Optoacoustic Imaging System Research Articles

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.

Trackerless panoramic optoacoustic imaging: a first feasibility evaluation

Optoacoustic imaging provides high spatial resolution and the possibility to image specific functional parameters in real-time, therefore positioning itself as a promising modality for various applications. However, despite these advantages, the applicability of real-time optoacoustic imaging is generally limited due to a relatively small field of view. With this work, we aim at presenting a path towards panoramic optoacoustic tomographic imaging without requiring additional sensors or position trackers. We propose a two-step seamless stitching method for the compounding of multiple datasets acquired with a real-time 3D optoacoustic imaging system within a panoramic scan. The employed workflow is specifically tailored to the image properties and respective challenges. A comparison of the presented alignment on in-vivo data shows a mean error of [Formula: see text] compared to ground truth tracking data. The presented compounding scheme integrates the physical resolution of optoacoustic data and hence can provide improved contrast in comparison with other compounding approaches based on addition or averaging. The proposed method can produce optoacoustic volumes with an enlarged field of view and improved quality compared to current methods in optoacoustic imaging. However, our study also shows challenges for panoramic scans. In this view, we discuss relevant properties, challenges, and opportunities and present an evaluation of the performance of the presented approach with different input data.

Efficient 3-D Model-Based Reconstruction Scheme for Arbitrary Optoacoustic Acquisition Geometries.

Optimal optoacoustic tomographic sampling is often hindered by the frequency-dependent directivity of ultrasound sensors, which can only be accounted for with an accurate 3-D model. Herein, we introduce a 3-D model-based reconstruction method applicable to optoacoustic imaging systems employing detection elements with arbitrary size and shape. The computational complexity and memory requirements are mitigated by introducing an efficient graphic processing unit (GPU)-based implementation of the iterative inversion. On-the-fly calculation of the entries of the model-matrix via a small look-up table avoids otherwise unfeasible storage of matrices typically occupying more than 300GB of memory. Superior imaging performance of the suggested method with respect to standard optoacoustic image reconstruction methods is first validated quantitatively using tissue-mimicking phantoms. Significant improvements in the spatial resolution, contrast to noise ratio and overall 3-D image quality are also reported in real tissues by imaging the finger of a healthy volunteer with a hand-held volumetric optoacoustic imaging system.

Terrylenediimide-Based Intrinsic Theranostic Nanomedicines with High Photothermal Conversion Efficiency for Photoacoustic Imaging-Guided Cancer Therapy.

Activatable theranostic nanomedicines involved in photothermal therapy (PTT) have received constant attention as promising alternatives to traditional therapies in clinic. However, the theranostic nanomedicines widely suffer from instability and complicated nanostructures, which hamper potential clinical applications. Herein, we demonstrated a terrylenediimide (TDI)-poly(acrylic acid) (TPA)-based nanomedicine (TNM) platform used as an intrinsic theranostic agent. As an exploratory paradigm in seeking biomedical applications, TDI was modified with poly(acrylic acid)s (PAAs), resulting in eight-armed, star-like TPAs composed of an outside hydrophilic PAA corona and an inner hydrophobic TDI core. TNMs were readily fabricated via spontaneous self-assembly. Without additional vehicle and cargo, the as-prepared TNMs possessed a robust nanostructure and high photothermal conversion efficiency up to approximately 41%. The intrinsic theranostic properties of TNMs for use in photoacoustic (PA) imaging by a multispectral optoacoustic tomography system and in mediating photoinduced tumor ablation were intensely explored. Our results suggested that the TNMs could be successfully exploited as intrinsic theranostic agents for PA imaging-guided efficient tumor PTT. Thus, these TNMs hold great potential for (pre)clinical translational development.

Abstract P4-02-08: Opto-acoustic imaging of the breast: Downclassification and upclassification of suspicious breast masses

Abstract Background Imagio® is an opto-acoustic (OA) breast imaging system designed to concomitantly collect OA images together with diagnostic ultrasound (CDU). The device is intended to improve distinction between benign and malignant masses. In this interim analysis of the post-market surveillance MAESTRO study we report the results of 75 patients who had breast masses classified as BI-RADS 4a and 4b by CDU. Aim We assessed OA's sensitivity, specificity, and its ability to downgrade benign masses and upgrade malignant masses based on the probability of malignancy (POM) and BI-RADS category. Methods Seventy-five patients with 78 breast masses were evaluated with OA prior to biopsy. For each mass, the radiologist scored 5 OA features, assigned a POM and BI-RADS category. OA sensitivity, specificity, and BI-RADS downgrade and upgrade percentages were assessed with and without contribution from a previously derived nomogram. Results The mean POM difference between malignant and benign masses was higher for OA (37%) than for CDU (27%). OA specificity was 43% without the nomogram and 68% with the nomogram. OA sensitivity was 97% with and without the nomogram. With OA 43% of benign masses could be downgraded and 47% of malignant masses could be upgraded in BIRADS category. Conclusion These results appear to confirm the previously reported ability of OA to improve the differentiation between benign and malignant masses compared to CDU alone, to potentially decrease negative biopsies, and to upgrade BI-RADS category in malignant masses. The MAESTRO study (n=200) may further confirm these results. Citation Format: Menezes GLG, Pijnappel RM, Meeuwis C, Bisschops B, Veltman J, Lavin PT, Mann RM. Opto-acoustic imaging of the breast: Downclassification and upclassification of suspicious breast masses [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-02-08.

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 (0.63mm and 0.7mm in diameter) inside blood vessel models (1.6mm and 2.4mm in diameter). Frame rate was 10Hz. High longitudinal spatial resolution of the system − 0.1mm − allows distinguishing the two walls of the vessel model and the position of the needle inside.

Towards Quantitative Evaluation of Tissue Absorption Coefficients Using Light Fluence Correction in Optoacoustic Tomography.

Optoacoustic tomography is a fast developing imaging modality, combining the high contrast available from optical excitation of tissue with the high resolution and penetration depth of ultrasound detection. Light is subject to both absorption and scattering when traveling through tissue; adequate knowledge of tissue optical properties and hence the spatial fluence distribution is required to create an optoacoustic image that is directly proportional to chromophore concentrations at all depths. Using data from a commercial multispectral optoacoustic tomography (MSOT) system, we implemented an iterative optimization for fluence correction based on a finite-element implementation of the delta-Eddington approximation to the Radiative Transfer Equation (RTE). We demonstrate a linear relationship between the image intensity and absorption coefficients across multiple wavelengths and depths in phantoms. We also demonstrate improved feature visibility and spectral recovery at depth in phantoms and with in vivo measurements, suggesting our approach could in the future enable quantitative extraction of tissue absorption coefficients in biological tissue.

Performance of a Multispectral Optoacoustic Tomography (MSOT) System equipped with 2D vs. 3D Handheld Probes for Potential Clinical Translation

A handheld approach to optoacoustic imaging is essential for the clinical translation. The first 2- and 3-dimensional handheld multispectral optoacoustic tomography (MSOT) probes featuring real-time unmixing have recently been developed. Imaging performance of both probes was determined in vitro and in a brain melanoma metastasis mouse model in vivo. T1-weighted MR images were acquired for anatomical reference. The limit of detection of melanoma cells in vitro was significantly lower using the 2D than the 3D probe. The signal decrease was more profound in relation to depth with the 3D versus the 2D probe. Both approaches were capable of imaging the melanoma tumors qualitatively at all time points. Quantitatively, the 2D approach enabled closer anatomical resemblance of the tumor compared to the 3D probe, particularly at depths beyond 3mm. The 3D probe was shown to be superior for rapid 3D imaging and, thus, holds promise for more superficial target structures.

Influence of the absorber dimensions on wavefront shaping based on volumetric optoacoustic feedback.

The recently demonstrated control over light distribution through turbid media based on real-time three-dimensional optoacoustic feedback has offered promising prospects to interferometrically focus light within scattering objects. Nevertheless, the focusing capacity of the feedback-based approach is strongly conditioned by the number of optical modes (speckle grains) enclosed in the volume that can be resolved with the optoacoustic imaging system. In this Letter, we experimentally tested the light intensity enhancement achieved with optoacoustic feedback measurements from different sizes of absorbing microparticles. The importance of the obtained results is discussed in the context of potential signal enhancement at deep locations within a scattering medium where the effective speckle grain sizes approach the minimum values dictated by optical diffraction.

Sparsity-based acoustic inversion in cross-sectional multiscale optoacoustic imaging.

With recent advancement in hardware of optoacoustic imaging systems, highly detailed cross-sectional images may be acquired at a single laser shot, thus eliminating motion artifacts. Nonetheless, other sources of artifacts remain due to signal distortion or out-of-plane signals. The purpose of image reconstruction algorithms is to obtain the most accurate images from noisy, distorted projection data. In this paper, the authors use the model-based approach for acoustic inversion, combined with a sparsity-based inversion procedure. Specifically, a cost function is used that includes the L1 norm of the image in sparse representation and a total variation (TV) term. The optimization problem is solved by a numerically efficient implementation of a nonlinear gradient descent algorithm. TV-L1 model-based inversion is tested in the cross section geometry for numerically generated data as well as for in vivo experimental data from an adult mouse. In all cases, model-based TV-L1 inversion showed a better performance over the conventional Tikhonov regularization, TV inversion, and L1 inversion. In the numerical examples, the images reconstructed with TV-L1 inversion were quantitatively more similar to the originating images. In the experimental examples, TV-L1 inversion yielded sharper images and weaker streak artifact. The results herein show that TV-L1 inversion is capable of improving the quality of highly detailed, multiscale optoacoustic images obtained in vivo using cross-sectional imaging systems. As a result of its high fidelity, model-based TV-L1 inversion may be considered as the new standard for image reconstruction in cross-sectional imaging.

Abstract 1481: Development of theranostic mesoporous silica nanoparticles for pancreatic cancer

Abstract Purpose: Modern methods of pancreatic cancer diagnosis and treatment are severely lacking and have failed to provide effectual treatment options for patients. The root cause of this inadequacy stems from the hypovascularized nature of pancreatic cancer, making traditional chemotherapeutics and cancer detecting contrast agents nearly obsolete. A potential solution for tumor-targeting difficulties is through the implementation of nanotechnology, specifically targeting ligand capped, theranostic nanoparticles. We hypothesize that silica based, mesoporous nanoparticles capped with the targeting ligand, urokinase plasminogen activator (UPA) will serve as theranostic agents for treatment and diagnosis of pancreatic cancer. Methods: Mesoporous, silica nanoparticles (MSNs) were synthesized through the covering of acetyl trimethylammonium bromide (CTAB) scaffolding with tetraethyl orthosilicate (TEOS) in an aqueous environment. Refluxing was later used to remove the CTAB scaffolding and construct the desired mesoporous structure of the nanoparticles. MSNs were chemically engineered to be sensitive to acidic pH, contain fluorescent dye, and utilize a protein targeting ligand UPA. Multiple characterization techniques were employed to analyze the synthesized MSNs including UV-Vis spectroscopy, Zeta Potential, and Transmission Electron Microscopy (TEM). Pancreatic cancer tumor cells, S2CP9 and S2VP10, were treated with UPA-targeted or no-ligand encapsulated MSNs at pH 7.4 and 6.5. Following treatment, cells were washed to remove unbound particles followed by particle visualization based upon fluorescent dyes rhodamine or indocyanine green. Alternatively, cells treated with indocyanine green (ICG) loaded MSNs were placed into a tissue phantom and viewed using the Multispectral Optoacoustic Tomography system (MSOT). Mice with S2CP9 orthotopic pancreatic tumors were i.v. injected with 100 μL of UPA-ICG loaded MSNs. Particle localization was conducted using MSOT after 4h post injection. Results: Zeta potential and UV-Vis analysis confirmed the MSNs chemical conjugation, attributing the MSNs properties of pH sensitivity and fluorescence. pH sensitivity tests on the MSNs displayed drug release quantities at pH 6.5 to be ∼x2 the quantities of pH 6.8 and 7.4. Fluorescence microscopy showed the MSN uptake into S2VP10 and S2CP9 cell lines, while NIH3T3 cells (negative control) showed less binding. Use of UPAR antibody on S2VP10 and S2CP9 cell lines (positive control) blocked cellular binding. In tissue phantoms, UPA-ICG loaded MSNs at pH 6.5 demonstrated 20X and 7X more cell signal than without ligand or at pH 7.4, respectively. In-vivo, UPA-ICG loaded MSNs were detected within the orthotopic pancreatic tumor. Conclusion: UPA targeted, pH sensitive MSNs were found to possess substantial tumor cell binding against multiple cell lines and demonstrate potential as a theranostic nanoparticle for pancreatic cancer. Citation Format: Dillon S. Pender, Anil Khanal, Michael E. Egger, Lacey R. McNally. Development of theranostic mesoporous silica nanoparticles for pancreatic cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1481. doi:10.1158/1538-7445.AM2015-1481

Three-Dimensional Optoacoustic and Laser-Induced Ultrasound Tomography System for Preclinical Research in Mice: Design and Phantom Validation.

In this work, we introduce a novel three-dimensional imaging system for in vivo high-resolution anatomical and functional whole-body visualization of small animal models developed for preclinical and other type of biomedical research. The system (LOUIS-3DM) combines a multiwavelength optoacoustic tomography (OAT) and laser-induced ultrasound tomography (LUT) to obtain coregistered maps of tissue optical absorption and speed of sound, displayed within the skin outline of the studied animal. The most promising applications of the LOUIS-3DM include 3D angiography, cancer research, and longitudinal studies of biological distributions of optoacoustic contrast agents.

Melanin nanoparticles as a novel contrast agent for optoacoustic tomography

We describe the synthesis and characterization of melanin-like nanoparticles (MNP) as novel contrast agents for optoacoustic tomography. Good dispersion stability of high concentration MNPs in different biological media was achieved with thiol-terminated methoxy-poly(ethylene glycol), which can be used for further functional conjugation. MNP-PEG were found biocompatible with human MCF-7 and 3T3 cells. Cell toxicity of MNPs was found lower than that of gold nanorods for concentrations that provide equal optical absorbance. Optoacoustic tomography images were obtained with Laser Optoacoustic Imaging System (LOIS-3D) from tubes filled with contrast agents and live mice. Imaging of tubes permitted verification of the system resolution <300μm and sensitivity Δμa=0.03/cm under safe laser fluence of 20mJ/cm2. Water suspensions of MNP demonstrated optoacoustic efficiency that is about equal to that of gold nanorods under conditions of equal optical absorption. We conclude that MNPs have the potential for biomedical imaging applications as optoacoustic contrast agents.

RGD-conjugated silica-coated gold nanorods on the surface of carbon nanotubes for targeted photoacoustic imaging of gastric cancer.

Herein, we reported for the first time that RGD-conjugated silica-coated gold nanorods on the surface of multiwalled carbon nanotubes were successfully used for targeted photoacoustic imaging of in vivo gastric cancer cells. A simple strategy was used to attach covalently silica-coated gold nanorods (sGNRs) onto the surface of multiwalled carbon nanotubes (MWNTs) to fabricate a hybrid nanostructure. The cross-linked reaction occurred through the combination of carboxyl groups on the MWNTs and the amino group on the surface of sGNRs modified with a silane coupling agent. RGD peptides were conjugated with the sGNR/MWNT nanostructure; resultant RGD-conjugated sGNR/MWNT probes were investigated for their influences on viability of MGC803 and GES-1 cells. The nude mice models loaded with gastric cancer cells were prepared, the RGD-conjugated sGNR/MWNT probes were injected into gastric cancer-bearing nude mice models via the tail vein, and the nude mice were observed by an optoacoustic imaging system. Results showed that RGD-conjugated sGNR/MWNT probes showed good water solubility and low cellular toxicity, could target in vivo gastric cancer cells, and obtained strong photoacoustic imaging in the nude model. RGD-conjugated sGNR/MWNT probes will own great potential in applications such as targeted photoacoustic imaging and photothermal therapy in the near future.

Optoacoustic characterization of prostate cancer in an in vivo transgenic murine model.

Optoacoustic (OA) imaging was employed to distinguish normal from neoplastic tissues in a transgenic murine model of prostate cancer. OA images of five tumor-bearing mice and five age-matched controls across a 14 mm × 14 mm region of interest (ROI) on the lower abdomen were acquired using a reverse-mode OA imaging system (Seno Medical Instruments Inc., San Antonio, Texas). Neoplastic prostate tissue was identified based on the OA signal amplitude in combination with spectral analysis of the OA radio frequency (RF) data. Integration of the signal amplitude images was performed to construct two-dimensional images of the ROI. The prostate tumors generated higher amplitude signals than those of the surrounding tissues, with contrast ratios ranging from 31 to 36 dB. The RF spectrum analysis showed significant differences between the tumor and the control mice. The midband fit was higher by 5 dB (62%), the intercept higher by 4 dB (57%) and the spectral slope higher by 0.4 dB/MHz (50%) for neoplastic prostate tissue compared to normal tissues in the control mice. The results demonstrate that OA offers high contrast imaging of prostate cancer in vivo.

Expediting model-based optoacoustic reconstructions with tomographic symmetries

Image quantification in optoacoustic tomography implies the use of accurate forward models of excitation, propagation, and detection of optoacoustic signals while inversions with high spatial resolution usually involve very large matrices, leading to unreasonably long computation times. The development of fast and memory efficient model-based approaches represents then an important challenge to advance on the quantitative and dynamic imaging capabilities of tomographic optoacoustic imaging. Herein, a method for simplification and acceleration of model-based inversions, relying on inherent symmetries present in common tomographic acquisition geometries, has been introduced. The method is showcased for the case of cylindrical symmetries by using polar image discretization of the time-domain optoacoustic forward model combined with efficient storage and inversion strategies. The suggested methodology is shown to render fast and accurate model-based inversions in both numerical simulations and post mortem small animal experiments. In case of a full-view detection scheme, the memory requirements are reduced by one order of magnitude while high-resolution reconstructions are achieved at video rate. By considering the rotational symmetry present in many tomographic optoacoustic imaging systems, the proposed methodology allows exploiting the advantages of model-based algorithms with feasible computational requirements and fast reconstruction times, so that its convenience and general applicability in optoacoustic imaging systems with tomographic symmetries is anticipated.

Enabling in vivo measurements of nanoparticle concentrations with three‐dimensional optoacoustic tomography

In this report, we demonstrate the feasibility of using optoacoustic tomography (OAT) to evaluate biodistributions of nanoparticles in animal models. The redistribution of single-walled carbon nanotubes (SWCNTs) was visualized in living mice. Nanoparticle concentrations in harvested organs were measured spectroscopically using the intrinsic optical absorption and fluorescence of SWCNTs. Observed increases in optoacoustic signal brightness in tissues were compared with increases in optical absorption coefficients caused by SWCNT accumulation. The methodology presented in this report can further be extended to calibrate the sensitivity of an optoacoustic imaging system for a range of changes in optical absorption coefficient values at specific locations or organs in a mouse body to enable noninvasive measurements of nanoparticle concentrations in vivo. Additionally, qualitative information provided by OAT and quantitative information obtained ex vivo may provide valuable feedback for advancing methods of quantitative analysis with OAT.

Three‐dimensional optoacoustic tomography using a conventional ultrasound linear detector array: Whole‐body tomographic system for small animals

Optoacoustic imaging relies on the detection of ultrasonic waves induced by laser pulse excitations to map optical absorption in biological tissue. A tomographic geometry employing a conventional ultrasound linear detector array for volumetric optoacoustic imaging is reported. The geometry is based on a translate-rotate scanning motion of the detector array, and capitalizes on the geometrical characteristics of the transducer assembly to provide a large solid angular detection aperture. A system for three-dimensional whole-body optoacoustic tomography of small animals is implemented. The detection geometry was tested using a 128-element linear array (5.0∕7.0 MHz, Acuson L7, Siemens), moved by steps with a rotation∕translation stage assembly. Translation and rotation range of 13.5 mm and 180°, respectively, were implemented. Optoacoustic emissions were induced in tissue-mimicking phantoms and ex vivo mice using a pulsed laser operating in the near-IR spectral range at 760 nm. Volumetric images were formed using a filtered backprojection algorithm. The resolution of the optoacoustic tomography system was measured to be better than 130 μm in-plane and 330 μm in elevation (full width half maximum), and to be homogenous along a 15 mm diameter cross section due to the translate-rotate scanning geometry. Whole-body volumetric optoacoustic images of mice were performed ex vivo, and imaged organs and blood vessels through the intact abdominal and head regions were correlated to the mouse anatomy. Overall, the feasibility of three-dimensional and high-resolution whole-body optoacoustic imaging of small animal using a conventional linear array was demonstrated. Furthermore, the scanning geometry may be used for other linear arrays and is therefore expected to be of great interest for optoacoustic tomography at macroscopic and mesoscopic scale. Specifically, conventional detector arrays with higher central frequencies may be investigated.

Laser optoacoustic tomography: Towards new technology for biomedical diagnostics

This paper provides a short review of physical principles, technology, biomedical applications and perspectives of the optoacoustic imaging. Ideas that made this rapidly developing field possible include the following: (1) laser pulses may be effectively used to produce acoustic pressure in biological tissues localized to the areas of increased optical absorption, (2) the resulting acoustic (ultrasonic) waves propagate in tissues with minimal distortions and attenuation, and (3) 2D and 3D maps (images) of the absorbed optical energy can be reconstructed with high resolution from the time-resolved optoacoustic signals. Modern optoacoustic imaging systems include scanning focused transducers and transducer arrays. The widely accepted linear arrays are employed either for real-time 2D optoacoustic imaging or for 3D imaging via translational or rotational scanning. A commercial prototype of a 3D OAT system was developed by TomoWave Labs for applications in preclinical research, where major biomedical applications include visualization of specific targeting using exogenous optoacoustic contrast agents and imaging of blood distribution and oxygentaion status.

Three-dimensional optoacoustic imaging as a new noninvasive technique to study long-term biodistribution of optical contrast agents in small animal models

We used a 3-D optoacoustic (OA) tomography system to create maps of optical absorbance of mice tissues contrasted with gold nanorods (GNRs). Nude mice were scanned before and after injection of GNRs at time periods varying from 1 to 192 h. Synthesized GNRs were purified from hexadecyltrimethylammonium bromide and coated with polyethylene glycol (PEG) to obtain GNR-PEG complexes suitable for in vivo applications. Intravenous administration of purified GNR-PEG complexes resulted in enhanced OA contrast of internal organs and blood vessels compared to the same mouse before injection of the contrast agent. Maximum enhancement of the OA images was observed 24 to 48 h postinjection, followed by a slow clearance trend for the remaining part of the studied period (eight days). We demonstrate that OA imaging with two laser wavelengths can be used for noninvasive, long-term studies of biological distribution of contrast agents.