KEYWORDS: Scattering, Photoacoustic spectroscopy, Tissue optics, Tissues, Connective tissue, In vivo imaging, Cervix, Infrared photography, Infrared radiation, Monte Carlo methods
Premature cervical remodeling is a critical precursor of spontaneous preterm birth, and the remodeling process is characterized by an increase in tissue hydration. Nevertheless, current clinical measurements of cervical remodeling are subjective and detect only late events, such as cervical effacement and dilation. Here, we present a photoacoustic endoscope that can quantify tissue hydration by measuring near-infrared cervical spectra. We quantify the water contents of tissue-mimicking hydrogel phantoms as an analog of cervical connective tissue. Applying this method to pregnant women in vivo, we observed an increase in the water content of the cervix throughout pregnancy. The application of this technique in maternal healthcare may advance our understanding of cervical remodeling and provide a sensitive method for predicting preterm birth.
Photoacoustic computed tomography (PACT) is an emerging computed imaging modality that exploits optical contrast and ultrasonic detection principles to form images of the photoacoustically induced initial pressure distribution within tissue. The PACT reconstruction problem corresponds to a time-domain inverse source problem, where the initial pressure distribution is recovered from the measurements recorded on an aperture outside the support of the source. A major challenge in transcranial PACT brain imaging is to compensate for aberrations in the measured data due to the propagation of the photoacoustic wavefields through the skull. To properly account for these effects, a wave equation-based inversion method should be employed that can model the heterogeneous elastic properties of the medium. In this study, an iterative image reconstruction method for 3D transcranial PACT is developed based on the elastic wave equation. To accomplish this, a forward model based on a finite-difference time-domain discretization of the elastic wave equation is established. Subsequently, gradient-based methods are employed for computing penalized least squares estimates of the initial source distribution that produced the measured photoacoustic data. The developed reconstruction algorithm is validated and investigated through computer-simulation studies.
We have enhanced photoacoustic computed tomography with dry acoustic coupling that eliminates water immersion anxiety and wrinkling of the animal and facilitates incorporating complementary modalities and procedures. The dry acoustic coupler is made of a tubular elastic membrane enclosed by a closed transparent water tank. The tubular membrane ensures water-free contact with the animal, and the closed water tank allows pressurization for animal stabilization. The dry coupler was tested using a whole-body small-animal ring-shaped photoacoustic computed tomography system. Dry coupling was found to provide image quality comparable to that of conventional water coupling.
While lasers have been commonly used as illumination sources in photoacoustic (PA) imaging, their high purchase and maintenance costs, as well as their bulkiness, have hindered the rapid clinical dissemination of PA imaging. With this in mind, we explore an alternative illumination source for PA tomography—a xenon flash lamp with high pulse energy and a microsecond pulse width. We demonstrate that, by using a single xenon flash lamp, we can image both a black latex cord placed in chicken breast tissue at a depth of up to 3.5 cm ex vivo and an entire mouse body in vivo. Our findings indicate that the xenon flash lamp, producing optical illumination that is safe for humans, can be potentially applied to human tissue imaging.
Capitalizing on endogenous hemoglobin contrast, photoacoustic-computed tomography (PACT), a deep-tissue high-resolution imaging modality, has drawn increasing interest in neuroimaging. However, most existing studies are limited to functional imaging on the cortical surface and the deep brain structural imaging capability of PACT has never been demonstrated. Here, we explicitly studied the limiting factors of deep brain PACT imaging. We found that the skull distorted the acoustic signal and blood suppressed the structural contrast from other chromophores. When the two effects are mitigated, PACT can potentially provide high-resolution label-free imaging of structures in the entire mouse brain. With 100-μm in-plane resolution, we can clearly identify major structures of the brain, which complements magnetic resonance microscopy for imaging small-animal brain structures. Spectral PACT studies indicate that structural contrasts mainly originate from cytochrome distribution and that the presence of lipid sharpens the image contrast; brain histology results provide further validation. The feasibility of imaging the structure of the brain in vivo is also discussed. Our results demonstrate that PACT is a promising modality for both structural and functional brain imaging.
Capitalizing on endogenous hemoglobin contrast, photoacoustic computed tomography (PACT), a deep-tissue highresolution imaging modality, has drawn increasing interest in neuro-imaging. However, most existing studies are limited to functional imaging on the cortical surface, and the deep-brain structural imaging capability of PACT has never been demonstrated. Here, we explicitly studied the limiting factors of deep-brain PACT imaging. We found that the skull distorted the acoustic signal and blood suppressed the structural contrast from other chromophores. When the two effects are mitigated, PACT can provide high-resolution label-free structural imaging through the entire mouse brain. With 100 μm in-plane resolution, we can clearly identify major structures of the brain, and the image quality is comparable to that of magnetic resonance microscopy. Spectral PACT studies indicate that structural contrasts mainly originate from cytochrome and lipid. The feasibility of imaging the structure of the brain in vivo has also been discussed. Our results demonstrate that PACT is a promising modality for both structural and functional brain imaging.
Super-resolution microscopy techniques—capable of overcoming the diffraction limit of light—have opened new opportunities to explore subcellular structures and dynamics not resolvable in conventional far-field microscopy. However, relying on staining with exogenous fluorescent markers, these techniques can sometimes introduce undesired artifacts to the image, mainly due to large tagging agent sizes and insufficient or variable labeling densities. By contrast, the use of endogenous pigments allows imaging of the intrinsic structures of biological samples with unaltered molecular constituents. Here, we report label-free photoacoustic (PA) nanoscopy, which is exquisitely sensitive to optical absorption, with an 88 nm resolution. At each scanning position, multiple PA signals are successively excited with increasing laser pulse energy. Because of optical saturation or nonlinear thermal expansion, the PA amplitude depends on the nonlinear incident optical fluence. The high-order dependence, quantified by polynomial fitting, provides super-resolution imaging with optical sectioning. PA nanoscopy is capable of super-resolution imaging of either fluorescent or nonfluorescent molecules.
KEYWORDS: Microscopy, Luminescence, Super resolution, Microscopes, Confocal microscopy, Diffraction, Image resolution, Signal to noise ratio, Point spread functions, Lithium
We present a generic sub-diffraction-limited imaging method – photobleaching imprinting microscopy (PIM) – for biological fluorescence imaging. A lateral resolution of 110 nm was measured, more than a two-fold improvement over the optical diffraction limit. Unlike other super-resolution imaging techniques, PIM does not require complicated illumination modules or specific fluorescent dyes. PIM is expected to facilitate the conversion of super-resolution imaging into a routine lab tool, making it accessible to a much broader biological research community.
The metabolic rate and oxygen consumption of the brain is reflected in jugular venous oxygen saturation. In many clinical conditions, such as head trauma, stroke, and low cardiac output states, the brain is at risk for hypoxic-ischemic injury. The current gold standard for monitoring brain oxygenation is invasive and requires jugular vein catheterization under fluoroscopic guidance; and therefore it is rarely used. Photo-acoustic tomography in combination with ultrasound can be used to estimate oxygen saturation of the internal jugular vein in real-time. This noninvasive method will enable earlier detection and prevention of impending hypoxic brain injury. A wavelength-tunable dye laser pumped by a Nd:YAG laser delivers light through an optical fiber bundle, and a modified commercial ultrasound imaging system (Philips iU22) detects both the pulse-echo ultrasound (US) and photoacoustic (PA) signals. A custom-built multichannel data acquisition system renders co-registered ultrasound and photoacoustic images at 5 frames per second. After the jugular vein was localized in healthy volunteers, dualwavelength PA images were used to calculate the blood hemoglobin oxygen saturation from the internal jugular vein in vivo. The preliminary results raise confidence that this emerging technology can be used clinically as an accurate, noninvasive indicator of cerebral oxygenation.
Many breast cancer patients receive neoadjuvant treatment to reduce tumor size and enable breast conserving therapy. Most imaging methods used to monitor response to neoadjuvant chemotherapy or hormone therapy depend on overall gross tumor morphology and size measurements, which may not be sensitive or specific, despite tumor response on a cellular level. A more sensitive and specific method of detecting response to therapy might allow earlier adjustments in treatment, and thus result in better outcomes while avoiding unnecessary morbidity. We developed an imaging system that combines spectral photoacoustic tomography and ultrasonography to predict breast neoadjuvant therapeutic response based on blood volume and blood oxygenation contrast. The system consists of a tunable dye laser pumped by a Nd:YAG laser, a commercial ultrasound imaging system (Philips iU22), and a multichannel data acquisition system which displays co-registered photoacoustic and ultrasound images in real time. Early studies demonstrate functional imaging capabilities, such as oxygen saturation and total concentration of hemoglobin, in addition to ultrasonography of tumor morphology. Further study is needed to determine if the co-registered photoacoustic tomography and ultrasonography system may provide an accurate tool to assess treatment efficacy by monitoring tumor response in vivo.
A novel method – photoacoustic recovery after photothermal bleaching (PRAP) – is proposed and implemented to study particle dynamics and medium properties at the micron scale via photoacoustic imaging. PRAP is an intuitive way to visualize as well as quantify dynamic processes in many kinds of media. We demonstrate PRAP first in a phantom study, and then in live cells. PRAP provides high signal-to-noise ratio imaging with minimal bleaching-induced artifacts during the recovery stage, ideal for monitoring the diffusive and kinetic phenomena inside a cell.
We studied the phenomenon of photothermal bleaching — a gradual reduction of contrast agent particles during repeated scans in photoacoustic microscopy. The dependence of the photothermal bleaching rate on the excitation pulse energy was determined while the laser focal diameter was held constant. Our results showed that, the dependence of the photothermal bleaching rate on the excitation pulse energy differed before and after the absorbers were raised to their melting point by the deposited laser energy. Based on this finding, we suggested an optimal excitation pulse energy, which balances the photothermal bleaching rate and signal amplitude, for time-lapse imaging applications.
Sentinel lymph node biopsy (SLNB) has emerged as an accurate, less invasive alternative to axillary lymph node dissection, and it has rapidly become the standard of care for patients with clinically node-negative breast cancer. The sentinel lymph node (SLN) hypothesis states that the pathological status of the axilla can be accurately predicted by determining the status of the first (i.e., sentinel) lymph nodes that drain from the primary tumor. Physicians use radio-labeled sulfur colloid and/or methylene blue dye to identify the SLN, which is most likely to contain metastatic cancer cells. However, the surgical procedure causes morbidity and associated expenses. To overcome these limitations, we developed a dual-modality photoacoustic and ultrasound imaging system to noninvasively detect SLNs based on the accumulation of methylene blue dye. Ultimately, we aim to guide percutaneous needle biopsies and provide a minimally invasive method for axillary staging of breast cancer. The system consists of a tunable dye laser pumped by a Nd:YAG laser, a commercial ultrasound imaging system (Philips iU22), and a multichannel data acquisition system which displays co-registered photoacoustic and ultrasound images in real-time. Our clinical results demonstrate that real-time photoacoustic imaging can provide sensitive and specific detection of methylene blue dye in vivo. While preliminary studies have shown that in vivo detection of SLNs by using co-registered photoacoustic and ultrasound imaging is feasible, further investigation is needed to demonstrate robust SLN detection.
We present an innovative method, photoacoustic recovery after photothermal bleaching (PRAP), for studying particle dynamics at micron scale via photoacoustic imaging. As an intuitive way to visualize and quantify dynamic processes, PRAP is demonstrated first in a simple phantom study and then in a more complex measurement involving live cells. Compared with the conventional fluorescence-based approach, PRAP provides high signal-to-noise ratio (SNR) imaging with minimal bleaching-induced artifacts during the recovery stage, ideal for monitoring the diffusive and kinetic processes inside a cell.
We report the development of photoacoustic microscopy capable of video-rate high-resolution in-vivo
imaging in deep tissue. A lightweight photoacoustic probe is made of a single-element
broadband ultrasound transducer, a compact photoacoustic beam combiner, and a bright-field light
delivery system. Focused broadband ultrasound detection provides a 44-μm lateral resolution and a
28-μm axial resolution. A multimode optical fiber is used to deliver laser pulses. The bright-field
light delivery system can effectively improve the illumination efficiency. The photoacoustic probe
weighs less than 40 grams and is mounted on a voice-coil scanner to acquire 40 cross-sectional
images per second over several-mm range. The fast speed can effectively improve imaging
throughput, reduce motion artifacts, and enable the visualization of highly dynamic biomedical
processes. High-resolution micro-vascular imaging is successfully demonstrated.
Photoacoustic tomography (PAT) of the human brain is challenging due to the fact that the skull strongly absorbs and scatters light, and attenuates and distorts ultrasound as well. For the first time, we demonstrated the feasibility of PAT through a whole adult human skull. A photon recycler (PR) was built to increase light transmittance through the skull. Both a graphite target and a canine brain were imaged through the skull. Use of the PR was found to improve the photoacoustic signal-to-noise ratio by a factor of 2.4. In addition, subtraction of photoacoustic signals that arise from light absorption within the skull significantly improved the contrast of the target. Our results indicate that PAT can potentially be applied to in vivo human brain imaging.
We report the development of functional photoacoustic microscopy capable of video-rate high-resolution in vivo imaging in deep tissue. A lightweight photoacoustic probe is made of a single-element broadband ultrasound transducer, a compact photoacoustic beam combiner, and a bright-field light delivery system. Focused broadband ultrasound detection provides a 44-μm lateral resolution and a 28-μm axial resolution based on the envelope (a 15-μm axial resolution based on the raw RF signal). Due to the efficient bright-field light delivery, the system can image as deep as 4.8 mm in vivo using low excitation pulse energy (28 μJ per pulse, 0.35 mJ/cm 2 on the skin surface). The photoacoustic probe is mounted on a fast-scanning voice-coil scanner to acquire 40 two-dimensional (2-D) B-scan images per second over a 9-mm range. High-resolution anatomical imaging is demonstrated in the mouse ear and brain. Via fast dual-wavelength switching, oxygen dynamics of mouse cardio-vasculature is imaged in realtime as well.
Photoacoustic (PA) microscopy (PAM) can image optical absorption contrast with ultrasonic spatial resolution in the optical diffusive regime. Conventionally, accurate quantification in PAM requires knowledge of the optical fluence attenuation, acoustic pressure attenuation, and detection bandwidth. We circumvent this requirement by quantifying the optical absorption coefficients from the acoustic spectra of PA signals acquired at multiple optical wavelengths. With the acoustic spectral method, the absorption coefficients of an oxygenated bovine blood phantom at 560, 565, 570, and 575 nm were quantified with errors of <3 % . We also quantified the total hemoglobin concentration and hemoglobin oxygen saturation in a live mouse. Compared with the conventional amplitude method, the acoustic spectral method provides greater quantification accuracy in the optical diffusive regime. The limitations of the acoustic spectral method was also discussed.
In surgical treatment of pancreatic cancers, the effectiveness of the procedures largely depends on the ability to
completely and precisely remove the malignant tumors. We present the ex-vivo use of oblique incidence diffuse
reflectance spectroscopy (OIRDS) to detect and differentiate normal from neoplastic tissue. An OIRDS probe has been
constructed to provide scattering and absorption information of the pancreatic tissue. To reveal the physiological origin
of the difference in these optical signatures, the optical scattering coefficients were extracted along the pancreatic duct
with 1-cm spacing. Experimental results show that OIDRS was able to successfully determinate the tumor margins
based on the higher optical scattering on malignant tissue.
KEYWORDS: Skin, Absorption, Melanoma, Diffuse reflectance spectroscopy, Scattering, Optical properties, In vivo imaging, Tissue optics, Spectroscopy, Monte Carlo methods
In this letter, we report the first use of oblique incidence diffuse reflectance spectrometry to conduct in-vivo measurements of optical properties of three different types of pigmented skin lesions, including melanoma, dysplastic, and common nevi. Both absorption and reduced scattering coefficient spectra were estimated from the spatially resolved diffuse reflectance within the wavelength range of 455-765 nm for 144 pigmented skin lesions including 16 melanomas. The absorption and reduced scattering spectra were found to change with the malignancy of the skin lesions, which were generally higher for the malignant cases than the benign ones. Based on the measurement results, the physiological origin leading to the change of the absorption and scattering properties is also discussed.
This paper presents the use of spatially resolved oblique-incidence diffuse reflectance spectroscopy for skin cancer
diagnosis. Spatio-spectral data from 166 pigmented skin lesions were collected for the wavelength range from 455 to
765 nm. A set of neural network based classifiers separates the pigmented malignant melanomas from the benign and
dysplastic subgroups. A total of 110 lesions were used as the training set and 56 lesions were used as the testing set.
This classifier performs with an overall 100% sensitivity and 92% specificity for the training set and 100% sensitivity
and 88% specificity for the testing set. The second classifier was designed to separate the benign from the dysplastic
subgroups. For the second classifier a total of 100 lesions were used as the training set and 51 lesions were used as the
testing set. The overall classification rates were 94% and 88% for the training and testing sets respectively.
This paper presents a study for in-vivo estimation of optical properties of pigmented skin tumor by oblique incidence
diffuse reflectance spectroscopy. The developed system has been tested in clinical conditions to compare the optical
properties of melanomas, dysplastic nevi and common nevi. The
spatio-spectral data are collected in the wavelength
range of 455 to 765 nm from 96 pigmented skin lesions including 10 histopathologically diagnosed as melanoma, 67 as
dysplastic nevi and 19 lesions as common nevi. The preliminary results indicate significantly larger average reduced
scattering coefficient spectra for malignant and dysplastic lesions than for benign common nevi.
This paper presents a study on non-invasive detection of two common epithelial cancers (skin and esophagus) based on oblique incidence diffuse reflectance spectroscopy (OIDRS). An OIDRS measurement system, which combines fiber optics and MEMS technologies, was developed. In our pilot studies, a total number of 137 cases have been measured in-vivo for skin cancer detection and a total number of 20 biopsy samples have been measured ex-vivo for esophageal cancer detection. To automatically differentiate the cancerous cases from benign ones, a statistical software classification program was also developed. An overall classification accuracy of 90% and 100% has been achieved for skin and esophageal cancer classification, respectively.
Optical phantoms are widely used for simulating optical properties of biological tissues. Their accurate design and
fabrication are important factors in validating and designing biomedical systems. We discuss fabrication and
measurement of optical phantoms in ultrasound-modulated optical tomography. The optical properties of the phantoms
are measured by an oblique-incidence diffuse reflectance spectrometer, which can accurately measure the wavelength-dependent
absorption and reduced scattering coefficients of optical phantoms. In addition, the acoustic properties of the
phantoms are discussed.
A study of in-vivo classification of pigmented skin lesions using oblique-incidence diffuse reflectance spectroscopy
is presented. Spatio-spectral data in the wavelength range from 455 to 765 nm are collected from 111 pigmented
lesions including 10 histopathologically diagnosed as melanoma. The first 67 lesions are used for training the
classifiers, and 44 lesions are used for testing. The first classifier separates (1) malignant melanoma and severe
dysplastic nevi from (2) moderate and mild dysplastic nevi, common nevi, actinic and seborrheic keratoses. The
second classifier next distinguishes between (a) moderate and mild dysplastic nevi, common nevi from (b) actinic
and seborrheic keratoses. The third classifier further separates (I) moderate and mild dysplastic nevi from (II)
common nevi. The first classifier performs with 100% sensitivity and 91% specificity with overall classification
rates of 93% and 95 % for the training and testing sets, respectively. The second classifier has classification rates of
95% and 97 % for the training and testing sets, respectively, whereas the third classifier has classification rates of
98% and 94 % for the training and testing sets, respectively.
This paper presents a study of the use of a single scaleable Monte Carlo simulation to estimate the physiological parameters of skin lesions from data collected in vivo using spectroscopic oblique-incidence reflectometry. Spatio-spectral data from 101 cases are separated into two groups based on their melanocytic conditions. Group-1 consists of (a) cancerous basal cell carcinomas and squamous cell carcinomas and (b) benign actinic keratoses and seborrheic keratoses. Group-2 consists of (a) dysplastic nevi and (b) benign common nevi. Several physiological parameters are estimated, such as the size distribution of the optical scatterers, the relative index of refraction of the scatterers, the total volume concentration of the scatterers, the concentration of the total hemoglobin and the oxygen saturation, and the relative changes related to the values calculated from the neighboring healthy tissues. The most significant features are then combined into one feature. The results show that for both groups the combined feature is significantly different for the benign and cancerous cases than for the dysplastic cases. The ROC area was 0.9 and 0.86 for group-1 and group-2, respectively.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.