Photoacoustic tomography (PAT) can create multiscale multicontrast images of living biological structures ranging from organelles to organs. This emerging technology overcomes the high degree of scattering of optical photons in biological tissue by making use of the photoacoustic effect. Light absorption by molecules creates a thermally induced pressure jump that launches ultrasonic waves, which are received by acoustic detectors to form images. Different implementations of PAT allow the spatial resolution to be scaled with the desired imaging depth in tissue while a high depth-to-resolution ratio is maintained. As a rule of thumb, the achievable spatial resolution is on the order of 1/200 of the desired imaging depth, which can reach up to 7 centimeters. PAT provides anatomical, functional, metabolic, molecular, and genetic contrasts of vasculature, hemodynamics, oxygen metabolism, biomarkers, and gene expression. We review the state of the art of PAT for both biological and clinical studies and discuss future prospects.

Photoacoustic Tomography: In Vivo Imaging from Organelles to Organs

Photoacoustic (PA) imaging, also called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade. It is a hybrid modality, combining the high-contrast and spectroscopic-based specificity of optical imaging with the high spatial resolution of ultrasound imaging. In essence, a PA image can be regarded as an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but its optical properties, specifically optical absorption. As a consequence, it offers greater specificity than conventional ultrasound imaging with the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores, but with greater penetration depth than purely optical imaging modalities that rely on ballistic photons. As well as visualizing anatomical structures such as the microvasculature, it can also provide functional information in the form of blood oxygenation, blood flow and temperature. All of this can be achieved over a wide range of length scales from micrometres to centimetres with scalable spatial resolution. These attributes lend PA imaging to a wide variety of applications in clinical medicine, preclinical research and basic biology for studying cancer, cardiovascular disease, abnormalities of the microcirculation and other conditions. With the emergence of a variety of truly compelling in vivo images obtained by a number of groups around the world in the last 2–3 years, the technique has come of age and the promise of PA imaging is now beginning to be realized. Recent highlights include the demonstration of whole-body small-animal imaging, the first demonstrations of molecular imaging, the introduction of new microscopy modes and the first steps towards clinical breast imaging being taken as well as a myriad of in vivo preclinical imaging studies. In this article, the underlying physical principles of the technique, its practical implementation, and a range of clinical and preclinical applications are reviewed.

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Biomedical photoacoustic imaging

The nonradiative conversion of light energy into heat (photothermal therapy, PTT) or sound energy (photoacoustic imaging, PAI) has been intensively investigated for the treatment and diagnosis of cancer, respectively. By taking advantage of nanocarriers, both imaging and therapeutic functions together with enhanced tumour accumulation have been thoroughly studied to improve the pre-clinical efficiency of PAI and PTT. In this review, we first summarize the development of inorganic and organic nano photothermal transduction agents (PTAs) and strategies for improving the PTT outcomes, including applying appropriate laser dosage, guiding the treatment via imaging techniques, developing PTAs with absorption in the second NIR window, increasing photothermal conversion efficiency (PCE), and also increasing the accumulation of PTAs in tumours. Second, we introduce the advantages of combining PTT with other therapies in cancer treatment. Third, the emerging applications of PAI in cancer-related research are exemplified. Finally, the perspectives and challenges of PTT and PAI for combating cancer, especially regarding their clinical translation, are discussed. We believe that PTT and PAI having noteworthy features would become promising next-generation non-invasive cancer theranostic techniques and improve our ability to combat cancers.

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Photothermal therapy and photoacoustic imaging via nanotheranostics in fighting cancer

Optical microscopy has been a fundamental tool of biological discovery for more than three centuries, but its in vivo tissue imaging ability has been restricted by light scattering to superficial investigations, even when confocal or multiphoton methods are used. Recent advances in optical and optoacoustic (photoacoustic) imaging now allow imaging at depths and resolutions unprecedented for optical methods. These abilities are increasingly important to understand the dynamic interactions of cellular processes at different systems levels, a major challenge of postgenome biology. This Review discusses promising photonic methods that have the ability to visualize cellular and subcellular components in tissues across different penetration scales. The methods are classified into microscopic, mesoscopic and macroscopic approaches, according to the tissue depth at which they operate. Key characteristics associated with different imaging implementations are described and the potential of these technologies in biological applications is discussed.

Going deeper than microscopy: the optical imaging frontier in biology

Photoacoustic tomography (PAT) is probably the fastest-growing area of biomedical imaging technology, owing to its capacity for high-resolution sensing of rich optical contrast in vivo at depths beyond the optical transport mean free path (~1 mm in human skin). Existing high-resolution optical imaging technologies, such as confocal microscopy and two-photon microscopy, have had a fundamental impact on biomedicine but cannot reach the penetration depths of PAT. By utilizing low ultrasonic scattering, PAT indirectly improves tissue transparency up to 1000-fold and consequently enables deeply penetrating functional and molecular imaging at high spatial resolution. Furthermore, PAT promises in vivo imaging at multiple length-scales; it can image subcellular organelles to organs with the same contrast origin — an important application in multiscale systems biology research.

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Multiscale photoacoustic microscopy and computed tomography.

Histological evaluation of tissues is currently a lengthy process that typically precludes intraoperative margin assessment. While numerous approaches have aimed to address the need for intraoperative virtual histology, none have yet proved sufficiently efficacious. We demonstrate the use of a new all-optical imaging modality, photoacoustic remote sensing (PARS), capable of virtual histopathological imaging, while simultaneously providing visualization of microvasculature in both freshly resected tissues and live animal subjects. We demonstrate high resolutions of 0.44µm and 1.2µm for 266-nm and 532-nm excitation wavelengths, respectively, as well as the characterization of maximum permissible exposure limits for both excitation wavelengths.

In vivo combined virtual histology and vascular imaging with dual-wavelength photoacoustic remote sensing microscopy

Photoacoustic (PA) imaging has been utilized to quantify the lifetime profile of exogenous agents using a series of pump-probe pulses with a varying time delay; however, current techniques typically lead to long acquisition times which are sensitive to motion and cause absorption or photobleaching. We introduce a technique called lifetime-weighted imaging, which uses only three laser pulses to preferentially weight signals from chromophores with long lifetimes (including exogenous contrast agents with triplet excited states such as methylene blue and porphyrins) while nulling chromophores with short picosecond- to nanosecond-scale lifetimes (including hemoglobin). This technique detects the PA signal from a probe pulse either with or without a pump pulse. By subtracting the probe-only signal from the pump-present probe signal, we effectively eliminate signals from chromophores with short lifetimes while preserving PA signals from chromophores with long-lifetimes. We demonstrate the oxygen-dependent lifetime of both methylene blue and porphyrin-lipids and demonstrate both ground-state recovery and excited-state lifetime-weighted imaging. Lifetime-weighted PA imaging may have applications in many molecular imaging application including: photodynamic therapy dosimetry guidance and oxygen sensing.

http://iopscience.iop.org/article/10.1088/2040-8978/18/12/124001/pdf

Lifetime-weighted photoacoustic imaging

Using a 0.8-mm-diameter image guide fiber bundle consisting of 30,000 single-mode fibers and an external linear array transducer, we demonstrate a dual-mode photoacoustic system capable of ultrasound-guided microendoscope insertion and photoacoustic imaging. The array optical resolution photoacoustic microendoscopy (AOR-PAME) system is designed to visualize the placement of the distal end of an endoscopy probe several centimeters into tissue, transmit scanning focused laser pulses into tissues via the fiber bundle, and acquire the generated photoacoustic signals. A ytterbium-doped fiber laser is tightly focused and is scanned across the proximal tip of the image guide fiber bundle using a two-dimensional galvanometer scanning mirror system. The end of the fiber bundle is used in contact mode with the object. The capabilities of AOR-PAME are demonstrated by imaging carbon fiber networks embedded in tissue-mimicking phantoms and the ears of a 60-g rat. The lateral resolution and signal-to-noise ratio are measured as 9 μm and 40 dB, respectively.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-18/issue-09/090502/Optical-resolution-photoacoustic-microendoscopy-with-ultrasound-guided-insertion-and-array/10.1117/1.JBO.18.9.090502.pdf

Optical resolution photoacoustic microendoscopy with ultrasound-guided insertion and array system detection

Crossed electrode arrays, wherein electrodes are connected in individually addressable rows and columns, have been previously investigated as a means to reduce the number of channels in two-dimensional ultrasound arrays for three-dimensional imaging Previously envisioned imaging schemes have been limited in their resolution and focusing ability due to their inability to perform active focusing in both the lateral and elevational direction for both transmit and receive As well, signal-to-noise ratio (SNR) can be compromised when transmitting or receiving on only one row or column at a time A new imaging method is proposed for this array architecture using capacitive micromachined ultrasound transducers (CMUTs) which can provide two-way focusing in both lateral and elevational directions while providing optimal SNR This method is a variation of synthetic aperture imaging wherein the columns active upon transmit and receive events are determined by applying a bias to specific columns upon transmission and reception in an S-sequence encoded pattern Mathematical derivations of this method are presented and simulations are done to compare this imaging method to previously published crossed-electrode array imaging schemes, as well as to fully-populated 2-D phased array imaging

Synthetic aperture 3D ultrasound imaging schemes with S-sequence bias-encoded top-orthogonal-to-bottom-electrode 2D CMUT arrays

We propose a theoretical framework for consecutively reconstructing absorption and scattering distributions in turbid soft tissue in an iterative manner. This approach takes advantage of the stability of a recently reported least-squares fixed-point iterative method for reconstructing an optical absorption coefficient map to iteratively update estimates of absorption and scattering for each iteration. Simulations demonstrate that this method converges to an accurate estimate of the optical properties within only a small number of iterations and is robust to noise at realistic signal-to-noise levels.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-19/issue-12/126009/Consecutively-reconstructing-absorption-and-scattering-distributions-in-turbid-media-with/10.1117/1.JBO.19.12.126009.pdf

Roger J. Zemp

Papers

In vivo combined virtual histology and vascular imaging with dual-wavelength photoacoustic remote sensing microscopy

Lifetime-weighted photoacoustic imaging

Optical resolution photoacoustic microendoscopy with ultrasound-guided insertion and array system detection

Synthetic aperture 3D ultrasound imaging schemes with S-sequence bias-encoded top-orthogonal-to-bottom-electrode 2D CMUT arrays

Consecutively reconstructing absorption and scattering distributions in turbid media with multiple-illumination photoacoustic tomography