Slip-ring-based multi-transducer photoacoustic tomography system

Whole-Body Photoacoustic Imaging Techniques for Preclinical Small Animal Studies

Photoacoustic imaging is a hybrid imaging technique that has received considerable attention in biomedical studies. In contrast to pure optical imaging techniques, photoacoustic imaging enables the visualization of optical absorption properties at deeper imaging depths. In preclinical small animal studies, photoacoustic imaging is widely used to visualize biodistribution at the molecular level. Monitoring the whole-body distribution of chromophores in small animals is a key method used in preclinical research, including drug-delivery monitoring, treatment assessment, contrast-enhanced tumor imaging, and gastrointestinal tracking. In this review, photoacoustic systems for the whole-body imaging of small animals are explored and summarized. The configurations of the systems vary with the scanning methods and geometries of the ultrasound transducers. The future direction of research is also discussed with regard to achieving a deeper imaging depth and faster imaging speed, which are the main factors that an imaging system should realize to broaden its application in biomedical studies.

Another decade of photoacoustic imaging

Photoacoustic imaging - a hybrid biomedical imaging modality finding its way to clinical practices. Although the photoacoustic phenomenon was known more than a century back, only in the last two decades it has been widely researched and used for biomedical imaging applications. In this review we focus on the development and progress of the technology in the last decade (2010-2020). From becoming more and more user friendly, cheaper in cost, portable in size, photoacoustic imaging promises a wide range of applications, if translated to clinic. The growth of photoacoustic community is steady, and with several new directions researchers are exploring, it is inevitable that photoacoustic imaging will one day establish itself as a regular imaging system in the clinical practices.

Optical Biopsy of Melanoma and Basal Cell Carcinoma Progression by Noncontact Photoacoustic and Optical Coherence Tomography: In Vivo Multi-Parametric Characterizing Tumor Microenvironment

Measuring the structural and functional status of tumor microenvironment for malignant melanoma (MM) and basal cell carcinoma (BCC) is of profound significance in understanding dermatological condition for biopsy. However, conventional optical imaging techniques are limited to visualize superficial skin features and parameter information is deficient to depict pathophysiology correlations of skin diseases. Here, we demonstrate a preclinical device, all-optically integrated photoacoustic and optical coherence tomography (AOPA/OCT), that, for the first time, can simultaneously provide label-free biomarkers of vascular patterns, temporal and spatial heterogeneity of blood flow, and tissue micro-structure changes during tumor growth with pathophysiological correlations in mice models. We found that tumor microenvironment of MM and BCC led to the alternation in spatial-temporal heterogeneity that affected morphological and functional parameters, performing the AOPA/OCT quantitative metrics. A robust correlation between imaging biomarkers derived from this in vivo technique and histopathology validation ex vivo in distinguishing benign from malignant is also presented. In receiver operating characteristics (ROC) analysis, multi-parametric AOPA/OCT yields improved diagnostic accuracy of 98.4% and 95.8% for MM and BCC respectively, which indicate that AOPA/OCT represents a high-performance and clinically translatable technique for accurate diagnosis and therapy monitoring in dermatology.

Photoacoustic Tomography with a Ring Ultrasound Transducer: A Comparison of Different Illumination Strategies

Photoacoustic (PA) imaging is a methodology that uses the absorption of short laser pulses by endogenous or exogenous chromophores within human tissue, and the subsequent generation of acoustic waves acquired by an ultrasound (US) transducer, to form an image that can provide functional and molecular information. Amongst the various types of PA imaging, PA tomography (PAT) has been proposed for imaging pathologies such as breast cancer. However, the main challenge for PAT imaging is the deliverance of sufficient light energy horizontally through an imaging cross-section as well as vertically. In this study, three different illumination methods are compared for a full-ring ultrasound (US) PAT system. The three distinct illumination setups are full-ring, diffused-beam, and point source illumination. The full-ring system utilizes a cone mirror and parabolic reflector to create the ringed-shaped beam for PAT, while the diffuse scheme uses a light diffuser to expand the beam, which illuminates tissue-mimicking phantoms. The results indicate that the full-ring illumination is capable of providing a more uniform fluence irrespective of the vertical depth of the imaged cross-section, while the point source and diffused illumination methods provide a higher fluence at regions closer to the point of entry, which diminishes with depth. In addition, a set of experiments was conducted to determine the optimum position of ring-illumination with respect to the position of the acoustic detectors to achieve the highest signal-to-noise ratio.

All-reflective ring illumination system for photoacoustic tomography

Given that breast cancer is the second leading cause of cancer-related deaths among women in the United States, it is necessary to continue improving the sensitivity and specificity of breast imaging systems that diagnose breast lesions. Photoacoustic (PA) imaging can provide functional information during in vivo studies and can augment the structural information provided by ultrasound (US) imaging. A full-ring, all-reflective, illumination system for photoacoustic tomography (PAT) coupled to a full-ring US receiver is developed and tested. The US/PA tomography system utilizes a cone mirror and conical reflectors to optimize light delivery for PAT imaging and has the potential to image objects that are placed within the ring US transducer. The conical reflector used in this system distributes the laser energy over a circular cross-sectional area, thereby reducing the overall fluence. This, in turn, allows the operator to increase the laser energy achieving better cross-sectional penetration depth. A proof-of-concept design utilizing a single cone mirror and a parabolic reflector is used for imaging cylindrical phantoms with light-absorbing objects. For the given phantoms, it has been shown that there was no restriction in imaging a given targeted cross-sectional area irrespective of vertical depth, demonstrating the potential of mirror-based, ring-illuminated PAT system. In addition, the all-reflective ring illumination method shows a uniform PA signal across the scanned cross-sectional area.

High-speed, low-cost, pulsed-laser-diode-based second-generation desktop photoacoustic tomography system

Bulky, expensive Nd:YAG lasers are used in conventional photoacoustic tomography (PAT) systems, making them difficult to translate into clinics. Moreover, real-time imaging is not feasible when a single-element ultrasound transducer is used with these low-pulse-repetition-rate lasers (10-100 Hz). Low-cost pulsed laser diodes (PLDs) can be used instead for photoacoustic imaging due to their high-pulse-repetition rates and compact size. Together with acoustic-reflector-based multiple single-element ultrasound transducers, a portable desktop PAT system was developed. This second-generation PLD-based PAT achieved 0.5 s cross-sectional imaging time with high spatial resolution of ∼165  μm and an imaging depth of 3 cm. The performance of this system was characterized using phantom and in vivo studies. Dynamic in vivo imaging was also demonstrated by monitoring the fast uptake and clearance of indocyanine green in small animal (rat) brain vasculature.

Enhancing sparse-view photoacoustic tomography with combined virtually parallel projecting and spatially adaptive filtering

To fully realize the potential of photoacoustic tomography (PAT) in preclinical and clinical applications, rapid measurements and robust reconstructions are needed. Sparse-view measurements have been adopted effectively to accelerate the data acquisition. However, since the reconstruction from the sparse-view sampling data is challenging, both the effective measurement and the appropriate reconstruction should be taken into account. In this study, we present an iterative sparse-view PAT reconstruction scheme, where a concept of virtual parallel-projection matching the measurement condition is introduced to aid the "compressive sensing" in the reconstruction procedure, and meanwhile, the non-local spatially adaptive filtering exploring the a priori information of the mutual similarities in natural images is adopted to recover the unknowns in the transformed sparse domain. Consequently, the reconstructed images with the proposed sparse-view scheme can be evidently improved in comparison to those with the universal back-projection method, for the cases of same sparse views. The proposed approach has been validated by the simulations and ex vivo experiments, which exhibits desirable performances in image fidelity even from a small number of measuring positions.

Fast photoacoustic imaging systems using pulsed laser diodes: a review

Photoacoustic imaging (PAI) is a newly emerging imaging modality for preclinical and clinical applications. The conventional PAI systems use Q-switched Nd:YAG/OPO (Optical Parametric Oscillator) nanosecond lasers as excitation sources. Such lasers are expensive, bulky, and imaging speed is limited because of low pulse repetition rate. In recent years, the semiconductor laser technology has advanced to generate high-repetitions rate near-infrared pulsed lasers diodes (PLDs) which are reliable, less-expensive, hand-held, and light-weight, about 200 g. In this article, we review the development and demonstration of PLD based PAI systems for preclinical and clinical applications reported in recent years.

Label-free photoacoustic imaging guided sclerotherapy for vascular malformations: a feasibility study

We used high-resolution photoacoustic imaging (PAI) to guide sclerotherapy of vascular malformations in an in vivo animal model. A focus-adjustable PAI system was developed. It can adapt to the imaging needs of different depths by adjusting the focus. Blood samples drawn before and after sclerosis were examined with PAI, which could distinguish whether or not the blood had been exposed to a sclerosing agent. Superficial and deep vessels in the animal model were examined in vivo to prove the feasibility of guiding sclerotherapy. We found that PAI can distinguish sclerotic vessels from normal vessels within a certain depth range. Our findings suggest the potential of PAI to find accurate injection points and to localize thrombi, making it possible to reduce the dosage of sclerosing agents.

Dynamic in vivo imaging of small animal brain using pulsed laser diode-based photoacoustic tomography system

We demonstrate dynamic in vivo imaging using a low-cost portable pulsed laser diode (PLD)-based photoacoustic tomography system. The system takes advantage of an 803-nm PLD having high-repetition rate ∼7000  Hz combined with a fast-scanning single-element ultrasound transducer leading to a 5 s cross-sectional imaging. Cortical vasculature is imaged in scan time of 5 s with high signal-to-noise ratio ∼48. To examine the ability for dynamic imaging, we monitored the fast uptake and clearance process of indocyanine green in the rat brain. The system will find applications to study neurofunctional activities, characterization of pharmacokinetic, and biodistribution profiles in the development process of drugs or imaging agents.

A High-performance Compact Photoacoustic Tomography System for In Vivo Small-animal Brain Imaging

In vivo small-animal imaging has an important role to play in preclinical studies. Photoacoustic tomography (PAT) is an emerging hybrid imaging modality that shows great potential for both preclinical and clinical applications. Conventional optical parametric oscillator-based PAT (OPO-PAT) systems are bulky and expensive and cannot provide high-speed imaging. Recently, pulsed-laser diodes (PLDs) have been successfully demonstrated as an alternative excitation source for PAT. Pulsed-laser diode PAT (PLD-PAT) has been successfully demonstrated for high-speed imaging on photoacoustic phantoms and biological tissues. This work provides a visualized experimental protocol for in vivo brain imaging using PLD-PAT. The protocol includes the compact PLD-PAT system configuration and its description, animal preparation for brain imaging, and a typical experimental procedure for 2D cross-sectional rat brain imaging. The PLD-PAT system is compact and cost-effective and can provide high-speed, high-quality imaging. Brain images collected in vivo at various scan speeds are presented.

Use of acoustic reflector to make a compact photoacoustic tomography system

A typical photoacoustic tomography (PAT) system uses a Q-switched Nd:YAG laser for irradiating the sample and a single-element ultrasound transducer (UST) for acquiring the photoacoustic data. Conventionally, in PAT systems, the UST is held in a horizontal position and moved in a circular motion around the sample in full 2 ? radians. Horizontal positioning of the UST requires a large water tank to house, and load on the motor is also high. To overcome this limitation, we used the UST in the vertical plane instead of the horizontal plane. The photoacoustic (PA) waves generated from the sample are directed to the detector surface using an acoustic reflector placed at 45 deg to the transducer body. Hence, we can reduce the scanning radius, which, in turn, will reduce the size of the water tank and load on the motor, and the overall conventional PAT system size can be minimized. In this work, we demonstrate that with this system configuration, we acquire nearly similar images for phantom and in vivo data as that of the conventional PAT system using both flat and focused USTs.