An optoacoustic imaging feature set to characterise blood vessels surrounding benign and malignant breast lesions

Combining optoacoustic (OA) imaging with ultrasound (US) enables visualisation of functional blood vasculature in breast lesions by OA to be overlaid with the morphological information of US. Here, we develop a simple OA feature set to differentiate benign and malignant breast lesions. 94 female patients with benign, indeterminate or suspicious lesions were recruited and underwent OA-US. An OA-US imaging feature set was developed using images from the first 38 patients, which contained 14 malignant and 8 benign solid lesions. Two independent radiologists blindly scored the OA-US images of a further 56 patients, which included 31 malignant and 13 benign solid lesions, with a sensitivity of 96.8% and specificity of 84.6%. Our findings indicate that OA-US can reveal vascular patterns of breast lesions that indicate malignancy using a simple feature set based on single wavelength OA data, which is therefore amenable to application in low resource settings for breast cancer management.

The IPASC data format: A consensus data format for photoacoustic imaging

Photoacoustic imaging (PAI) is an emerging modality that has shown promise for improving patient management in a range of applications. Unfortunately, the current lack of uniformity in PAI data formats compromises inter-user data exchange and comparison, which impedes: technological progress; effective research collaboration; and efforts to deliver multi-centre clinical trials. To overcome this challenge, the International Photoacoustic Standardisation Consortium (IPASC) has established a data format with a defined consensus metadata structure and developed an open-source software application programming interface (API) to enable conversion from proprietary file formats into the IPASC format. The format is based on Hierarchical Data Format 5 (HDF5) and designed to store photoacoustic raw time series data. Internal quality control mechanisms are included to ensure completeness and consistency of the converted data. By unifying the variety of proprietary data and metadata definitions into a consensus format, IPASC hopes to facilitate the exchange and comparison of PAI data.

Deep Learning for Automatic Segmentation of Hybrid Optoacoustic Ultrasound (OPUS) Images

The highly complementary information provided by multispectral optoacoustics and pulse-echo ultrasound have recently prompted development of hybrid imaging instruments bringing together the unique contrast advantages of both modalities. In the hybrid optoacoustic ultrasound (OPUS) combination, images retrieved by one modality may further be used to improve the reconstruction accuracy of the other. In this regard, image segmentation plays a major role as it can aid improving the image quality and quantification abilities by facilitating modeling of light and sound propagation through the imaged tissues and surrounding coupling medium. Here, we propose an automated approach for surface segmentation in whole-body mouse OPUS imaging using a deep convolutional neural network (CNN). The method has shown robust performance, attaining accurate segmentation of the animal boundary in both optoacoustic and pulse-echo ultrasound images, as evinced by quantitative performance evaluation using Dice coefficient metrics.

Assessment of hessian-based Frangi vesselness filter in optoacoustic imaging

The Hessian-based Frangi vesselness filter is commonly used to enhance vasculature in optoacoustic (photoacoustic) images, but its accuracy and limitations have never been rigorously assessed. Here we validate the ability of the filter to enhance vessel-like structures in phantoms, and we introduce an experimental approach that uses measurements before and after the administration of gold nanorods (AuNRs) to examine filter performance in vivo. We evaluate the influence of contrast, filter scales, angular tomographic coverage, out-of-plane signals and light fluence on image quality, and gain insight into the performance of the filter. We observe the generation of artifactual structures that can be misinterpreted as vessels and provide recommendations to ensure appropriate use of Frangi and other vesselness filters and avoid misinterpretation of post-processed optoacoustic images.

Optoacoustic Imaging Detects Hormone-Related Physiological Changes of Breast Parenchyma

PURPOSE

 Optoacoustic imaging with ultrasound (OPUS) can assess in-vivo perfusion/oxygenation through surrogate measures of oxy, deoxy and total hemoglobin content in tissues. The primary aim of our study was to evaluate the ability of OPUS to detect physiological changes in the breast during the menstrual cycle and to determine qualitative/quantitative metrics of normal parenchymal tissue in pre-/post-menopausal women. The secondary aim was to assess the technique's repeatability.

MATERIALS AND METHODS

 We performed a prospective ethically approved study in volunteers using OPUS (700, 800 and 850 nm wavelengths) in the proliferative/follicular and secretory phase of the menstrual cycle. Regions of interest (ROIs) were drawn on the most superficial region of fibroglandular tissue and same-day intra-observer repeatability was assessed. We used t-tests to interrogate differences in the OPUS measurements due to hormonal changes and interclass correlation coefficients/Bland-Altman plots to evaluate the repeatability of mean ROI signal intensities.

RESULTS

 22 pre-menopausal and 8 post-menopausal volunteers were recruited. 21 participants underwent repeatability examinations. OPUS intensity values were significantly higher (p < 0.0001) at all excitation wavelengths in the secretory compared to the proliferative/follicular phase. Post-menopausal volunteers showed similar optoacoustic values to the proliferative/follicular phase of pre-menopausal volunteers. The repeatability of the technique was comparable to other handheld ultrasound modalities.

CONCLUSION

 OPUS detects changes in perfusion/vascularity related to the menstrual cycle and menopausal status of breast parenchyma.

A light-fluence-independent method for the quantitative analysis of dynamic contrast-enhanced multispectral optoacoustic tomography (DCE MSOT)

MultiSpectral Optoacoustic Tomography (MSOT) is an emerging imaging technology that allows for data acquisition at high spatial and temporal resolution. These imaging characteristics are advantageous for Dynamic Contrast Enhanced (DCE) imaging that can assess the combination of vascular flow and permeability. However, the quantitative analysis of DCE MSOT data has not been possible due to complications caused by wavelength-dependent light attenuation and variability in light fluence at different anatomical locations. In this work we present a new method for the quantitative analysis of DCE MSOT data that is not biased by light fluence. We have named this method the two-compartment linear standard model (2C-LSM) for DCE MSOT.

Detection of microspheres in vivo using multispectral optoacoustic tomography

We introduce a new approach to detect individual microparticles that contain NIR fluorescent dye by multispectral optoacoustic tomography in the context of the hemoglobin-rich environment within murine liver. We encapsulated a near infrared (NIR) fluorescent dye within polystyrene microspheres, then injected them into the ileocolic vein, which drains to the liver. NIR absorption was determined using multispectral optoacoustic tomography. To quantitate the minimum diameter of microspheres, we used both colorimetric and spatial information to segment the regions in which the microspheres appear. Regional diameter was estimated by doubling the maximum regional distance. We found that the minimum microsphere size threshold for detection by multispectral optoacoustic tomography images is 78.9 µm.

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.

Hybrid optoacoustic tomography and pulse-echo ultrasonography using concave arrays

Implementation of hybrid imaging using optoacoustic tomography (OAT) and ultrasound (US) brings together the important advantages and complementary features of both methods. However, the fundamentally different physical contrast mechanisms of the two modalities may impose significant difficulties in the optimal tomographic data acquisition and image formation strategies. We investigate the applicability of the commonly applied imaging geometries for acquisition and reconstruction of hybrid optoacoustic tomography and pulse-echo ultrasound (OPUS) images. Optimization of the ultrasound image formation strategy using concave array geometry was implemented using a synthetic aperture method combined with spatial compounding. Experimental validation was performed using a custom-made multiplexer unit executing switching between the two modalities employing the same transducer array. A variety of array probes with different angular coverages were subsequently tested, including arrays for clinical hand-held imaging as well as stationary arrays for tomographic small animal imaging. The results demonstrate that acquisition of OAT data by mere addition of an illumination source to the common US linear array geometry may result in significant limited-view artifacts and overall loss of image quality. On the other hand, unsatisfactory US image quality is achieved with tomographic arrays solely optimized for OAT image acquisition without considering the optimal transmit-receive beamforming parameters. Optimal selection of the array pitch size, tomographic coverage and spatial compounding parameters has achieved here an accurate hybrid imaging performance, which was experimentally showcased in tissuemimicking phantoms, post-mortem mice, and hand-held imaging of a healthy volunteer. The efficient combination of the two modalities in a single imaging device reveals the true power of functional and molecular imaging capacities of OAT in addition to the morphological and functional imaging capabilities of US.

Abstract 5254: Sentinel lymph node detection and in vivo/ex vivo assessment of melanin distribution by means of multispectral optoacoustic tomography (MSOT) in patients with malignant melanoma

Melanoma accounts for less than 5% of skin cancer cases, yet it causes more than 75% of skin cancer death, and its incidence is growing faster than any other cancer in the world. Because melanoma metastasizes early into regional/sentinel lymph nodes (SLN), SLN excision (SLNE) is probably the most important diagnostic procedure for melanoma patients, as histology provides the most relevant prognostic factor for the survival of melanoma patients. However, 50% of excised lymph nodes show no evidence of metastasis; and, current histological protocols involve sampling only a small portion of the SLN, resulting in a relatively high false negative rate. Furthermore, SLNE exposes the patients to complications such as swelling, edema and future risk of infection. Therefore, a clear need exists to improve the sensitivity and specificity of SLN analysis. Multispectral optoacoustic tomography (MSOT) ulitizes the molecular specificity of optical imaging, while capitalizing on the high temporal and spatial resolution of ultrasound imaging. This method allows sensitive detection of optical markers such as melanin, which would identify potentially metastatic lymph nodes, and indocyanine green (ICG), which can potentially label SLN. In this study, 148 lymph nodes were excised from 65 melanoma patients (stage I - IV), and the lymph nodes were examined by MSOT ex vivo to guide the pathologist to examination of melanin-containing regions of the lymph node, thereby increasing the detection rate in histological analysis. Compared to standard histology, MSOT demonstrated a superior 100% sensitivity/47% specificity. Further, 21 melanoma patients were scanned with a handheld MSOT device in vivo. MSOT was able to detect sentinel lymph nodes using ICG specific contrast, with the added ability of non-invasive assessment of the melanin status prior to excision. In vivo MSOT measurements using a 2D and a 3D detector were compared with ultrasound, SPECT/CT, planar fluorescence imaging and ex vivo histology, with a promising concordance between in vivo MSOT measurements and in vivo and ex vivo gold standard assessments. MSOT represents a viability modality to improve histological analysis of excised SLN in melanoma patients, and it offers the ability to stage lymph nodes noninvasively, potentially reducing the necessity to excise lymph nodes in some melanoma patients.

Citation Format: Stefan Morscher, Ingo Stoffels, Neal C. Burton, Jing Claussen, Thomas Sardella, Iris Helfrich, Uwe Hillen, Julia Leyh, Dirk Schadendorf, Matthias Gunzer, Joachim Klode. Sentinel lymph node detection and in vivo/ex vivo assessment of melanin distribution by means of multispectral optoacoustic tomography (MSOT) in patients with malignant melanoma. [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 5254. doi:10.1158/1538-7445.AM2015-5254

Deep-tissue reporter-gene imaging with fluorescence and optoacoustic tomography: a performance overview

PURPOSE

A primary enabling feature of near-infrared fluorescent proteins (FPs) and fluorescent probes is the ability to visualize deeper in tissues than in the visible. The purpose of this work is to find which is the optimal visualization method that can exploit the advantages of this novel class of FPs in full-scale pre-clinical molecular imaging studies.

PROCEDURES

Nude mice were stereotactically implanted with near-infrared FP expressing glioma cells to from brain tumors. The feasibility and performance metrics of FPs were compared between planar epi-illumination and trans-illumination fluorescence imaging, as well as to hybrid Fluorescence Molecular Tomography (FMT) system combined with X-ray CT and Multispectral Optoacoustic (or Photoacoustic) Tomography (MSOT).

RESULTS

It is shown that deep-seated glioma brain tumors are possible to visualize both with fluorescence and optoacoustic imaging. Fluorescence imaging is straightforward and has good sensitivity; however, it lacks resolution. FMT-XCT can provide an improved rough resolution of ∼1 mm in deep tissue, while MSOT achieves 0.1 mm resolution in deep tissue and has comparable sensitivity.

CONCLUSIONS

We show imaging capacity that can shift the visualization paradigm in biological discovery. The results are relevant not only to reporter gene imaging, but stand as cross-platform comparison for all methods imaging near infrared fluorescent contrast agents.

Abstract 4310: Assessing PK parameters using dynamic contrast enhanced multispectral optoacoustic tomography (DCE-MSOT)

Pharmacokinetic imaging is a powerful platform for evaluating new candidate drugs and imaging agents, and multitude of applications have been demonstrated in DCE MRI. Interesting parameters such as Ktrans, half-life and Tmax can be retrieved for a tumor region after injection of a perfusion agent, allowing one to draw important conclusions on tumor growth, vascularization or therapy response. DCE MRI however is to a certain extent limited to magnetic agents and provides limited spatial and temporal resolution.

Multispectral Optoacoustic Imaging (MSOT) is an emerging modality that combines ultrasound resolution of 150 µm and acquisition times of a few microseconds with optical contrast in the near infrared (NIR) spectral region. Multispectral imaging allows the localization of injected fluorophores without necessity of a baseline scan before injection, while a core imaging rate of 10 images/second allows the acquisition of a multispectral image within a second or even less. This enables fast image acquisition to support pharmacokinetic imaging, where the temporal profile of individual pixels is fit to a model equation and resulting parameters are plotted as parametric maps. Using targeted agents, binding specificity and its kinetics can be evaluated, while perfusion agents allow the assessment of perfusion and tissue uptake through Ktrans. One advantage in regards to DCE-MRI is the ability to extract the arterial input function from the data itself by monitoring a single cross-section with maximal temporal resolution. Another important feature is the use of intrinsic contrast that allows blood oxygenation quantification, enabling the co-registration of functional oxygenation measurements with DCE perfusion measurements.

In particular, various algorithms can be applied in order to visualize blood oxygenation as a result from tissue intrinsic optoacoustic contrast without the injection of additional agents. These can be evaluated for pixel dependent temporal changes using the fast image acquisition techniques described above, which allows for tumor delineation and assessment of perfusion in a CO2 challenge experiment by visualizing the localized change in blood oxygenation. This can be cross-validated by the subsequent injection of a perfusion agent and evaluation of its transfer coefficient using DCE-MSOT techniques to paint a complete picture of the evaluated tumor microenvironment. The presented work uses the U87-MG glioblastoma and 4T1 tumors to illustrate the abilities of the technique in both subcutaneous and orthotopic settings.

The extension of contrast-enhanced kinetic modeling to the optoacoustic imaging regime allows access to a library of optical probes that are otherwise unavailable to traditional DCE-MRI, while maintaining high spatial resolution and providing access to functional hemodynamic parameters.

Citation Format: Stefan Morscher, Wouter HP Driessen, Neal C. Burton, Thomas Sardella, Daniel Razansky, Vasilis Ntziachristos. Assessing PK parameters using dynamic contrast enhanced multispectral optoacoustic tomography (DCE-MSOT). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4310. doi:10.1158/1538-7445.AM2014-4310

Semi-quantitative Multispectral Optoacoustic Tomography (MSOT) for volumetric PK imaging of gastric emptying

A common side effect of medication is gastrointestinal intolerance. Symptoms can include reduced appetite, diarrhea, constipation, GI inflammation, nausea and vomiting. Such effects often have a dramatic impact on compliance with a treatment regimen. Therefore, characterization of GI tolerance is an important step when establishing a novel therapeutic approach. In this study, Multispectral Optoacoustic Tomography (MSOT) is used to monitor gastrointestinal motility by in vivo whole body imaging in mice. MSOT combines high spatial and temporal resolution based on ultrasound detection with strong optical contrast in the near infrared. Animals were given Indocyanine Green (ICG) by oral gavage and imaged by MSOT to observe the fate of ICG in the gastrointestinal tract. Exponential decay of ICG signal was observed in the stomach in good correlation with ex vivo validation. We discuss how kinetic imaging in MSOT allows visualization of parameters unavailable to other imaging methods, both in 2D and 3D.

Unmixing Molecular Agents From Absorbing Tissue in Multispectral Optoacoustic Tomography

Detection of intrinsic or extrinsically administered chromophores and photo-absorbing nanoparticles has been achieved by multi-spectral optoacoustic tomography (MSOT). The detection sensitivity of MSOT depends not only on the signal to noise ratio considerations, as in conventional optoacoustic (photoacoustic) tomography implementations, but also on the ability to resolve the molecular targets of interest from the absorbing tissue background by means of spectral unmixing or sub-pixel detection methods. However, it is not known which unmixing methods are optimally suited for the characteristics of multispectral optoacoustic images. In this work we investigated the performance of different sub-pixel detection methods, typically used in remote sensing hyperspectral imaging, within the context of MSOT. A quantitative comparison of the different algorithmic approaches was carried out in an effort to identify methods that operate optimally under the particulars of molecular imaging applications. We find that statistical sub-pixel detection methods can demonstrate a unique detection performance with up to five times enhanced sensitivity as compared to linear unmixing approximations, under the condition that the optical agent of interest is sparsely present within the tissue volume, as common when using targeted agents and reporter genes.

Abstract 737: Multispectral Optoacoustic Tomography (MSOT) imaging and quantification of apoptosis in vivo

Apoptosis is an important mechanism in cellular homeostasis and imbalances in the apoptotic process are associated with various disease states. An important example is the acquired ability of cancer cells to resist their own programmed cell death and therefore it is the aim of many tumor therapies to either reestablish pro-apoptotic signaling pathways or induce apoptosis through activation of existing mechanisms within the cell. Therefore, visualizing and quantifying the apoptotic process in vivo has great value in monitoring therapy response, diagnosis and staging disease.

By being able to detect subtle changes in perfusion and tissue oxygenation, the small animal Multispectral Optoacoustic Tomography (MSOT) scanner offers unprecedented performance in cross-sectional imaging of tumor heterogeneity as well as a powerful capacity to simultaneously image molecular processes such as apoptosis by utilizing novel molecular probes. In this study, apoptotic regions within a mouse mammary tumor where visualized using a dye-conjugated caspase probe and compared to the hypoxia status of each tumor region.

The temporal resolution of the MSOT small animal scanner (generation of multispectral cross-sectional data in less than 1 second) allows for the dynamic imaging of targeted and control probe simultaneously. To illustrate this point apoptosis probe was co-injected with a control dye with similar physical and chemical properties, but a different absorbance maximum after which the tumor region was imaged over 1 hour with multi-spectral image acquisition. Next, mice were imaged by MSOT immediately before and 24 hours post treatment with the chemotherapeutic Doxorubicin and the induction of apoptosis was visualized and quantified. Doxorubicin treatment led to a significant increase in signal resulting from the DyLight 747-conjugated apoptosis probe. In summary, MSOT can be used to determine the extent of apoptosis in tumors in vivo and thus has great value in monitoring therapy response, diagnosis and staging disease.

Citation Format: Wouter H. Driessen, Neal C. Burton, Thomas Sardella, Stefan Morscher, Daniel Razansky, Vasilis Ntziachristos. Multispectral Optoacoustic Tomography (MSOT) imaging and quantification of apoptosis in vivo. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 737. doi:10.1158/1538-7445.AM2013-737

Multispectral opto-acoustic tomography (MSOT) of the brain and glioblastoma characterization

Brain research depends strongly on imaging for assessing function and disease in vivo. We examine herein multispectral opto-acoustic tomography (MSOT), a novel technology for high-resolution molecular imaging deep inside tissues. MSOT illuminates tissue with light pulses at multiple wavelengths and detects the acoustic waves generated by the thermoelastic expansion of the environment surrounding absorbing molecules. Using spectral unmixing analysis of the data collected, MSOT can then differentiate the spectral signatures of oxygenated and deoxygenated hemoglobin and of photo-absorbing agents and quantify their concentration. By being able to detect absorbing molecules up to centimeters deep in the tissue it represents an ideal modality for small animal brain imaging, simultaneously providing anatomical, hemodynamic, functional, and molecular information. In this work we examine the capacity of MSOT in cross-sectional brain imaging of mice. We find unprecedented optical imaging performance in cross-sectional visualization of anatomical and physiological parameters of the mouse brain. For example, the potential of MSOT to characterize ischemic brain areas was demonstrated through the use of a carbon dioxide challenge. In addition, indocyanine green (ICG) was injected intravenously, and the kinetics of uptake and clearance in the vasculature of the brain was visualized in real-time. We further found that multiparameter, multispectral imaging of the growth of U87 tumor cells injected into the brain could be visualized through the intact mouse head, for example through visualization of deoxygenated hemoglobin in the growing tumor. We also demonstrate how MSOT offers several compelling features for brain research and allows time-dependent detection and quantification of brain parameters that are not available using other imaging methods without invasive procedures.

Abstract 2445: High-resolution imaging of orthotopic glioblastoma in mice using multispectral optoacoustic tomography

High grade glioblastomas are aggressive and highly invasive tumors that rarely are curable. To investigate novel treatments, accurate orthotopic tumor-models are crucial to evaluate efficacy. Because caliper measurements are not possible in this setting, accurate imaging strategies are needed for temporal tumor-tracking. Here we present a novel imaging strategy that leverages the recent development of fluorescent proteins absorbing light in the near infrared region, enabling their application in visualizing biological processes in deep seated tissues such as orthotopic glioblastomas. While purely optical in vivo imaging methods can only provide insufficient resolution limited by scattering, multispectral optoacoustic tomography (MSOT) is a very well suited hybrid imaging modality that can provide noninvasive imaging of optical absorbers at ultrasound resolution. MSOT is based on the photoacoustic effect; the generation of ultrasound waves as a result of the transient heating and expansion that takes place after the absorption of nanosecond laser pulses. We have developed a preclinical MSOT whole animal scanner that makes use of a tomographic ultrasound transducer array to achieve fast image rates of 10Hz, while high penetration depths can be achieved using light in the near infrared (NIR) optical window. Optical absorption of intrinsic tissue chromophores enable rich anatomical contrast, while multispectral imaging permits the separation of distinct optical absorbers and promotes the applicability of MSOT for functional and molecular imaging. The utilized imaging setup consists of a tunable NIR laser (680-950nm) and a 64 element ultrasound transducer array covering 172° around the sample (center frequency 5Mhz) to achieve an in-plane resolution of ∼150µm; further detailed in A. Buehler et al, 2011, OpticsLetters. For the presented study 8-10 weeks old CD1 nude mice were stereotactically implanted with 105 U87MG cells stably expressing iRFP (Filonov, G.S. et al., NatBiotechnol, 2011) in depths of 1.5mm and 3.5mm. MSOT imaging was performed in vivo 12 and 25 days after implantation. Excellent accordance was observed between in vivo MSOT imaging and subsequent post-mortem epi-fluorescent cryoslicing, demonstrating the unique performance of optoacoustic imaging in the presented scenario. MSOT was able to discriminate the iRFP signal over strong background absorbers such as oxygenated and deoxygenated hemoglobin as early as 12 days post implantation of the tumor cells. MSOT offers considerable advantages over MRI and bioluminescent imaging, such as high spatial resolution, ease of use and cost-efficiency. The presented study illustrates the capabilities of this emerging imaging modality for the tracking of deep seated tumors with a multitude of settings and applications being feasible.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2445. doi:1538-7445.AM2012-2445

Abstract 2440: Glioblastoma molecular imaging in vivo using multi-spectral optoacoustic tomography

Brain research has been revolutionized by imaging technologies. X-ray CT and MRI provide high spatial resolution, revealing anatomical anomalies indicative of disease. PET and optical imaging have target specificity, allowing the visualization of molecular events. Intravital microscopy has specificity and resolution, providing key information on pathological micro-events, but lacks penetration depth. Multi-spectral optoacoustic tomography (MSOT) combines high resolution, molecular specificity and depth to achieve non-invasive in vivo anatomical, functional and molecular imaging in deep tissue. MSOT illuminates tissue with light pulses at multiple wavelengths and detects the acoustic waves generated by the thermoelastic expansion following light absorption. Using spectral analysis of the data collected, MSOT can then differentiate the spectral signatures of endogenous biomarkers such as oxy-/deoxy-hemoglobin and of photo-absorbing agents and quantify their concentration. In this work we explore the potential of MSOT in cross-sectional imaging of the mouse brain and contrast these results with MRI and ex vivo brain imaging to validate the MSOT in vivo findings. 8 week old nude CD-1 mice were used for stereotactic implantation of U87 glioblastoma cells and for imaging of hemoglobin contrast and ICG biodistribution. In vivo MSOT of the intact mouse head yielded unprecedented performance in cross-sectional imaging of the mouse brain by visualizing the overall brain outline and anatomy, and imaging temporal arteries and blood vessels beneath the skull. Additionally, NIR probes were injected into the 3rd ventricle, with an excellent correlation between MSOT and fluorescence imaging of cryoslices, demonstrating the capacity of MSOT to localize NIR probes in the brain through intact skin and skull with high accuracy. In addition, spectral decomposition of hemoglobin confirmed the MSOT ability to visualize well perfused and ischemic brain conditions following a CO2 challenge. Additionally, MSOT accurately visualized ICG bio-distribution injected into the tail vein, and followed in real time the ICG kinetics and clearance. Finally, spectral decomposition of deoxygenated hemoglobin allowed the observation of hypoxia related to the growth of U87 tumor cells injected into the striatum. Multispectral processing allowed the visualization of the true organ distribution of IntegriSense and AngioSense in the brain, with planar fluorescence imaging used for a reference comparison. The application of MSOT in in vivo brain imaging is demonstrated. MSOT can be used to follow changes in blood oxygenation, as well as the distribution of near-infrared probes. With the advent of new molecular probes, MSOT could also track molecular features of neurological disease and cancer in mouse models.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2440. doi:1538-7445.AM2012-2440

Abstract 58: Comparative determination of compound delivery to orthotopic and subcutaneous tumors by non-invasive imaging

Cancer is fundamentally a disease of aberrant tissue growth and determining accurate progression in malignancies is of significant importance for the understanding of the heterogeneous growth patterns, and irregular nature of malignant tumors. Here we used two different imaging modalities to detect tumor progression, and highlight the inherent challenges with accurate determination of growth characteristics. The main focus in this study was to examine the differences in compound deposition and uptake into orthotopically and subcutaneously implanted tumors. Several perfusion studies with Indocyanine green (ICG) were performed and compound kinetics and distribution within the tumors were determined by MSOT and FMT-XCT. Multi-Spectral Optoacoustic Tomography (MSOT) is a powerful novel imaging modality that decomposes the spectral responses of endogenous and exogenous chromophores in vivo, with high resolution and at depths ranging from several millimeters to centimeters. Therefore, it can simultaneously detect and separate the signal of endogenous chromophores such as (oxy)hemoglobin and extrinsically administered photo-absorbing agents such as ICG and nanoparticles. FMT-XCT is an imaging system that integrates X-ray computed tomography (XCT) and fluorescence molecular tomography (FMT), enabling quantitative, volumetric detection of fluorescent agents with co-registration of anatomical features. For the subcutaneous model, HT29 human colon adenocarcinoma cells (∼106) were injected subcutaneously in the hind limb of CD1 nude mice (Charles River Laboratories, Germany). For the syngeneic, orthotopic model, Balb/c mice (Charles River Laboratories, Germany) were injected with 4T1 mouse mammary tumor cells (∼0.5x106) into the mammary fat pad. With MSOT imaging after ICG injection (50µg) we were able to show that the vascular perfusion of subcutaneous tumors was limited to the outer edges of the tumor, caused by necrosis within these tumors. The orthotopically implanted tumors were much better perfused and compound delivery throughout the entire tumor was achieved. A major advantage of MSOT imaging compared to FMT-XCT is that contrast agents and oxygenated vs. deoxygenated hemoglobin can be visualized non-invasively at the tumor site simultaneously with high spatial resolution, in real time, and throughout the entire period of tumor growth. Using FMT-XCT we were able to assess further information about the three-dimensional distribution of ICG within the entire animal. These findings show that the heterogeneity of tumors can be visualized non-invasively using MSOT and FMT-XCT. MSOT has the added advantage that endogenous chromophores such as hemoglobin can be resolved simultaneously. These imaging strategies are of critical importance to monitor tumor progression in order to evaluate appropriate treatment regimens and/ or stratify tumors based on oxygenation status.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 58. doi:1538-7445.AM2012-58

Fast multispectral optoacoustic tomography (MSOT) for dynamic imaging of pharmacokinetics and biodistribution in multiple organs

The characterization of pharmacokinetic and biodistribution profiles is an essential step in the development process of new candidate drugs or imaging agents. Simultaneously, the assessment of organ function related to the uptake and clearance of drugs is of great importance. To this end, we demonstrate an imaging platform capable of high-rate characterization of the dynamics of fluorescent agents in multiple organs using multispectral optoacoustic tomography (MSOT). A spatial resolution of approximately 150 µm through mouse cross-sections allowed us to image blood vessels, the kidneys, the liver and the gall bladder. In particular, MSOT was employed to characterize the removal of indocyanine green from the systemic circulation and its time-resolved uptake in the liver and gallbladder. Furthermore, it was possible to track the uptake of a carboxylate dye in separate regions of the kidneys. The results demonstrate the acquisition of agent concentration metrics at rates of 10 samples per second at a single wavelength and 17 s per multispectral sample with 10 signal averages at each of 5 wavelengths. Overall, such imaging performance introduces previously undocumented capabilities of fast, high resolution in vivo imaging of the fate of optical agents for drug discovery and basic biological research.