Frederik Beekman

Optimizing multi-pinhole SPECT geometries using an analytical model

State-of-the-art multi-pinhole SPECT devices allow for sub-mm resolution imaging of radio-molecule distributions in small laboratory animals. The optimization of multi-pinhole and detector geometries using simulations based on ray-tracing or Monte Carlo algorithms is time-consuming, particularly because many system parameters need to be varied. As an efficient alternative we develop a continuous analytical model of a pinhole SPECT system with a stationary detector set-up, which we apply to focused imaging of a mouse. The model assumes that the multi-pinhole collimator and the detector both have the shape of a spherical layer, and uses analytical expressions for effective pinhole diameters, sensitivity and spatial resolution. For fixed fields-of-view, a pinhole-diameter adapting feedback loop allows for the comparison of the system resolution of different systems at equal system sensitivity, and vice versa. The model predicts that (i) for optimal resolution or sensitivity the collimator layer with pinholes should be placed as closely as possible around the animal given a fixed detector layer, (ii) with high-resolution detectors a resolution improvement up to 31% can be achieved compared to optimized systems, (iii) high-resolution detectors can be placed close to the collimator without significant resolution losses, (iv) interestingly, systems with a physical pinhole diameter of 0 mm can have an excellent resolution when high-resolution detectors are used.

High-Resolution 4D Preclinical Single-Photon Emission Computed Tomography/X-ray Computed Tomography Imaging of Technetium Transport within a Heterogeneous Porous Media

A dynamic 99mTc tracer experiment was performed to investigate the capabilities of combined preclinical single photon emission computed tomography (SPECT) and X-ray computed tomography (CT) for investigating transport in a heterogeneous porous medium. The experiment was conducted by continuously injecting a 99mTc solution into a column packed with eight layers (i.e., soil, silica gel, and 0.2-4 mm glass beads). Within the imaging results it was possible to correlate observed features with objects as small as 2 mm for the SPECT and 0.2 mm for the CT. Time-lapse SPECT imaging results illustrated both local and global nonuniform transport phenomena and the high-resolution CT data were found to be useful for interpreting the cause of variations in the 99mTc concentration associated with structural features within the materials, such as macropores. The results of this study demonstrate SPECT/CT as a novel tool for 4D (i.e., transient three-dimensional) noninvasive imaging of fate and transport processes in porous media. Despite its small scale, an experiment with such high resolution data allows us to better understand the pore scale transport which can then be used to inform larger scale studies.

Continuous model of multi-pinhole SPECT devices

Recently multi-pinhole SPECT systems have been proposed that allow for sub-mm resolution imaging in mice. The optimization of multi-pinhole systems using ray-tracing or Monte Carlo simulation is extremely time consuming. As an alternative we develop an approximate continuous model of a SPECT system which we apply to focused pinhole imaging of the mouse. In order to facilitate the use of analytical mathematical calculations the model assumes that the system consists of spherical layers. It uses analytical expressions of effective pinhole diameters, sensitivity, and resolution. With a pinhole-diameter adapting feedback loop that allows to compare system resolution at equal system sensitivity and vice versa the model predicts that (i) for optimal resolution pinholes should be as close as possible to the animal, (ii) with high resolution (<0.1 mm) gamma cameras a resolution improvement of at least 20% can be reached compared to a system with with standard low resolution (3.2 mm) NaI detectors in a compact system.