Optical coherence photoacoustic microscopy: accomplishing optical coherence tomography and photoacoustic microscopy with a single light source

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.

Functional optical coherence tomography and photoacoustic microscopy imaging for zebrafish larvae

We present a dual modality functional optical coherence tomography and photoacoustic microscopy (OCT-PAM) system. The photoacoustic modality employs an akinetic optical sensor with a large imaging window. This imaging window enables direct reflection mode operation, and a seamless integration of optical coherence tomography (OCT) as a second imaging modality. Functional extensions to the OCT-PAM system include Doppler OCT (DOCT) and spectroscopic PAM (sPAM). This functional and non-invasive imaging system is applied to image zebrafish larvae, demonstrating its capability to extract both morphological and hemodynamic parameters in vivo in small animals, which are essential and critical in preclinical imaging for physiological, pathophysiological and drug response studies.

Optical coherence tomography-guided dynamic focusing for combined optical and mechanical scanning multimodal photoacoustic microscopy

To achieve fast imaging and large field of view (FOV), we improved our multimodal imaging system, which integrated optical resolution photoacoustic microscopy, optical coherence tomography (OCT), and confocal fluorescence microscopy in one platform, by combining optical scanning with mechanical scanning. To ensure good focusing of the objective lens over all the imaged area, we employed OCT-guided dynamic focusing. Different from our previous point-by-point dynamic focusing, we employed an area-by-area focusing adjustment strategy, in which each fast optical scanning area has a fixed focusing depth. We have demonstrated the performance of the system by imaging biological samples ex vivo (plant leaf) and in vivo (mouse ear). The system has achieved uniform resolution in an FOV of 10  mm  ×  10  mm with an imaging time of about 5 min.

A handheld microscope integrating photoacoustic microscopy and optical coherence tomography

The combination of optical resolution photoacoustic microscopy (ORPAM) and optical coherence tomography (OCT) is capable of providing complementary imaging contrasts. Unfortunately, the miniaturization of ORPAM remains a major challenge in the development of a handheld dual-modality imaging microscope with OCT. Here, we report the design and evaluation of an integrated ORPAM and OCT imaging probe using a two-dimensional MEMS (micro-electro-mechanical-system)-based optical scanner. This microscope, weighting 35.4 g, has an ultracompact size of 65×30×18 mm³, and an effective imaging area of 2×2 mm². The experimental lateral resolutions are 3.7 μm (ORPAM) and 5.6 μm (OCT), and the axial resolutions are measured as 120 μm (ORPAM) and 7.3 μm (OCT). Besides phantom and animal experiments, we carried out oral imaging of a healthy volunteer to show the clinical feasibility of this technique.

Dual modality reflection mode optical coherence and photoacoustic microscopy using an akinetic sensor

This Letter presents a novel dual modality reflection mode optical coherence and photoacoustic microscopy (OC-PAM) system. The optical coherence microscopy modality features a broadband source to accomplish 5 μm axial resolution. The photoacoustic microscopy modality uses a rigid akinetic Fabry-Perot etalon encapsulated in an optically transparent medium, which forms a 2  mm×11  mm translucent imaging window, permitting reflection mode dual modality imaging. After characterization, the OC-PAM system was applied to image zebrafish larvae in vivo, demonstrating its capability in biomedical imaging with complementary optical scattering and absorption contrasts by revealing morphology in the fish larvae.

Visible-light optical coherence tomography-based multimodal retinal imaging for improvement of fluorescent intensity quantification

We developed a spectral-domain visible-light optical coherence tomography (VIS-OCT) based multimodal imaging technique which can accomplish simultaneous OCT and fluorescence imaging with a single broadband light source. Phantom experiments showed that by using the simultaneously acquired OCT images as a reference, the effect of light attenuation on the intensity of the fluorescent images by materials in front of the fluorescent target can be compensated. This capability of the multimodal imaging technique is of high importance for achieving quantification of the true intensities of autofluorescence (AF) imaging of the retina. We applied the technique in retinal imaging including AF imaging of the retinal pigment epithelium and fluorescein angiography (FA). We successfully demonstrated the effect of compensation on AF and FA images with the simultaneously acquired VIS-OCT images.

Dual Modality Noncontact Photoacoustic and Spectral Domain OCT Imaging

We developed a multimodal imaging system, combining noncontact photoacoustic imaging and optical coherence tomography (OCT). Photoacoustic signals are recorded without contact to the specimens' surface by using an interferometric technique. The interferometer is realized within a fiber-optic network using a fiber laser at 1550 nm as source. The fiber-optic network allows the integration of a fiber-based OCT system operating at a wavelength region around 1310 nm. Light from the fiber laser and the OCT source are multiplexed into one fiber using wavelength-division multiplexing. The same focusing optics is used for both modalities. Back-reflected light from the sample is demultiplexed and guided to the respective imaging systems. As the same optical components are used for OCT and photoacoustic imaging, the obtained images are co-registered intrinsically in lateral direction. Three-dimensional imaging is implemented by hybrid galvanometer and mechanical scanning. To allow fast B-scan measurements, scanning of the interrogation beam along one dimension is executed by a galvanometer scanner. Slow-axis scanning, perpendicular to the fast axis, is performed utilizing a linear translational stage. We demonstrate two-dimensional and three-dimensional imaging on agarose phantoms.

Noncontact photoacoustic tomography of in vivo chicken chorioallantoic membrane based on all-fiber heterodyne interferometry

We present three-dimensional (3-D) in vivo photoacoustic (PA) images of the blood vasculature of a chicken chorioallantoic membrane (CAM) obtained by using a fiber-based noncontact PA tomography system. With a fiber-optic heterodyne interferometer, the system measures the surface displacement of a sample, induced by the PA wave, which overcomes the disadvantage of physical-contact of ultrasonic transducer in a conventional system. The performance of an implemented system is analyzed and its capability of in vivo 3-D bioimaging is presented. At a depth of 2.5 mm in a phantom experiment, the lateral and axial resolutions were measured as 100 and 30  μm, respectively. The lateral resolution became doubled at a depth of 7.0 mm; however, interestingly, the axial resolution was not noticeably deteriorated with the depth. With the CAM experiment, performed under the American National Standards Institute laser safety standard condition, blood vessel structures placed as deep as 3.5 mm were clearly recognized.

Fast subcellular optical coherence photoacoustic microscopy for pigment cell imaging

We developed a fast ultrahigh resolution optical coherence photoacoustic microscopy (FU-OCPAM) system by combining two complementary imaging modes of optical coherence microscopy (OCM) and photoacoustic microscopy (PAM) for cellular/subcellular imaging. The system used optical scanning to realize fast imaging speed and provided ultrahigh resolution of 1.24 and 0.59 μm for OCM and PAM, respectively. We imaged the retinal pigment epithelium (RPE) to demonstrate the subcellular imaging capability of the FU-OCPAM system. The OCM and PAM images clearly showed the RPE cell morphology and reflected the complementary optical properties of scattering and absorption. A quantitative analysis of the RPE cells was made based on photoacoustic (PA) signals. The cell area mainly ranged from 80 to 300  μm2, and had a linear relationship with the sum intensity of PA signals which mainly reflected the melanin content of the cells. The morphology and the PA signal could be used to identify qualitatively and quantitatively the aging and healthy states of the RPE cells. The results show the potential applications in studying the real-time cellular response to external stimulations and the progress of aging and diseases at the cellular level with FU-OCPAM.

All-optically integrated photo-acoustic microscopy and optical coherence tomography based on a single Michelson detector

An all-optically noncontact dual-mode imaging system using a single Michelson detector that simultaneously achieved photo-acoustic microscopy (PAM) and optical coherence tomography (OCT) is presented. The pulse laser-induced photo-acoustic signals and the back-scattered photons were alternately detected by a single Michelson detector. The spatial resolution and imaging capability of the dual-mode imaging system were testified by scattering phantoms. Furthermore, in vivo images of the mouse ear demonstrated that the PAM-OCT can provide complementary anatomical and functional information for imaging of biological tissues, which could be the best co-imaging strategy of PAM and OCT in biomedicine.

Optical coherence photoacoustic microscopy for in vivo multimodal retinal imaging

We developed an optical coherence photoacoustic microscopy (OC-PAM) system, which can accomplish optical coherence tomography (OCT) and photoacoustic microscopy (PAM) simultaneously by using a single pulsed broadband light source. With a center wavelength of 800 nm and a bandwidth of 30 nm, the system is suitable for imaging the retina. Generated from the same group of photons, the OCT and PAM images are intrinsically registered in the lateral directions. To test the capabilities of the system on multimodal ophthalmic imaging, we imaged the retina of pigmented rats. The OCT images showed the retinal structures with quality similar to conventional OCT, while the PAM images revealed the distribution of absorbers in the retina. Since the absorption of hemoglobin is relatively weak at around 800 nm, the NIR PAM signals are generated mainly from melanin in the posterior segment of the eye, thus providing melanin-specific imaging of the retina.

Multimodal noncontact photoacoustic and optical coherence tomography imaging using wavelength-division multiplexing

We present multimodal noncontact photoacoustic (PA) and optical coherence tomography (OCT) imaging. PA signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as the source. In the same fiber-optic network, a spectral-domain OCT system is implemented. The OCT system utilizes a supercontinuum light source at 1310 nm and a spectrometer with an InGaAs line array detector. Light from the fiber laser and the OCT source is multiplexed into one fiber using a wavelength-division multiplexer; the same objective is used for both imaging modalities. Reflected light is spectrally demultiplexed and guided to the respective imaging systems. We demonstrate two-dimensional and three-dimensional imaging on a tissue-mimicking sample and a chicken skin phantom. The same fiber network and same optical components are used for PA and OCT imaging, and the obtained images are intrinsically coregistered.

Photoacoustic imaging platforms for multimodal imaging

Photoacoustic (PA) imaging is a hybrid biomedical imaging method that exploits both acoustical Epub ahead of print and optical properties and can provide both functional and structural information. Therefore, PA imaging can complement other imaging methods, such as ultrasound imaging, fluorescence imaging, optical coherence tomography, and multi-photon microscopy. This article reviews techniques that integrate PA with the above imaging methods and describes their applications.

An overview of optical coherence tomography for ovarian tissue imaging and characterization

Ovarian cancer has the lowest survival rate among all the gynecologic cancers because it is predominantly diagnosed at late stages due to the lack of reliable symptoms and efficacious screening techniques. Optical coherence tomography (OCT) is an emerging technique that provides high-resolution images of biological tissue in real time, and demonstrates great potential for imaging of ovarian tissue. In this article, we review OCT studies for visualization and diagnosis of human ovaries as well as quantitative extraction of ovarian tissue optical properties for classifying normal and malignant ovaries. OCT combined with other imaging modalities to further improve ovarian tissue diagnosis is also reviewed.

A low-cost photoacoustic microscopy system with a laser diode excitation

Photoacoustic microscopy (PAM) is capable of mapping microvasculature networks in biological tissue and has demonstrated great potential for biomedical applications. However, the clinical application of the PAM system is limited due to the use of bulky and expensive pulsed laser sources. In this paper, a low-cost optical-resolution PAM system with a pulsed laser diode excitation has been introduced. The lateral resolution of this PAM system was estimated to be 7 µm by imaging a carbon fiber. The phantoms made of polyethylene tubes filled with blood and a mouse ear were imaged to demonstrate the feasibility of this PAM system for imaging biological tissues.

Integrated photoacoustic, ultrasound and fluorescence platform for diagnostic medical imaging-proof of concept study with a tissue mimicking phantom

The structural and molecular heterogeneities of biological tissues demand the interrogation of the samples with multiple energy sources and provide visualization capabilities at varying spatial resolution and depth scales for obtaining complementary diagnostic information. A novel multi-modal imaging approach that uses optical and acoustic energies to perform photoacoustic, ultrasound and fluorescence imaging at multiple resolution scales from the tissue surface and depth is proposed in this paper. The system comprises of two distinct forms of hardware level integration so as to have an integrated imaging system under a single instrumentation set-up. The experimental studies show that the system is capable of mapping high resolution fluorescence signatures from the surface, optical absorption and acoustic heterogeneities along the depth (>2cm) of the tissue at multi-scale resolution (<1µm to <0.5mm).

Full-range Fourier domain Doppler optical coherence tomography based on sinusoidal phase modulation

A novel full-range Fourier domain Doppler optical coherence tomography (full-range FD-DOCT) using sinusoidal phase modulation for B-M scan is proposed. In this sinusoidal B-M scan, zero optical path difference (OPD) position does not move corresponding to lateral scanning points in contrast to linear B-M scan. Since high phase sensitivity arises around the zero OPD position, the proposed full-range FD-DOCT can achieve easily high velocity sensitivity without mirror image around the zero OPD position. Velocity sensitivity dependent on the OPD and the interval of scanning points is examined, and flow velocity detection capability is verified through Doppler imaging of a flow phantom and an in vivo biological sample.

Combined photoacoustic and optical coherence tomography using a single near-infrared supercontinuum laser source

We developed an integrated dual-modal photoacoustic and optical coherence tomography (PA-OCT) system using a single near-infrared supercontinuum laser source to simultaneously provide both optical absorption and scattering contrasts. A pulsed broadband supercontinuum source was generated by a pulsed Nd:YAG laser and a photonic-crystal fiber. When we imaged two colored hairs, the black hair was visible in both PA and OCT images, whereas the white hair was only mapped in the OCT image. The single laser source will potentially allow us to implement relatively simple, cheap, and compact dual-modal PA-OCT systems, which are key criteria for fast clinical translation and commercialization.

In vivo three dimensional dual wavelength photoacoustic tomography imaging of the far red fluorescent protein E2-Crimson expressed in adult zebrafish

For the first time the far red fluorescent protein (FP) E2-Crimson genetically expressed in the exocrine pancreas of adult zebrafish has been non-invasively mapped in 3D in vivo using photoacoustic tomography (PAT). The distribution of E2-Crimson in the exocrine pancreas acquired by PAT was confirmed using epifluorescence imaging and histology, with optical coherence tomography (OCT) providing complementary structural information. This work demonstrates the depth advantage of PAT to resolve FP in an animal model and establishes the value of E2-Crimson for PAT studies of transgenic models, laying the foundation for future longitudinal studies of the zebrafish as a model of diseases affecting inner organs.

Non-contact photoacoustic imaging using a fiber based interferometer with optical amplification

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications contact with the tissue should be avoided. As alternatives to contacting transducers interferometric means can be used to acquire photoacoustic signals remotely. In this paper we report on non-contact three and two dimensional photoacoustic imaging using an optical fiber-based Mach-Zehnder interferometer. A detection beam is transmitted through an optical fiber network onto the surface of the specimen. Back reflected light is collected and coupled into the same optical fiber. To achieve a high signal/noise ratio the reflected light is amplified by means of optical amplification with an erbium doped fiber amplifier before demodulation. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain reconstruction algorithm. We present remote photoacoustic imaging of a tissue mimicking phantom and on chicken skin.

Intrasweep phase-sensitive optical coherence tomography for noncontact optical photoacoustic imaging

We introduce a method to extract the photoacoustic (PA) signal from the phase time evolution of an optical coherence tomography (OCT) swept source spectral sweep. This all-optical detection is achieved in a noncontact fashion directly on the sample surface by using its specular reflection. High-speed measurement and referencing allow for close to shot noise limited phase-sensitive detection. It offers a simple way to perform OCT and PA imaging by sharing the same system components.

Near-infrared light photoacoustic ophthalmoscopy

We achieved photoacoustic ophthalmoscopy (PAOM) imaging of the retina with near-infrared (NIR) light illumination. A PAOM imaging system with dual-wavelength illumination at 1064 nm and 532 nm was built. We compared in vivo imaging results of both albino and pigmented rat eyes at the two wavelengths. The results show that the bulk optical absorption of the retinal pigment epithelium (RPE) is only slightly higher than that of the retinal vessels at 532 nm while it becomes more than an order of magnitude higher than that of the retinal vessels at 1064 nm. These studies suggest that although visible light illumination is suitable for imaging both the retinal vessels and the RPE, NIR light illumination, being more comfortable to the eye, is better suited for RPE melanin related investigations and diagnoses.