Un-collimated single-photon imaging system for high-sensitivity small animal and plant imaging

A Technique to Quantify Very Low Activities in Regions of Interest With a Collimatorless Detector

We present a new method to measure sub-microcurie activities of photon-emitting radionuclides in organs and lesions of small animals in vivo. Our technique, named the collimator-less likelihood fit, combines a very high sensitivity collimatorless detector with a Monte Carlo-based likelihood fit in order to estimate the activities in previously segmented regions of interest along with their uncertainties. This is done directly from the photon projections in our collimatorless detector and from the region of interest segmentation provided by an x-ray computed tomography scan. We have extensively validated our approach with 225Ac experimentally in spherical phantoms and mouse phantoms, and also numerically with simulations of a realistic mouse anatomy. Our method yields statistically unbiased results with uncertainties smaller than 20% for activities as low as ~111Bq (3nCi) and for exposures under 30 minutes. We demonstrate that our method yields more robust recovery coefficients when compared to SPECT imaging with a commercial pre-clinical scanner, specially at very low activities. Thus, our technique is complementary to traditional SPECT/CT imaging since it provides a more accurate and precise organ and tumor dosimetry, with a more limited spatial information. Finally, our technique is specially significant in extremely low-activity scenarios when SPECT/CT imaging is simply not viable.

Compton and proximity imaging of 225Ac in vivo with a CZT gamma camera: a proof of principle with simulations

In vivo imaging of 225Ac is a major challenge in the development of targeted alpha therapy radiopharmaceuticals due to the extremely low injected doses. In this paper, we present the design of a multi-modality gamma camera that integrates both proximity and Compton imaging in order to achieve the demanding sensitivities required to image 225Ac with good image quality. We consider a dual-head camera, each of the heads consisting of two planar cadmium zinc telluride detectors acting as scatterer and absorber for Compton imaging, and with the scatterer practically in contact with the subject to allow for proximity imaging. We optimize the detector's design and characterize the detector's performance using Monte Carlo simulations. We show that Compton imaging can resolve features of up to 1.5 mm for hot rod phantoms with an activity of 1 μCi, and can reconstruct 3D images of a mouse injected with 0.5 μCi after a 15 minutes exposure and with a single bed position, for both 221Fr and 213Bi. Proximity imaging is able to resolve two 1 mm-radius sources of less than 0.1 μCi separated by 1 cm and at 1 mm from the detector, as well as it can provide planar images of 221Fr and 213Bi biodistributions of the mouse phantom in 5 minutes.

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.

Preclinical SPECT and SPECT-CT in Oncology

Molecular imaging enables both spatial and temporal understanding of the complex biologic systems underlying carcinogenesis and malignant spread. Single-photon emission tomography (SPECT) is a versatile nuclear imaging-based technique with ideal properties to study these processes in vivo in small animal models, as well as to identify potential drug candidates and characterize their antitumor action and potential adverse effects. Small animal SPECT and SPECT-CT (single-photon emission tomography combined with computer tomography) systems continue to evolve, as do the numerous SPECT radiopharmaceutical agents, allowing unprecedented sensitivity and quantitative molecular imaging capabilities. Several of these advances, their specific applications in oncology as well as new areas of exploration are highlighted in this chapter.

Imaging Salt Uptake Dynamics in Plants Using PET

Soil salinity is a global environmental challenge for crop production. Understanding the uptake and transport properties of salt in plants is crucial to evaluate their potential for growth in high salinity soils and as a basis for engineering varieties with increased salt tolerance. Positron emission tomography (PET), traditionally used in medical and animal imaging applications for assessing and quantifying the dynamic bio-distribution of molecular species, has the potential to provide useful measurements of salt transport dynamics in an intact plant. Here we report on the feasibility of studying the dynamic transport of ²²Na in millet using PET. Twenty-four green foxtail (Setaria viridis L. Beauv.) plants, 12 of each of two different accessions, were incubated in a growth solution containing ²²Na⁺ ions and imaged at 5 time points over a 2-week period using a high-resolution small animal PET scanner. The reconstructed PET images showed clear evidence of sodium transport throughout the whole plant over time. Quantitative region-of-interest analysis of the PET data confirmed a strong correlation between total ²²Na activity in the plants and time. Our results showed consistent salt transport dynamics within plants of the same variety and important differences between the accessions. These differences were corroborated by independent measurement of Na⁺ content and expression of the NHX transcript, a gene implicated in sodium transport. Our results demonstrate that PET can be used to quantitatively evaluate the transport of sodium in plants over time and, potentially, to discern differing salt-tolerance properties between plant varieties. In this paper, we also address the practical radiation safety aspects of working with ²²Na in the context of plant imaging and describe a robust pipeline for handling and incubating plants. We conclude that PET is a promising and practical candidate technology to complement more traditional salt analysis methods and provide insights into systems-level salt transport mechanisms in intact plants.

Real-time whole-plant dynamics of heavy metal transport in Arabidopsis halleri and Arabidopsis thaliana by gamma-ray imaging

Heavy metals such as zinc are essential for plant growth, but toxic at high concentrations. Despite our knowledge of the molecular mechanisms of heavy metal uptake by plants, experimentally addressing the real-time whole-plant dynamics of heavy metal uptake and partitioning has remained a challenge. To overcome this, we applied a high sensitivity gamma-ray imaging system to image uptake and transport of radioactive 65Zn in whole-plant assays of Arabidopsis thaliana and the Zn hyperaccumulator Arabidopsis halleri. We show that our system can be used to quantitatively image and measure uptake and root-to-shoot translocation dynamics of zinc in real time. In the metal hyperaccumulator Arabidopsis halleri, 65Zn uptake and transport from its growth media to the shoot occurs rapidly and on time scales similar to those reported in rice. In transgenic A. halleri plants in which expression of the zinc transporter gene HMA4 is suppressed by RNAi, 65Zn uptake is completely abolished.

Theory and realization of a 2D high resolution and high sensitivity SPECT system with an angle-encoding attenuator pattern

The camera of the conventional SPECT system requires a collimator to allow incoming photons from a specific range of incident angle to reach the detector. It is the major factor that determines the spatial resolution of the camera. Moreover, it also greatly reduces the number of detected photons and hence increases statistical fluctuations in the acquired image data. The goal of this paper is to propose a theory and design for a novel high resolution and high sensitivity SPECT system without conventional collimators. The key is to resolve the incident photons from all directional angles and detected by every detector bin. Special 'attenuators' were designed to 'encode' the incoming photons from different directions similar to coded aperture to form projection data for image reconstruction. Each encoded angular pattern of detected photons was recorded as one measurement. Different angular patterns were achieved by changing the configurations of the attenuators so that angular pattern of different measurements or measurement matrix (MM) is invertible, which guarantee a unique reconstructed image. In simulation, the attenuators were fitted on a virtual full-ring gamma camera, as an alternative to the collimators in conventional SPECT systems. To evaluate the performance of the new SPECT system, analytical simulated projection data in 2D scenario were generated from the XCAT phantom. Noisy simulation using 100 noise realizations suggests that the new attenuator design provides much improved image quality in terms of contrast-noise trade-offs (~30% improvement). The results suggest that the new design of using attenuators to replace collimator is feasible and could potentially improve sensitivity without sacrificing resolution in today's SPECT systems.

Plant-PET Scans: In Vivo Mapping of Xylem and Phloem Functioning

Medical imaging techniques are rapidly expanding in the field of plant sciences. Positron emission tomography (PET) is advancing as a powerful functional imaging technique to decipher in vivo the function of xylem water flow (with (15)O or (18)F), phloem sugar flow (with (11)C or (18)F), and the importance of their strong coupling. However, much remains to be learned about how water flow and sugar distribution are coordinated in intact plants, both under present and future climate regimes. We propose to use PET analysis of plants (plant-PET) to visualize and generate these missing data about integrated xylem and phloem transport. These insights are crucial to understanding how a given environment will affect plant physiological processes and growth.