Quality analysis of in vivo near-infrared fluorescence and conventional gamma images acquired using a dual-labeled tumor-targeting probe

Targeted Dual-Modal PET/SPECT-NIR Imaging: From Building Blocks and Construction Strategies to Applications

Molecular imaging is an emerging non-invasive method to qualitatively and quantitively visualize and characterize biological processes. Among the imaging modalities, PET/SPECT and near-infrared (NIR) imaging provide synergistic properties that result in deep tissue penetration and up to cell-level resolution. Dual-modal PET/SPECT-NIR agents are commonly combined with a targeting ligand (e.g., antibody or small molecule) to engage biomolecules overexpressed in cancer, thereby enabling selective multimodal visualization of primary and metastatic tumors. The use of such agents for (i) preoperative patient selection and surgical planning and (ii) intraoperative FGS could improve surgical workflow and patient outcomes. However, the development of targeted dual-modal agents is a chemical challenge and a topic of ongoing research. In this review, we define key design considerations of targeted dual-modal imaging from a topological perspective, list targeted dual-modal probes disclosed in the last decade, review recent progress in the field of NIR fluorescent probe development, and highlight future directions in this rapidly developing field.

Research progress of near-infrared fluorescence probes based on indole heptamethine cyanine dyes in vivo and in vitro

Near-infrared (NIR) fluorescence imaging is a noninvasive technique that provides numerous advantages for the real-time in vivo monitoring of biological information in living subjects without the use of ionizing radiation. Near-infrared fluorescent (NIRF) dyes are widely used as fluorescent imaging probes. These fluorescent dyes remarkably decrease the interference caused by the self-absorption of substances and autofluorescence, increase detection selectivity and sensitivity, and reduce damage to the human body. Thus, they are beneficial for bioassays. Indole heptamethine cyanine dyes are widely investigated in the field of near-infrared fluorescence imaging. They are mainly composed of indole heterocyclics, heptamethine chains, and N-substituent side chains. With indole heptamethine cyanine dyes as the parent, introducing reactive groups to the parent compounds or changing their structures can make fluorescent probes have different functions like labeling protein and tumor, detecting intracellular metal cations, which has become the hotspot in the field of fluorescence imaging of biological research. Therefore, this study reviewed the applications of indole heptamethine cyanine fluorescent probes to metal cation detection, pH, molecules, tumor imaging, and protein in vivo. The distribution, imaging results, and metabolism of the probes in vivo and in vitro were described. The biological application trends and existing problems of fluorescent probes were discussed.

Application of Metallic Nanomaterials in Nanomedicine

In this chapter, we explain why metallic nanomaterials are used in nanomedicine. We have shown that the electron density in metallic nanomaterials oscillates and creates electron density waves. When laser light falls on metallic nanoparticles, light interacts with electron density waves. According to Einstein, light, which is electromagnetic waves, consists of particles called photons. Similarly, electron density waves are also made of particles called surface plasmons. Therefore, photons from laser light and surface plasmons from metallic nanostructures interact with each other and create new particles called surface plasmon polaritons. These new particles produce an intense light near the surface of metallic nanomaterials. We showed that this intense light is important in the application of metallic nanomaterials in nanomedicine. Further, we have applied metallic nanoparticles, single metallic nanoshells and double metallic nanoshells for treatment of cancer and detection of smaller tumors.

Development of a high quantum yield dye for tumour imaging

A fluorescent dye, FEB, with high fluorescence quantum yield for tumour imaging is reported. FEB dyes can be efficiently synthesized in three steps and then easily modified with either PEG or PEG-iRGD to yield FEB-2000 or FEB-2000-iRGD, respectively. Both modified dyes showed negligible toxicity and were thus able to be adopted for in vivo tumour imaging. PEG modification endowed the dye FEB-2000 with both long circulating times and good tumour targeting properties in a MDA-MB-231 xenograft model. Further conjugation with iRGD to generate FEB-2000-iRGD showed minimal targeting enhancement. These results provide a template for the efficient preparation of FEB dyes for use in tumour imaging, thus providing a foundation for future modifications.

Surface biotinylation of cytotoxic T lymphocytes for in vivo tracking of tumor immunotherapy in murine models

Currently, there is no stable and flexible method to label and track cytotoxic T lymphocytes (CTLs) in vivo in CTL immunotherapy. We aimed to evaluate whether the sulfo-hydroxysuccinimide (NHS)-biotin-streptavidin (SA) platform could chemically modify the cell surface of CTLs for in vivo tracking. CD8+ T lymphocytes were labeled with sulfo-NHS-biotin under different conditions and then incubated with SA-Alexa647. Labeling efficiency was proportional to sulfo-NHS-biotin concentration. CD8+ T lymphocytes could be labeled with higher efficiency with sulfo-NHS-biotin in DPBS than in RPMI (P < 0.05). Incubation temperature was not a key factor. CTLs maintained sufficient labeling for at least 72 h (P < 0.05), without altering cell viability. After co-culturing labeled CTLs with mouse glioma stem cells (GSCs) engineered to present biotin on their surface, targeting CTLs could specifically target biotin-presenting GSCs and inhibited cell proliferation (P < 0.01) and tumor spheres formation. In a biotin-presenting GSC brain tumor model, targeting CTLs could be detected in biotin-presenting gliomas in mouse brains but not in the non-tumor-bearing contralateral hemispheres (P < 0.05). In vivo fluorescent molecular tomography imaging in a subcutaneous U87 mouse model confirmed that targeting CTLs homed in on the biotin-presenting U87 tumors but not the control U87 tumors. PET imaging with ⁸⁹Zr-deferoxamine-biotin and SA showed a rapid clearance of the PET signal over 24 h in the control tumor, while only minimally decreased in the targeted tumor. Thus, sulfo-NHS-biotin-SA labeling is an efficient method to noninvasively track the migration of adoptive transferred CTLs and does not alter CTL viability or interfere with CTL-mediated cytotoxic activity.

Combination of an Integrin-Targeting NIR Tracer and an Ultrasensitive Spectroscopic Device for Intraoperative Detection of Head and Neck Tumor Margins and Metastatic Lymph Nodes

Despite major advances in targeted drug therapy and radiation therapy, surgery remains the most effective treatment for most solid tumors. The single most important predictor of patient survival is a complete surgical resection of the primary tumor, draining lymph nodes, and metastatic lesions. Presently, however, 20%–30% of patients with head and neck cancer who undergo surgery still leave the operating room without complete resection because of missed lesions. Thus, major opportunities exist to develop advanced imaging tracers and intraoperative instrumentation that would allow surgeons to visualize microscopic tumors during surgery. The cell adhesion molecule integrin αvβ3 is specifically expressed by tumor neovasculature and invading tumor cells, but not by quiescent vessels or normal cells. Here we report the combined use of an integrin-targeting near-infrared tracer (RGD-IRDye800CW) and a handheld spectroscopic device, an integrated point spectroscopy with wide-field imaging system, for highly sensitive detection of integrin overexpression on infiltrating cancer cells. By using an orthotopic head and neck cancer animal model, we show that this tracer–device combination allows intraoperative detection of not only invasive tumor margins but also metastatic lymph nodes. Correlated histological analysis further reveals that microscopic clusters of 50–100 tumor cells can be detected intraoperatively with high sensitivity and specificity, raising new possibilities in guiding surgical resection of microscopic tumors and metastatic lymph nodes.

Influence of chelator and near-infrared dye labeling on biocharacteristics of dual-labeled trastuzumab-based imaging agents

Objective

To investigate the effect of fluorescent dye labeling on the targeting capabilities of 111In- (DTPA)n-trastuzumab-(IRDye 800)m.

Methods

Trastuzumab-based conjugates were synthesized and conjugated with diethylenetriaminepentaacetic acid (DTPA) at molar ratios of 1, 2, 3 and 5 and with a fluorescent dye (IRDye 800CW) at molar ratios of 1, 3 and 5. Immunoreactivity and internalization were assessed on SKBR-3 cells, overexpressing human epidermal growth factor receptor 2. The stability in human serum and phosphate-buffered saline (PBS) was evaluated. The biodistribution of dual-labeled conjugates was compared with that of 111In-(DTPA)2-trastuzumab in a SKBR-3 xenograft model to evaluate the effect of dye-to-protein ratio.

Results

All trastuzumab-based conjugates exhibited a high level of chemical and optical purity. Flow cytometry results showed that increasing dye-to-protein ratios were associated with decreased immunoreactivity. Stability studies revealed that the conjugate was stable in PBS, while in human serum, increased degradation and protein precipitation were observed with increasing dye-to-protein ratios. At 4 h, the percentages of internalization of dual-labeled conjugates normalized by dye-to-protein ratio (m) were 24.88%±2.10%, 19.99%±0.59%, and 17.47%±1.26% for "m" equal to 1, 3, and 5, respectively. A biodistribution study revealed a progressive decrease in tumor uptake with an increase in the dye-to-protein ratios. The liver, spleen and kidney showed a marked uptake with increased dye-to-protein ratios, particularly in the latter.

Conclusions

With non-specific-site conjugation of the fluorescent dye with a protein based on imaging agent, the increase in dye-to-protein ratios negatively impacted the immunoreactivity and stability, indicating a reduced tumor uptake.

Advancing Molecular-Guided Surgery through probe development and testing in a moderate cost evaluation pipeline

Molecular guided oncology surgery has the potential to transform the way decisions about resection are done, and can be critically important in areas such as neurosurgery where the margins of tumor relative to critical normal tissues are not readily apparent from visual or palpable guidance. Yet there are major financial barriers to advancing agents into clinical trials with commercial backing. We observe that development of these agents in the standard biological therapeutic paradigm is not viable, due to the high up front financial investment needed and the limitations in the revenue models of contrast agents for imaging. The hypothesized solution to this problem is to develop small molecular biologicals tagged with an established fluorescent reporter, through the chemical agent approval pathway, targeting a phase 0 trials initially, such that the initial startup phase can be completely funded by a single NIH grant. In this way, fast trials can be completed to de-risk the development pipeline, and advance the idea of fluorescence-guided surgery (FGS) reporters into human testing. As with biological therapies the potential successes of each agent are still moderate, but this process will allow the field to advance in a more stable and productive manner, rather than relying upon isolated molecules developed at high cost and risk. The pathway proposed and tested here uses peptide synthesis of an epidermal growth factor receptor (EGFR)-binding Affibody molecules, uniquely conjugated to IRDye 800CW, developed and tested in academic and industrial laboratories with well-established records for GMP production, fill & finish, toxicity testing, and early phase clinical trials with image guidance.

Targeted multimodal imaging modalities

Molecular imaging non-invasively visualizes and characterizes the biologic functions and mechanisms in living organisms at a molecular level. In recent years, advances in imaging instruments, imaging probes, assay methods, and quantification techniques have enabled more refined and reliable images for more accurate diagnoses. Multimodal imaging combines two or more imaging modalities into one system to produce details in clinical diagnostic imaging that are more precise than conventional imaging. Multimodal imaging offers complementary advantages: high spatial resolution, soft tissue contrast, and biological information on the molecular level with high sensitivity. However, combining all modalities into a single imaging probe involves problems yet to be solved due to the requirement of high dose contrast agents for a component of imaging modality with low sensitivity. The introduction of targeting moieties into the probes enhances the specific binding of targeted multimodal imaging modalities and selective accumulation of the imaging agents at a disease site to provide more accurate diagnoses. An extensive list of prior reports on the targeted multimodal imaging probes categorized by each modality is presented and discussed. In addition to accurate diagnosis, targeted multimodal imaging agents carrying therapeutic medications make it possible to visualize the theranostic effect and the progress of disease. This will facilitate the development of an imaging-guided therapy, which will widen the application of the targeted multimodal imaging field to experiments in vivo.

Multi-modality imaging to determine the cellular heterogeneity of nasopharyngeal carcinoma components

Nasopharyngeal carcinoma (NPC) is an endemic public health problem in South and Southeast Asian countries. The disease components at the molecular level are unclear and need exploration for the development of future individualized molecular medicine. The purpose of this study was to test the feasibility of target-specific agents to detect different components of NPC. The binding capability of human NPC cell lines was determined by incubation with either agents that specifically target the metabolic status, host cytokines, and stroma. Mice bearing human NPC xenografts were injected with the same test agents plus a clinical molecular imaging agent (18F-fluorodeoxyglucose) and computer tomography (CT) contrast agent. In vitro cell studies have demonstrated that target-specific agents bind to NPC cells with significantly higher signal intensities. Those agents not only bound to the cell membrane but also penetrated into the cytosol and cell nuclei. In vivo imaging demonstrated that the human NPC xenografts revealed high glucose uptake and a profound vasculature in the tumor. All agents were bound to the tumor regions with a high tumor-to-muscle ratio. Finally, all imaging data were validated by histopathological results. Multiple, target-specific agents determine the dynamic and heterogeneous components of NPC at the molecular level.

Bimodal imaging probes for combined PET and OI: recent developments and future directions for hybrid agent development

Molecular imaging—and especially positron emission tomography (PET)—has gained increasing importance for diagnosis of various diseases and thus experiences an increasing dissemination. Therefore, there is also a growing demand for highly affine PET tracers specifically accumulating and visualizing target structures in the human body. Beyond the development of agents suitable for PET alone, recent tendencies aim at the synthesis of bimodal imaging probes applicable in PET as well as optical imaging (OI), as this combination of modalities can provide clinical advantages. PET, due to the high tissue penetration of the γ-radiation emitted by PET nuclides, allows a quantitative imaging able to identify and visualize tumors and metastases in the whole body. OI on the contrary visualizes photons exhibiting only a limited tissue penetration but enables the identification of tumor margins and infected lymph nodes during surgery without bearing a radiation burden for the surgeon. Thus, there is an emerging interest in bimodal agents for PET and OI in order to exploit the potential of both imaging techniques for the imaging and treatment of tumor diseases. This short review summarizes the available hybrid probes developed for dual PET and OI and discusses future directions for hybrid agent development.

Non-invasive fluorescence imaging under ambient light conditions using a modulated ICCD and laser diode

One limitation of fluorescence molecular imaging that can limit clinical implementation and hamper small animal imaging is the inability to eliminate ambient light. Herein, we demonstrate the ability to conduct rapid non-invasive, far-red and near-infrared fluorescence imaging in living animals and a phantom under ambient light conditions using a modulated image intensified CCD (ICCD) and a laser diode operated in homodyne detection. By mapping AC amplitude from three planar images at varying phase delays, we show improvement in target-to-background ratios (TBR) and reasonable signal-to-noise ratios (SNR) over continuous wave measurements. The rapid approach can be used to accurately collect fluorescence in situations where ambient light cannot be spectrally conditioned or controlled, such as in the case of fluorescent molecular image-guided surgery.

Effects of nanoencapsulation and PEGylation on biodistribution of indocyanine green in healthy mice: quantitative fluorescence imaging and analysis of organs

Near-infrared nanoconstructs present a potentially effective platform for site-specific and deep tissue optical imaging and phototherapy. We have engineered a polymeric nanocapsule composed of polyallylamine hydrochloride (PAH) chains cross-linked with sodium phosphate and doped with indocyanine green (ICG) toward such endeavors. The ICG-doped nanocapsules were coated covalently with polyethylene glycol (5000 daltons) through reductive amination. We administrated the constructs by tail vein injection to healthy mice. To characterize the biodistribution of the constructs, we performed in vivo quantitative fluorescence imaging and subsequently analyzed the various extracted organs. Our results suggest that encapsulation of ICG in these PEGylated constructs is an effective approach to prolong the circulation time of ICG and delay its hepatic accumulation. Increased bioavailability of ICG, due to encapsulation, offers the potential of extending the clinical applications of ICG, which are currently limited due to rapid elimination of ICG from the vasculature. Our results also indicate that PAH and ICG-doped nanocapsules (ICG-NCs) are not cytotoxic at the levels used in this study.

Multiple target-specific molecular imaging agents detect liver cancer in a preclinical model

Liver cancer is the fifth most common cause of cancer deaths worldwide. Noninvasive diagnosis is difficult and the disease heterogeneity reduces the accuracy of pathological assays. Improvement in diagnostic imaging of specific molecular disease markers has provided hope for accurate and early noninvasive detection of liver cancer. However, all current imaging technologies, including ultrasonography, computed tomography (CT), positron emission tomography (PET), and magnetic resonance imaging, are not specific targets for detection of liver cancer. The aim of this study was to test the feasibility of injecting a cocktail of specific molecular imaging agents to noninvasively image liver cancer. The target-specific cocktail contained agents for imaging the neovasculature (RGD peptide), matrix metalloproteinase (MMP), and glucose transport (¹⁸F-fluorodeoxyglucose [¹⁸F-FDG]). Imaging studies were performed in liver cancer cells and xenograft models. The distribution of MMP at the intracellular level was imaged by confocal microscopy. RGD, MMP, and ¹⁸F-FDG were imaged on tumor-bearing mice using PET, CT, X-ray, and multi-wavelength optical imaging modalities. Image data demonstrated that each agent bound to a specific disease target component. The same liver cancer xenograft contained multiple disease markers. Those disease markers were heterogenetically distributed in the same tumor nodule. The molecular imaging agents had different distributions in the whole body and inside the tumor nodule. All target-specific agents yielded high tumor-to-background ratios after injection. In conclusion, target-specific molecular imaging agents can be used to study liver cancer in vitro and in vivo. Noninvasive multimodal/multi-target-specific molecular imaging agents could provide tools to simultaneously study multiple liver cancer components.

Multimodal chelation platform for near-infrared fluorescence/nuclear imaging

Dual-labeled compounds containing nuclear and near-infrared fluorescence contrast have the potential to molecularly guide surgical resection of cancer by extending whole-body diagnostic imaging findings into the surgical suite. To simplify the dual labeling process for antibody-based agents, we designed a multimodality chelation (MMC) scaffold which combined a radiometal chelating agent and fluorescent dye into a single moiety. Three dye-derivatized MMC compounds were synthesized and radiolabeled. The IRDye 800CW conjugate, 4, had favorable optical properties and showed rapid clearance in vivo. Using 4, an epithelial cell adhesion molecule (EpCAM) targeting MMC-immunoconjugate was prepared and dual-labeled with (64)Cu. In vitro binding activity was confirmed after MMC conjugation. Multimodal imaging studies showed higher tumor accumulation of (64)Cu-7 compared to nontargeted (64)Cu-4 in a prostate cancer model. Further evaluation in different EpCAM-expressing cell lines is warranted as well as application of the MMC dual labeling approach with other monoclonal antibodies.

Advancing the translation of optical imaging agents for clinical imaging

Despite the development of a large number of promising candidates, few contrast agents for established medical imaging modalities have successfully been translated over the past decade. The emergence of new imaging contrast agents that employ biomedical optics is further complicated by the relative infancy of the field and the lack of approved imaging devices compared to more established clinical modalities such as nuclear medicine. Herein, we propose a navigational approach (as opposed to a fixed "roadmap") for translation of optical imaging agents that is (i) proposed through consensus by four academic research programs that are part of the cooperative U54 NCI Network for Translational Research, (ii) developed through early experiences for translating optical imaging agents in order to meet distinctly varied needs in cancer diagnostics, and (iii) adaptable to the rapidly changing environment of academic medicine. We describe the pathways by which optical imaging agents are synthesized, qualified, and validated for preclinical testing, and ultimately translated for "first-in-humans" studies using investigational optical imaging devices. By identifying and adopting consensus approaches for seemingly disparate optical imaging modalities and clinical indications, we seek to establish a systematic method for navigating the ever-changing "roadmap" to most efficiently arrive at the destination of clinical adoption and improved outcome and survivorship for cancer patients.

A compact frequency-domain photon migration system for integration into commercial hybrid small animal imaging scanners for fluorescence tomography

The work presented herein describes the system design and performance evaluation of a miniaturized near-infrared fluorescence (NIRF) frequency-domain photon migration (FDPM) system with non-contact excitation and homodyne detection capability for small animal fluorescence tomography. The FDPM system was developed specifically for incorporation into a Siemens micro positron emission tomography/computed tomography (microPET/CT) commercial scanner for hybrid small animal imaging, but could be adapted to other systems. Operating at 100 MHz, the system noise was minimized and the associated amplitude and phase errors were characterized to be ±0.7% and ±0.3°, respectively. To demonstrate the tomographic ability, a commercial mouse-shaped phantom with 50 µM IRDye800CW and ⁶⁸Ga containing inclusion was used to associate PET and NIRF tomography. Three-dimensional mesh generation and anatomical referencing was accomplished through CT. A third-order simplified spherical harmonics approximation (SP₃) algorithm, for efficient prediction of light propagation in small animals, was tailored to incorporate the FDPM approach. Finally, the PET-NIRF target co-localization accuracy was analyzed in vivo with a dual-labeled imaging agent targeting orthotopic growth of human prostate cancer. The obtained results validate the integration of time-dependent fluorescence tomography system within a commercial microPET/CT scanner for multimodality small animal imaging.

Integrin αvβ3-targeted IRDye 800CW near-infrared imaging of glioblastoma

PURPOSE

Integrin α(v)β(3) plays an important role in tumor angiogenesis, growth, and metastasis. We have tested a targeted probe to visualize integrin receptor expression in glioblastomas using near-infrared fluorescent (NIRF) imaging.

EXPERIMENTAL DESIGN

A transgenic glioblastoma mouse model (RCAS-PDGF-driven/tv-a glioblastoma, which mimics the infiltrative growth pattern of human glioblastomas) and two human orthotopic glioblastoma models (U-87 MG with high integrin β(3) expression and TS543 with low integrin β(3) expression) were studied. An integrin-targeting NIRF probe, IRDye 800CW-cyclic-RGD peptide (IRDye 800CW-RGD), was tested by in vivo and ex vivo NIRF imaging.

RESULTS

We show that the IRDye 800CW-RGD peptide: (i) specifically binds to integrin receptors; (ii) is selectively localized to glioblastoma tissue with overexpressed integrin receptors and is retained over prolonged periods of time; (iii) is associated with minimal autofluorescence and photobleaching because of imaging at 800 nm; (iv) provides delineation of tumor tissue with high precision because of a high tumor-to-normal brain fluorescence ratio (79.7 ± 6.9, 31.2 ± 2.8, and 16.3 ± 1.3) in the U-87 MG, RCAS-PDGF, and TS543 models, respectively; P < 0.01); and (v) enables fluorescence-guided glioblastoma resection. Importantly, small foci of residual fluorescence were observed after resection was completed using white light imaging alone, and these fluorescent foci were shown to represent residual tumor tissue by histology.

CONCLUSIONS

NIRF imaging with the IRDye 800CW-RGD probe provides a simple, rapid, low-cost, nonradioactive, and highly translatable approach for improved intraoperative glioblastoma visualization and resection. It also has the potential to serve as an imaging platform for noninvasive cancer detection and drug efficacy evaluation studies.

Dual-labeling strategies for nuclear and fluorescence molecular imaging: a review and analysis

Molecular imaging is used for the detection of biochemical processes through the development of target-specific contrast agents. Separately, modalities such as nuclear and near-infrared fluorescence (NIRF) imaging have been shown to non-invasively monitor disease. More recently, merging of these modalities has shown promise owing to their comparable detection sensitivity and benefited from the development of dual-labeled imaging agents. Dual-labeled agents hold promise for whole-body and intraoperative imaging and could bridge the gap between surgical planning and image-guided resection with a single, molecularly targeted agent. In this review, we summarized the literature for dual-labeled antibodies and peptides that have been developed and have highlighted key considerations for incorporating NIRF dyes into nuclear labeling strategies. We also summarized our findings on several commercially available NIRF dyes and offer perspectives for developing a toolkit to select the optimal NIRF dye and radiometal combination for multimodality imaging.

DOTAGA-anhydride: a valuable building block for the preparation of DOTA-like chelating agents

A DOTA derivative that contains an anhydride group was readily synthesized by reacting DOTAGA with acetic anhydride and its reactivity was investigated. Opening the anhydride with propylamine led to the selective formation of one of two possible regioisomers. The structure of the obtained isomer was unambiguously determined by 1D and 2D NMR experiments, including COSY, HMBC, and NOESY techniques. This bifunctional chelating agent offers a convenient and attractive approach for labeling biomolecules and, more generally, for the synthesis of a large range of DOTA derivatives. The scope of the reaction was extended to prepare DOTA-like compounds that contained various functional groups, such as isothiocyanate, thiol, ester, and amino acid moieties. This versatile building block was also used for the synthesis of a bimodal tag for SPECT or PET/optical imaging.

Stroma targeting nuclear imaging and radiopharmaceuticals

Malignant transformation of tumor accompanies profound changes in the normal neighboring tissue, called tumor stroma. The tumor stroma provides an environment favoring local tumor growth, invasion, and metastatic spreading. Nuclear imaging (PET/SPECT) measures biochemical and physiologic functions in the human body. In oncology, PET/SPECT is particularly useful for differentiating tumors from postsurgical changes or radiation necrosis, distinguishing benign from malignant lesions, identifying the optimal site for biopsy, staging cancers, and monitoring the response to therapy. Indeed, PET/SPECT is a powerful, proven diagnostic imaging modality that displays information unobtainable through other anatomical imaging, such as CT or MRI. When combined with coregistered CT data, [(18)F]fluorodeoxyglucose ([(18)F]FDG)-PET is particularly useful. However, [(18)F]FDG is not a target-specific PET tracer. This paper will review the tumor microenvironment targeting oncologic imaging such as angiogenesis, invasion, hypoxia, growth, and homing, and also therapeutic radiopharmaceuticals to provide a roadmap for additional applications of tumor imaging and therapy.

Multimodal interventional molecular imaging of tumor margins and distant metastases by targeting αvβ3 integrin

α(v)β(3) integrin is involved in (tumor-induced) angiogenesis and is a promising candidate for the specific visualization of both primary tumors and of their distant metastases. Combination of radioactive and fluorescent imaging labels in a single multimodal, or rather hybrid, RGD-based imaging agent enables integration of pre-, intra-, and postoperative angiogenesis imaging. A hybrid imaging agent targeting the α(v)β(3) integrin--(111)In-MSAP-RGD (MSAP = multifunctional single-attachment-point reagent), which contains a targeting moiety, a pentetic acid (DTPA) chelate, and a cyanine dye--was evaluated for its potential value in combined lesion detection and interventional molecular imaging in a 4T1 mouse breast cancer model. SPECT/CT and fluorescence imaging were used to visualize the tumor in vivo. Tracer distribution was evaluated ex vivo down to the microscopic level. The properties of (111)In-MSAP-RGD were compared with those of (111)In-DTPA-RGD. Biodistribution studies revealed a prolonged retention and increased tumor accumulation of (111)In-MSAP-RGD relative to (111)In-DTPA-RGD. With (111)In-MSAP-RGD, identical features could be visualized preoperatively (SPECT/CT) and intraoperatively (fluorescence imaging). As well as the primary tumor, (111)In-MSAP-RGD also enabled detection and accurate excision of distant metastases in the head and neck region of the mice. Therefore, the hybrid RGD derivative (111)In-MSAP-RGD shows potential in preoperative planning and fluorescence-based surgical intervention.

Imaging prostate cancer lymph node metastases with a multimodality contrast agent

BACKGROUND

Methods to detect lymph node (LN) metastases in prostate cancer (PCa) are limited. Pelvic LN dissection is commonly performed during prostatectomy, but often followed by morbid complications. More refined methods for detecting LN invasion are needed.

METHODS

We developed a dual-labeled targeting agent having a near-infrared (NIR) fluorophore for intraoperative guidance, and a conventional radiotracer for detection of LN metastasis. Nu/Nu mice were orthotopically implanted with DsRed-expressing human PCa (PC3) cells. Antibody (Ab) specific for epithelial cell adhesion molecule was conjugated to DOTA, IRDye 800CW, and radiolabeled with (64) Cu. Dual-labeled Ab was administered intravenously at 10-12 weeks post-implantation, and positron emission tomography/computed tomography (PET/CT) and fluorescence imaging were performed within 18-24 hr.

RESULTS

Metastasis to lumbar LNs was detected by DsRed fluorescence imaging, as well as pathology, in 75% of mice having pathology-confirmed primary prostate tumors. These metastases were also detected by NIR fluorescence imaging. In some cases, metastases to sciatic, medial, renal, and axillary nodes were also detected. For all LNs examined, no significant differences were found between the percentages of metastases detected by NIR imaging (63%) and µPET/CT (64%) (P = 0.93), or between those detected by DsRed imaging (25%) and pathological examination (19%) (P = 0.12).

CONCLUSION

This study demonstrates that a multimodality contrast agent is useful for early detection of metastatic disease, and has applications for intraoperative PCa treatment. Further agent optimization is necessary to enhance specificity, and provide validation for prostate and other LN metastasizing epithelial cancers.

The role of lymphatics in cancer as assessed by near-infrared fluorescence imaging

The lymphatic system is the secondary circulatory system responsible for fluid homeostasis and protein transport in the body. In addition, because the lymphatic system provides a primary pathway for cancer metastasis, lymph node involvement is routinely used as a determinant in cancer staging. Despite their importance, the lymphatics remain poorly understood, in part because of the historic lack of imaging modalities with sufficient spatial and/or temporal resolution to visualize the fine lymphatic structure and subtle contractile function. In recent years, near-infrared fluorescence (NIRF) imaging has emerged as a new imaging modality to non-invasively visualize the lymphatics and assess contractile lymphatic function in humans following administration of microdose amounts of a NIRF contrast agent. In this contribution, we first review NIRF imaging and its clinical application in sentinel lymph node mapping, intraoperative guidance, and assessing the architecture and contractile function of the lymphatics in health and in cancer-related lymphedema. We then present recent NIRF lymphatic imaging for non-invasive assessment of lymphatics both in preclinical melanoma models and in human subjects with melanoma.

Translation of near-infrared fluorescence imaging technologies: emerging clinical applications

Technical developments in near-infrared fluorescence (NIRF) imaging and tomography have enabled recent translation into investigational human studies. Noninvasive imaging of the lymphatic vasculature for diagnosis and assessment of function has been uniquely accomplished with NIR using indocyanine green (ICG), a nonspecific dye that has comparatively poor fluorescent properties compared to emerging dyes. Adjunct use of NIRF-ICG for (a) intraoperative sentinel lymph node mapping for cancer staging, (b) video-angiography during surgery, and (c) discrimination of malignant from benign breast lesions detected by mammography and ultrasongraphy also evidences the clinical utility of NIRF. Future NIRF imaging agents that consist of bright fluorescent dyes conjugated to disease-targeting moieties promise molecular imaging and image-guided surgery. In this review, emerging NIRF imaging is described within the context of nuclear imaging technologies that remain the "gold standard" of molecular imaging.

Simultaneous PET and multispectral 3-dimensional fluorescence optical tomography imaging system

Integrated PET and 3-dimensional (3D) fluorescence optical tomography (FOT) imaging has unique and attractive features for in vivo molecular imaging applications. We have designed, built, and evaluated a simultaneous PET and 3D FOT system. The design of the FOT system is compatible with many existing small-animal PET scanners.

METHODS

The 3D FOT system comprises a novel conical mirror that is used to view the whole-body surface of a mouse with an electron-multiplying charge-coupled device camera when a collimated laser beam is projected on the mouse to stimulate fluorescence. The diffusion equation was used to model the propagation of optical photons inside the mouse body, and 3D fluorescence images were reconstructed iteratively from the fluorescence intensity measurements measured from the surface of the mouse. Insertion of the conical mirror into the gantry of a small-animal PET scanner allowed simultaneous PET and 3D FOT imaging.

RESULTS

The mutual interactions between PET and 3D FOT were evaluated experimentally. PET has negligible effects on 3D FOT performance. The inserted conical mirror introduces a reduction in the sensitivity and noise-equivalent count rate of the PET system and increases the scatter fraction. PET-FOT phantom experiments were performed. An in vivo experiment using both PET and FOT was also performed.

CONCLUSION

Phantom and in vivo experiments demonstrate the feasibility of simultaneous PET and 3D FOT imaging. The first in vivo simultaneous PET-FOT results are reported.

Matrix metalloproteinase-9 is a diagnostic marker of heterotopic ossification in a murine model

Heterotopic ossification (HO) is a serious disorder that occurs when there is aberrant bone morphogenic protein (BMP) signaling in soft tissues. Currently, there are no methods to detect HO before mineralization occurs. Yet once mineralization occurs, there are no effective treatments, short of surgery, to reverse HO. Herein, we used in vivo molecular imaging and confirmatory ex vivo tissue analyses of an established murine animal model of BMP-induced HO to show that matrix metalloproteinase-9 (MMP-9) can be detected as an early-stage biomarker before mineralization. Ex vivo analyses show that active MMP-9 protein is significantly elevated within tissues undergoing HO as early as 48 h after BMP induction, with its expression co-localizing to nerves and vessels. In vivo molecular imaging with a dual-labeled near-infrared fluorescence and micro-positron emission tomography (μPET) agent specific to MMP-2/-9 expression paralleled the ex vivo observations and reflected the site of HO formation as detected from microcomputed tomography 7 days later. The results suggest that the MMP-9 is a biomarker of the early extracellular matrix (ECM) re-organization and could be used as an in vivo diagnostic with confirmatory ex vivo tissue analysis for detecting HO or conversely for monitoring the success of tissue-engineered bone implants that employ ECM biology for engraftment.

NEAR-INFRARED DYES: Probe Development and Applications in Optical Molecular Imaging

The recent emergence of optical imaging has brought forth a unique challenge for chemists: development of new biocompatible dyes that fluoresce in the near-infrared (NIR) region for optimal use in biomedical applications. This review describes the synthesis of NIR dyes and the design of probes capable of noninvasively imaging molecular events in small animal models.

Assessment of lymphatic contractile function after manual lymphatic drainage using near-infrared fluorescence imaging

OBJECTIVE

To investigate the feasibility of assessing the efficacy of manual lymphatic drainage (MLD), a method for lymphedema (LE) management, by using near-infrared (NIR) fluorescence imaging.

DESIGN

Exploratory pilot study.

SETTING

Primary care unit.

PARTICIPANTS

Subjects (N=10; age, 18-68y) with a diagnosis of grade I or II LE and 12 healthy control subjects (age, 22-59y).

INTERVENTION

Indocyanine green (25 μg in 0.1 mL each) was injected intradermally in bilateral arms or legs of subjects. Diffused excitation light illuminated the limbs, and NIR fluorescence images were collected by using custom-built imaging systems. Subjects received MLD therapy, and imaging was performed pre- and posttherapy.

MAIN OUTCOME MEASURES

Apparent lymph velocities and periods between lymphatic propulsion events were computed from fluorescence images. Data collected pre- and post-MLD were compared and evaluated for differences.

RESULTS

By comparing pre-MLD lymphatic contractile function against post-MLD lymphatic function, results showed that average apparent lymph velocity increased in both the symptomatic (+23%) and asymptomatic (+25%) limbs of subjects with LE and control limbs (+28%) of healthy subjects. The average lymphatic propulsion period decreased in symptomatic (-9%) and asymptomatic (-20%) limbs of subjects with LE, as well as in control limbs (-23%).

CONCLUSIONS

We showed that NIR fluorescence imaging could be used to quantify immediate improvement of lymphatic contractile function after MLD.

Characterization of chemical, radiochemical and optical properties of a dual-labeled MMP-9 targeting peptide

Optical imaging possesses similar sensitivity to nuclear imaging and has led to the emergence of multimodal approaches with dual-labeled nuclear/near-infrared (NIR) agents. The growing impact of (68)Ga (t(1/2)=68 min) labeled peptides on preclinical and clinical research offers a promising opportunity to merge the high spatial resolution of NIR imaging with the clinically-accepted positron emission tomography (PET). Previously, dual-labeled agents have been prepared with longer-lived radiometals and showed no detrimental effects on optical properties as a result of radiolabeling. In this study, we selected a peptide (M(2)) that targets MMP-2/9 and is dual-labeled with IRDye 800 CW and (68)Ga. Since (68)Ga chelation typically requires low pH (3.5-4) and elevated heating temperatures (95 °C), we sought to evaluate the impact of (68)Ga labeling on the optical properties of M(2). An efficient method for preparation of (68)Ga-M(2) was developed and reaction conditions were optimized. Stability studies in PBS, DTPA, and serum were performed and high levels of intact agent were evident under each condition. The addition of multiple reporters to a targeting agent adds further complexity to the characterization and validation and thus requires not only testing to ensure the agent is stable chemically and radiochemically, but also optically. Therefore, fluorescence properties were evaluated using a spectrofluorometer as well as by fluorescence detection via HPLC. It was determined that (68)Ga-labeling conditions did not impair the fluorescent properties of the agent. The agent was then used for in vivo imaging in a mouse model of heterotopic ossification (HO) with activated MMP-9 expression as an early biomarker which precedes mineralization. Although (68)Ga-complexation greatly reduced binding affinity of the peptide and negated tracer uptake on PET, NIR imaging showed consistent fluorescent signal that correlated to MMP-9 expression. This attests to the feasibility of using (68)Ga/NIR for dual-labeling of other peptides or small molecules for multimodality molecular imaging.

Single-dose intravenous toxicity study of IRDye 800CW in Sprague-Dawley rats

Objective

Fluorophore-labeled contrast imaging agents are moving toward clinical use for a number of applications. The near-infrared dye IRDye 800CW is frequently used in its N-hydroxysuccinamide (NHS) ester form for labeling these agents. Following conjugation or breakdown of a labeled ligand, excess NHS ester is converted to the carboxylate form. To prepare for clinical use as a near-infrared fluorophore, a toxicity study was conducted on IRDye 800CW carboxylate.

Methods

Male and female Sprague–Dawley rats were given a single intravenous or intradermal administration of IRDye 800CW carboxylate; Indocyanine Green was used as a comparative control. Animals were injected with varying doses of the test and control articles and observed for up to 14 days. Clinical chemistry, hematological, and pharmacokinetic analyses were performed on subgroups of animals. Organs were analyzed for content of the test article. Tissues were analyzed microscopically for pathological changes.

Results

Based on hematologic, clinical chemistry, and histopathologic evaluation, single administration of IRDye 800CW carboxylate intravenously at dose levels of 1, 5, and 20 mg/kg or 20 mg/kg intradermally produced no pathological evidence of toxicity.

Conclusion

A dose of 20 mg/kg was identified as the no observed adverse effect level following IV or ID routes of administration of IRDye 800CW.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-010-0317-x) contains supplementary material, which is available to authorized users.

Multimodality Imaging of Integrin α(v)β(3) Expression

Over the last decade, integrin α_vβ₃ has been studied with every single molecular imaging modality. Since no single modality is perfect and sufficient to obtain all the necessary information for a particular question, combination of certain molecular imaging modalities can offer synergistic advantages over any modality alone. This review will focus on multimodality imaging of integrin α_vβ₃ expression, where the contrast agent used can be detected by two or more imaging modalities, such as combinations of PET and optical, SPECT and fluorescence, PET and MRI, SPECT and MRI, and lastly, MRI and fluorescence. Most of these agents are based on certain type(s) of nanoparticles. Contrast agents that can be detected by more than two imaging modalities are expected to emerge in the future and a PET/MRI/fluorescence agent will likely find the most future biomedical/clinical applications. Big strides have been made over the last decade for imaging integrin α_vβ₃ expression and several PET/SPECT probes have been tested in human studies. For dualmodality and multimodality imaging applications, a number of proof-of-principle studies have been reported which opened up many new avenues for future research. The next decade will likely witness further growth and continued prosperity of molecular imaging studies focusing on integrin α_vβ₃, which can eventually impact patient management.

Multimodal tumor-targeting peptides functionalized with both a radio- and a fluorescent label

The use of monolabeled tumor-targeting peptides for molecular imaging is widespread. However, it is often desirable to use the same compound for different clinical applications, e.g., combined pre- and intraoperative tumor detection. On the basis of their detection sensitivity, the combination of radioactivity and fluorescence is probably the most valuable in multimodal molecular imaging. In this review, we compare multimodal peptide derivatives and discuss the influence of the diagnostic labels on receptor affinity and biodistribution. On the basis of the described constructs, we propose improvements for the design of future multimodal tumor-targeting peptide derivatives.

Multimodal radio- (PET/SPECT) and fluorescence imaging agents based on metallo-radioisotopes: current applications and prospects for development of new agents

This perspective focuses on complexes of radioactive metal ions applied in multimodal radio- and optical imaging. The application of metal ions in radioimaging techniques such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) has advantages over lighter nuclei in terms of half-life, but there are particular issues related to their speciation (particularly leaching from complexes) and with the combination of certain ions with fluorescent systems. The basic coordination chemistry of the ions involved and issues relating to biological conditions and their compatibility with optical imaging techniques are reviewed, the current literature presented in context, and the prospect of exploiting the intrinsic luminescence of certain metal-ligand complexes is discussed.

Perspectives and potential applications of nanomedicine in breast and prostate cancer

Nanomedicine is a branch of nanotechnology that includes the development of nanostructures and nanoanalytical systems for various medical applications. Among these applications, utilization of nanotechnology in oncology has captivated the attention of many research endeavors in recent years. The rapid development of nano-oncology raises new possibilities in cancer diagnosis and treatment. It also holds great promise for realization of point-of-care, theranostics, and personalized medicine. In this article, we review advances in nano-oncology, with an emphasis on breast and prostate cancer because these organs are amenable to the translation of nanomedicine from small animals to humans. As new drugs are developed, the incorporation of nanotechnology approaches into medicinal research becomes critical. Diverse aspects of nano-oncology are discussed, including nanocarriers, targeting strategies, nanodevices, as well as nanomedical diagnostics, therapeutics, and safety. The review concludes by identifying some limitations and future perspectives of nano-oncology in breast and prostate cancer management.

Detection of Cancer Metastases with a Dual-labeled Near-Infrared/Positron Emission Tomography Imaging Agent

By dual labeling a targeting moiety with both nuclear and optical probes, the ability for noninvasive imaging and intraoperative guidance may be possible. Herein, the ability to detect metastasis in an immunocompetent animal model of human epidermal growth factor receptor 2 (HER-2)-positive cancer metastases using positron emission tomography (PET) and near-infrared (NIR) fluorescence imaging is demonstrated.

METHODS

((64)Cu-DOTA)(n)-trastuzumab-(IRDye800)(m) was synthesized, characterized, and administered to female Balb/c mice subcutaneously inoculated with highly metastatic 4T1.2neu/R breast cancer cells. ((64)Cu-DOTA)(n)-trastuzumab-(IRDye800)(m) (150 µg, 150 µCi, m = 2, n = 2) was administered through the tail vein at weeks 2 and 6 after implantation, and PET/computed tomography and NIR fluorescence imaging were performed 24 hours later. Results were compared with the detection capabilities of F-18 fluorodeoxyglucose ((18)FDG-PET).

RESULTS

Primary tumors were visualized with (18)FDG and ((64)Cu-DOTA)(n)-trastuzumab-(IRDye800)(m), but resulting metastases were identified only with the dual-labeled imaging agent. (64)Cu-PET imaging detected lung metastases, whereas ex vivo NIR fluorescence showed uptake in regions of lung, skin, skeletal muscle, and lymph nodes, which corresponded with the presence of cancer cells as confirmed by histologic hematoxylin and eosin stains. In addition to detecting the agent in lymph nodes, the high signal-to-noise ratio from NIR fluorescence imaging enabled visualization of channels between the primary tumor and the axillary lymph nodes, suggesting a lymphatic route for trafficking cancer cells. Because antibody clearance occurs through the liver, we could not distinguish between nonspecific uptake and liver metastases.

CONCLUSION

((64)Cu-DOTA)(n)-trastuzumab-(IRDye800)(m) may be an effective diagnostic imaging agent for staging HER-2-positive breast cancer patients and intraoperative resection.

Exogenous Molecular Probes for Targeted Imaging in Cancer: Focus on Multi-modal Imaging

Cancer is one of the major causes of mortality and morbidity in our health care system. Molecular imaging is an emerging methodology for the early detection of cancer, and the development of exogenous molecular probes that can be labeled for multi-modality imaging is critical to this process. Today, molecular imaging is at crossroad, and new targeted imaging agents are expected to broadly expand our ability to detect pre-malignant lesions. This integrated imaging strategy will permit clinicians to not only localize lesions within the body, but also to visualize the expression and activity of specific molecules. This information is expected to have a major impact on diagnosis, therapy, drug development and understanding of basic cancer biology. At this time, a number of molecular probes have been developed by conjugating various labels to affinity ligands for targeting in different imaging modalities. This review will describe the current status of exogenous molecular probes for optical, nuclear and MRI imaging platforms. Furthermore, we will also shed light on how these techniques can be used synergistically in multi-modal platforms and how these techniques are being employed in current research.

A dual-labeled knottin peptide for PET and near-infrared fluorescence imaging of integrin expression in living subjects

Previously, we used directed evolution to engineer mutants of the Ecballium elaterium trypsin inhibitor (EETI-II) knottin that bind to αvβ3 and αvβ5 integrin receptors with low nanomolar affinity, and showed that Cy5.5- or (64)Cu-DOTA-labeled knottin peptides could be used to image integrin expression in mouse tumor models using near-infrared fluorescence (NIRF) imaging or positron emission tomography (PET). Here, we report the development of a dual-labeled knottin peptide conjugated to both NIRF and PET imaging agents for multimodality imaging in living subjects. We created an orthogonally protected peptide-based linker for stoichiometric coupling of (64)Cu-DOTA and Cy5.5 onto the knottin N-terminus and confirmed that conjugation did not affect binding to αvβ3 and αvβ5 integrins. NIRF and PET imaging studies in tumor xenograft models showed that Cy5.5 conjugation significantly increased kidney uptake and retention compared to the knottin peptide labeled with (64)Cu-DOTA alone. In the tumor, the dual-labeled (64)Cu-DOTA/Cy5.5 knottin peptide showed decreased wash-out leading to significantly better retention (p < 0.05) compared to the (64)Cu-DOTA-labeled knottin peptide. Tumor uptake was significantly reduced (p < 0.05) when the dual-labeled knottin peptide was coinjected with an excess of unlabeled competitor and when tested in a tumor model with lower levels of integrin expression. Finally, plots of tumor-to-background tissue ratios for Cy5.5 versus (64)Cu uptake were well-correlated over several time points post injection, demonstrating pharmacokinetic cross validation of imaging labels. This dual-modality NIRF/PET imaging agent is promising for further development in clinical applications where high sensitivity and high resolution are desired, such as detection of tumors located deep within the body and image-guided surgical resection.

Dual-modality molecular imaging using antibodies labeled with activatable fluorescence and a radionuclide for specific and quantitative targeted cancer detection

Multimodality molecular imaging should have potential for compensating the disadvantages and enhancing the advantages of each modality. Nuclear imaging is superior to optical imaging in whole body imaging and in quantification due to good tissue penetration of gamma rays. However, target specificity can be compromised by high background signal due to the always signal ON feature of nuclear probes. In contrast, optical imaging can be superior in target-specific imaging by employing target-specific signal activation systems, although it is not quantitative because of signal attenuation. In this study, to take advantage of the mutual cooperation of each modality, multimodality imaging was performed by a combination of quantitative radiolabeled probe and an activatable optical probe. The monoclonal antibodies, panitumumab (anti-HER1) and trastuzumab (anti-HER2), were labeled with 111In and ICG and tested in both HER1 and HER2 tumor bearing mice by the cocktail injection of radiolabeled and optical probes and by the single injection of a dual-labeled probe. The optical and nuclear images were obtained over 6 days after the conjugates injection. The fluorescence activation properties of ICG labeled antibodies were also investigated by in vitro microscopy. In vitro microscopy demonstrated that there was no fluorescence signal with either panitumumab-ICG or trastuzumab-ICG, when the probes were bound to cell surface antigens but were not yet internalized. After the conjugates were internalized into the cells, both conjugates showed bright fluorescence signal only in the target cells. These results show that both conjugates work as activatable probes. In in vivo multimodality imaging by injection of a cocktail of radio-optical probes, only the target specific tumor was visualized by optical imaging. Meanwhile, the biodistribution profile of the injected antibody was provided by nuclear imaging. Similar results were obtained with radio and optical dual-labeled probes, and it is confirmed that pharmacokinetic properties did not affect the results above. Here, we could characterize the molecular targets by activatable optical probes and visualize the delivery of targeting molecules quantitatively by radioactive probes. Multimodality molecular imaging combining activatable optical and radioactive probes has great potential for simultaneous visualization, characterization, and measurement of biological processes.

Optimal parameters for near infrared fluorescence imaging of amyloid plaques in Alzheimer's disease mouse models

Amyloid-beta plaques are an Alzheimer's disease biomarker which present unique challenges for near-infrared fluorescence tomography because of size (<50 microm diameter) and distribution. We used high-resolution simulations of fluorescence in a digital Alzheimer's disease mouse model to investigate the optimal fluorophore and imaging parameters for near-infrared fluorescence tomography of amyloid plaques. Fluorescence was simulated for amyloid-targeted probes with emission at 630 and 800 nm, plaque-to-background ratios from 1-1000, amyloid burden from 0-10%, and for transmission and reflection measurement geometries. Fluorophores with high plaque-to-background contrast ratios and 800 nm emission performed significantly better than current amyloid imaging probes. We tested idealized fluorophores in transmission and full-angle tomographic measurement schemes (900 source-detector pairs), with and without anatomical priors. Transmission reconstructions demonstrated strong linear correlation with increasing amyloid burden, but underestimated fluorescence yield and suffered from localization artifacts. Full-angle measurements did not improve upon the transmission reconstruction qualitatively or in semi-quantitative measures of accuracy; anatomical and initial-value priors did improve reconstruction localization and accuracy for both transmission and full-angle schemes. Region-based reconstructions, in which the unknowns were reduced to a few distinct anatomical regions, produced highly accurate yield estimates for cortex, hippocampus and brain regions, even with a reduced number of measurements (144 source-detector pairs).

Fluorescence enhancement by Au nanostructures: nanoshells and nanorods

Metallic nanoparticles influence the quantum yield and lifetime of adjacent fluorophores in a manner dependent on the properties of the nanostructure. Here we directly compare the fluorescence enhancement of the near-infrared fluorophore IR800 by Au nanoshells (NSs) and Au nanorods (NRs), where human serum albumin (HSA) serves as a spacer layer between the nanoparticle and the fluorophore. Our measurements reveal that the quantum yield of IR800 is enhanced from approximately 7% as an isolated fluorophore to 86% in a NSs-HSA-IR800 complex and 74% in a NRs-HSA-IR800 complex. This dramatic increase in fluorescence shows tremendous potential for contrast enhancement in fluorescence-based bioimaging.