Simultaneous photoacoustic and optically mediated ultrasound microscopy: an in vivo study

Dual-modal Photoacoustic and Ultrasound Imaging: from preclinical to clinical applications

Photoacoustic imaging is a novel biomedical imaging modality that has emerged over the recent decades. Due to the conversion of optical energy into the acoustic wave, photoacoustic imaging offers high-resolution imaging in depth beyond the optical diffusion limit. Photoacoustic imaging is frequently used in conjunction with ultrasound as a hybrid modality. The combination enables the acquisition of both optical and acoustic contrasts of tissue, providing functional, structural, molecular, and vascular information within the same field of view. In this review, we first described the principles of various photoacoustic and ultrasound imaging techniques and then classified the dual-modal imaging systems based on their preclinical and clinical imaging applications. The advantages of dual-modal imaging were thoroughly analyzed. Finally, the review ends with a critical discussion of existing developments and a look toward the future.

Axial accuracy and signal enhancement in acoustic-resolution photoacoustic microscopy by laser jitter effect correction and pulse energy compensation

In recent years, photoacoustic imaging has found vast applications in biomedical imaging. Photoacoustic imaging has high optical contrast and high ultrasound resolution allowing deep tissue non-invasive imaging beyond the optical diffusion limit. Q-switched lasers are extensively used in photoacoustic imaging due to the availability of high energy and short laser pulses, which are essential for high-resolution photoacoustic imaging. In most cases, this type of light source suffers from pulse peak-power energy variations and timing jitter noise, resulting in uncertainty in the output power and arrival time of the laser pulses. These problems cause intensity degradation and temporal displacement of generated photoacoustic signals which in turn deteriorate the quality of the acquired photoacoustic images. In this study, we used a high-speed data acquisition system in combination with a fast photodetector and a software-based approach to capture laser pulses precisely in order to reduce the effect of timing jitter and normalization of the photoacoustic signals based on pulse peak-powers simultaneously. In the experiments, maximum axial accuracy enhancement of 14 µm was achieved in maximum-amplitude projected images on XZ and YZ planes with ±13.5 ns laser timing jitter. Furthermore, photoacoustic signal enhancement of 77% was obtained for 75% laser pulses peak-power stability.

Integrating photoacoustic microscopy with other imaging technologies for multimodal imaging

As a hybrid optical microscopic imaging technology, photoacoustic microscopy images the optical absorption contrasts and takes advantage of low acoustic scattering of biological tissues to achieve high-resolution anatomical and functional imaging. When combined with other imaging modalities, photoacoustic microscopy-based multimodal technologies can provide complementary contrast mechanisms to reveal complementary information of biological tissues. To achieve intrinsically and precisely registered images in a multimodal photoacoustic microscopy imaging system, either the ultrasonic transducer or the light source can be shared among the different imaging modalities. These technologies are the major focus of this minireview. It also covered the progress of the recently developed penta-modal photoacoustic microscopy imaging system featuring a novel dynamic focusing technique enabled by OCT contour scan.

Optoacoustic mesoscopy for biomedicine

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

Performance of optoacoustic and fluorescence imaging in detecting deep-seated fluorescent agents

Fluorescent contrast agents are widely employed in biomedical research. While many studies have reported deep tissue imaging of fluorescent moieties using either fluorescence-based or absorption-based (optoacoustic) imaging systems, no systematic comparison has been performed regarding the actual performance of these imaging modalities in detecting deep-seated fluorescent agents. Herein, an integrated imager combining epi-fluorescence and volumetric optoacoustic imaging capabilities has been employed in order to evaluate image degradation with depth for several commonly-used near-infrared dyes in both modes. We performed controlled experiments in tissue-mimicking phantoms containing deeply embedded targets filled with different concentrations of Alexa Fluor 700, Alexa Fluor 750, indocyanine green (ICG) and IRDye 800CW. The results are further corroborated by multi-modal imaging of ICG through mouse tissues in vivo. It is shown that optoacoustics consistently provides better sensitivity in differentiating fluorescent targets located at depths beyond 2 mm in turbid tissues, as quantified by evaluating image contrast, signal to noise ratio and spatial resolution performance.

Virtual craniotomy for high-resolution optoacoustic brain microscopy

Ultrasound-mediated transcranial images of the brain often suffer from acoustic distortions produced by the skull bone. In high-resolution optoacoustic microscopy, the skull-induced acoustic aberrations are known to impair image resolution and contrast, further skewing the location and intensity of the different absorbing structures. We present a virtual craniotomy deconvolution algorithm based on an ultrasound wave propagation model that corrects for the skull-induced distortions in optically-resolved optoacoustic transcranial microscopy data. The method takes advantage of the geometrical and spectral information of a pulse-echo ultrasound image of the skull simultaneously acquired by our multimodal imaging system. Transcranial mouse brain imaging experiments confirmed the ability to accurately account for the signal amplitude decay, temporal delay and pulse broadening introduced by the rodent's skull. Our study is the first to demonstrate skull-corrected transcranial optoacoustic imaging in vivo.

Miniature all-optical probe for large synthetic aperture photoacoustic-ultrasound imaging

A miniature all-optical probe for high-resolution photoacoustic (PA)-ultrasound (US) imaging using a large synthetic aperture is developed. The probe consists of three optical fibers for PA excitation, US generation, and detection of acoustic waves, respectively. The fiber for PA excitation has a large numerical aperture (NA) for wide-angle laser illumination. On the other hand, the fiber with a carbon black-polydimethylsiloxane composite coated on the end face of the optical fiber is used for wide-angle US transmission through laser-US conversion. Both the excited PA and backscattered US signals are detected by a fiber-tip Fabry-Perot cavity for wide-angle acoustic detection. The probe outer diameter is only ~2 mm. The synergy of the three optical fibers makes a large-NA synthetic aperture focusing technique for high-resolution PA and US imaging possible. High PA lateral resolutions of 104-154 μm and high US lateral resolutions of 64-112 μm over a depth range of > 4 mm are obtained. Compared with other existing miniature PA-US probes, to our knowledge, our probe achieves by far the best performance in terms of lateral resolutions and imaging depth range. The constructed probe has potential for endoscopic and intravascular imaging applications that require PA and US contrasts with high resolutions over a large depth range.

On the use of an optoacoustic and laser ultrasonic imaging system for assessing peripheral intravenous access

We describe a universal system for research in combined real-time optoacoustic (OA) and laser-ultrasonic (LU) imaging. The results of its testing on the task of needle insertion into the blood vessel model diagnostics are presented. In OA mode, where laser light is absorbed directly in the sample, the contents of blood vessel model is clearly visible. In LU mode, where the short ultrasonic probe pulse scattered on the sample is detected, the needle is clearly visible. The developed solution combining OA and LU imaging modalities due to the common detection system allowed real-time diagnostics of the position of medical needles (0.63 mm and 0.7 mm in diameter) inside blood vessel models (1.6 mm and 2.4 mm in diameter). Frame rate was 10 Hz. High longitudinal spatial resolution of the system - 0.1 mm - allows distinguishing the two walls of the vessel model and the position of the needle inside.

Simultaneous in vivo imaging of diffuse optical reflectance, optoacoustic pressure and ultrasonic scattering

We present reflection-mode bioimaging system providing complementary optical, photoacsoutic and acoustic measurements by acoustic detector after each laser pulse. While the photons absorbed within the sample provide optoacoustic (OA) signals, the photons absorbed by the external electrode of a detector provide the measurable diffuse reflectance (DR) from the sample and the probing ultrasonic (US) pulse. To demonstrate the in vivo capabilities of the system we present the results of complementary DR/OA/US imaging of a mouse tumor, head of a newborn rat, and the back of a newborn rat with 3.5mm/50μm/35μm lateral resolution. Trimodal approach allows visualization of mechanical structures in healthy and pathological tissues along with peculiarities of blood supply. The system may be used for diagnostics of diseases accompanied by the defects of vascularization as well as for assessing the mechanisms of vascular changes when monitoring response to therapy.

Optimal wavelengths for optoacoustic measurements of blood oxygen saturation in biological tissues

The non-invasive measurement of blood oxygen saturation in blood vessels is a promising clinical application of optoacoustic imaging. Nevertheless, precise optoacoustic measurements of blood oxygen saturation are limited because of the complexities of calculating the spatial distribution of the optical fluence. In the paper error in the determination of blood oxygen saturation, associated with the use of approximate methods of optical fluence evaluation within the blood vessel, was investigated for optoacoustic measurements at two wavelengths. The method takes into account both acoustic pressure noise and the error in determined values of the optical scattering and absorption coefficients used for the calculation of the fluence. It is shown that, in conditions of an unknown (or partially known) spatial distribution of fluence at depths of 2 to 8 mm, minimal error in the determination of blood oxygen saturation is achieved at wavelengths of 658 ± 40 nm and 1069 ± 40 nm.

Miniature probe integrating optical-resolution photoacoustic microscopy, optical coherence tomography, and ultrasound imaging: proof-of-concept

In this Letter, we present a novel tri-modal miniature side-view probe, through which optical-resolution photoacoustic microscopy (OR-PAM), optical coherence tomography (OCT), and pulse-echo ultrasound (US) images can be coaxially acquired and displayed simultaneously. The probe consists of a common optical path for OR-PAM (light delivery) and OCT (light delivery/detection), and a 40-MHz unfocused ultrasound transducer for OR-PAM (photoacoustic detection) and US (ultrasound transmission/receiving) with an overall diameter of 2 mm. Combining OR-PAM, OCT, and US would provide complementary information including optical absorption (OR-PAM), optical back-scattering (OCT), and deep tissue structures (US) about biological tissue. Based on an integrated imaging system consisting of OR-PAM, time-domain OCT, and US, phantom images and in vivo images of rat ear were acquired to demonstrate the capabilities of the integrated tri-modality imaging probe. The probe yields a lateral resolution of 13.6 μm for OR-PAM and OCT, and an axial resolution of 43 μm for OR-PAM and US. Currently, for a scanning area of 1 ×1  mm, it took ∼25  min to acquire data for tri-modal volumetric imaging.