14C autoradiography with an energy-sensitive silicon pixel detector

Monte Carlo simulation of the bremsstrahlung X-rays emitted from H-3 and C-14 for the in-vivo imaging of small animals

For the imaging using low energy pure beta-emitting radionuclides, autoradiography is used by slicing the subjects because the range of beta particles is short and thought to be impossible to detect beta particles from outside the subjects. Contrary to this scientific consensus, we recently found that the distributions of C-14 could be measured by detecting the bremsstrahlung X-rays emitted from the solution of C-14 and may also be applicable to lower energy pure beta-emitting radionuclide, H-3. Although the detection of bremsstrahlung X-rays emitted from H-3 and C-14 may be a possible method for in-vivo imaging of small animals, the absorption of the bremsstrahlung X-rays in the subjects are significant because the energy of bremsstrahlung X-rays is relatively low. In addition, the generations of bremsstrahlung X-rays are lower for low energy beta particles. They may make the in-vivo imaging of these beta radionuclides difficult. To clarify these points for the in-vivo imaging of bremsstrahlung X-rays emitted from H-3 and C-14, we used Monte Carlo simulation to calculate the numbers of counts and the energy spectra of the bremsstrahlung X-rays emitted from H-3 and C-14 in water. The simulation results showed that the fraction of detected bremsstrahlung X-rays by a 4 cm × 4 cm detector in all emitted beta particles was 3.5 × 10^-6 at 0.1 mm from the source. Thus, with a 10 M Bq of H-3, we will detect ~35 cps at 0.1 mm from the source so in-vivo imaging at surface area will be possible. For C-14, the fraction of detected bremsstrahlung X-rays by the detector without and with collimator were 7.0 × 10^-5 and 1.1 × 10^-6 at 10 mm from the source, respectively. Thus, with a 10 M Bq of C-14, we will detect ~700 cps and ~11 cps at 10 mm from the source without and with collimator, respectively. The count rate without collimator is easy to form an image in a short time using a low energy X-ray detector. With collimator, in-vivo imaging of distribution of C-14 will be possible. We conclude that in-vivo imaging of small animals by detecting the bremsstrahlung X-rays emitted from H-3 and C-14 is possible and promising for a new molecular imaging technology.

Subcellular Targeting of Theranostic Radionuclides

The last decade has seen rapid growth in the use of theranostic radionuclides for the treatment and imaging of a wide range of cancers. Radionuclide therapy and imaging rely on a radiolabeled vector to specifically target cancer cells. Radionuclides that emit β particles have thus far dominated the field of targeted radionuclide therapy (TRT), mainly because the longer range (μm-mm track length) of these particles offsets the heterogeneous expression of the molecular target. Shorter range (nm-μm track length) α- and Auger electron (AE)-emitting radionuclides on the other hand provide high ionization densities at the site of decay which could overcome much of the toxicity associated with β-emitters. Given that there is a growing body of evidence that other sensitive sites besides the DNA, such as the cell membrane and mitochondria, could be critical targets in TRT, improved techniques in detecting the subcellular distribution of these radionuclides are necessary, especially since many β-emitting radionuclides also emit AE. The successful development of TRT agents capable of homing to targets with subcellular precision demands the parallel development of quantitative assays for evaluation of spatial distribution of radionuclides in the nm-μm range. In this review, the status of research directed at subcellular targeting of radionuclide theranostics and the methods for imaging and quantification of radionuclide localization at the nanoscale are described.

Radiation Imagers for Quantitative, Single-particle Digital Autoradiography of Alpha- and Beta-particle Emitters

Promising therapies are being developed or are in early-stage clinical trials that employ the use of alpha- and beta-emitting radionuclides to cure hematologic malignancies. However, these targeted radionuclide therapies have not yet met their expected potential for cancer treatment. A primary reason is lack of biodistribution, dosimetry, and dose-response information at cellular levels, which are directly related to optimal targeting, achieving a requisite therapeutic dose, and assessing the safety profile in normal organs and tissues. The current set of imaging tools, such as film autoradiography, scintigraphy, and SPECT/CT, available to researchers and clinicians do not allow the effective assessment of radiation absorbed dose distributions at cellular levels because resolutions are poor, measurement and analytical times are long, and the spatial resolutions are low-generally resulting in poor signal-to-noise ratios. Recently, new radiation digital autoradiography imaging tools have been developed that promise to address these challenges. They include scintillation-, gaseous-, and semiconductor-based radiation-detection technologies that localize the emission location of charged particles on an event-by-event basis at resolutions up to 20 µm FWHM for alpha and beta emitters. These imaging systems allow radionuclide activity concentrations to be quantified to unprecedented levels (mBq/µg) and provide real-time imaging and simultaneous imaging capabilities of both high- and low-activity samples without dynamic range limitations that plague traditional autoradiography. Additionally, large-area imagers are available (>20 × 20 cm²) to accommodate high-throughput imaging studies. This article reviews the various detector classes and their associated performance trade-offs to provide researchers with an overview of the current technologies available for selecting an optimal detector configuration to meet imaging requirement needs.

Performance evaluation of 18 F radioluminescence microscopy using computational simulation

PURPOSE

Radioluminescence microscopy can visualize the distribution of beta-emitting radiotracers in live single cells with high resolution. Here, we perform a computational simulation of ¹⁸ F positron imaging using this modality to better understand how radioluminescence signals are formed and to assist in optimizing the experimental setup and image processing.

METHODS

First, the transport of charged particles through the cell and scintillator and the resulting scintillation is modeled using the GEANT4 Monte-Carlo simulation. Then, the propagation of the scintillation light through the microscope is modeled by a convolution with a depth-dependent point-spread function, which models the microscope response. Finally, the physical measurement of the scintillation light using an electron-multiplying charge-coupled device (EMCCD) camera is modeled using a stochastic numerical photosensor model, which accounts for various sources of noise. The simulated output of the EMCCD camera is further processed using our ORBIT image reconstruction methodology to evaluate the endpoint images.

RESULTS

The EMCCD camera model was validated against experimentally acquired images and the simulated noise, as measured by the standard deviation of a blank image, was found to be accurate within 2% of the actual detection. Furthermore, point source simulations found that a reconstructed spatial resolution of 18.5 μm can be achieved near the scintillator. As the source is moved away from the scintillator, spatial resolution degrades at a rate of 3.5 μm per μm distance. These results agree well with the experimentally measured spatial resolution of 30-40 μm (live cells). The simulation also shows that the system sensitivity is 26.5%, which is also consistent with our previous experiments. Finally, an image of a simulated sparse set of single cells is visually similar to the measured cell image.

CONCLUSIONS

Our simulation methodology agrees with experimental measurements taken with radioluminescence microscopy. This in silico approach can be used to guide further instrumentation developments and to provide a framework for improving image reconstruction.

Multiscale Framework for Imaging Radiolabeled Therapeutics

The resistance of a tumor to a drug is the result of bulk properties of the tumor tissue as well as phenotypic variations displayed by single cells. Here, we show that radioisotopic detection methods, commonly used for tracking the tissue distribution of drug compounds, can be extended to the single-cell level to image the same molecule over a range of physical scales. The anticancer drug rituximab was labeled with short-lived radionuclides (⁸⁹Zr/⁶⁴Cu) and its accumulation at the organ level was imaged using PET in a humanized transgenic mouse model of non-Hodgkin’s lymphoma. To capture the distribution of the drug at a finer scale, tissue sections and single living cells were imaged using radioluminescence microscopy (RLM), a novel method that can detect radionuclides with single-cell resolution. In vivo PET images (24 h postinjection) showed that [⁸⁹Zr]rituximab targeted the intended site of human CD20 expression, the spleen. Within this organ, RLM was used to resolve radiotracer accumulation in the splenic red pulp. In a separate study, RLM highlighted marked differences between single cells, with binding of the radiolabeled antibody ranging from background levels to 1200 radionuclides per cell. Overall, RLM images demonstrated significantly higher spatial resolution and sensitivity than conventional storage-phosphor autoradiography. In conclusion, this combination of PET and RLM provides a unique opportunity for exploring the molecular mechanism of drugs by tracking the same molecule over multiple physical scales, ranging from single living cells to organs substructures and entire living subjects.

Characterization of a double-sided silicon strip detector autoradiography system

PURPOSE

The most commonly used technology currently used for autoradiography is storage phosphor screens, which has many benefits such as a large field of view but lacks particle-counting detection of the time and energy of each detected radionuclide decay. A number of alternative designs, using either solid state or scintillator detectors, have been developed to address these issues. The aim of this study is to characterize the imaging performance of one such instrument, a double-sided silicon strip detector (DSSD) system for digital autoradiography. A novel aspect of this work is that the instrument, in contrast to previous prototype systems using the same detector type, provides the ability for user accessible imaging with higher throughput. Studies were performed to compare its spatial resolution to that of storage phosphor screens and test the implementation of multiradionuclide ex vivo imaging in a mouse preclinical animal study.

METHODS

Detector background counts were determined by measuring a nonradioactive sample slide for 52 h. Energy spectra and detection efficiency were measured for seven commonly used radionuclides under representative conditions for tissue imaging. System dead time was measured by imaging (18)F samples of at least 5 kBq and studying the changes in count rate over time. A line source of (58)Co was manufactured by irradiating a 10 μm nickel wire with fast neutrons in a research reactor. Samples of this wire were imaged in both the DSSD and storage phosphor screen systems and the full width at half maximum (FWHM) measured for the line profiles. Multiradionuclide imaging was employed in a two animal study to examine the intratumoral distribution of a (125)I-labeled monoclonal antibody and a (131)I-labeled engineered fragment (diabody) injected in the same mouse, both targeting carcinoembryonic antigen.

RESULTS

Detector background was 1.81 × 10(-6) counts per second per 50 × 50 μm pixel. Energy spectra and detection efficiency were successfully measured for seven radionuclides. The system dead time was measured to be 59 μs, and FWHM for a (58)Co line source was 154 ± 14 μm for the DSSD system and 343 ± 15 μm for the storage phosphor system. Separation of the contributions from (125)I and (131)I was performed on autoradiography images of tumor sections.

CONCLUSIONS

This study has shown that a DSSD system can be beneficially applied for digital autoradiography with simultaneous multiradionuclide imaging capability. The system has a low background signal, ability to image both low and high activity samples, and a good energy resolution.

Autoradiography imaging in targeted alpha therapy with Timepix detector

There is a lack of data related to activity uptake and particle track distribution in targeted alpha therapy. These data are required to estimate the absorbed dose on a cellular level as alpha particles have a limited range and traverse only a few cells. Tracking of individual alpha particles is possible using the Timepix semiconductor radiation detector. We investigated the feasibility of imaging alpha particle emissions in tumour sections from mice treated with Thorium-227 (using APOMAB), with and without prior chemotherapy and Timepix detector. Additionally, the sensitivity of the Timepix detector to monitor variations in tumour uptake based on the necrotic tissue volume was also studied. Compartmental analysis model was used, based on the obtained imaging data, to assess the Th-227 uptake. Results show that alpha particle, photon, electron, and muon tracks were detected and resolved by Timepix detector. The current study demonstrated that individual alpha particle emissions, resulting from targeted alpha therapy, can be visualised and quantified using Timepix detector. Furthermore, the variations in the uptake based on the tumour necrotic volume have been observed with four times higher uptake for tumours pretreated with chemotherapy than for those without chemotherapy.