Combined multi-modal photoacoustic tomography, optical coherence tomography (OCT) and OCT angiography system with an articulated probe for in vivo human skin structure and vasculature imaging

Cutaneous blood flow accounts for approximately 5% of cardiac output in human and plays a key role in a number of a physiological and pathological processes. We show for the first time a multi-modal photoacoustic tomography (PAT), optical coherence tomography (OCT) and OCT angiography system with an articulated probe to extract human cutaneous vasculature in vivo in various skin regions. OCT angiography supplements the microvasculature which PAT alone is unable to provide. Co-registered volumes for vessel network is further embedded in the morphologic image provided by OCT. This multi-modal system is therefore demonstrated as a valuable tool for comprehensive non-invasive human skin vasculature and morphology imaging in vivo.

Whole-body multispectral photoacoustic imaging of adult zebrafish

The zebrafish, an ideal vertebrate for studying developmental biology and genetics, is increasingly being used to understand human diseases, due to its high similarity to the human genome and its optical transparency during embryonic stages. Once the zebrafish has fully developed, especially wild-type breeds, conventional optical imaging techniques have difficulty in imaging the internal organs and structures with sufficient resolution and penetration depth. Even with established mutant lines that remain transparent throughout their life cycle, it is still challenging for purely optical imaging modalities to visualize the organs of juvenile and adult zebrafish at a micro-scale spatial resolution. In this work, we developed a non-invasive three-dimensional photoacoustic imaging platform with an optimized illumination pattern and a cylindrical-scanning-based data collection system to image entire zebrafish with micro-scale resolutions of 80 μm and 600 μm in the lateral and axial directions, respectively. In addition, we employed a multispectral strategy that utilized excitation wavelengths from 690 nm to 930 nm to statistically quantify the relative optical absorption spectrum of major organs.

Synthetic-aperture based photoacoustic re-beamforming (SPARE) approach using beamformed ultrasound data

Photoacoustic (PA) imaging has been developed for various clinical and pre-clinical applications, and acquiring pre-beamformed channel data is necessary to reconstruct these images. However, accessing these pre-beamformed channel data requires custom hardware to enable parallel beamforming, and is available for a limited number of research ultrasound platforms. To broaden the impact of clinical PA imaging, our goal is to devise a new PA reconstruction approach that uses ultrasound post-beamformed radio frequency (RF) data rather than raw channel data, because this type of data is readily available in both clinical and research ultrasound systems. In our proposed Synthetic-aperture based photoacoustic re-beamforming (SPARE) approach, post-beamformed RF data from a clinical ultrasound scanner are considered as input data for an adaptive synthetic aperture beamforming algorithm. When receive focusing is applied prior to obtaining these data, the focal point is considered as a virtual element, and synthetic aperture beamforming is implemented assuming that the photoacoustic signals are received at the virtual element. The resolution and SNR obtained with the proposed method were compared to that obtained with conventional delay-and-sum beamforming with 99.87% and 91.56% agreement, respectively. In addition, we experimentally demonstrated feasibility with a pulsed laser diode setup. Results indicate that the post-beamformed RF data from any commercially available ultrasound platform can potentially be used to create PA images.

In vivo demonstration of reflection artifact reduction in photoacoustic imaging using synthetic aperture photoacoustic-guided focused ultrasound (PAFUSion)

Reflection artifacts caused by acoustic inhomogeneities are a critical problem in epi-mode biomedical photoacoustic imaging. High light fluence beneath the probe results in photoacoustic transients, which propagate into the tissue and reflect back from echogenic structures. These reflection artifacts cause problems in image interpretation and significantly impact the contrast and imaging depth. We recently proposed a method called PAFUSion (Photoacoustic-guided focused ultrasound) to identify such reflection artifacts in photoacoustic imaging. In its initial version, PAFUSion mimics the inward-travelling wavefield from small blood vessel-like PA sources by applying ultrasound pulses focused towards these sources, and thus provides a way to identify the resulting reflection artifacts. In this work, we demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on human volunteers. In view of the spatially distributed PA sources that are found in clinical applications, we implemented an improved version of PAFUSion where photoacoustic signals are backpropagated to imitate the inward travelling wavefield and thus the reflection artifacts. The backpropagation is performed in a synthetic way based on the pulse-echo acquisitions after transmission on each single element of the transducer array. The results provide a direct confirmation that reflection artifacts are prominent in clinical epi-photoacoustic imaging, and that PAFUSion can strongly reduce these artifacts to improve deep-tissue photoacoustic imaging.

Frequency domain photoacoustic and fluorescence microscopy

We report on simultaneous frequency domain optical-resolution photoacoustic and fluorescence microscopy with sub-µm lateral resolution. With the help of a blood smear, we show that photoacoustic and fluorescence images provide complementary information. Furthermore, we compare theoretically predicted signal-to-noise ratios of sinusoidal modulation in frequency domain with pulsed excitation in time domain.

Velocity measurements in whole blood using acoustic resolution photoacoustic Doppler

Acoustic resolution photoacoustic Doppler velocimetry promises to overcome the spatial resolution and depth penetration limitations of current blood flow measuring methods. Despite successful implementation using blood-mimicking fluids, measurements in blood have proved challenging, thus preventing in vivo application. A common explanation for this difficulty is that whole blood is insufficiently heterogeneous relative to detector frequencies of tens of MHz compatible with deep tissue photoacoustic measurements. Through rigorous experimental measurements we provide new insight that refutes this assertion. We show for the first time that, by careful choice of the detector frequency and field-of-view, and by employing novel signal processing methods, it is possible to make velocity measurements in whole blood using transducers with frequencies in the tens of MHz range. These findings have important implications for the prospects of making deep tissue measurements of blood flow relevant to the study of microcirculatory abnormalities associated with cancer, diabetes, atherosclerosis and other conditions.

Characterizing intestinal inflammation and fibrosis in Crohn's disease by photoacoustic imaging: feasibility study

The pathology of Crohn's disease (CD) is characterized by obstructing intestinal strictures because of inflammation (with high levels of hemoglobin), fibrosis (high levels of collagen), or a combination of both. The accurate characterization of the strictures is critical for the management of CD. This study examines the feasibility of characterizing intestinal strictures by Photoacoustic imaging (PAI) without extrapolation from superficial biopsies. Ex vivo normal rat colon tissue, inflammatory and fibrotic intestinal strictures in rat trinitrobenzene sulfonic acid (TNBS) model were first differentiated by a PA-US parallel imaging system. Surgically removed human intestinal stricture specimens were afterwards imaged by a multiwavelength acoustic resolution PA microscope (ARPAM). The experiment results suggest that PAI is a potential tool for the diagnosis of the diseased conditions in intestinal strictures.

Noninvasive and high-resolving photoacoustic dermoscopy of human skin

We proposed and developed a photoacoustic (PA) dermoscope equipped with an integrated PA probe to achieve quantification and high-resolution, high-contrast deep imaging of human skin. The PA probe, with light-sound confocal excitation and reception, is specially designed, and integrated with an objective lens, an ultrasound transducer, and an inverted-triangle coupling cup to facilitate convenient implementation in a clinical setting. The PA dermoscope was utilized for noninvasive and high-resolution imaging of epidermal and dermal structure in volunteers. The imaging results demonstrated that the characteristic parameters of skin disease, including pigment distribution and thickness, vascular diameter, and depth, can be obtained by the PA dermoscope, confirming that PA dermoscopy can serve as a potential tool for the diagnosis and curative effect evaluation of human skin disease.

Ultrasound-guided photoacoustic imaging for the selective detection of EGFR-expressing breast cancer and lymph node metastases

We assessed the use of ultrasound (US)-guided photoacoustic imaging (PAI) and anti-EGFR antibody-conjugated gold nanorods (anti-EGFR-GNs) to non-invasively detect EGFR-expressing primary tumor masses and regional lymph node (LN) metastases in breast tumor mice generated by injecting MCF-7 (EGFR-negative) or MDA-MB-231 (EGFR-positive) human breast cells using a preclinical Vevo 2100 LAZR Imaging system. Anti-EGFR-GNs provided a significant enhancement in the PA signal in MDA-MB-231 tumor and the axillary LN metastases relative to MCF-7 tumor and non-LN metastases. We demonstrated that US-guided PAI using anti-EGFR-GNs is highly sensitive for the selective visualization of EGFR-expressing breast primary tumors as well as LN micrometastases.

Targeted Aucore-Agshell nanorods as a dual-functional contrast agent for photoacoustic imaging and photothermal therapy

Optimizing contrast enhancement is essential for producing specific signals in biomedical imaging and therapy. The potential of using Aucore-Agshell nanorods (Au@Ag NRs) as a dual-functional theranostic contrast agent is demonstrated for effective cancer imaging and treatments. Due to its strong NIR absorption and high efficiency of photothermal conversion, effects of both photoacoustic tomography (PAT) and photothermal therapy (PTT) are enhanced significantly. The PAT signal grows by 45.3% and 82% in the phantom and in vivo experiments, respectively, when compared to those using Au NRs. In PTT, The maximum increase of tissue temperature treated with Au@Ag NRs is 22.8 °C, twice that with Au NRs. Results of the current study show the feasibility of using Au@Ag NRs for synergetic PAT with PTT. And it will enhance the potential application on real-time PAT guided PTT, which will greatly benefit the customized PTT treatment of cancer.

In vivo deconvolution acoustic-resolution photoacoustic microscopy in three dimensions

Acoustic-resolution photoacoustic microscopy (ARPAM) provides a spatial resolution on the order of tens of micrometers, and is becoming an essential tool for imaging fine structures, such as the subcutaneous microvasculature. High lateral resolution of ARPAM is achieved using high numerical aperture (NA) of acoustic transducer; however, the depth of focus and working distance will be deteriorated correspondingly, thus sacrificing the imaging range and accessible depth. The axial resolution of ARPAM is limited by the transducer's bandwidth. In this work, we develop deconvolution ARPAM (D-ARPAM) in three dimensions that can improve the lateral resolution by 1.8 and 3.7 times and the axial resolution by 1.7 and 2.7 times, depending on the adopted criteria, using a 20-MHz focused transducer without physically increasing its NA and bandwidth. The resolution enhancement in three dimensions by D-ARPAM is also demonstrated by in vivo imaging of the microvasculature of a chick embryo. The proposed D-ARPAM has potential for biomedical imaging that simultaneously requires high spatial resolution, extended imaging range, and long accessible depth.

Real-time GPU-accelerated processing and volumetric display for wide-field laser-scanning optical-resolution photoacoustic microscopy

Fast signal processing and real-time displays are essential for practical imaging modality in various fields of applications. However, the imaging speed in optical-resolution photoacoustic microscopy (OR-PAM), in particular, depends on factors such as the pulse repetition rate of the laser, scanning method, field of view (FOV), and signal processing time. In the past, efforts to increase acquisition speed either focused on developing new scanning methods or using lasers with higher pulse repetition rates. However, high-speed signal processing is also important for real-time volumetric display in OR-PAM. In this study, we carried out parallel signal processing using a graphics processing unit (GPU) to enable fast signal processing and wide-field real-time displays in laser-scanning OR-PAM. The average total GPU processing time for a B-mode PAM image was approximately 1.35 ms at a display speed of 480 fps when the data samples were acquired with 736 (axial) × 500 (lateral) points/B-mode-frame at a pulse repetition rate of 300 kHz. In addition, we successfully displayed maximum amplitude projection images of a mouse's ear as volumetric images with an FOV of 3 mm × 3 mm (500 × 500 pixels) at 1.02 s, corresponding to 0.98 fps.

Mechanical characterization of intraluminal tissue with phase-resolved photoacoustic viscoelasticity endoscopy

We developed a phase-sensitive side-scanning photoacoustic viscoelasticity endoscopy (PAVEE) for mechanical characterization of intraluminal tissues. In PAVEE, the PA phase can be extracted from the optical absorption induced ultrasonic waves and provides an index of viscoelasticity that is closely linked to tissue compositions. The transverse resolution of the PAVEE measured by carbon fiber was about 32 μm. The imaging capability of the PAVEE was verified using a vessel-mimicking phantom with different agar density. Moreover, PAVEE was investigated in processed lumen-shaped vascular tissues to evaluate the biomechanical features, which was highly consistent with the histology. The results demonstrated that the PAVEE can obtain viscoelastic properties of intraluminal tissues, which puts a new insight into the intravascular disease and holds great promise for plaque vulnerability detection.

High-speed intravascular photoacoustic imaging at 1.7 μm with a KTP-based OPO

Lipid deposition inside the arterial wall is a hallmark of plaque vulnerability. Based on overtone absorption of C-H bonds, intravascular photoacoustic (IVPA) catheter is a promising technology for quantifying the amount of lipid and its spatial distribution inside the arterial wall. Thus far, the clinical translation of IVPA technology is limited by its slow imaging speed due to lack of a high-pulse-energy high-repetition-rate laser source for lipid-specific first overtone excitation at 1.7 μm. Here, we demonstrate a potassium titanyl phosphate (KTP)-based optical parametric oscillator with output pulse energy up to 2 mJ at a wavelength of 1724 nm and with a repetition rate of 500 Hz. Using this laser and a ring-shape transducer, IVPA imaging at speed of 1 frame per sec was demonstrated. Performance of the IVPA imaging system's resolution, sensitivity, and specificity were characterized by carbon fiber and a lipid-mimicking phantom. The clinical utility of this technology was further evaluated ex vivo in an excised atherosclerotic human femoral artery with comparison to histology.

Performance characterization of low-cost, high-speed, portable pulsed laser diode photoacoustic tomography (PLD-PAT) system

Photoacoustic tomography systems that uses Q-switched Nd:YAG/OPO pulsed lasers are expensive, bulky, and hence limits its use in clinical applications. The low pulse repetition rate of these lasers makes it unsuitable for real-time imaging when used with single-element ultrasound detector. In this work, we present a pulsed laser diode photoacoustic tomography (PLD-PAT) system that integrates a compact PLD inside a single-detector circular scanning geometry. We compared its performance against the traditional Nd:YAG/OPO based PAT system in terms of imaging depth, resolution, imaging time etc. The PLD provides near-infrared pulses at ~803 nm wavelength with pulse energy ~1.4 mJ/pulse at 7 kHz repetition rate. The PLD-PAT system is capable of providing 2D image in scan time as small as 3 sec with a signal-to-noise ratio ~30. High-speed and deep-tissue imaging is demonstrated on phantoms and biological samples. The PLD-PAT system is inexpensive, portable, allows high-speed PAT imaging, and its performance is as good as traditional expensive OPO based PAT system. Therefore, it holds promises for future translational biomedical imaging applications.

Photoacoustic imaging of salivary glands

In this work, we utilized photoacoustic imaging (PAI) with co-registered ultrasound (US) to non-invasively assess salivary gland function in vivo. A significant increase in salivary gland oxygen saturation was observed on PAI within minutes after gustatory stimulation of healthy mice reflective of the hyperemic response associated with secretion of saliva. Good correlation was seen between PAI and Doppler sonography. Salivary adenoid cystic carcinomas showed higher oxygen saturation compared to surrounding salivary gland tissue. Our results demonstrate the potential clinical utility of PAI for visualization of salivary gland physiology and pathology.

Photoacoustic imaging of cerebral hypoperfusion during acupuncture

Using acupuncture to treat cerebral hypoperfusion is a hot topic. However, there is a lack of effective tools to clarify the therapeutic effect of acupuncture on cerebral hypoperfusion. Here, we show in a mouse model of cerebral hypoperfusion that photoacoustic tomography (PAT) can noninvasively image cerebral vasculature and track total hemoglobin (HbT) concentration changes in cerebral hypoperfusion with acupuncture stimulation on the YangLingQuan (GB34) point. We measured the changes of HbT concentration and found that the HbT concentration in hypoperfusion regions was clearly lower than that in the control regions when the acupuncture was absent; however, it was significantly increased when the acupuncture was implemented on the GB34 point. We also observed the increase of vessel size and the generation of new vessels in cerebral hypoperfusion during acupuncture. Laser speckle imaging (LSI) was employed to validate some of the PAT findings.

In vivo dual-modality photoacoustic and optical coherence tomography imaging of human dermatological pathologies

Vascular abnormalities serve as a key indicator for many skin diseases. Currently available methods in dermatology such as histopathology and dermatoscopy analyze underlying vasculature in human skin but are either invasive, time-consuming, and laborious or incapable of providing 3D images. In this work, we applied for the first time dual-modality photoacoustic and optical coherence tomography that provides complementary information about tissue morphology and vasculature of patients with different types of dermatitis. Its noninvasiveness and relatively short imaging time and the wide range of diseases that it can detect prove the merits of the dual-modality imaging system and show the great potential of its clinical use in the future.

Dual-pulse nonlinear photoacoustic technique: a practical investigation

The dual-pulse nonlinear photoacoustic technique is a recently developed technology based on temperature dependence of the Grüneisen parameter and involves consecutive excitations of biological tissue using two laser pulses with a short time delay. Here we review the principle of the technique and give a discussion about its technical aspects, including selection and combination of excitation laser wavelengths, determination of laser fluence, estimation of thermal relaxation function and probability of photoablation or cavitation. Comparisons between the dual-pulse technique and conventional photoacoustics as well as thermal photoacoustics are also presented. These investigations are supported by experimental results and will give a practical reference and guide for further developments of the technique.

Photoacoustic imaging of fluorophores using pump-probe excitation

A pump-probe technique for the detection of fluorophores in tomographic PA images is introduced. It is based on inducing stimulated emission in fluorescent molecules, which in turn modulates the amount of thermalized energy, and hence the PA signal amplitude. A theoretical model of the PA signal generation in fluorophores is presented and experimentally validated on cuvette measurements made in solutions of Rhodamine 6G, a fluorophore of known optical and molecular properties. The application of this technique to deep tissue tomographic PA imaging is demonstrated by determining the spatial distribution of a near-infrared fluorophore in a tissue phantom.

Multigrid-based reconstruction algorithm for quantitative photoacoustic tomography

This paper proposes a multigrid inversion framework for quantitative photoacoustic tomography reconstruction. The forward model of optical fluence distribution and the inverse problem are solved at multiple resolutions. A fixed-point iteration scheme is formulated for each resolution and used as a cost function. The simulated and experimental results for quantitative photoacoustic tomography reconstruction show that the proposed multigrid inversion can dramatically reduce the required number of iterations for the optimization process without loss of reliability in the results.

Photoacoustic radar phase-filtered spatial resolution and co-registered ultrasound image enhancement for tumor detection

Co-registered ultrasound (US) and frequency-domain photoacoustic radar (FD-PAR) imaging is reported for the first time in this paper. The merits of ultrasound and cross-correlation (radar) frequency-domain photoacoustic imaging are leveraged for accurate tumor detection. Commercial US imagers possess sophisticated, optimized software for rapid image acquisition that could dramatically speed-up PA imaging. The PAR image generated from the amplitude of the cross-correlation between detected and input signals was filtered by the standard deviation (SD) of the phase of the correlation signal, resulting in strong improvement of image spatial resolution, signal-to-noise ratio (SNR) and contrast. Application of phase-mediated image improvement is illustrated by imaging a cancer cell-injected mouse. A 14-15 dB SNR gain was recorded for the phase-filtered image compared to the amplitude and phase independently, while ~340 μm spatial resolution was seen for the phase PAR image compared to ~840 μm for the amplitude image.

Optical-resolution photoacoustic endomicroscopy in vivo

Optical-resolution photoacoustic microscopy (OR-PAM) has become a major experimental tool of photoacoustic tomography, with unique imaging capabilities for various biological applications. However, conventional imaging systems are all table-top embodiments, which preclude their use in internal organs. In this study, by applying the OR-PAM concept to our recently developed endoscopic technique, called photoacoustic endoscopy (PAE), we created an optical-resolution photoacoustic endomicroscopy (OR-PAEM) system, which enables internal organ imaging with a much finer resolution than conventional acoustic-resolution PAE systems. OR-PAEM has potential preclinical and clinical applications using either endogenous or exogenous contrast agents.

Phthalocyanine photosensitizers as contrast agents for in vivo photoacoustic tumor imaging

There is a need for contrast agents for non-invasive diagnostic imaging of tumors. Herein, Multispectral Optoacoustic Tomography (MSOT) was employed to evaluate phthalocyanines commonly used in photodynamic therapy as photoacoustic contrast agents. We studied the photoacoustic activity of three water-soluble phthalocyanine photosensitizers: phthalocyanine tetrasulfonic acid (PcS4), Zn(II) phthalocyanine tetrasulfonic acid (ZnPcS4) and Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4) in phantom and in tumor-bearing mice to investigate the biodistribution and fate of the phthalocyanines in the biological tissues. PcS4 was observed to grant good contrast between the different reticuloendothelial organs and accumulate in the tumor within an hour of post-administration. ZnPcS4 and AlPcS4 offered little contrast in photoacoustic signals between the organs. PcS4 is a promising photoacoustic contrast agent and can be exploited as a photodiagnostic agent.

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

We propose the use of thermoelastic (TE) excitation of an ultrasonic (US) detector by backscattered laser radiation as a means of upgrading a single-modality photoacoustic (PA) microscope to dual-modality PA/US imaging at minimal cost. The upgraded scanning head of our dual-modality microscope consists of a fiber bundle with 14 output arms and a 32MHz polyvinylidene difluoride (PVDF) detector with a 34 MHz bandwidth (-6 dB level), 12.7 mm focal length, and a 0.25 numerical aperture. A single optical pulse delivered through the fiber bundle to the biotissue being investigated, in combination with a metalized surface on the PVDF detector allows us to obtain both PA and US A-scans. To demonstrate the in vivo capabilities of the proposed method we present the results of bimodal imaging of the brain of a newborn rat, a mouse tail and a mouse tumor.