Towards continuous-to-continuous 3D imaging in the real world.

Imaging systems are often modeled as continuous-to-discrete mappings that map the object (i.e. a function of continuous variables such as space, time, energy, wavelength, etc) to a finite set of measurements. When it comes to reconstruction, some discretized version of the object is almost always assumed, leading to a discrete-to-discrete representation of the imaging system. In this paper, we discuss a method for single-photon emission computed tomography (SPECT) imaging that avoids discrete representations of the object or the imaging system, thus allowing reconstruction on an arbitrarily fine set of points.

Design and performance of a small-animal imaging system using synthetic collimation.

This work outlines the design and construction of a single-photon emission computed tomography imaging system based on the concept of synthetic collimation. A focused multi-pinhole collimator is constructed using rapid-prototyping and casting techniques. The collimator projects the centre of the field of view (FOV) through 46 pinholes when the detector is adjacent to the collimator, with the number reducing towards the edge of the FOV. The detector is then moved further from the collimator to increase the magnification of the system. The object distance remains constant, and each new detector distance is a new system configuration. The level of overlap of the pinhole projections increases as the system magnification increases, but the number of projections subtended by the detector is reduced. There is no rotation in the system; a single tomographic angle is used in each system configuration. Image reconstruction is performed using maximum-likelihood expectation-maximization and an experimentally measured system matrix. The system matrix is measured for each configuration on a coarse grid, using a point source. The pinholes are individually identified and tracked, and a Gaussian fit is made to each projection. The coefficients of these fits are used to interpolate the system matrix. The system is validated experimentally with a hot-rod phantom. The Fourier crosstalk matrix is also measured to provide an estimate of the average spatial resolution along each axis over the entire FOV. The 3D synthetic-collimator image is formed by estimating the activity distribution within the FOV and summing the activities in the voxels along the axis perpendicular to the collimator face.

ADAPTIVE SMALL-ANIMAL SPECT/CT.

We are exploring the concept of adaptive multimodality imaging, a form of non-linear optimization where the imaging configuration is automatically adjusted in response to the object. Preliminary studies suggest that substantial improvement in objective, task-based measures of image quality can result. We describe here our work to add motorized adjustment capabilities and a matching CT to our existing FastSPECT II system to form an adaptive small-animal SPECT/CT.

Fast maximum-likelihood estimation methods for scintillation cameras and other optical sensors.

Maximum-likelihood estimation methods offer many advantages for processing experimental data to extract information, especially when combined with carefully measured calibration data. There are many tasks relevant to x-ray and gamma-ray detection that can be addressed with a new, fast ML-search algorithm that can be implemented in hardware or software. Example applications include gamma-ray event position, energy, and timing estimation, as well as general applications in optical testing and wave-front sensing.

Maximum-likelihood Estimation of 3D Event Position in Monolithic Scintillation Crystals: Experimental Results.

We present a simple 3D event position-estimation method using raw list-mode acquisition and maximum-likelihood estimation in a modular gamma camera with a thick (25mm) monolithic scintillation crystal. This method involves measuring 2D calibration scans with a well-collimated 511 keV source and fitting each point to a simple depth-dependent light distribution model. Preliminary results show that angled collimated beams appear properly reconstructed.

Compact low-cost detection electronics for optical coherence imaging.

A compact and low-cost detection electronics scheme for optical coherence imaging is demonstrated. The performance of the designed electronics is analyzed in comparison to a commercial lock-in amplifier of equal bandwidth. Images of a fresh-onion sample are presented for each detection configuration.

Real-time Data Acquisition and Maximum-Likelihood Estimation for Gamma Cameras.

We have developed modular gamma-ray cameras for biomedical imaging that acquire data with a raw list-mode acquisition architecture. All observations associated with a gamma-ray event, such as photomultiplier (PMT) signals and time, are assembled into an event packet and added to an ordered list of event entries that comprise the acquired data. In this work we present the design of the data-acquisition system, and discuss algorithms for a specialized computing engine to reside in the data path between the front and back ends of each camera and carry out maximum-likelihood position and energy estimations in real time while data was being acquired..

Tomographic Small-Animal Imaging Using a High-Resolution Semiconductor Camera.

We have developed a high-resolution, compact semiconductor camera for nuclear medicine applications. The modular unit has been used to obtain tomographic images of phantoms and mice. The system consists of a 64 x 64 CdZnTe detector array and a parallel-hole tungsten collimator mounted inside a 17 cm x 5.3 cm x 3.7 cm tungsten-aluminum housing. The detector is a 2.5 cm x 2.5 cm x 0.15 cm slab of CdZnTe connected to a 64 x 64 multiplexer readout via indium-bump bonding. The collimator is 7 mm thick, with a 0.38 mm pitch that matches the detector pixel pitch. We obtained a series of projections by rotating the object in front of the camera. The axis of rotation was vertical and about 1.5 cm away from the collimator face. Mouse holders were made out of acrylic plastic tubing to facilitate rotation and the administration of gas anesthetic. Acquisition times were varied from 60 sec to 90 sec per image for a total of 60 projections at an equal spacing of 6 degrees between projections. We present tomographic images of a line phantom and mouse bone scan and assess the properties of the system. The reconstructed images demonstrate spatial resolution on the order of 1-2 mm.

A new design for a SPECT small-animal imager.

We demonstrate, using computer models, the feasibility of a new SPECT system for imaging small animals such as mice. This system consists of four modular scintillation cameras, four multiple-pinhole apertures, electronics, and tomographic reconstruction software. All of these constituents have been designed in our laboratory. The cameras are 120mm×120mm with a resolution of approximately 2mm, the apertures can have either single or multiple pinholes, and reconstruction is performed using the OS-EM algorithm. One major advantage of this system is the design flexibility it offers, as the cameras are easy to move and the aperture s are simple to modify. We explored a number of possible configurations. One promising configuration had the four camera faces forming four sides of a cube with multiple-pinhole apertures employed to focus the incoming high-energy photons. This system is rotated three times, so that data are collected from a total of sixteen camera angles. It is shown that this hybrid system has some superior properties to single-aperture-type systems. We conclude that this proposed system offers advantages over current imaging systems in terms of flexibility, simplicity, and performance.

Spatial Pileup Considerations for Pixellated Gamma -ray Detectors.

High-spatial-resolution solid-state detectors being developed for gamma-ray applications benefit from having pixel dimensions substantially smaller than detector slab thickness. This leads to an enhanced possibility of charge partially spreading to neighboring pixels as a result of diffusion (and secondary photon emission) transverse to the drift direction. An undesirable consequence is the effective magnification of the event "size" and the spatial overlap issues which result when two photons are absorbed in close proximity within the integration time of the detector/readout system. In this work, we develop the general statistics of spatial pileup in imaging systems and apply the results to detectors we are developing based on pixellated cadmium zinc telluride (CdZnTe) and a multiplexing application-specific integrated circuit (ASIC) readout. We consider the limitations imposed on total count rate capacity and explore in detail the consequences for the LISTMODE data-acquisition strategy. Algorithms are proposed for identifying and, where possible, resolving overlapping events by maximum-likelihood estimation. The efficacy and noise tolerance of these algorithms will be tested with a combination of simulated and experimental data in future work.

Use of X-ray Absorption Spectroscopy and Esterification to Investigate Cr(III) and Ni(II) Ligands in Alfalf a Biomass.

Previously performed studies have shown that alfalfa shoot biomass can bind an appreciable amount of nickel(II) and chromium(III) ions from aqueous solution. Direct and indirect approaches were applied to study the possible mechanis ms involved in metal binding by the alfalfa biomass. The direct approach involves investigations of the metal-bound alfal fa shoot biomass by X-ray absorption spectroscopic analysis (XANES and EXAFS). Results from these studies suggest that ni ckel(II) and chromium(III) binding mostly occurs through coordination with oxygen ligands. Indirect approaches consist of chemical modification of carboxylate groups that have been shown to play an important role in metal binding to the alfal fa biomass. An appreciable decrease in metal binding resulted after acidic methanol esterification of the biomass, indica ting that carboxyl groups are entailed in the metal binding by the alfalfa biomass. In addition, base hydrolysis of the a lfalfa biomass increased the binding of these metals, which further indicates that carboxyl groups play an important role in the binding of these metal ions from solution. Therefore, by combining two different techniques, our results indicate that carboxylate groups are the major ligands responsible for the binding of nickel(II) and chromium(III) by alfalfa bio mass.

Silicon Detector System for High Rate EXAFS Applications.

A multichannel silicon pad detector for EXAFS (Extended X-ray Absorption Fine Structure) applications has been designed and built. The X-ray spectroscopic measurements demonstrate that an adequate energy resolution of 230 eV FWHM (corresponding to 27 rms electrons in silicon) can be achieved reliably at -35 °C. A resolution of 190 eV FWHM (corresponding to 22 rms electrons) has been obtained from individual pads at -35 °C. At room temperature (25 °C) an average energy resolution of 380 eV FWHM is achieved and a resolution of 350 eV FWHM (41 rms electrons) is the best performance. A simple cooling system constituted of Peltier cells is sufficient to reduce the reverse currents of the pads and their related shot noise contribution, in order to achieve resolutions better than 300 eV FWHM which is adequate for the EXAFS applications.

The NSLS 100 element solid state array detector.

X-ray absorption studies of dilute samples require fluorescence detection techniques. Since signal-to-noise ratios are governed by the ratio of fluorescent to scattered photons counted by a detector, solid state detectors which can discriminate between fluorescence and scattered photons have become the instruments of choice for trace element measurements. Commercially available 13 element Ge array detectors permitting total count rates < 500000 counts per second are now in routine use. Since X-ray absorption beamlines at high brightness synchrotron sources can already illuminate most dilute samples with enough flux to saturate the current generation of solid state detectors, the development of next-generation instruments with significantly higher total count rates is essential. We present the design and current status of the 100 element Si array detector being developed in a collaboration between the NSLS and the Instrumentation Division at Brookhaven National Laboratory. The detecting array consists of a 10×10 matrix of 4 mm×4 mm elements laid out on a single piece of ultrahigh purity silicon mounted at the front end of a liquid nitrogen dewar assembly. A matrix of charge sensitive integrating preamplifiers feed signals to an array of shaping amplifiers, single channel analyzers, and scalers. An electronic switch, delay amplifier, linear gate, digital scope, peak sensing A/D converter, and histogramining memory module provide for complete diagnostics and channel calibration. The entire instrument is controlled by a LabView 2 application on a MacII ci; the software also provides full control over beamline hardware and performs the data collection.