In vivo ¹⁸F-FDG tumour uptake measurements in small animals using Cerenkov radiation

Prospective testing of clinical Cerenkov luminescence imaging against standard-of-care nuclear imaging for tumour location

In oncology, the feasibility of Cerenkov luminescence imaging (CLI) has been assessed by imaging superficial lymph nodes in a few patients undergoing diagnostic ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT). However, the weak luminescence signal requires the removal of ambient light. Here we report the development of a clinical CLI fiberscope with a lightproof enclosure, and the clinical testing of the setup using five different radiotracers. In an observational prospective trial (ClinicalTrials.gov identifier NCT03484884 ) involving 96 patients with existing or suspected tumours, scheduled for routine clinical FDG PET or ¹³¹I therapy, the level of agreement of CLI with standard-of-care imaging (PET or planar single-photon emission CT) for tumour location was 'acceptable' or higher (≥3 in the 1-5 Likert scale) for 90% of the patients. CLI correlated with the concentration of radioactive activity, and captured therapeutically relevant information from patients undergoing targeted radiotherapy or receiving the alpha emitter ²²³Ra, which cannot be feasibly imaged clinically. CLI could supplement radiological scans, especially when scanner capacity is limited.

PET/Fluorescence Imaging: An Overview of the Chemical Strategies to Build Dual Imaging Tools

Molecular imaging is a biomedical research discipline that has quickly emerged to afford the observation, characterization, monitoring, and quantification of biomarkers and biological processes in living organism. It covers a large array of imaging techniques, each of which provides anatomical, functional, or metabolic information. Multimodality, as the combination of two or more of these techniques, has proven to be one of the best options to boost their individual properties, hence offering unprecedented tools for human health. In this review, we will focus on the combination of positron emission tomography and fluorescence imaging from the specific perspective of the chemical synthesis of dual imaging agents. Based on a detailed analysis of the literature, this review aims at giving a comprehensive overview of the chemical strategies implemented to build adequate imaging tools considering radiohalogens and radiometals as positron emitters, fluorescent dyes mostly emitting in the NIR window and all types of targeting vectors.

Molecular Probes for Autofluorescence-Free Optical Imaging

Optical imaging is an indispensable tool in clinical diagnostics and fundamental biomedical research. Autofluorescence-free optical imaging, which eliminates real-time optical excitation to minimize background noise, enables clear visualization of biological architecture and physiopathological events deep within living subjects. Molecular probes especially developed for autofluorescence-free optical imaging have been proven to remarkably improve the imaging sensitivity, penetration depth, target specificity, and multiplexing capability. In this Review, we focus on the advancements of autofluorescence-free molecular probes through the lens of particular molecular or photophysical mechanisms that produce long-lasting luminescence after the cessation of light excitation. The versatile design strategies of these molecular probes are discussed along with a broad range of biological applications. Finally, challenges and perspectives are discussed to further advance the next-generation autofluorescence-free molecular probes for in vivo imaging and in vitro biosensors.

Beta radioguided surgery: towards routine implementation?

INTRODUCTION

In locally or locally advanced solid tumors, surgery still remains a fundamental treatment method. However, conservative resection is associated with high collateral damage and functional limitations of the patient. Furthermore, the presence of residual tumor tissue following conservative surgical treatment is currently a common cause of locally recurrent cancer or of distant metastases. Reliable intraoperative detection of small cancerous tissue would allow surgeons to selectively resect malignant areas: this task can be achieved by means of image-guided surgery, such as beta radioguided surgery (RGS).

EVIDENCE ACQUISITION

In this paper, a comprehensive review of beta RGS is given, starting from the physical principles that differentiate beta from gamma radiation, that has already its place in nuclear medicine current practice. Also, the recent clinical feasibility of using Cerenkov radiation is discussed.

EVIDENCE SYNTHESIS

Despite being first proposed several decades ago, only in the last years a remarkable interest in beta RGS has been observed, probably driven by the diffusion of PET radio tracers. Today several different approaches are being pursued to assess the effectiveness of such a technique, including both beta+ and beta- emitting radiopharmaceuticals.

CONCLUSIONS

Beta RGS shows some peculiarities that can present it as a very promising complementary technique to standard procedures. Good results are being obtained in several tests, both ex vivo and in vivo. This might however be the time to initiate the trials to demonstrate the real clinical value of these technologies with seemingly clear potential.

Surgical Advances in Osteosarcoma

Simple Summary

Osteosarcoma (OS) is the most common bone cancer in children. OS most commonly arises in the legs, but can arise in any bone, including the spine, head or neck. Along with chemotherapy, surgery is a mainstay of OS treatment and in the 1990s, surgeons began to shift from amputation to limb-preserving surgery. Since then, improvements in imaging, surgical techniques and implant design have led to improvements in functional outcomes without compromising on the cancer outcomes for these patients. This paper summarises these advances, along with a brief discussion of future technologies currently in development.

Abstract

Osteosarcoma (OS) is the most common primary bone cancer in children and, unfortunately, is associated with poor survival rates. OS most commonly arises around the knee joint, and was traditionally treated with amputation until surgeons began to favour limb-preserving surgery in the 1990s. Whilst improving functional outcomes, this was not without problems, such as implant failure and limb length discrepancies. OS can also arise in areas such as the pelvis, spine, head, and neck, which creates additional technical difficulty given the anatomical complexity of the areas. We reviewed the literature and summarised the recent advances in OS surgery. Improvements have been made in many areas; developments in pre-operative imaging technology have allowed improved planning, whilst the ongoing development of intraoperative imaging techniques, such as fluorescent dyes, offer the possibility of improved surgical margins. Technological developments, such as computer navigation, patient specific instruments, and improved implant design similarly provide the opportunity to improve patient outcomes. Going forward, there are a number of promising avenues currently being pursued, such as targeted fluorescent dyes, robotics, and augmented reality, which bring the prospect of improving these outcomes further.

Recent Advances in Nanostrategies Capable of Overcoming Biological Barriers for Tumor Management

Engineered nanomaterials have been extensively employed as therapeutics for tumor management. Meanwhile, the complex tumor niche along with multiple barriers at the cellular level collectively hinders the action of nanomedicines. Here, the advanced strategies that hold promise for overcoming the numerous biological barriers facing nanomedicines are summarized. Starting from tumor entry, methods that promote tissue penetration of nanomedicine and address the hypoxia issue are also highlighted. Then, emphasis is given to the significance of overcoming both physical barriers, such as membrane-associated efflux pumps, and biological features, such as resistance to apoptosis. The pros and cons for an individual approach are presented. In addition, the associated technical problems are discussed, along with the importance of balancing the therapeutic merits and the additional cost of sophisticated nanomedicine designs.

Imaging techniques in nanomedical research

About twenty years ago, nanotechnology began to be applied to biomedical issues giving rise to the research field called nanomedicine. Thus, the study of the interactions between nanomaterials and the biological environment became of primary importance in order to design safe and effective nanoconstructs suitable for diagnostic and/or therapeutic purposes. Consequently, imaging techniques have increasingly been used in the production, characterisation and preclinical/clinical application of nanomedical tools. This work aims at making an overview of the microscopy and imaging techniques in vivo and in vitro in their application to nanomedical investigation, and to stress their contribution to this developing research field.

Radionuclide-Activated Nanomaterials and Their Biomedical Applications

Radio-nanomedicine, or the use of radiolabeled nanoparticles in nuclear medicine, has attracted much attention in the last few decades. Since the discovery of Cerenkov radiation and its employment in Cerenkov luminescence imaging, the combination of nanomaterials and Cerenkov radiation emitters has been revolutionizing the way nanomaterials are perceived in the field: from simple inert carriers of radioactivity to activatable nanomaterials for both diagnostic and therapeutic applications. Herein, we provide a comprehensive review on the types of nanomaterials that have been used to interact with Cerenkov radiation and the gamma and beta scintillation of radionuclides, as well as on their biological applications.

A Monte Carlo study of pinhole collimated Cerenkov luminescence imaging integrated with radionuclide treatment

Cerenkov luminescence imaging (CLI) is an emerging optical imaging technique, which has been widely investigated for biological imaging. In this study, we proposed to integrate the CLI technique with the radionuclide treatment as a "see-and-treat" approach, and evaluated the performance of the pinhole collimator-based CLI technique. The detection of Cerenkov luminescence during radionuclide therapy was simulated using the Monte Carlo technique for breast cancer treatment as an example. Our results show that with the pinhole collimator-based configuration, the location, size and shape of the tumors can be clearly visualized on the Cerenkov luminescence images of the breast phantom. In addition, the CLI of multiple tumors can reflect the relative density of radioactivity among tumors, indicating that the intensity of Cerenkov luminescence is independent of the size and shape of a tumor. The current study has demonstrated the high-quality performance of the pinhole collimator-based CLI in breast tumor imaging for the "see-and-treat" multi-modality treatment.

Innovations in Nuclear Imaging Instrumentation: Cerenkov Imaging

Cerenkov luminescence (CL) is blue glow light produced by charged subatomic particles travelling faster than the phase velocity of light in a dielectric medium such as water or tissue. CL was first discovered in 1934, but for biomedical research it was recognized only in 2009 after advances in optical camera sensors brought the required high sensitivity. Recently, applications of CL from clinical radionuclides have been rapidly expanding to include not only preclinical and clinical biomedical imaging but also an approach to therapy. Cerenkov Luminescence Imaging (CLI) utilizes CL generated from clinically relevant radionuclides alongside optical imaging instrumentation. CLI is advantageous over traditional nuclear imaging methods in terms of infrastructure cost, resolution, and imaging time. Furthermore, CLI is a truly multimodal imaging method where the same agent can be detected by two independent modalities, with optical (CL) imaging and with positron emission tomography (PET) imaging. CL has been combined with small molecules, biomolecules and nanoparticles to improve diagnosis and therapy in cancer research. Here, we cover the fundamental breakthroughs and recent advances in reagents and instrumentation methods for CLI as well as therapeutic application of CL.

Theranostic Role of 32P-ATP as Radiopharmaceutical for the Induction of Massive Cell Death within Avascular Tumor Core

Drug inaccessibility to vast areas of the tumor parenchyma is amongst the major hurdles for conventional therapies. Treatment efficacy rapidly decreases with distance from vessels and most of the tumor cells survive therapy. Also, between subsequent cycles of treatment, spared cancer cells replace those killed near the vessels, improving their access to nutrients, boosting their proliferation rate, and thus enabling tumor repopulation. Because of their property of "acting at a distance," radioisotopes are believed to overcome the physical barrier of vascular inaccessibility. Methods A novel molecular imaging tool called Cerenkov Luminescence Imaging (CLI) was employed for the detection of Cerenkov radiation emitted by beta particles, allowing in vivo tracking of beta-emitters. More precisely we investigated using a xenograft model of colon carcinoma the potential use of 32P-ATP as a novel theranostic radiopharmaceutical for tracing tumor lesions while simultaneously hampering their growth. Results Our analyses demonstrated that 32P-ATP injected into tumor-bearing mice reaches tumor lesions and persists for days and weeks within the tumor parenchyma. Also, the high-penetrating beta particles of 32P-ATP exert a "cross-fire" effect that induces massive cell death throughout the entire tumor parenchyma including core regions. Conclusion Our findings suggest 32P-ATP treatment as a potential approach to complement conventional therapies that fail to reach the tumor core and to prevent tumor repopulation.

Quantitative Measurement of the Thyroid Uptake Function of Mouse by Cerenkov Luminescence Imaging

Cerenkov luminescence imaging (CLI) has been an evolutional and alternative approach of nuclear imaging in basic research. This study aimed to measure the ¹³¹I thyroid uptake of mouse using CLI for assessment of thyroid function. Quantification of ¹³¹I thyroid uptake of mice in euthyroid, hypothyroid and hyperthyroid status was performed by CLI and γ-scintigraphy at 24 hours after injection of ¹³¹I. The ¹³¹I thyroid uptake was calculated using the equation: (thyroid counts − background counts)/(counts of injected dose of ¹³¹I) × 100%. Serum T4 concentration was determined to evaluate the thyroid function. The radioactivity of ¹³¹I was linearly correlated with the CL signals in both in vitro and in vivo measurements. CLI showed a significant decrease and increase of ¹³¹I thyroid uptake in the mice in hypo- and hyperfunctioning status, respectively, and highly correlated with that measured by γ-scintigraphy. However, the percent thyroid uptake measured by CLI were one-fifth of those measured by γ-scintigraphy due to insufficient tissue penetration of CL. These results indicate that CLI, in addition to nuclear imaging, is able to image and evaluate the ¹³¹I thyroid uptake function in mice in preclinical and research settings.

Cerenkov luminescence imaging: physics principles and potential applications in biomedical sciences

Cerenkov luminescence imaging (CLI) is a novel imaging modality to study charged particles with optical methods by detecting the Cerenkov luminescence produced in tissue. This paper first describes the physical processes that govern the production and transport in tissue of Cerenkov luminescence. The detectors used for CLI and their most relevant specifications to optimize the acquisition of the Cerenkov signal are then presented, and CLI is compared with the other optical imaging modalities sharing the same data acquisition and processing methods. Finally, the scientific work related to CLI and the applications for which CLI has been proposed are reviewed. The paper ends with some considerations about further perspectives for this novel imaging modality.

Utilizing the power of Cerenkov light with nanotechnology

The characteristic blue glow of Cerenkov luminescence (CL) arises from the interaction between a charged particle travelling faster than the phase velocity of light and a dielectric medium, such as water or tissue. As CL emanates from a variety of sources, such as cosmic events, particle accelerators, nuclear reactors and clinical radionuclides, it has been used in applications such as particle detection, dosimetry, and medical imaging and therapy. The combination of CL and nanoparticles for biomedicine has improved diagnosis and therapy, especially in oncological research. Although radioactive decay itself cannot be easily modulated, the associated CL can be through the use of nanoparticles, thus offering new applications in biomedical research. Advances in nanoparticles, metamaterials and photonic crystals have also yielded new behaviours of CL. Here, we review the physics behind Cerenkov luminescence and associated applications in biomedicine. We also show that by combining advances in nanotechnology and materials science with CL, new avenues for basic and applied sciences have opened.

Recent Advances of Light-Mediated Theranostics

Currently, precision theranostics have been extensively demanded for the effective treatment of various human diseases. Currently, efficient therapy at the targeted disease areas still remains challenging since most available drug molecules lack of selectivity to the pathological sites. Among different approaches, light-mediated therapeutic strategy has recently emerged as a promising and powerful tool to precisely control the activation of therapeutic reagents and imaging probes in vitro and in vivo, mostly attributed to its unique properties including minimally invasive capability and highly spatiotemporal resolution. Although it has achieved initial success, the conventional strategies for light-mediated theranostics are mostly based on the light with short wavelength (e.g., UV or visible light), which may usually suffer from several undesired drawbacks, such as limited tissue penetration depth, unavoidable light absorption/scattering and potential phototoxicity to healthy tissues, etc. Therefore, a near-infrared (NIR) light-mediated approach on the basis of long-wavelength light (700-1000 nm) irradiation, which displays deep-tissue penetration, minimized photo-damage and low autofluoresence in living systems, has been proposed as an inspiring alternative for precisely phototherapeutic applications in the last decades. Despite numerous NIR light-responsive molecules have been currently proposed for clinical applications, several inherent drawbacks, such as troublesome synthetic procedures, low water solubility and limited accumulation abilities in targeted areas, heavily restrict their applications in deep-tissue therapeutic and imaging studies. Thanks to the amazing properties of several nanomaterials with large extinction coefficient in the NIR region, the construction of NIR light responsive nanoplatforms with multifunctions have become promising approaches for deep-seated diseases diagnosis and therapy. In this review, we summarized various light-triggered theranostic strategies and introduced their great advances in biomedical applications in recent years. Moreover, some other promising light-assisted techniques, such as photoacoustic and Cerenkov radiation, were also systemically discussed. Finally, the potential challenges and future perspectives for light-mediated deep-tissue diagnosis and therapeutics were proposed.

Cerenkov Luminescence Imaging as a Modality to Evaluate Antibody-Based PET Radiotracers

Antibodies, and engineered antibody fragments, labeled with radioisotopes are being developed as radiotracers for the detection and phenotyping of diseases such as cancer. The development of antibody-based radiotracers requires extensive characterization of their in vitro and in vivo properties, including their ability to target tumors in an antigen-selective manner. In this study, we investigated the use of Cerenkov luminescence imaging (CLI) as compared with PET as a modality for evaluating the in vivo behavior of antibody-based radiotracers.

METHODS

The anti-prostate-specific membrane antigen (PSMA) huJ591 antibody (IgG; 150 kDa) and its minibody (Mb; 80 kDa) format were functionalized with the chelator 1,4,7-triazacyclononane-1-glutaric acid-4,7-diacetic acid (NODAGA) and radiolabeled with the positron-emitting radionuclide ⁶⁴Cu (half-life, 12.7 h). Immunoreactive preparations of the radiolabeled antibodies were injected into NCr nu/nu mice harboring PSMA-positive CWR22Rv1 and PSMA-negative PC-3 tumor xenografts. Tumor targeting was evaluated by both PET and CLI.

RESULTS

⁶⁴Cu-NODAGA-PSMA-IgG and ⁶⁴Cu-NODAGA-PSMA-Mb retained the ability to bind cell surface PSMA, and both radiotracers exhibited selective uptake into PSMA-positive tumors. Under the experimental conditions used, PSMA-selective uptake of ⁶⁴Cu-NODAGA-PSMA-IgG and ⁶⁴Cu-NODAGA-PSMA-Mb was observed by CLI as early as 3 h after injection, with tumor-to-background ratios peaking at 24 (IgG) and 16 (Mb) h after injection. Targeting data generated by CLI correlated with that generated by PET and necropsy.

CONCLUSION

CLI provided a rapid and simple assessment of the targeting specificity and pharmacokinetics of the antibody-based PET radiotracers that correlated well with the behavior observed by standard PET imaging. Moreover, CLI provided clear discrimination between uptake kinetics of an intact IgG and its small-molecular-weight derivative Mb. These data support the use of CLI for the evaluation of radiotracer performance.

Cerenkov and radioluminescence imaging of brain tumor specimens during neurosurgery

We presented the first example of Cerenkov luminescence imaging (CLI) and radioluminescence imaging (RLI) of human tumor specimens. A patient with a brain meningioma localized in the left parietal region was injected with 166 MBq of 90Y-DOTATOC the day before neurosurgery. The specimens of the tumor removed during surgery were imaged using both CLI and RLI using an optical imager prototype developed in our laboratory. The system is based on a cooled electron multiplied charge coupled device coupled with an f ∕0.95 17-mm C-mount lens. We showed for the first time the possibility of obtaining CLI and RLI images of fresh human brain tumor specimens removed during neurosurgery.

Cherenkov radiation fluence estimates in tissue for molecular imaging and therapy applications

Cherenkov radiation has recently emerged as an interesting phenomenon for a number of applications in the biomedical sciences. Its unique properties, including broadband emission spectrum, spectral weight in the ultraviolet and blue wavebands, and local generation of light within a given tissue, have made it an attractive new source of light within tissue for molecular imaging and phototherapy applications. While several studies have investigated the total Cherenkov light yield from radionuclides in units of [photons/decay], further consideration of the light propagation in tissue is necessary to fully consider the utility of this signal in vivo. Therefore, to help further guide the development of this novel field, quantitative estimates of the light fluence rate of Cherenkov radiation from both radionuclides and radiotherapy beams in a biological tissue are presented for the first time. Using Monte Carlo simulations, these values were found to be on the order of 0.01-1 nW cm(-2) per MBq g(-1) for radionuclides, and 1-100 μW cm(-2) per Gy s(-1) for external radiotherapy beams, dependent on the given waveband, optical properties, and radiation source. For phototherapy applications, the total light fluence was found to be on the order of nJ cm(-2) for radionuclides, and mJ cm(-2) for radiotherapy beams. The results indicate that diagnostic potential is reasonable for Cherenkov excitation of molecular probes, but phototherapy may remain elusive at such exceedingly low fluence values. The results of this study are publicly available for distribution online at www.dartmouth.edu/optmed/.

High resolution Cerenkov light imaging of induced positron distribution in proton therapy

PURPOSE

In proton therapy, imaging of the positron distribution produced by fragmentation during or soon after proton irradiation is a useful method to monitor the proton range. Although positron emission tomography (PET) is typically used for this imaging, its spatial resolution is limited. Cerenkov light imaging is a new molecular imaging technology that detects the visible photons that are produced from high-speed electrons using a high sensitivity optical camera. Because its inherent spatial resolution is much higher than PET, the authors can measure more precise information of the proton-induced positron distribution with Cerenkov light imaging technology. For this purpose, they conducted Cerenkov light imaging of induced positron distribution in proton therapy.

METHODS

First, the authors evaluated the spatial resolution of our Cerenkov light imaging system with a (22)Na point source for the actual imaging setup. Then the transparent acrylic phantoms (100 × 100 × 100 mm(3)) were irradiated with two different proton energies using a spot scanning proton therapy system. Cerenkov light imaging of each phantom was conducted using a high sensitivity electron multiplied charge coupled device (EM-CCD) camera.

RESULTS

The Cerenkov light's spatial resolution for the setup was 0.76 ± 0.6 mm FWHM. They obtained high resolution Cerenkov light images of the positron distributions in the phantoms for two different proton energies and made fused images of the reference images and the Cerenkov light images. The depths of the positron distribution in the phantoms from the Cerenkov light images were almost identical to the simulation results. The decay curves derived from the region-of-interests (ROIs) set on the Cerenkov light images revealed that Cerenkov light images can be used for estimating the half-life of the radionuclide components of positrons.

CONCLUSIONS

High resolution Cerenkov light imaging of proton-induced positron distribution was possible. The authors conclude that Cerenkov light imaging of proton-induced positron is promising for proton therapy.

Cerenkov imaging

Cerenkov luminescence (CL) has been used recently in a plethora of medical applications like imaging and therapy with clinically relevant medical isotopes. The range of medical isotopes used is fairly large and expanding. The generation of in vivo light is useful since it circumvents depth limitations for excitation light. Cerenkov luminescence imaging (CLI) is much cheaper in terms of infrastructure than positron emission tomography (PET) and is particularly useful for imaging of superficial structures. Imaging can basically be done using a sensitive camera optimized for low-light conditions, and it has a better resolution than any other nuclear imaging modality. CLI has been shown to effectively diagnose disease with regularly used PET isotope ((18)F-FDG) in clinical setting. Cerenkov luminescence tomography, Cerenkov luminescence endoscopy, and intraoperative Cerenkov imaging have also been explored with positive conclusions expanding the current range of applications. Cerenkov has also been used to improve PET imaging resolution since the source of both is the radioisotope being used. Smart imaging agents have been designed based on modulation of the Cerenkov signal using small molecules and nanoparticles giving better insight of the tumor biology.

Unified approach for bioluminescence, Cerenkov, β, X and γ rays imaging

The goal of this work is to demonstrate that a CCD-based system can be used as a unified device which allows visible, β, X and γ rays imaging. A system composed of a CCD coupled with lens mounted on a black light-tight box and a high resolution intensifying screen for the radiations conversion were used. In order to investigate the detection of different type of radiations in vitro and in vivo experiments were performed. The comparison of the results obtained with our prototype and those obtained with dedicated commercial devices showed a good agreement.

Feasibility study of novel endoscopic Cerenkov luminescence imaging system in detecting and quantifying gastrointestinal disease: first human results

OBJECTIVES

Cerenkov luminescence imaging (CLI) provides potential to use clinical radiotracers for optical imaging. The goal of this study was to present a newly developed endoscopic CLI (ECLI) system and illustrate its feasibility and potential in distinguishing and quantifying cancerous lesions of the GI tract.

METHODS

The ECLI system was established by integrating an electron-multiplying charge-coupled device camera with a flexible fibre endoscope. Phantom experiments and animal studies were conducted to test and illustrate the system in detecting and quantifying the presence of radionuclide in vitro and in vivo. A pilot clinical study was performed to evaluate our system in clinical settings.

RESULTS

Phantom and mice experiments demonstrated its ability to acquire both the luminescent and photographic images with high accuracy. Linear quantitative relationships were also obtained when comparing the ECLI radiance with the radiotracer activity (r (2) = 0.9779) and traditional CLI values (r (2) = 0.9025). Imaging of patients revealed the potential of ECLI in the identification and quantification of cancerous tissue from normal, which showed good consistence with the clinical PET examination.

CONCLUSIONS

The new ECLI system shows good consistence with the clinical PET examination and has great potential for clinical translation and in aiding detection of the GI tract disease.

KEY POINTS

• CLI preserves the characteristics of both optical and radionuclide imaging. • CLI provides great potential for clinical translation of optical imaging. • The newly developed endoscopic CLI (ECLI) has quantification and imaging capacities. • GI tract has accessible open surfaces, making ECLI a potentially suitable technique. • Cerenkov endoscopy has great clinical potential in detecting GI disease.

Novel biomedical applications of Cerenkov radiation and radioluminescence imaging

The main goals of this review is to provide an up-to-date account of the different uses of Cerenkov radiation (CR) and radioluminescence imaging for pre-clinical small animal imaging. We will focus on new emerging applications such as the use of Cerenkov imaging for monitoring radionuclide and external radiotherapy in humans. Another novel application that will be described is the monitoring of radiochemical synthesis using microfluidic chips. Several pre-clinical aspects of CR will be discussed such as the development of 3D reconstruction methods for Cerenkov images and the use of CR as excitation source for nanoparticles or for endoscopic imaging. We will also include a discussion on radioluminescence imaging that is a more general method than Cerenkov imaging for the detection using optical methods of alpha and gamma emitters.

Performance evaluation of endoscopic Cerenkov luminescence imaging system: in vitro and pseudotumor studies

By integrating the clinically used endoscope with the emerging Cerenkov luminescence imaging (CLI) technology, a new endoscopic Cerenkov luminescence imaging (ECLI) system was developed. The aim is to demonstrate the potential of translating CLI to clinical studies of gastrointestinal (GI) tract diseases. We systematically evaluated the feasibility and performance of the developed ECLI system with a series of in vitro and pseudotumor experiments. The ECLI system is comprised of an electron multiplying charge coupled device (EMCCD) camera coupled with a clinically used endoscope via an optical adapter. A 1951-USAF test board was used to measure the white-light lateral resolution, while a homemade test chart filled with (68)Ga was employed to measure the CL lateral resolution. Both in vitro and pseudotumor experiments were conducted to obtain the sensitivity of the ECLI system. The results were validated with that of CLI using EMCCD only, and the relative attenuation ratio of the ECLI system was calculated. Results showed that The white-light lateral resolution of the ECLI system was 198 µm, and the luminescent lateral resolution was better than 1 mm. Sensitivity experiments showed a theoretical sensitivity of [Formula: see text] ([Formula: see text]) and [Formula: see text] ([Formula: see text]) for the in vitro and pseudotumor studies, respectively. The relative attenuation ratio of ECLI to CLI was about 96%. The luminescent lateral resolution of the ECLI system was comparable with that of positron emission tomography (PET). The pseudotumor study illustrated the feasibility and applicability of the ECLI system in living organisms, indicating the potential for translating the CLI technology to the clinic.

Ultrahigh-resolution Cerenkov-light imaging system for positron radionuclides: potential applications and limitations

Objective

Cerenkov-light imaging provides inherently high resolution because the light is emitted near the positron radionuclide. However, the magnitude for the high spatial resolution of Cerenkov-light imaging is unclear. Its potential molecular imaging applications also remain unclear. We developed an ultrahigh-resolution Cerenkov-light imaging system, measured its spatial resolution, and explored its applications to molecular imaging research.

Methods

Our Cerenkov-light imaging system consists of a high-sensitivity charged-coupled device camera (Hamamatsu Photonics ORCA2-ER) and a bright lens (Xenon 0.95/25). An extension ring was inserted between them to magnify the subject. A ~100-μm-diameter ²²Na point source was made and imaged by the system. For applications of Cerenkov-light imaging, we conducted ¹⁸F-FDG administered in vivo, ex vivo whole brain, and sliced brain imaging of rats.

Results

We obtained spatial resolution of ~220 μm for a ²²Na point source with our developed imaging system. The ¹⁸F-FDG rat head images showed high light intensity in the eyes for the Cerenkov-light images, although there was no accumulation in these parts in the PET images. The sliced rat brain showed much higher spatial resolution for the Cerenkov-light images compared with CdWO₄ scintillator-based autoradiography, although some contrast decrease was observed for them.

Conclusion

Even though the Cerenkov-light images showed ultrahigh resolution of ~220 μm, their distribution and contrast were sometimes different from the actual positron accumulation in the subjects. Care must be taken when evaluating positron distribution from Cerenkov-light images. However, the ultrahigh resolution of Cerenkov-light imaging will be useful for transparent subjects including phantom studies.

Probability method for Cerenkov luminescence tomography based on conformance error minimization

Cerenkov luminescence tomography (CLT) was developed to reconstruct a three-dimensional (3D) distribution of radioactive probes inside a living animal. Reconstruction methods are generally performed within a unique framework by searching for the optimum solution. However, the ill-posed aspect of the inverse problem usually results in the reconstruction being non-robust. In addition, the reconstructed result may not match reality since the difference between the highest and lowest uptakes of the resulting radiotracers may be considerably large, therefore the biological significance is lost. In this paper, based on the minimization of a conformance error, a probability method is proposed that consists of qualitative and quantitative modules. The proposed method first pinpoints the organ that contains the light source. Next, we developed a 0-1 linear optimization subject to a space constraint to model the CLT inverse problem, which was transformed into a forward problem by employing a region growing method to solve the optimization. After running through all of the elements used to grow the sources, a source sequence was obtained. Finally, the probability of each discrete node being the light source inside the organ was reconstructed. One numerical study and two in vivo experiments were conducted to verify the performance of the proposed algorithm, and comparisons were carried out using the hp-finite element method (hp-FEM). The results suggested that our proposed probability method was more robust and reasonable than hp-FEM.

Imaging of luminescence induced by beta and gamma emitters in conventional non-scintillating materials

Detecting radioluminescence in tissues in the optical and near infrared regions of the electromagnetic spectrum has recently emerged as a new research field for preclinical bioluminescent imaging.

Enhancement of Cerenkov luminescence imaging by dual excitation of Er(3+),Yb(3+)-doped rare-earth microparticles

Cerenkov luminescence imaging (CLI) has been successfully utilized in various fields of preclinical studies; however, CLI is challenging due to its weak luminescent intensity and insufficient penetration capability. Here, we report the design and synthesis of a type of rare-earth microparticles (REMPs), which can be dually excited by Cerenkov luminescence (CL) resulting from the decay of radionuclides to enhance CLI in terms of intensity and penetration.

METHODS

Yb(3+)- and Er(3+)- codoped hexagonal NaYF4 hollow microtubes were synthesized via a hydrothermal route. The phase, morphology, and emission spectrum were confirmed for these REMPs by power X-ray diffraction (XRD), scanning electron microscopy (SEM), and spectrophotometry, respectively. A commercial CCD camera equipped with a series of optical filters was employed to quantify the intensity and spectrum of CLI from radionuclides. The enhancement of penetration was investigated by imaging studies of nylon phantoms and nude mouse pseudotumor models.

RESULTS

the REMPs could be dually excited by CL at the wavelengths of 520 and 980 nm, and the emission peaks overlaid at 660 nm. This strategy approximately doubled the overall detectable intensity of CLI and extended its maximum penetration in nylon phantoms from 5 to 15 mm. The penetration study in living animals yielded similar results.

CONCLUSIONS

this study demonstrated that CL can dually excite REMPs and that the overlaid emissions in the range of 660 nm could significantly enhance the penetration and intensity of CL. The proposed enhanced CLI strategy may have promising applications in the future.

Optimization of microfluidic PET tracer synthesis with Cerenkov imaging

Microfluidic technologies provide an attractive platform for the synthesis of radiolabeled compounds. Visualization of radioisotopes on chip is critical for synthesis optimization and technological development. With Cerenkov imaging, beta particle emitting isotopes can be localized with a sensitive CCD camera. In order for Cerenkov imaging to also serve as a quantitative tool, it is necessary to understand how material properties relevant to Cerenkov emission, namely, index of refraction and beta particle stopping power, affect Cerenkov light output. In this report, we investigate the fundamental physical characteristics of Cerenkov photon yield at different stages of [(18)F]FDG synthesis on the electrowetting on dielectric (EWOD) microfluidic platform. We also demonstrate how Cerenkov imaging has enabled synthesis optimization. Geant4, a Monte Carlo program applied extensively in high energy physics, is used to simulate Cerenkov photon yield from (18)F beta particles traversing materials of interest during [(18)F]FDG synthesis on chip. Our simulations show that the majority (approximately two-thirds) of the (18)F beta particle energy available to produce Cerenkov photons is deposited on the glass plates of the EWOD chip. This result suggests the possibility of using a single calibration factor to convert Cerenkov signal to radioactivity, independent of droplet composition. We validate our simulations with a controlled measurement examining varying ratios of [(18)O]H2O, dimethyl sulfoxide (DMSO), and acetonitrile (MeCN), and find a consistent calibration independent of solvent composition. However, the calibration factor may underestimate the radioactivity in actual synthesis due to discoloration of the droplet during certain steps of probe synthesis. In addition to the attractive quantitative potential of Cerenkov imaging, this imaging strategy provides indispensable qualitative data to guide synthesis optimization. We are able to use this imaging technique to optimize the mixing protocol as well as identify and correct for loss of radioactivity due to the migration of radioactive vapor outside of the EWOD heater, enabling an overall increase in the crude radiochemical yield from 50 ± 3% (n = 3) to 72 ± 13% (n = 5).

Cerenkov Luminescence Imaging (CLI) for cancer therapy monitoring

In molecular imaging, positron emission tomography (PET) and optical imaging (OI) are two of the most important and thus most widely used modalities. PET is characterized by its excellent sensitivity and quantification ability while OI is notable for non-radiation, relative low cost, short scanning time, high throughput, and wide availability to basic researchers. However, both modalities have their shortcomings as well. PET suffers from poor spatial resolution and high cost, while OI is mostly limited to preclinical applications because of its limited tissue penetration along with prominent scattering optical signals through the thickness of living tissues. Recently a bridge between PET and OI has emerged with the discovery of Cerenkov Luminescence Imaging (CLI). CLI is a new imaging modality that harnesses Cerenkov Radiation (CR) to image radionuclides with OI instruments. Russian Nobel laureate Alekseyevich Cerenkov and his colleagues originally discovered CR in 1934. It is a form of electromagnetic radiation emitted when a charged particle travels at a superluminal speed in a dielectric medium. The charged particle, whether positron or electron, perturbs the electromagnetic field of the medium by displacing the electrons in its atoms. After passing of the disruption photons are emitted as the displaced electrons return to the ground state. For instance, one (18)F decay was estimated to produce an average of 3 photons in water. Since its emergence, CLI has been investigated for its use in a variety of preclinical applications including in vivo tumor imaging, reporter gene imaging, radiotracer development, multimodality imaging, among others. The most important reason why CLI has enjoyed much success so far is that this new technology takes advantage of the low cost and wide availability of OI to image radionuclides, which used to be imaged only by more expensive and less available nuclear imaging modalities such as PET. Here, we present the method of using CLI to monitor cancer drug therapy. Our group has recently investigated this new application and validated its feasibility by a proof-of-concept study. We demonstrated that CLI and PET exhibited excellent correlations across different tumor xenografts and imaging probes. This is consistent with the overarching principle of CR that CLI essentially visualizes the same radionuclides as PET. We selected Bevacizumab (Avastin; Genentech/Roche) as our therapeutic agent because it is a well-known angiogenesis inhibitor. Maturation of this technology in the near future can be envisioned to have a significant impact on preclinical drug development, screening, as well as therapy monitoring of patients receiving treatments.

Čerenkov radiation emission and excited luminescence (CREL) sensitivity during external beam radiation therapy: Monte Carlo and tissue oxygenation phantom studies

Radiotherapy generates Čerenkov radiation emission in tissue, and spectral absorption features appearing in the emission spectrum can be used to quantify blood oxygen saturation (S(t)O(2)) from the known absorptions of hemoglobin. Additionally, the Čerenkov light can be used to excite oxygen-sensitive phosphorescence of probe PtG4, whose emission lifetime directly reports on tissue oxygen partial pressure (pO(2)). Thus, it is feasible to probe both hemoglobin S(t)O(2) and pO(2) using external radiation therapy beam to create as an internal light source in tumor tissue. In this study, the sensitivity and spatial origins of these two signals were examined. Emission was detected using a fiber-optic coupled intensifier-gated CCD camera interfaced to a spectrometer. The phosphorescence lifetimes were quantified and compared with S(t)O(2) changes previously measured. Monte Carlo simulations of the linear accelerator beam were used together with tracking of the optical signals, to predict the spatial distribution and zone sensitivity within the phantom. As the fiber-to-beam distance (FBD) varied from 0 to 30 mm, i.e. the distance from the fiber tip to the nearest side of the radiotherapy beam, the effective sampling depth for CR emission changed from 4 to 29 mm for the wavelengths in the range of 600-1000 nm. For the secondary emission (phosphorescence) the effective sampling depth was determined to be in the range of 9 to 19 mm. These results indicate that sampling of S(t)O(2) and pO(2) in tissue should be feasible during radiation therapy, and that the radiation beam and fiber sampling geometry can be set up to acquire signals that originate as deep as a few centimeters in the tissue.

Intraoperative imaging of tumors using Cerenkov luminescence endoscopy: a feasibility experimental study

Cerenkov luminescence imaging (CLI) is an emerging new molecular imaging modality that is relatively inexpensive, easy to use, and has high throughput. CLI can image clinically available PET and SPECT probes using optical instrumentation. Cerenkov luminescence endoscopy (CLE) is one of the most intriguing applications that promise potential clinical translation. We developed a prototype customized fiberscopic Cerenkov imaging system to investigate the potential in guiding minimally invasive surgical resection.

METHODS

All experiments were performed in a dark chamber. Cerenkov luminescence from (18)F-FDG samples containing decaying radioactivity was transmitted through an optical fiber bundle and imaged by an intensified charge-coupled device camera. Phantoms filled with (18)F-FDG were used to assess the imaging spatial resolution. Finally, mice bearing subcutaneous C6 glioma cells were injected intravenously with (18)F-FDG to determine the feasibility of in vivo imaging. The tumor tissues were exposed, and CLI was performed on the mouse before and after surgical removal of the tumor using the fiber-based imaging system and compared with a commercial optical imaging system.

RESULTS

The sensitivity of this particular setup was approximately 45 kBq (1.21 μCi)/300 μL. The 3 smallest sets of cylindric holes in a commercial SPECT phantom were identifiable via this system, demonstrating that the system has a resolution better than 1.2 mm. Finally, the in vivo tumor imaging study demonstrated the feasibility of using CLI to guide the resection of tumor tissues.

CONCLUSION

This proof-of-concept study explored the feasibility of using fiber-based CLE for the detection of tumor tissue in vivo for guided surgery. With further improvements of the imaging sensitivity and spatial resolution of the current system, CLE may have a significant application in the clinical setting in the near future.

Three-dimensional noninvasive monitoring iodine-131 uptake in the thyroid using a modified Cerenkov luminescence tomography approach

Background

Cerenkov luminescence tomography (CLT) provides the three-dimensional (3D) radiopharmaceutical biodistribution in small living animals, which is vital to biomedical imaging. However, existing single-spectral and multispectral methods are not very efficient and effective at reconstructing the distribution of the radionuclide tracer. In this paper, we present a semi-quantitative Cerenkov radiation spectral characteristic-based source reconstruction method named the hybrid spectral CLT, to efficiently reconstruct the radionuclide tracer with both encouraging reconstruction results and less acquisition and image reconstruction time.

Methodology/Principal Findings

We constructed the implantation mouse model implanted with a 400 µCi Na¹³¹I radioactive source and the physiological mouse model received an intravenous tail injection of 400 µCi radiopharmaceutical Iodine-131 (I-131) to validate the performance of the hybrid spectral CLT and compared the reconstruction results, acquisition, and image reconstruction time with that of single-spectral and multispectral CLT. Furthermore, we performed 3D noninvasive monitoring of I-131 uptake in the thyroid and quantified I-131 uptake in vivo using hybrid spectral CLT. Results showed that the reconstruction based on the hybrid spectral CLT was more accurate in localization and quantification than using single-spectral CLT, and was more efficient in the in vivo experiment compared with multispectral CLT. Additionally, 3D visualization of longitudinal observations suggested that the reconstructed energy of I-131 uptake in the thyroid increased with acquisition time and there was a robust correlation between the reconstructed energy versus the gamma ray counts of I-131 (). The ex vivo biodistribution experiment further confirmed the I-131 uptake in the thyroid for hybrid spectral CLT.

Conclusions/Significance

Results indicated that hybrid spectral CLT could be potentially used for thyroid imaging to evaluate its function and monitor its treatment for thyroid cancer.

Optimizing in vivo small animal Cerenkov luminescence imaging

In vivo Cerenkov luminescence imaging is a rapidly growing molecular imaging research field based on the detection of Cerenkov radiation induced by beta particles when traveling though biological tissues. We investigated theoretically the possibility of enhancing the number of the detected Cerenkov photons in the near infrared (NIR) region of the spectrum. The analysis is based on applying a photon propagation diffusion model to Cerenkov photons in the tissue. Results show that despite the smaller number of Cerenkov photons in the NIR region, the fraction exiting the tissues is greater than in the visible range, and thus, a charge-coupled device detector optimized for the NIR range will allow to obtain a higher signal. The comparison was performed considering Cerenkov point sources located at different depths inside the animal. We concluded that the improvement can be up to 35% and is more significant when the Cerenkov source to be imaged is located deeper inside the animal.

Proof-of-concept study of monitoring cancer drug therapy with cerenkov luminescence imaging

Cerenkov luminescence imaging (CLI) has emerged as a less expensive, easier-to-use, and higher-throughput alternative to other nuclear imaging modalities such as PET. It is expected that CLI will find many applications in biomedical research such as cancer detection, probe development, drug screening, and therapy monitoring. In this study, we explored the possibility of using CLI to monitor drug efficacy by comparisons against PET. To assess the performance of both modalities in therapy monitoring, 2 murine tumor models (large cell lung cancer cell line H460 and prostate cancer cell line PC3) were given bevacizumab versus vehicle treatments. Two common radiotracers, 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) and (18)F-FDG, were used to monitor bevacizumab treatment efficacy.

METHODS

One group of mice (n = 6) was implanted with H460 xenografts bilaterally in the shoulder region, divided into treatment and control groups (n = 3 each), injected with (18)F-FLT, and imaged with PET immediately followed by CLI. The other group of mice (n = 6) was implanted with PC3 xenografts in the same locations, divided into treatment and control groups (n = 3 each), injected with (18)F-FDG, and imaged by the same modalities. Bevacizumab treatment was performed by 2 injections of 20 mg/kg at days 0 and 2.

RESULTS

On (18)F-FLT scans, both CLI and PET revealed significantly decreased signals from H460 xenografts in treated mice from pretreatment to day 3. Moderately increased to unchanged signals were observed in untreated mice. On (18)F-FDG scans, both CLI and PET showed relatively unchanged signals from PC3 tumors in both treated and control groups. Quantifications of tumor signals of Cerenkov luminescence and PET images showed that the 2 modalities had excellent correlations (R(2) > 0.88 across all study groups).

CONCLUSION

CLI and PET exhibit excellent correlations across different tumor xenografts and radiotracers. This is the first study, to our knowledge, demonstrating the use of CLI for monitoring cancer treatment. The findings warrant further exploration and optimization of CLI as an alternative to PET in preclinical therapeutic monitoring and drug screening.

Unsupervised analysis of small animal dynamic Cerenkov luminescence imaging

Clustering analysis (CA) and principal component analysis (PCA) were applied to dynamic Cerenkov luminescence images (dCLI). In order to investigate the performances of the proposed approaches, two distinct dynamic data sets obtained by injecting mice with (32)P-ATP and (18)F-FDG were acquired using the IVIS 200 optical imager. The k-means clustering algorithm has been applied to dCLI and was implemented using interactive data language 8.1. We show that cluster analysis allows us to obtain good agreement between the clustered and the corresponding emission regions like the bladder, the liver, and the tumor. We also show a good correspondence between the time activity curves of the different regions obtained by using CA and manual region of interest analysis on dCLIT and PCA images. We conclude that CA provides an automatic unsupervised method for the analysis of preclinical dynamic Cerenkov luminescence image data.

Optical imaging of alpha emitters: simulations, phantom, and in vivo results

There has been growing interest in investigating both the in vitro and in vivo detection of optical photons from a plethora of beta emitters using optical techniques. In this paper we have investigated an alpha particle induced fluorescence signal by using a commercial CCD-based small animal optical imaging system. The light emission of a (241)Am source was simulated using GEANT4 and tested in different experimental conditions including the imaging of in vivo tissue. We believe that the results presented in this work can be useful to describe a possible mechanism for the in vivo detection of alpha emitters used for therapeutic purposes.

In vivo Cerenkov luminescence imaging: a new tool for molecular imaging

Cerenkov radiation is a phenomenon where optical photons are emitted when a charged particle moves faster than the speed of light for the medium in which it travels. Recently, we and others have discovered that measurable visible light due to the Cerenkov effect is produced in vivo following the administration of β-emitting radionuclides to small animals. Furthermore, the amounts of injected activity required to produce a detectable signal are consistent with small-animal molecular imaging applications. This surprising observation has led to the development of a new hybrid molecular imaging modality known as Cerenkov luminescence imaging (CLI), which allows the spatial distribution of biomolecules labelled with β-emitting radionuclides to be imaged in vivo using sensitive charge-coupled device cameras. We review the physics of Cerenkov radiation as it relates to molecular imaging, present simulation results for light intensity and spatial distribution, and show an example of CLI in a mouse cancer model. CLI allows many common radiotracers to be imaged in widely available in vivo optical imaging systems, and, more importantly, provides a pathway for directly imaging β(-)-emitting radionuclides that are being developed for therapeutic applications in cancer and that are not readily imaged by existing methods.

Harnessing the power of radionuclides for optical imaging: Cerenkov luminescence imaging

Over the past several years, nuclear imaging modalities such as PET and SPECT have received much attention because they have been instrumental not only in preclinical cancer research but also in nuclear medicine. Yet nuclear imaging is limited by high instrumentation cost and subsequently low availability to basic researchers. Cerenkov radiation, a relativistic physical phenomenon that was discovered 70 years ago, has recently become an intriguing subject of study in molecular imaging because of its potential in augmenting nuclear imaging, particularly in preclinical small-animal studies. The intrinsic capability of radionuclides emitting luminescent light from decay is promising because of the possibility of bridging nuclear imaging with optical imaging-a modality that is much less expensive, is easier to use, and has higher throughput than its nuclear counterpart. Thus, with the maturation of this novel imaging technology using Cerenkov radiation, which is termed Cerenkov luminescence imaging, it is foreseeable that advances in both nuclear imaging and preclinical research involving radioisotopes will be significantly accelerated in the near future.