Cherenkov Light Emission in Pure Cherenkov Emitters for Prompt Gamma Imaging

Proton range verification (PRV) in proton therapy by means of prompt-gamma detection is a promising but challenging approach. High count rates, energies ranging between 1 MeV and 7 MeV, and a strong background complicate the detection of such particles. In this work, the Cherenkov light generated by prompt-gammas in the pure Cherenkov emitters TlBr, TlCl and PbF2 was studied. Cherenkov light in these crystals can provide a very fast timing signal with the potential to achieve very high count rates and to discern between prompt-gammas and background signals. Crystals of 1×1 cm2 and thicknesses of 1 cm, 2 cm, 3 cm and 4 cm were simulated. Different photodetector configurations were studied for 2.3 MeV, 4.4 MeV, and 6.1 MeV prompt-gammas. TlCl achieved the greatest number of detected Cherenkov photons for all energies, detector dimensions, and photodetector efficiency modeling. For the highest prompt-gamma energy simulated, TlCl yielded approximately 250 Cherenkov detected photons, using a hypothetical high-performance photodetector. Results show the crystal blocks of 1 cm × 1 cm × 1 cm have greater prompt-gamma detection efficiency per volume and a comparable average number of detected Cherenkov photons per event.

Efficient and multiplexed tracking of single cells using whole-body PET/CT

In vivo molecular imaging tools are crucially important for elucidating how cells move through complex biological systems, however, achieving single-cell sensitivity over the entire body remains challenging. Here, we report a highly sensitive and multiplexed approach for tracking upwards of 20 single cells simultaneously in the same subject using positron emission tomography (PET). The method relies on a new tracking algorithm (PEPT-EM) to push the cellular detection threshold to below 4 Bq/cell, and a streamlined workflow to reliably label single cells with over 50 Bq/cell of 18F-fluorodeoxyglucose (FDG). To demonstrate the potential of method, we tracked the fate of over 70 melanoma cells after intracardiac injection and found they primarily arrested in the small capillaries of the pulmonary, musculoskeletal, and digestive organ systems. This study bolsters the evolving potential of PET in offering unmatched insights into the earliest phases of cell trafficking in physiological and pathological processes and in cell-based therapies.

The SNR of Positron Emission Data With Gaussian and Non-Gaussian Time-of-Flight Kernels, With Application to Prompt Photon Coincidence

It is well known that measurement of the time-of-flight (TOF) increases the information provided by coincident events in positron emission tomography (PET). This information increase propagates through the reconstruction and improves the signal-to-noise ratio in the reconstructed images. Takehiro Tomitani has analytically computed the gain in variance in the reconstructed image, provided by a particular TOF resolution, for the center of a uniform disk and for a Gaussian TOF kernel. In this paper we extend this result, by computing the signal-to-noise ratio (SNR) contributed by individual coincidence events for two different tasks. One task is the detection of a hot spot in the center of a uniform cylinder. The second one is the same as that considered by Tomitani, i.e. the reconstruction of the central voxel in the image of a uniform cylinder. In addition, we extend the computation to non-Gaussian TOF kernels. It is found that a modification of the TOF-kernel changes the SNR for both tasks in almost exactly the same way. The proposed method can be used to compare TOF-systems with different and possibly event-dependent TOF-kernels, as encountered when prompt photons, such as Cherenkov photons are present, or when the detector is composed of different scintillators. The method is validated with simple 2D simulations and illustrated by applying it to PET detectors producing optical photons with event-dependent timing characteristics.

Potential of Depth-of-Interaction-Based Detection Time Correction in Cherenkov Emitter Crystals for TOF-PET

Cherenkov light can improve the timing resolution of Positron Emission Tomography (PET) radiation detectors, thanks to its prompt emission. Coincidence time resolutions (CTR) of ~30 ps were recently reported when using 3.2 mm-thick Cherenkov emitters. However, sufficient detection efficiency requires thicker crystals, causing the timing resolution to be degraded by the optical propagation inside the crystal. We report on depth-of-interaction (DOI) correction to mitigate the time-jitter due to the photon time spread in Cherenkov-based radiation detectors. We simulated the Cherenkov and scintillation light generation and propagation in 3 × 3 mm2 lead fluoride, lutetium oxyorthosilicate, bismuth germanate, thallium chloride, and thallium bromide. Crystal thicknesses varied from 9 to 18 mm with a 3-mm step. A DOI-based time correction showed a 2-to-2.5-fold reduction of the photon time spread across all materials and thicknesses. Results showed that highly refractive crystals, though producing more Cherenkov photons, were limited by an experimentally obtained high-cutoff wavelength and refractive index, restricting the propagation and extraction of Cherenkov photons mainly emitted at shorter wavelengths. Correcting the detection time using DOI information shows a high potential to mitigate the photon time spread. These simulations highlight the complexity of Cherenkov-based detectors and the competing factors in improving timing resolution.

Preclinical evaluation of 68Ga- and 177Lu-labeled integrin αvβ6-targeting radiotheranostic peptides

Rationale: The integrin α_vβ₆, an epithelial-specific cell surface receptor, is overexpressed on numerous malignancies, including the highly lethal pancreatic ductal adenocarcinomas (PDAC). Here, we developed and tested a novel α_vβ₆-targeting peptide, DOTA-5G (1) radiolabeled with gallium-68 for positron emission tomography/computed tomography (PET/CT) imaging, and lutetium-177 for treatment. With the goal to develop a radiotheranostic, further modifications were made for increased circulation time, renal recycling, and tumor uptake, yielding DOTA-ABM-5G (2). Methods: Peptides 1 and 2 were synthesized on solid phase and their affinity for αvβ6 assessed by ELISA. The peptides were radiolabeled with gallium-68 and lutetium-177. In vitro cell binding, internalization, and efflux of ⁶⁸Ga-1 and ¹⁷⁷Lu-2 were evaluated in α_vβ₆ (+) BxPC-3 human pancreatic cancer cells. PET/CT imaging of ⁶⁸Ga-1 and ⁶⁸Ga-2 was performed in female nu/nu mice bearing subcutaneous BxPC-3 tumors. Biodistribution was performed for ⁶⁸Ga-1 (1 and 2 h p.i.), ⁶⁸Ga-2 (2 and 4 h p.i.), and ¹⁷⁷Lu-1 and ¹⁷⁷Lu-2 (1, 24, 48, and 72 h p.i.). The ¹⁷⁷Lu-2 biodistribution data were extrapolated for human dosimetry data estimates using OLINDA/EXM 1.1. Therapeutic efficacy of ¹⁷⁷Lu-2 was evaluated in mice bearing BxPC-3 tumors. Results: Peptides 1 and 2 demonstrated high affinity for α_vβ₆ by ELISA. ⁶⁸Ga-1, ⁶⁸Ga-2, ¹⁷⁷Lu-1 and ¹⁷⁷Lu-2 were synthesized in high radiochemical purity (RCP). Rapid in vitro binding and internalization of ⁶⁸Ga-1 and ¹⁷⁷Lu-2 were observed in BxPC-3 cells. PET/CT imaging and biodistribution studies demonstrated uptake in BxPC-3 tumors. Introduction of the ABM in ¹⁷⁷Lu-2 resulted in a 5-fold increase in tumor uptake and retention over time. Based on the extended dosimetry data the dose-limiting organ for ¹⁷⁷Lu-2 are the kidneys. Treatment with ¹⁷⁷Lu-2 prolonged survival. Conclusion: ⁶⁸Ga-1 and ¹⁷⁷Lu-2 demonstrated high affinity for the integrin α_vβ₆ both in vitro and in vivo, were rapidly internalized into BxPC-3 cells, and were stable in mouse and human serum. Both radiotracers showed favorable pharmacokinetics in pre-clinical studies with predominantly renal excretion and good tumor-to-normal tissue ratios. Favorable human dosimetry data suggest the potential of ¹⁷⁷Lu-2 as a treatment for PDAC.

The OpenGATE ecosystem for Monte Carlo simulation in medical physics

This paper reviews the ecosystem of GATE, an open-source Monte Carlo toolkit for medical physics. Based on the shoulders of Geant4, the principal modules (geometry, physics, scorers) are described with brief descriptions of some key concepts (Volume, Actors, Digitizer). The main source code repositories are detailed together with the automated compilation and tests processes (Continuous Integration). We then described how the OpenGATE collaboration managed the collaborative development of about one hundred developers during almost 20 years. The impact of GATE on medical physics and cancer research is then summarized, and examples of a few key applications are given. Finally, future development perspectives are indicated.

Radioembolization Dosimetry with Total-Body 90Y PET

Transarterial radioembolization (TARE) is a locoregional radiopharmaceutical therapy based on the delivery of radioactive ⁹⁰Y microspheres to liver tumors. The importance of personalized dosimetry to make TARE safer and more effective has been demonstrated in recent clinical studies, stressing the need for quantification of the dose-response relationship to ultimately optimize the administered activity before treatment and image it after treatment. ⁹⁰Y dosimetric studies are challenging because of the lack of accurate and precise methods but are best realized with PET combined with Monte Carlo simulations and other image modalities to calculate a segmental dose distribution. The aim of this study was to assess the suitability of imaging ⁹⁰Y PET patients with the total-body PET/CT uEXPLORER and to investigate possible improvements in TARE ⁹⁰Y PET-based dosimetry. The uEXPLORER is the first commercially available ultra-high-resolution (171 cps/kBq) total-body digital PET/CT device with a 194-cm axial PET field of view that enables the whole body to be scanned at a single bed position. Methods: Two PET/CT scanners were evaluated in this study: the Biograph mCT and the total-body uEXPLORER. Images of a National Electrical Manufacturers Association (NEMA) image-quality phantom and 2 patients were reconstructed using our standard clinical oncology protocol. A late portal phase contrast-enhanced CT scan was used to contour the liver segments and create corresponding volumes of interest. To calculate the absorbed dose, Monte Carlo simulations were performed using Geant4 Application for Tomographic Emission (GATE). The absorbed dose and dose-volume histograms were calculated for all 6 spheres (diameters ranging from 10 to 37 mm) of the NEMA phantom, the liver segments, and the entire liver. Differences between the phantom doses and an analytic ground truth were quantified through the root mean squared error. Results: The uEXPLORER showed a higher signal-to-noise ratio at 10- and 13-mm diameters, consistent with its high spatial resolution and system sensitivity. The total liver-absorbed dose showed excellent agreement between the uEXPLORER and the mCT for both patients, with differences lower than 0.2%. Larger differences of up to 60% were observed when comparing the liver segment doses. All dose-volume histograms were in good agreement, with narrower tails for the uEXPLORER in all segments, indicating lower image noise. Conclusion: This patient study is compelling for the use of total-body ⁹⁰Y PET for liver dosimetry. The uEXPLORER scanner showed a better signal-to-noise ratio than mCT, especially in lower-count regions of interest, which is expected to improve dose quantification and tumor dosimetry.

The Impact of Injection Distance to Bifurcations on Yttrium-90 Distribution in Liver Cancer Radioembolization

PURPOSE

To model the effect of the injection location on the distribution of yttrium-90 (⁹⁰Y) microspheres in the liver during radioembolization using computational simulation and to determine the potential effects of radial movements of the catheter tip.

MATERIALS AND METHODS

Numerical studies were conducted using images from a representative patient with hepatocellular carcinoma. The right hepatic artery (RHA) was segmented from contrast-enhanced cone-beam computed tomography scans. The blood flow was investigated in the trunk of the RHA using numerical simulations for 6 injection position scenarios at 2 sites located at a distance of approximately 5 and 20 mm upstream of the first bifurcation (RHA diameters of approximately 4.6 mm). The ⁹⁰Y delivery to downstream vessels was calculated from the simulated hepatic artery hemodynamics.

RESULTS

Varying the injection location along the RHA and across the vessel cross-section resulted in different simulated microsphere distributions in the downstream vascular bed. When the catheter tip was 5 mm upstream of the bifurcation, ⁹⁰Y distribution in the downstream branches varied by as much as 53% with a 1.5-mm radial movement of the tip. However, the catheter radial movement had a weaker effect on the microsphere distribution when the injection plane was farther from the first bifurcation (20 mm), with a maximum delivery variation of 9% to a downstream branch.

CONCLUSIONS

An injection location far from bifurcations is recommended to minimize the effect of radial movements of the catheter tip on the microsphere distribution.

The Accuracy of Cerenkov Photons Simulation in Geant4/Gate Depends on the Parameterization of Primary Electron Propagation

Energetic electrons traveling in a dispersive medium can produce Cerenkov radiation. Cerenkov photons' prompt emission, combined with their predominantly forward emission direction with respect to the parent electron, makes them extremely promising to improve radiation detector timing resolution. Triggering gamma detections based on Cerenkov photons to achieve superior timing resolution is challenging due to the low number of photons produced per interaction. Monte Carlo simulations are fundamental to understanding their behavior and optimizing their pathway to detection. Therefore, accurately modeling the electron propagation and Cerenkov photons emission is crucial for reliable simulation results. In this work, we investigated the physics characteristics of the primary electrons (velocity, energy) and those of all emitted Cerenkov photons (spatial and timing distributions) generated by 511 keV photoelectric interactions in a bismuth germanate crystal using simulations with Geant4/GATE. Geant4 uses a stepwise particle tracking approach, and users can limit the electron velocity change per step. Without limiting it (default Geant4 settings), an electron mean step length of ~250 μm was obtained, providing only macroscopic modeling of electron transport, with all Cerenkov photons emitted in the forward direction with respect to the incident gamma direction. Limiting the electron velocity change per step reduced the electron mean step length (~0.200 μm), leading to a microscopic approach to its transport which more accurately modeled the electron physical properties in BGO at 511 keV. The electron and Cerenkov photons rapidly lost directionality, affecting Cerenkov photons' transport and, ultimately, their detection. Results suggested that a deep understanding of low energy physics is crucial to perform accurate optical Monte Carlo simulations and ultimately use them in TOF PET detectors.

Integration of polarization in the LUTDavis model for optical Monte Carlo simulation in radiation detectors

OBJECTIVE

Cerenkov photons have distinctive features from scintillation photons. Among them is their polarization: their electric field is always perpendicular to the direction of propagation of light and parallel to the plane of incidence. Scintillation photons are instead considered unpolarized.

APPROACH

This study aims at understanding and optimizing the reflectance of polarized Cerenkov photons for optical Monte Carlo simulation of scintillation detectors with Geant4/GATE. First, the Cerenkov emission spectrum and polarization were implemented in the previously developed look-up-table Davis model of crystal reflectance. Next, we modified Geant4/GATE source code to account for scintillation and Cerenkov photons LUTs simultaneously. Then, we performed optical Monte Carlo simulations in BGO using GATE to show the effect of Cerenkov features on the photons' momentum at the photodetector face, using two surface finishes, with and without reflector.

MAIN RESULTS

In this work, we describe the new features added to the algorithm and GATE. We showed that Cerenkov characteristics affect their probability to be reflected/refracted and thus their travel path within a material.

SIGNIFICANCE

We showed the importance of accounting for accurate Cerenkov photons reflectance while performing advanced optical Monte Carlo simulations.

Study of Čerenkov Light Emission in the Semiconductors TlBr and TlCl for TOF-PET

Thallium bromide (TlBr) and thallium chloride (TlCl) are semiconductor materials with high transparency to visible light, high index of refraction, and high detection efficiency for gamma rays and annihilation photons. This manuscript reports on measurements of the light intensity and timing response of Čerenkov light emitted in one 3 mm × 3 mm × 5 mm slab of each of these materials operated in coincidence with a lutetium fine silicate (LFS) crystal with dimensions of 3 mm × 3 mm × 20 mm. A 22Na radioactive source was used. The measured average number of detected photons per event was 1.5 photons for TlBr and 2.8 photons for TlCl when these materials were coupled to a silicon photomultiplier. Simulation predicts these results with an overestimation of 12%. The best coincidence time resolution (CTR) for events in TlBr and TlCl were 329 ± 9 ps and 316 ± 9 ps, respectively, when events with 4 photons and >7 photons were selected. Simulation showed the CTR degraded from 120 ps to 405 ps in TlCl, and from 160 ps to 700 ps in TlBr when the first or second Čerenkov photon were selected. Results of this work show TlCl has a stronger Čerenkov light emission compared to TlBr and a greater potential to obtain the best timing measurements. Results also stress the importance of improving detection efficiency and transport of light to capture the first Čerenkov photon in timing measurements.

Optimization of scintillator-reflector optical interfaces for the LUT Davis model

PURPOSE

Designing and optimizing scintillator-based gamma detector using Monte Carlo simulation is of great importance in nuclear medicine and high energy physics. In scintillation detectors, understanding the light transport in the scintillator and the light collection by the photodetector plays a crucial role in achieving high performance. Thus, accurately modeling them is critical.

METHODS

In previous works, we developed a model to compute crystal reflectance from the crystal 3D surface measurement and store it in look-up tables to be used in the Monte Carlo simulation software GATE. The relative light output comparison showed excellent agreement between simulations and experiments for both polished and rough surfaces in several configurations, that is, without and with reflector. However, when comparing them at the irradiation depth closest to the photodetector face, rough crystals with a reflector overestimated the predicted light output. Investigating the cause of this overestimation, we optimized the LUT algorithm to improve the reflectance computation accuracy, especially for rough surfaces. However, optical Monte Carlo simulations carried out with these newly generated LUTs still overestimate the light output. Based on previous observations, one probable cause is the erroneous assumption of perfect couplings between the reflector and crystal and between the crystal and photodetector, which likely results in an important overestimation of the light output compared to experimental values. In practice, several factors could degrade it. Here, we investigated possible suboptimal optical experimental configurations that could lead to a degraded light collection when using Teflon or ESR reflectors coupled to the crystal with air or grease. We generated look-up tables with a mixture of air and grease and showed the effect of three possible sources of light loss: the presence of a small gap between the crystal and the reflector edges close to the photodetector face, the infiltration of grease in the crystal-reflector coupling, and the presence of inhomogeneities in the photodetector-crystal interface.

RESULTS

The strongest effect is linked to the presence of a small gap of grease between the edges of the reflector material and the crystal (light loss of 10%-12% for 0.2 mm gap). The optical grease infiltrating the crystal-reflector air coupling decreases the light output, depending on the infiltration's extent and the amount of grease infiltrated. Five percent of air in the crystal-photodetector coupling can cause a light output decrease of 2% to 4%. The individual and combined effect of these advanced models can explain the discrepancy of the relative light output obtained with ESR in simulations and experiments. With Teflon, the study indicates that the light output loss strongly depends on the reflectance deterioration caused by grease absorption.

CONCLUSIONS

Our results indicate that when studying scintillation detector performance with different finishes, performing simulations in ideal coupling conditions can lead to light output overestimation. To perform an accurate light output comparison and ultimately have a reliable detector performance estimation, all potential sources of practical limitations must be carefully considered. To broadly enable high-fidelity modeling, we developed an interface for users to compute their own LUTs, using their surface, scintillator, and reflector characteristics.

Realistic boundary conditions in SimVascular through inlet catheter modeling

OBJECTIVE

This study aims at developing a pipeline that provides the capability to include the catheter effect in the computational fluid dynamics (CFD) simulations of the cardiovascular system and other human vascular flows carried out with the open-source software SimVascular. This tool is particularly useful for CFD simulation of interventional radiology procedures such as tumor embolization where estimation of a therapeutic agent distribution is of interest.

RESULTS

A pipeline is developed that generates boundary condition files which can be used in SimVascular CFD simulations. The boundary condition files are modified such that they simulate the effect of catheter presence on the flow field downstream of the inlet. Using this pipeline, the catheter flow, velocity profile, radius, wall thickness, and deviation from the vessel center can be defined. Since our method relies on the manipulation of the boundary condition that is imposed on the inlet, it is sensitive to the mesh density. The finer the mesh is (especially around the catheter wall), the more accurate the velocity estimations are. In this study, we also utilized this pipeline to qualitatively investigate the effect of catheter presence on the flow field in a truncated right hepatic arterial tree of a liver cancer patient.

Advanced Monte Carlo simulations of emission tomography imaging systems with GATE

Built on top of the Geant4 toolkit, GATE is collaboratively developed for more than 15 years to design Monte Carlo simulations of nuclear-based imaging systems. It is, in particular, used by researchers and industrials to design, optimize, understand and create innovative emission tomography systems. In this paper, we reviewed the recent developments that have been proposed to simulate modern detectors and provide a comprehensive report on imaging systems that have been simulated and evaluated in GATE. Additionally, some methodological developments that are not specific for imaging but that can improve detector modeling and provide computation time gains, such as Variance Reduction Techniques and Artificial Intelligence integration, are described and discussed.

Technical Note: Standalone application to generate custom reflectance Look-Up Table for advanced optical Monte Carlo simulation in GATE/Geant4

PURPOSE

The need for high-fidelity modeling of radiation detectors to perform reliable detector performance optimization using Monte Carlo simulations requires to accurately simulate the light transport in the scintillator and the light collection by the photodetector. In this work, we implement our well-validated crystal reflectance model computed from three-dimensional (3D) crystal surface measurement in a standalone open-source application to allow researchers to generate fully customized crystal reflectance look-up-tables (LUTs) to be used in optical Monte Carlo simulation.

METHODS

The LUTDavisModel application can be installed in a few minutes on Windows, macOS, and Linux, using 26 MB of space. MATLAB Runtime is required and is automatically installed with the application. The core algorithm has been previously validated experimentally and implemented in GATE v8.0. The standalone is divided into five panels, each of which performing a specific task: generate LUTs from a combination of surface type, scintillator, and coupling medium available in the database (such as LSO or BGO) or custom; compute LUTs with the reflectors available and custom coupling thickness; create a mixture of coupling media to account for possible defects in the optical coupling; plot precomputed LUTs for visual comparison. Tooltips and errors/warnings facilitate the navigation. The reported computational times were obtained with an Intel Core i7 MacBook Pro.

RESULTS

LUTs can be generated with computational time ranging from a few minutes to several hours depending on the selected surface, sampling, and computational power. A longer time is needed when using rough surfaces and thick coupling media (hundreds of μ m ) due to increased photon tracking.

CONCLUSIONS

We developed a user-friendly standalone application to generate LUTs that can be used inside GATE Monte Carlo simulations. It can be easily downloaded, installed, and used. Future optimizations will expand the database, decrease the computational time through greater parallelization, and include the generation of LUTs to study Cerenkov photons transport.

Quantitative PET in the 2020s: a roadmap

Positron emission tomography (PET) plays an increasingly important role in research and clinical applications, catalysed by remarkable technical advances and a growing appreciation of the need for reliable, sensitive biomarkers of human function in health and disease. Over the last 30 years, a large amount of the physics and engineering effort in PET has been motivated by the dominant clinical application during that period, oncology. This has led to important developments such as PET/CT, whole-body PET, 3D PET, accelerated statistical image reconstruction, and time-of-flight PET. Despite impressive improvements in image quality as a result of these advances, the emphasis on static, semi-quantitative 'hot spot' imaging for oncologic applications has meant that the capability of PET to quantify biologically relevant parameters based on tracer kinetics has not been fully exploited. More recent advances, such as PET/MR and total-body PET, have opened up the ability to address a vast range of new research questions, from which a future expansion of applications and radiotracers appears highly likely. Many of these new applications and tracers will, at least initially, require quantitative analyses that more fully exploit the exquisite sensitivity of PET and the tracer principle on which it is based. It is also expected that they will require more sophisticated quantitative analysis methods than those that are currently available. At the same time, artificial intelligence is revolutionizing data analysis and impacting the relationship between the statistical quality of the acquired data and the information we can extract from the data. In this roadmap, leaders of the key sub-disciplines of the field identify the challenges and opportunities to be addressed over the next ten years that will enable PET to realise its full quantitative potential, initially in research laboratories and, ultimately, in clinical practice.

Multiscale Computational Fluid Dynamics Modeling for Personalized Liver Cancer Radioembolization Dosimetry

Yttrium-90 (90Y) radioembolization is a minimally invasive procedure increasingly used for advanced liver cancer treatment. In this method, radioactive microspheres are injected into the hepatic arterial bloodstream to target, irradiate, and kill cancer cells. Accurate and precise treatment planning can lead to more efficient and safer treatment by delivering a higher radiation dose to the tumor while minimizing the exposure of the surrounding liver parenchyma. Treatment planning primarily relies on the estimated radiation dose delivered to tissue. However, current methods used to estimate the dose are based on simplified assumptions that make the dosimetry results unreliable. In this work, we present a computational model to predict the radiation dose from the 90Y activity in different liver segments to provide a more realistic and personalized dosimetry. Computational fluid dynamics (CFD) simulations were performed in a 3D hepatic arterial tree model segmented from cone-beam CT angiographic data obtained from a patient with hepatocellular carcinoma (HCC). The microsphere trajectories were predicted from the velocity field. 90Y dose distribution was then calculated from the volumetric distribution of the microspheres. Two injection locations were considered for the microsphere administration, a lobar and a selective injection. Results showed that 22% and 82% of the microspheres were delivered to the tumor, after each injection, respectively, and the combination of both injections ultimately delivered 49% of the total administered 90Y microspheres to the tumor. Results also illustrated the nonhomogeneous distribution of microspheres between liver segments, indicating the importance of developing patient-specific dosimetry methods for effective radioembolization treatment.

Estimation of Yttrium-90 Distribution in Liver Radioembolization using Computational Fluid Dynamics and Deep Neural Networks

Yttrium-90 (⁹⁰Y) radioembolization is a liver cancer therapy based on ⁹⁰Y microspheres injected into the hepatic artery. Current dosimetry methods used to estimate the absorbed dose in order to prescribe the ⁹⁰Y activity to inject are not accurate, which can affect the treatment effectiveness. A new dosimetry based on the hemodynamics simulation of the hepatic arterial tree, CFDose, aimed at overcoming some of the limitations of the current methods. However, due to the expensive computational cost of computational fluid dynamics (CFD) simulations, this method needs to be accelerated before it can be used in real-time during treatment planning. In this paper, we introduce a convolutional neural network model trained with the CFD results of a patient with hepatocellular carcinoma to predict the ⁹⁰Y distribution under different downstream vasculature resistance conditions. The model performance was evaluated using two metrics, the mean squared error and prediction accuracy. The prediction accuracy showed that the average difference between the actual and predicted data was less than 1%. The proposed model could estimate the ⁹⁰Y distribution significantly faster than a CFD simulation.

Current Status of Radiopharmaceutical Therapy

In radiopharmaceutical therapy (RPT), a radionuclide is systemically or locally delivered with the goal of targeting and delivering radiation to cancer cells while minimizing radiation exposure to untargeted cells. Examples of current RPTs include thyroid ablation with the administration of ¹³¹I, treatment of liver cancer with ⁹⁰Y microspheres, the treatment of bony metastases with ²²³Ra, and the treatment of neuroendocrine tumors with ¹⁷⁷Lu-DOTATATE. New RPTs are being developed where radionuclides are incorporated into systemic targeted therapies. To assure that RPT is appropriately implemented, advances in targeting need to be matched with advances in quantitative imaging and dosimetry methods. Currently, radiopharmaceutical therapy is administered by intravenous or locoregional injection, and the treatment planning has typically been implemented like chemotherapy, where the activity administered is either fixed or based on a patient's body weight or body surface area. RPT pharmacokinetics are measurable by quantitative imaging and are known to vary across patients, both in tumors and normal tissues. Therefore, fixed or weight-based activity prescriptions are not currently optimized to deliver a cytotoxic dose to targets while remaining within the tolerance dose of organs at risk. Methods that provide dose estimates to individual patients rather than to reference geometries are needed to assess and adjust the injected RPT dose. Accurate doses to targets and organs at risk will benefit the individual patients and decrease uncertainties in clinical trials. Imaging can be used to measure activity distribution in vivo, and this information can be used to determine patient-specific treatment plans where the dose to the targets and organs at risk can be calculated. The development and adoption of imaging-based dosimetry methods is particularly beneficial in early clinical trials. In this work we discuss dosimetric accuracy needs in modern radiation oncology, uncertainties in the dosimetry in RPT, and best approaches for imaging and dosimetry of internal radionuclide therapy.

Computational Modeling of the Liver Arterial Blood Flow for Microsphere Therapy: Effect of Boundary Conditions

Transarterial embolization is a minimally invasive treatment for advanced liver cancer using microspheres loaded with a chemotherapeutic drug or radioactive yttrium-90 (90Y) that are injected into the hepatic arterial tree through a catheter. For personalized treatment, the microsphere distribution in the liver should be optimized through the injection volume and location. Computational fluid dynamics (CFD) simulations of the blood flow in the hepatic artery can help estimate this distribution if carefully parameterized. An important aspect is the choice of the boundary conditions imposed at the inlet and outlets of the computational domain. In this study, the effect of boundary conditions on the hepatic arterial tree hemodynamics was investigated. The outlet boundary conditions were modeled with three-element Windkessel circuits, representative of the downstream vasculature resistance. Results demonstrated that the downstream vasculature resistance affected the hepatic artery hemodynamics such as the velocity field, the pressure field and the blood flow streamline trajectories. Moreover, the number of microspheres received by the tumor significantly changed (more than 10% of the total injected microspheres) with downstream resistance variations. These findings suggest that patient-specific boundary conditions should be used in order to achieve a more accurate drug distribution estimation with CFD in transarterial embolization treatment planning.

Personalized Dosimetry for Liver Cancer Y-90 Radioembolization Using Computational Fluid Dynamics and Monte Carlo Simulation

Yttrium-90 (Y-90) transarterial radioembolization uses radioactive microspheres injected into the hepatic artery to irradiate liver tumors internally. One of the major challenges is the lack of reliable dosimetry methods for dose prediction and dose verification. We present a patient-specific dosimetry approach for personalized treatment planning based on computational fluid dynamics (CFD) simulations of the microsphere transport combined with Y-90 physics modeling called CFDose. The ultimate goal is the development of a software to optimize the amount of activity and injection point for optimal tumor targeting. We present the proof-of-concept of a CFD dosimetry tool based on a patient's angiogram performed in standard-of-care planning. The hepatic arterial tree of the patient was segmented from the cone-beam CT (CBCT) to predict the microsphere transport using multiscale CFD modeling. To calculate the dose distribution, the predicted microsphere distribution was convolved with a Y-90 dose point kernel. Vessels as small as 0.45 mm were segmented, the microsphere distribution between the liver segments using flow analysis was predicted, the volumetric microsphere and resulting dose distribution in the liver volume were computed. The patient was imaged with positron emission tomography (PET) 2 h after radioembolization to evaluate the Y-90 distribution. The dose distribution was found to be consistent with the Y-90 PET images. These results demonstrate the feasibility of developing a complete framework for personalized Y-90 microsphere simulation and dosimetry using patient-specific input parameters.

Cerenkov light transport in scintillation crystals explained: realistic simulation with GATE

PURPOSE

We are investigating the use of promptly emitted Cerenkov photons to improve scintillation detector timing resolution for time-of-flight (TOF) positron emission tomography (PET). Bismuth germanate (BGO) scintillator was used in most commercial PET scanners until the emergence of lutetium oxyorthosilicate, which allowed for TOF PET by triggering on the fast and bright scintillation signal. Yet BGO is also a candidate to generate fast timing triggers based on Cerenkov light produced in the first few picoseconds following a gamma interaction. Triggering on the Cerenkov light produces excellent timing resolution in BGO but is complicated by the very low number of photons produced. A better understanding of the transport and collection of Cerenkov photons is needed to optimize their use for effective triggering of the detectors.

METHODS

We simultaneously generated and tracked Cerenkov and scintillation photons with a new model of light transport that we have released in GATE V8.0. This crystal reflectance model was used to study photon detection and timing properties, building realistic waveforms as measured with silicon photomultipliers.

RESULTS

We compared the behavior and effect of detecting Cerenkov and scintillation photons at several levels, including detection time stamps, travel time, and coincidence resolving time in 3 × 3 × 20 mm3 BGO crystals. Simulations showed excellent agreement with experimental results and indicated that Cerenkov photons constitute the majority of the signal rising edge. They are therefore critical to provide early triggering and improved the coincidence timing resolution by 50%.

POTENTIAL APPLICATIONS

To our knowledge, this is the first complete simulation of the generation, transport, and detection of the combination of Cerenkov and scintillation photons for TOF detectors. This simulation framework will allow for quantitative study of the factors influencing timing resolution, including the photodetector characteristics, and ultimately aid the development of BGO and other Cerenkov-based detectors for TOF PET.

Modelling the transport of optical photons in scintillation detectors for diagnostic and radiotherapy imaging

Computational modelling of radiation transport can enhance the understanding of the relative importance of individual processes involved in imaging systems. Modelling is a powerful tool for improving detector designs in ways that are impractical or impossible to achieve through experimental measurements. Modelling of light transport in scintillation detectors used in radiology and radiotherapy imaging that rely on the detection of visible light plays an increasingly important role in detector design. Historically, researchers have invested heavily in modelling the transport of ionizing radiation while light transport is often ignored or coarsely modelled. Due to the complexity of existing light transport simulation tools and the breadth of custom codes developed by users, light transport studies are seldom fully exploited and have not reached their full potential. This topical review aims at providing an overview of the methods employed in freely available and other described optical Monte Carlo packages and analytical models and discussing their respective advantages and limitations. In particular, applications of optical transport modelling in nuclear medicine, diagnostic and radiotherapy imaging are described. A discussion on the evolution of these modelling tools into future developments and applications is presented.

An integrated model of scintillator-reflector properties for advanced simulations of optical transport

Accurately modeling the light transport in scintillation detectors is essential to design new detectors for nuclear medicine or high energy physics. Optical models implemented in software such as Geant4 and GATE suffer from important limitations that we addressed by implementing a new approach in which the crystal reflectance was computed from 3D surface measurements. The reflectance was saved in a look-up-table (LUT) then used in Monte Carlo simulation to determine the fate of optical photons. Our previous work using this approach demonstrated excellent agreement with experimental characterization of crystal light output in a limited configuration, i.e. when using no reflector. As scintillators are generally encapsulated in a reflector, it is essential to include the crystal-reflector interface in the LUT. Here we develop a new LUT computation and apply it to several reflector types. A second LUT that contains transmittance data is also saved to enable modeling of optical crosstalk. LUTs have been computed for rough and polished crystals coupled to a Lambertian (e.g. Teflon tape) or a specular reflector (e.g. ESR) using air or optical grease, and the light output was computed using a custom Monte Carlo code. 3  ×  3  ×  20 mm³ lutetium oxyorthosilicate crystals were prepared using these combinations, and the light output was measured experimentally at different irradiation depths. For all reflector and surface finish combinations, the measured and simulated light output showed very good agreement. The behavior of optical photons at the interface crystal-reflector was studied using these simulations, and results highlighted the large difference in optical properties between rough and polished crystals, and Lambertian and specular reflectors. These simulations also showed how the travel path of individual scintillation photons was affected by the reflector and surface finish. The ultimate goal of this work is to implement this model in Geant4 and GATE, and provide a database of scintillators combined with a variety of reflectors.

Reaching 200-ps timing resolution in a time-of-flight and depth-of-interaction positron emission tomography detector using phosphor-coated crystals and high-density silicon photomultipliers

Current research in the field of positron emission tomography (PET) focuses on improving the sensitivity of the scanner with thicker detectors, extended axial field-of-view, and time-of-flight (TOF) capability. These create the need for depth-of-interaction (DOI) encoding to correct parallax errors. We have proposed a method to encode DOI using phosphor-coated crystals. Our initial work using photomultiplier tubes (PMTs) demonstrated the possibilities of the proposed method, however, a major limitation of PMTs for this application is poor quantum efficiency in yellow light, corresponding to the wavelengths of the converted light by the phosphor coating. In contrast, the red-green-blue-high-density (RGB-HD) silicon photomultipliers (SiPMs) have a high photon detection efficiency across the visible spectrum. Excellent coincidence resolving time (CRT; [Formula: see text]) was obtained by coupling RGB-HD SiPMs and [Formula: see text] lutetium fine silicate crystals coated on a third of one of their lateral sides. Events were classified in three DOI bins ([Formula: see text] width) with an average sensitivity of 83.1%. A CRT of [Formula: see text] combined with robust DOI encoding is a marked improvement in the phosphor-coated approach that we pioneered. For the first time, we read out these crystals with SiPMs and clearly demonstrated the potential of the RGB-HD SiPMs for this TOF-DOI PET detector.

Improving Depth, Energy and Timing Estimation in PET Detectors with Deconvolution and Maximum Likelihood Pulse Shape Discrimination

In a scintillation detector, the light generated in the scintillator by a gamma interaction is converted to photoelectrons by a photodetector and produces a time-dependent waveform, the shape of which depends on the scintillator properties and the photodetector response. Several depth-of-interaction (DOI) encoding strategies have been developed that manipulate the scintillator's temporal response along the crystal length and therefore require pulse shape discrimination techniques to differentiate waveform shapes. In this work, we demonstrate how maximum likelihood (ML) estimation methods can be applied to pulse shape discrimination to better estimate deposited energy, DOI and interaction time (for time-of-flight (TOF) PET) of a gamma ray in a scintillation detector. We developed likelihood models based on either the estimated detection times of individual photoelectrons or the number of photoelectrons in discrete time bins, and applied to two phosphor-coated crystals (LFS and LYSO) used in a previously developed TOF-DOI detector concept. Compared with conventional analytical methods, ML pulse shape discrimination improved DOI encoding by 27% for both crystals. Using the ML DOI estimate, we were able to counter depth-dependent changes in light collection inherent to long scintillator crystals and recover the energy resolution measured with fixed depth irradiation (~11.5% for both crystals). Lastly, we demonstrated how the Richardson-Lucy algorithm, an iterative, ML-based deconvolution technique, can be applied to the digitized waveforms to deconvolve the photodetector's single photoelectron response and produce waveforms with a faster rising edge. After deconvolution and applying DOI and time-walk corrections, we demonstrated a 13% improvement in coincidence timing resolution (from 290 to 254 ps) with the LFS crystal and an 8% improvement (323 to 297 ps) with the LYSO crystal.

A combined time-of-flight and depth-of-interaction detector for total-body positron emission tomography

PURPOSE

In support of a project to build a total-body PET scanner with an axial field-of-view of 2 m, the authors are developing simple, cost-effective block detectors with combined time-of-flight (TOF) and depth-of-interaction (DOI) capabilities.

METHODS

This work focuses on investigating the potential of phosphor-coated crystals with conventional PMT-based block detector readout to provide DOI information while preserving timing resolution. The authors explored a variety of phosphor-coating configurations with single crystals and crystal arrays. Several pulse shape discrimination techniques were investigated, including decay time, delayed charge integration (DCI), and average signal shapes.

RESULTS

Pulse shape discrimination based on DCI provided the lowest DOI positioning error: 2 mm DOI positioning error was obtained with single phosphor-coated crystals while 3-3.5 mm DOI error was measured with the block detector module. Minimal timing resolution degradation was observed with single phosphor-coated crystals compared to uncoated crystals, and a timing resolution of 442 ps was obtained with phosphor-coated crystals in the block detector compared to 404 ps without phosphor coating. Flood maps showed a slight degradation in crystal resolvability with phosphor-coated crystals; however, all crystals could be resolved. Energy resolution was degraded by 3%-7% with phosphor-coated crystals compared to uncoated crystals.

CONCLUSIONS

These results demonstrate the feasibility of obtaining TOF-DOI capabilities with simple block detector readout using phosphor-coated crystals.

Evaluation of Matrix9 silicon photomultiplier array for small-animal PET

PURPOSE

The MatrixSL-9-30035-OEM (Matrix9) from SensL is a large-area silicon photomultiplier (SiPM) photodetector module consisting of a 3 × 3 array of 4 × 4 element SiPM arrays (total of 144 SiPM pixels) and incorporates SensL's front-end electronics board and coincidence board. Each SiPM pixel measures 3.16 × 3.16 mm(2) and the total size of the detector head is 47.8 × 46.3 mm(2). Using 8 × 8 polished LSO/LYSO arrays (pitch 1.5 mm) the performance of this detector system (SiPM array and readout electronics) was evaluated with a view for its eventual use in small-animal positron emission tomography (PET).

METHODS

Measurements of noise, signal, signal-to-noise ratio, energy resolution, flood histogram quality, timing resolution, and array trigger error were obtained at different bias voltages (28.0-32.5 V in 0.5 V intervals) and at different temperatures (5 °C-25 °C in 5 °C degree steps) to find the optimal operating conditions.

RESULTS

The best measured signal-to-noise ratio and flood histogram quality for 511 keV gamma photons were obtained at a bias voltage of 30.0 V and a temperature of 5 °C. The energy resolution and timing resolution under these conditions were 14.2% ± 0.1% and 4.2 ± 0.1 ns, respectively. The flood histograms show that all the crystals in the 1.5 mm pitch LSO array can be clearly identified and that smaller crystal pitches can also be resolved. Flood histogram quality was also calculated using different center of gravity based positioning algorithms. Improved and more robust results were achieved using the local 9 pixels for positioning along with an energy offset calibration. To evaluate the front-end detector readout, and multiplexing efficiency, an array trigger error metric is introduced and measured at different lower energy thresholds. Using a lower energy threshold greater than 150 keV effectively eliminates any mispositioning between SiPM arrays.

CONCLUSIONS

In summary, the Matrix9 detector system can resolve high-resolution scintillator arrays common in small-animal PET with adequate energy resolution and timing resolution over a large detector area. The modular design of the Matrix9 detector allows it to be used as a building block for simple, low channel-count, yet high performance, small animal PET or PET/MRI systems.

Optimizing light transport in scintillation crystals for time-of-flight PET: an experimental and optical Monte Carlo simulation study

Achieving excellent timing resolution in gamma ray detectors is crucial in several applications such as medical imaging with time-of-flight positron emission tomography (TOF-PET). Although many factors impact the overall system timing resolution, the statistical nature of scintillation light, including photon production and transport in the crystal to the photodetector, is typically the limiting factor for modern scintillation detectors. In this study, we investigated the impact of surface treatment, in particular, roughening select areas of otherwise polished crystals, on light transport and timing resolution. A custom Monte Carlo photon tracking tool was used to gain insight into changes in light collection and timing resolution that were observed experimentally: select roughening configurations increased the light collection up to 25% and improved timing resolution by 15% compared to crystals with all polished surfaces. Simulations showed that partial surface roughening caused a greater number of photons to be reflected towards the photodetector and increased the initial rate of photoelectron production. This study provides a simple method to improve timing resolution and light collection in scintillator-based gamma ray detectors, a topic of high importance in the field of TOF-PET. Additionally, we demonstrated utility of our Monte Carlo simulation tool to accurately predict the effect of altering crystal surfaces on light collection and timing resolution.

Timing properties of phosphor-coated polished LSO crystals

This study investigates a time-of-flight (TOF)-depth-of-interaction (DOI) detector design for positron emission tomography (PET), based on phosphor-coated lutetium oxyorthosilicate (LSO) scintillator crystals coupled to fast single channel photomultiplier tubes. Interaction of the scintillation light with the phosphor coating changes the pulse shape in a depth-dependent manner. 3 × 3 × 10 mm(3) LSO scintillation crystals with polished surfaces were characterized, with and without phosphor coating, to assess DOI capability and timing properties. Two different phosphor coating geometries were studied: coating of the top surface of the crystal, and the top plus half of the crystal sides. There was negligible depth dependency in the decay time when coating only the top surface, however there was a ∼10 ns difference in end-to-end decay time when coating the top plus half of the crystal sides, sufficient to support the use of three DOI bins (3.3 mm DOI bin width). The rise time of the half-coated phosphor crystal was slightly faster at all depths, compared to uncoated crystals, however the signal amplitude was lower. Phosphor coating resulted in depth-dependent photopeak positions with an energy resolution of 13.7%, at a depth of 1 mm, and 15.3%, at a depth of 9 mm, for the half-coated crystal. Uncoated LSO crystals showed no change in photopeak position as a function of depth, with an energy resolution of 10.4%. The head-on coincidence timing resolution (CTR) of two uncoated LSO crystals was 287 ps using constant fraction discrimination for time pick-off. With phosphor coating, the CTR of the top-coated crystal was 314 ps, compared to 384 ps for the half-coated crystal. We demonstrate that the trade-off between timing resolution and DOI resolution can be controlled by the phosphor coating geometry. Here we present preliminary results demonstrating that good DOI resolution can be achieved with only a modest 26% degradation in CTR.

Predicting the timing properties of phosphor-coated scintillators using Monte Carlo light transport simulation

Current developments in positron emission tomography focus on improving timing performance for scanners with time-of-flight (TOF) capability, and incorporating depth-of-interaction (DOI) information. Recent studies have shown that incorporating DOI correction in TOF detectors can improve timing resolution, and that DOI also becomes more important in long axial field-of-view scanners. We have previously reported the development of DOI-encoding detectors using phosphor-coated scintillation crystals; here we study the timing properties of those crystals to assess the feasibility of providing some level of DOI information without significantly degrading the timing performance. We used Monte Carlo simulations to provide a detailed understanding of light transport in phosphor-coated crystals which cannot be fully characterized experimentally. Our simulations used a custom reflectance model based on 3D crystal surface measurements. Lutetium oxyorthosilicate crystals were simulated with a phosphor coating in contact with the scintillator surfaces and an external diffuse reflector (teflon). Light output, energy resolution, and pulse shape showed excellent agreement with experimental data obtained on 3 × 3 × 10 mm³ crystals coupled to a photomultiplier tube. Scintillator intrinsic timing resolution was simulated with head-on and side-on configurations, confirming the trends observed experimentally. These results indicate that the model may be used to predict timing properties in phosphor-coated crystals and guide the coating for optimal DOI resolution/timing performance trade-off for a given crystal geometry. Simulation data suggested that a time stamp generated from early photoelectrons minimizes degradation of the timing resolution, thus making this method potentially more useful for TOF-DOI detectors than our initial experiments suggested. Finally, this approach could easily be extended to the study of timing properties in other scintillation crystals, with a range of treatments and materials attached to the surface.

Simulation of light transport in scintillators based on 3D characterization of crystal surfaces

In the development of positron emission tomography (PET) detectors, understanding and optimizing scintillator light collection is critical for achieving high performance, particularly when the design incorporates depth-of-interaction (DOI) encoding or time-of-flight information. Monte-Carlo simulations play an important role in guiding research in detector designs and popular software such as GATE now include models of light transport in scintillators. Although current simulation toolkits are able to provide accurate models of perfectly polished surfaces, they do not successfully predict light output for other surface finishes, for example those often used in DOI-encoding detectors. The lack of accuracy of those models mainly originates from a simplified description of rough surfaces as an ensemble of micro-facets determined by the distribution of their normal, typically a gaussian distribution. The user can specify the standard deviation of this distribution, but this parameter does not provide a full description of the surface reflectance properties. We propose a different approach based on 3D measurements of the surface using atomic force microscopy. Polished and rough (unpolished) crystals were scanned to compute the surface reflectance properties. The angular distributions of reflectance and reflected rays were computed and stored in look-up tables (LUTs). The LUTs account for the effect of incidence angle and were integrated in a light transport model. Crystals of different sizes were simulated with and without reflector. The simulated maximum light output and the light output as a function of DOI showed very good agreement with experimental characterization of the crystals, indicating that our approach provides an accurate model of polished and rough surfaces and could be used to predict light collection in scintillators. This model is based on a true 3D representation of the surface, makes no assumption about the surface and provides insight on the optical behaviour of rough crystals that can play a critical role in optimizing the design of PET detectors. This approach is also compatible with existing simulation toolkits and next steps include the implementation in GATE.

Pulse shape discrimination and classification methods for continuous depth of interaction encoding PET detectors

In previous work we demonstrated the potential of positron emission tomography (PET) detectors with depth-of-interaction (DOI) encoding capability based on phosphor-coated crystals. A DOI resolution of 8 mm full-width at half-maximum was obtained for 20 mm long scintillator crystals using a delayed charge integration linear regression method (DCI-LR). Phosphor-coated crystals modify the pulse shape to allow continuous DOI information determination, but the relationship between pulse shape and DOI is complex. We are therefore interested in developing a sensitive and robust method to estimate the DOI. Here, linear discriminant analysis (LDA) was implemented to classify the events based on information extracted from the pulse shape. Pulses were acquired with 2×2×20 mm(3) phosphor-coated crystals at five irradiation depths and characterized by their DCI values or Laguerre coefficients. These coefficients were obtained by expanding the pulses on a Laguerre basis set and constituted a unique signature for each pulse. The DOI of individual events was predicted using LDA based on Laguerre coefficients (Laguerre-LDA) or DCI values (DCI-LDA) as discriminant features. Predicted DOIs were compared to true irradiation depths. Laguerre-LDA showed higher sensitivity and accuracy than DCI-LDA and DCI-LR and was also more robust to predict the DOI of pulses with higher statistical noise due to low light levels (interaction depths further from the photodetector face). This indicates that Laguerre-LDA may be more suitable to DOI estimation in smaller crystals where lower collected light levels are expected. This novel approach is promising for calculating DOI using pulse shape discrimination in single-ended readout depth-encoding PET detectors.

New device for real-time bioluminescence imaging in moving rodents

Bioluminescence imaging (BLI) allows detection of biological functions in genetically modified cells, bacteria, or animals expressing a luciferase (i.e., firefly, Renilla, or aequorin). Given the high sensitivity and minimal toxicity of BLI, in vivo studies on molecular events can be performed noninvasively in living rodents. To date, detection of bioluminescence in living animals has required long exposure times that are incompatible with studies on dynamic signaling pathways or nonanaesthetised freely moving animals. Here we develop an imaging system that allows: 1. bioluminescence to be recorded at a rate of 25 images/s using a third generation intensified charge-coupled device (CCD) camera running in a photon counting mode, and 2. coregistration of a video image from a second CCD camera under infrared lighting. The sensitivity of this instrument permits studies with subsecond temporal resolution in nonanaesthetized and unrestrained mice expressing firefly luciferase and imaging of calcium signaling in transgenic mice expressing green fluorescent protein (GFP) aequorin. This imaging system enables studies on signal transduction, tumor growth, gene expression, or infectious processes in nonanaesthetized and freely moving animals.

Non-invasive in vivo imaging of calcium signaling in mice

Rapid and transient elevations of Ca(2+) within cellular microdomains play a critical role in the regulation of many signal transduction pathways. Described here is a genetic approach for non-invasive detection of localized Ca(2+) concentration ([Ca(2+)]) rises in live animals using bioluminescence imaging (BLI). Transgenic mice conditionally expressing the Ca(2+)-sensitive bioluminescent reporter GFP-aequorin targeted to the mitochondrial matrix were studied in several experimental paradigms. Rapid [Ca(2+)] rises inside the mitochondrial matrix could be readily detected during single-twitch muscle contractions. Whole body patterns of [Ca(2+)] were monitored in freely moving mice and during epileptic seizures. Furthermore, variations in mitochondrial [Ca(2+)] correlated to behavioral components of the sleep/wake cycle were observed during prolonged whole body recordings of newborn mice. This non-invasive imaging technique opens new avenues for the analysis of Ca(2+) signaling whenever whole body information in freely moving animals is desired, in particular during behavioral and developmental studies.