High-Resolution Anamorphic SPECT Imaging.

We have developed a gamma-ray imaging system that combines a high-resolution silicon detector with two sets of movable, half-keel-edged copper-tungsten blades configured as crossed slits. These apertures can be positioned independently between the object and detector, producing an anamorphic image in which the axial and transaxial magnifications are not constrained to be equal. The detector is a 60 mm × 60 mm, one-millimeter-thick, one-megapixel silicon double-sided strip detector with a strip pitch of 59 µm. The flexible nature of this system allows the application of adaptive imaging techniques. We present system details; calibration, acquisition, and reconstruction methods; and imaging results.

Adaptive SPECT imaging with crossed-slit apertures.

Preclinical single-photon emission computed tomography (SPECT) is an essential tool for studying the progression, response to treatment, and physiological changes in small animal models of human disease. The wide range of imaging applications is often limited by the static design of many preclinical SPECT systems. We have developed a prototype imaging system that replaces the standard static pinhole aperture with two sets of movable, keel-edged copper-tungsten blades configured as crossed (skewed) slits. These apertures can be positioned independently between the object and detector, producing a continuum of imaging configurations in which the axial and transaxial magnifications are not constrained to be equal. We incorporated a megapixel silicon double-sided strip detector to permit ultrahigh-resolution imaging. We describe the configuration of the adjustable slit aperture imaging system and discuss its application toward adaptive imaging, and reconstruction techniques using an accurate imaging forward model, a novel geometric calibration technique, and a GPU-based ultra-high-resolution reconstruction code.

High-resolution, anamorphic, adaptive small-animal SPECT imaging with silicon double-sided strip detectors.

We are developing a prototype gamma-ray imaging system that consists of two sets of movable, keel-edged copper-tungsten blades configured as crossed slits. These apertures can be positioned independently between the object and detector, producing an anamorphic image in which the axial and transaxial magnifications are not constrained to be equal. The detector is a 60 mm × 60 mm, millimeter thick, one-megapixel silicon double-sided strip detector. The flexible nature of this system allows the application of adaptive imaging techniques. We will discuss system details, calibration and acquisition methods, and our progress towards biological imaging applications.

DR with a DSLR: Digital Radiography with a Digital Single-Lens Reflex camera.

An inexpensive, portable digital radiography (DR) detector system for use in remote regions has been built and evaluated. The system utilizes a large-format digital single-lens reflex (DSLR) camera to capture the image from a standard fluorescent screen. The large sensor area allows relatively small demagnification factors and hence minimizes the light loss. The system has been used for initial phantom tests in urban hospitals and Himalayan clinics in Nepal, and it has been evaluated in the laboratory at the University of Arizona by additional phantom studies. Typical phantom images are presented in this paper, and a simplified discussion of the detective quantum efficiency of the detector is given.

Thick Silicon Double-Sided Strip Detectors for Low-Energy Small-Animal SPECT.

This work presents characterization studies of thick silicon double-sided strip detectors for a high-resolution small-animal SPECT. The dimension of these detectors is 60.4 mm × 60.4 mm × 1 mm. There are 1024 strips on each side that give the coordinates of the photon interaction, with each strip processed by a separate ASIC channel. Our measurement shows that intrinsic spatial resolution equivalent to the 59 µm strip pitch is attainable. Good trigger uniformity can be achieved by proper setting of a 4-bit DAC in each ASIC channel to remove trigger threshold variations. This is particularly important for triggering at low energies. The thick silicon DSSD (Double-sided strip detector) shows high potential for small-animal SPECT.

Data-Processing Strategies for Crossed-Strip Gamma-Ray Detectors.

Crossed-strip gamma-ray detectors are an attractive option for small-animal SPECT imagers due to their high space-bandwidth product. In systems with independent triggering of the two sides of the detector, advanced data-processing techniques are required to accurately determine gamma-ray interaction locations and energy deposition. Optimal detector operation further relies on rigorous detector characterization in order to achieve detector triggering uniformity and best timing resolution and to permit position and energy estimation with maximum-likelihood methods. We describe algorithms and methods developed for calibrating and characterizing a recently fabricated system based on 1024-strips-per-side 1-mm-thick silicon detectors.

Performance Characteristics of Thick Silicon Double-sided Strip Detectors.

This work presents characterization studies of thick silicon double-sided strip detectors for a high-resolution small-animal SPECT. The dimension of these detectors is 60 mm × 60 mm × 1 mm. There are 1024 strips on each side that give the coordinates of the photon interaction, with each strip processed by a separate ASIC channel. Our measurement shows that intrinsic spatial resolution equivalent to the 59 µm strip pitch is attainable. Good flood uniformity can be achieved by proper setting of a 4-bit DAC in each ASIC channel to remove trigger threshold variations. This is particularly important for triggering at low energies. The thick silicon DSSD shows high potential for small-animal SPECT imaging.

A Silicon SPECT System for Molecular Imaging of the Mouse Brain.

We previously demonstrated the feasibility of using silicon double-sided strip detectors (DSSDs) for SPECT imaging of the activity distribution of iodine-125 using a 300-micrometer thick detector. Based on this experience, we now have developed fully customized silicon DSSDs and associated readout electronics with the intent of developing a multi-pinhole SPECT system. Each DSSD has a 60.4 mm × 60.4 mm active area and is 1 mm thick. The strip pitch is 59 micrometers, and the readout of the 1024 strips on each side gives rise to a detector with over one million pixels. Combining four high-resolution DSSDs into a SPECT system offers an unprecedented space-bandwidth product for the imaging of single-photon emitters. The system consists of two camera heads with two silicon detectors stacked one behind the other in each head. The collimator has a focused pinhole system with cylindrical-shaped pinholes that are laser-drilled in a 250 µm tungsten plate. The unique ability to collect projection data at two magnifications simultaneously allows for multiplexed data at high resolution to be combined with lower magnification data with little or no multiplexing. With the current multi-pinhole collimator design, our SPECT system will be capable of offering high spatial resolution, sensitivity and angular sampling for small field-of-view applications, such as molecular imaging of the mouse brain.

Influence of the longitudinal mode field in grating scattering from weakly guided optical fiber waveguides.

We measured the polarization dependence of light scattered from a tilted fiber grating and found disagreement with previous volume-current perturbation analysis. However, by including the longitudinal E field of the guided wave we were able to obtain good agreement, demonstrating that, although it is small, this component cannot be neglected when scattering of weakly guided waves is considered. A first-order approximation formula for the polarization dependence was also obtained and is shown to be accurate within most of the resonance band of scattering.