI and fusing functional and morphological imaging. Our newly developed PAI system prototype not only enhances the PA image resolution but also acquires ultrasonic (US) B-mode images at continuous positions in the same coordinate axes. In addition, the pulse-to-pulse alternating laser irradiation shortens the measurement time difference between two wavelengths. We scanned extremities and breasts in an imaging region 140 mm in diameter and obtained 3D-PA images of fine blood vessels, including arterioles and venules. We could estimate whether a vessel was an artery or a vein by using the S-factor obtained from the PA images at two wavelengths, which corresponds approximately to the haemoglobin oxygen saturation. Furthermore, we observed tumour-related blood vessels around breast tumours with unprecedented resolution. In the future, clinical studies with our new PAI system will help to elucidate various mechanisms of vascular-associated diseases and events.

Compact and low-cost handheld quasibright-field linear-array probe design in photoacoustic computed tomography

The optimal photoacoustic probe design is the key to obtain highest imaging sensitivity in photoacoustic computed tomography. Two commonly used probe design types are dark- and bright-field designs. We proposed a design for photoacoustic probe called quasibright-field illumination and compared the performance of all three kinds of probes theoretically and experimentally. Our conclusion is that the proposed quasibright-field illumination photoacoustic probe is superior compared to the existing probe designs as demonstrated. However, each type of illumination should still have its own advantages under certain circumstances. The dark-field illumination is capable of minimizing surface interference signals and reducing their contributions to the background of deeper signals. Hence, it should perform better when imaging samples with high optical absorbance at the surface layer. The bright field may perform better under circumstance when phase distortion is less. We also designed and fabricated three kinds of probes using a single multimode optical fiber for laser energy delivery instead of fiber bundle. Single fiber probes are low cost, transmit laser energy efficiently, and are compact for easy handling. Thus, our study not only provides a method for probe design but also a guidance for cost-effective transducer array-based photoacoustic probe design and manufacturing in the future.

Clinically-Applicable Perfluorocarbon-Loaded Nanoparticles For In vivo Photoacoustic, 19F Magnetic Resonance And Fluorescent Imaging

Photoacoustic imaging (PAI) is an emerging biomedical imaging technique that is now coming to the clinic. It has a penetration depth of a few centimeters and generates useful endogenous contrast, particularly from melanin and oxy-/deoxyhemoglobin. Indocyanine green (ICG) is a Food and Drug Administration-approved contrast agents for human applications, which can be also used in PAI. It is a small molecule dye with limited applications due to its fast clearance, rapid protein binding, and bleaching effect. Methods: Here, we entrap ICG in a poly(lactic-co-glycolic acid) nanoparticles together with a perfluorocarbon (PFC) using single emulsion method. These nanoparticles and nanoparticle-loaded dendritic cells were imaged with PA, 19F MR, and fluorescence imaging in vitro and in vivo. Results: We formulated particles with an average diameter of 200 nm. The encapsulation of ICG within nanoparticles decreased its photobleaching and increased the retention of the signal within cells, making it available for applications such as cell imaging. As little as 0.1x106 cells could be detected in vivo with PAI using automated spectral unmixing. Furthermore, we observed the accumulation of ICG signal in the lymph node after subcutaneous injection of nanoparticles. Conclusion: We show that we can label primary human dendritic cells with the nanoparticles and image them in vitro and in vivo, in a multimodal manner. This work demonstrates the potential of combining PAI and 19F MRI for cell imaging and lymph node detection using nanoparticles that are currently produced at GMP-grade for clinical use.

Handheld Real-Time LED-Based Photoacoustic and Ultrasound Imaging System for Accurate Visualization of Clinical Metal Needles and Superficial Vasculature to Guide Minimally Invasive Procedures

Ultrasound imaging is widely used to guide minimally invasive procedures, but the visualization of the invasive medical device and the procedure’s target is often challenging. Photoacoustic imaging has shown great promise for guiding minimally invasive procedures, but clinical translation of this technology has often been limited by bulky and expensive excitation sources. In this work, we demonstrate the feasibility of guiding minimally invasive procedures using a dual-mode photoacoustic and ultrasound imaging system with excitation from compact arrays of light-emitting diodes (LEDs) at 850 nm. Three validation experiments were performed. First, clinical metal needles inserted into biological tissue were imaged. Second, the imaging depth of the system was characterized using a blood-vessel-mimicking phantom. Third, the superficial vasculature in human volunteers was imaged. It was found that photoacoustic imaging enabled needle visualization with signal-to-noise ratios that were 1.2 to 2.2 times higher than those obtained with ultrasound imaging, over insertion angles of 26 to 51 degrees. With the blood vessel mimicking phantom, the maximum imaging depth was 38 mm. The superficial vasculature of a human middle finger and a human wrist were clearly visualized in real-time. We conclude that the LED-based system is promising for guiding minimally invasive procedures with peripheral tissue targets.

Context encoding enables machine learning-based quantitative photoacoustics

Real-time monitoring of functional tissue parameters, such as local blood oxygenation, based on optical imaging could provide groundbreaking advances in the diagnosis and interventional therapy of various diseases. Although photoacoustic (PA) imaging is a modality with great potential to measure optical absorption deep inside tissue, quantification of the measurements remains a major challenge. We introduce the first machine learning-based approach to quantitative PA imaging (qPAI), which relies on learning the fluence in a voxel to deduce the corresponding optical absorption. The method encodes relevant information of the measured signal and the characteristics of the imaging system in voxel-based feature vectors, which allow the generation of thousands of training samples from a single simulated PA image. Comprehensive in silico experiments suggest that context encoding-qPAI enables highly accurate and robust quantification of the local fluence and thereby the optical absorption from PA images.

Laser Generated Leaky Acoustic Waves for Needle Visualization

Ultrasound (US)-guided needle operation is usually used to visualize both tissue and needle position such as tissue biopsy and localized drug delivery. However, the transducer-needle orientation is limited due to reflection of the acoustic waves. We proposed a leaky acoustic wave method to visualize the needle position and orientation. Laser pulses are emitted on top of the needle to generate acoustic waves; then, these acoustic waves propagate along the needle surface. Leaky wave signals are detected by the US array transducer. The needle position can be calculated by phase velocities of two different wave modes and their corresponding emission angles. In our experiments, a series of needles was inserted into a tissue mimicking phantom and porcine tissue to evaluate the accuracy of the proposed method. The results show that the detection depth is up to 51 mm and the insertion angle is up to 40° with needles of different diameters. It is demonstrated that the proposed approach outperforms the conventional B-mode US-guided needle operation in terms of the detection range while achieving similar accuracy. The proposed method reveals the potentials for further clinical applications.

Linear array-based real-time photoacoustic imaging system with a compact coaxial excitation handheld probe for noninvasive sentinel lymph node mapping

We developed a linear ultrasound array-based real-time photoacoustic imaging system with a compact coaxial excitation handheld photoacoustic imaging probe for guiding sentinel lymph node (SLN) needle biopsy. Compared with previous studies, our system and probe have the following advantages: (1) the imaging probe is quite compact and user-friendly; (2) laser illumination and ultrasonic detection are achieved coaxially, enabling high signal-to-noise ratio; and (3) GPU-based image reconstruction enables real-time imaging and displaying at a frame rate of 20 Hz. With the system and probe, clear visualization of the SLN at the depth of 2 cm (~human SLN depth) was demonstrated on a living rat. A fine needle was pushed towards the SLN based on the guidance of real-time photoacoustic imaging. The proposed photoacoustic imaging system and probe was shown to have great potential to be used in clinics for guiding SLN needle biopsy, which may reduce the high morbidity rate related to the current gold standard clinical SLN biopsy procedure.

Internal-illumination photoacoustic computed tomography

We report a photoacoustic computed tomography (PACT) system using a customized optical fiber with a cylindrical diffuser to internally illuminate deep targets. The traditional external light illumination in PACT usually limits the penetration depth to a few centimeters from the tissue surface, mainly due to strong optical attenuation along the light propagation path from the outside in. By contrast, internal light illumination, with external ultrasound detection, can potentially detect much deeper targets. Different from previous internal illumination PACT implementations using forward-looking optical fibers, our internal-illumination PACT system uses a customized optical fiber with a 3-cm-long conoid needle diffuser attached to the fiber tip, which can homogeneously illuminate the surrounding space and substantially enlarge the field of view. We characterized the internal illumination distribution and PACT system performance. We performed tissue phantom and in vivo animal studies to further demonstrate the superior imaging depth using internal illumination over external illumination. We imaged a 7.5-cm-deep leaf target embedded in optically scattering medium and the beating heart of a mouse overlaid with 3.7-cm-thick chicken tissue. Our results have collectively demonstrated that the internal light illumination combined with external ultrasound detection might be a useful strategy to improve the penetration depth of PACT in imaging deep organs of large animals and humans.

Non-invasive sentinel lymph node mapping and needle guidance using clinical handheld photoacoustic imaging system in small animal

Translating photoacoustic imaging (PAI) into clinical setup is a challenge. Handheld clinical real-time PAI systems are not common. In this work, we report an integrated photoacoustic (PA) and clinical ultrasound imaging system by combining light delivery with the ultrasound probe for sentinel lymph node imaging and needle guidance in small animal. The open access clinical ultrasound platform allows seamless integration of PAI resulting in the development of handheld real-time PAI probe. Both methylene blue and indocyanine green were used for mapping the sentinel lymph node using 675 and 690 nm wavelength illuminations, respectively. Additionally, needle guidance with combined ultrasound and PAI was demonstrated using this imaging system. Up to 1.5 cm imaging depth was observed with a 10 Hz laser at an imaging frame rate of 5 frames per second, which is sufficient for future translation into human sentinel lymph node imaging and needle guidance for fine needle aspiration biopsy.

Hand-held Clinical Photoacoustic Imaging System for Real-time Non-invasive Small Animal Imaging

Translation of photoacoustic imaging into the clinic is a major challenge. Handheld real-time clinical photoacoustic imaging systems are very rare. Here, we report a combined photoacoustic and clinical ultrasound imaging system by integrating an ultrasound probe with light delivery for small animal imaging. We demonstrate this by showing sentinel lymph node imaging in small animals along with minimally invasive real-time needle guidance. A clinical ultrasound platform with access to raw channel data allows the integration of photoacoustic imaging leading to a handheld real-time clinical photoacoustic imaging system. Methylene blue was used for sentinel lymph node imaging at 675 nm wavelength. Additionally, needle guidance with dual modal ultrasound and photoacoustic imaging was shown using the imaging system. Depth imaging of up to 1.5 cm was demonstrated with a 10 Hz laser at a photoacoustic imaging frame rate of 5 frames per second.

Real-time Triple-modal Photoacoustic, Ultrasound, and Magnetic Resonance Fusion Imaging of Humans

Imaging that fuses multiple modes has become a useful tool for diagnosis and therapeutic monitoring. As a next step, real-time fusion imaging has attracted interest as for a tool to guide surgery. One widespread fusion imaging technique in surgery combines real-time ultrasound (US) imaging and pre-acquired magnetic resonance (MR) imaging. However, US imaging visualizes only structural information with relatively low contrast. Here, we present a photoacoustic (PA), US, and MR fusion imaging system which integrates a clinical PA and US imaging system with an optical tracking-based navigation sub-system. Through co-registration of pre-acquired MR and real-time PA/US images, overlaid PA, US, and MR images can be concurrently displayed in real time. We successfully acquired fusion images from a phantom and a blood vessel in a human forearm. This fusion imaging can complementarily delineate the morphological and vascular structure of tissues with good contrast and sensitivity, has a well-established user interface, and can be flexibly integrated with clinical environments. As a novel fusion imaging, the proposed triple-mode imaging can provide comprehensive image guidance in real time, and can potentially assist various surgeries.

Photoacoustic imaging of dental implants in a porcine jawbone ex vivo

Currently, x-ray-based imaging is used before and after the dental implant treatment, but the ionizing radiation is potentially harmful to patients and operators. Here, we demonstrate ex vivo photoacoustic imaging of a dental implant embedded in a porcine jawbone. By layering biological tissue over the jawbone to mimic a clinical environment, we demonstrate 10 mm deep imaging. Our results show that photoacoustic imaging can provide jawbone anatomical information, the location of an embedded implant fixture, and the thickness of the soft tissue above the jawbone.

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.

Real-time sentinel lymph node biopsy guidance using combined ultrasound, photoacoustic, fluorescence imaging: in vivo proof-of-principle and validation with nodal obstruction

Precise sentinel lymph node (SLN) identification is crucial not only for accurate diagnosis of micro-metastases at an early stage of cancer progression but also for reducing the number of SLN biopsies (SLNB) to minimize their severe side effects. Furthermore, it is desirable that an SLNB guidance should be as safe as possible in routine clinical use. Although there are currently various SLNB guidance methods for pre-operative or intra-operative assessment, none are ideal. We propose a real-time SLNB guidance method using contrast-enhanced tri-modal images (i.e., ultrasound, photoacoustic, and fluorescence) acquired by a recently developed hand-held tri-modal probe. The major advantage of tri-modal imaging is demonstrated here through an in vivo study of the technically-difficult case of nodal obstruction that frequently leads to false-negative results in patients. The results in a tumor model in rabbits and normal controls showed that tri-modal imaging is capable of clearly identifying obstructed SLNs and of indicating their metastatic involvement. Based on these findings, we propose an SLNB protocol to help surgeons take full advantage of the complementary information obtained from tri-modal imaging, including for pre-operative localization, intra-operative biopsy guidance and post-operative analysis.

How Can New Imaging Modalities Help in the Practice of Radiology?

The purpose of this article was to provide an up-to-date review on the spectrum of new imaging applications in the practice of radiology. New imaging techniques have been developed with the objective of obtaining structural and functional analyses of different body systems. Recently, new imaging modalities have aroused the interest of many researchers who are studying the applicability of these modalities in the evaluation of different organs and diseases. In this review article, we present the efficiency and utilization of current imaging modalities in daily radiological practice.

Identification and removal of laser-induced noise in photoacoustic imaging using singular value decomposition

Singular value decomposition (SVD) was used to identify and remove laser-induced noise in photoacoustic images acquired with a clinical ultrasound scanner. This noise, which was prominent in the radiofrequency data acquired in parallel from multiple transducer elements, was induced by the excitation light source. It was modelled by truncating the SVD matrices so that only the first few largest singular value components were retained, and subtracted prior to image reconstruction. The dependency of the signal amplitude and the number of the largest singular value components used for noise modeling was investigated for different photoacoustic source geometries. Validation was performed with simulated data and measured noise, and with photoacoustic images acquired from the human forearm and finger in vivo using L14-5/38 and L40-8/12 linear array clinical imaging probes. The use of only one singular value component was found to be sufficient to achieve near-complete removal of laser-induced noise from reconstructed images. This method has strong potential to increase image quality for a wide range of photoacoustic imaging systems with parallel data acquisition.

64 CuS-labeled nanoparticles: a new sentinel-lymph-node-mapping agent for PET-CT and photoacoustic tomography

Determining sentinel lymph node (SLN) status is critical to cancer staging and treatment decisions. Currently, in clinical practice, ^99m Tc-radiocolloid-mediated planar scintigraphy and single-photon emission computed tomography (SPECT) are used to guide the biopsy and resection of SLNs. Recently, an emerging technique that combines positron emission tomography (PET) and photoacoustic tomography (PAT; PET-PAT) may offer accurate information in detecting SLNs. Herein, we report a kind of ⁶⁴ CuS-labeled nanoparticle (⁶⁴ CuS-NP) for the detection of SLNs with PET-PAT. We subcutaneously injected ⁶⁴ CuS-NPs into the rats' forepaw pads. After 24 h, the rats' first draining axillary lymph nodes (i.e. the SLNs) could be clearly visualized with micro-PET (μPET)-CT. Rats were sacrificed after μPET-CT imaging, their axillary lymph nodes were surgically identified, and then PAT was employed to discover ⁶⁴ CuS-NP-avid SLNs, which were embedded inside tissues. Biodistribution, autoradiography, and copper staining analyses confirmed the SLNs' high uptake of ⁶⁴ CuS-NPs. Our study indicates that ⁶⁴ CuS-NPs are a promising dual-function agent for both PET-CT and PAT and could be used with multi-modal imaging strategies such as PET-PAT to identify SLNs in a clinical setting. Copyright © 2016 John Wiley & Sons, Ltd.

Photoacoustic Imaging in Oncology: Translational Preclinical and Early Clinical Experience

Photoacoustic imaging has evolved into a clinically translatable platform with the potential to complement existing imaging techniques for the management of cancer, including detection, characterization, prognosis, and treatment monitoring. In photoacoustic imaging, tissue is optically excited to produce ultrasonographic images that represent a spatial map of optical absorption of endogenous constituents such as hemoglobin, fat, melanin, and water or exogenous contrast agents such as dyes and nanoparticles. It can therefore provide functional and molecular information that allows noninvasive soft-tissue characterization. Photoacoustic imaging has matured over the years and is currently being translated into the clinic with various clinical studies underway. In this review, the current state of photoacoustic imaging is presented, including techniques and instrumentation, followed by a discussion of potential clinical applications of this technique for the detection and management of cancer. (©) RSNA, 2016.