Unbiased and biased estimators in coded aperture imaging for far field standoff detection at low count rates.

In general, the reconstructed image in coded aperture imaging is affected by the source configuration. Fenimore's balanced convolution method in conjunction with the uniformly redundant array can remove the interference due to the source configuration. As an extension of Fenimore's balanced convolution method, we present general conditions for designing an unbiased mean estimator for a far-field coded aperture imaging system with a random binary mask. As part of the general conditions, we propose decoding arrays whose elements are variable with respect to source directions. We also show that the unbiased mean estimator from Fenimore's balanced convolution method is a special case of the general conditions. We also present a practical example of designing restoring arrays for a coded aperture system with a random mask.

A high-resolution PET demonstrator using a silicon "magnifying glass".

To assist ongoing investigations of the limits of the tradeoff between spatial resolution and noise in PET imaging, several PET instruments based on silicon-pad detectors have been developed. The latest is a segment of a dual-ring device to demonstrate that excellent reconstructed image resolution can be achieved with a scanner that uses high-resolution detectors placed close to the object of interest or surrounding a small field-of-view in combination with detectors having modest resolution at larger radius. The outer ring of our demonstrator comprises conventional BGO block detectors scavenged from a clinical PET scanner and located at a 500mm radius around a 50mm diameter field-of-view. The inner detector-in contrast to the high-Z scintillator typically used in PET-is based on silicon-pad detectors located at 70mm nominal radius. Each silicon detector has 512 1.4mm x 1.4mm x 1mm detector elements in a 16 x 32 array and is read out using VATA GP7 ASICs (Gamma Medica-Ideas, Northridge, CA). Even though virtually all interactions of 511 keV annihilation photons in silicon are Compton-scatter, both high spatial resolution and reasonable sensitivity appears possible. The system has demonstrated resolution of ~0.7mm FWHM with Na-22 for coincidences having the highest intrinsic resolution (silicon-silicon) and 5-6mm FWHM for the lowest resolution BGO-BGO coincidences. Spatial resolution for images reconstructed from the mixed silicon-BGO coincidences is ~1.5mm FWHM demonstrating the "magnifying-glass" concept.

Statistical performance evaluation and comparison of a Compton medical imaging system and a collimated Anger camera for higher energy photon imaging.

In radionuclide treatment, tumor cells are primarily destroyed by charged particles emitted by the compound while associated higher energy photons are used to image the tumor in order to determine radiation dose and monitor shrinkage. However, the higher energy photons are difficult to image with conventional collimated Anger cameras, since a tradeoff exists between resolution and sensitivity, and the collimator septal penetration and scattering is increased due to the high energy photons. This research compares imaging performance of the conventional Anger camera to a Compton imaging system that can have improved spatial resolution and sensitivity for high energy photons because this tradeoff is decoupled, and the effect of Doppler broadening at higher gamma energies is decreased. System performance is analyzed by the modified uniform Cramer-Rao bound (M-UCRB) algorithms based on the developed system modeling. The bound shows that the effect of Doppler broadening is the limiting factor for Compton camera performance for imaging 364.4 keV photons emitted from 131I. According to the bound, the Compton camera outperforms the collimated system for an equal number of detected events when the desired spatial resolution for a 26 cm diameter uniform disk object is better than 12 mm FWHM. For a 3D cylindrical phantom, the lower bound on variance for the collimated camera is greater than for the Compton imaginer over the resolution range from 0.5 to 2 cm FWHM. Furthermore, the detection sensitivity of the proposed Compton imaging system is about 15-20 times higher than that of the collimated Anger camera.

A prototype of very high resolution small animal PET scanner using silicon pad detectors.

A very high resolution small animal positron emission tomograph (PET) which can achieve sub-millimeter spatial resolution is being developed using silicon pad detectors. The prototype PET for a single slice instrument consists of two 1 mm thick silicon pad detectors, each containing a 32 x 16 array of 1.4 mm x 1.4 mm pads read out with four VATAGP3 chips which have 128 channels low-noise self triggering ASIC in each chip, coincidence units, a source turntable and tungsten slice collimator. The silicon detectors were located edgewise on opposite sides of a 4 cm field-of-view to maximize efficiency. Energy resolution is dominated by electronic noise, which is 0.98% (1.38 keV) FWHM at 140.5 keV. Coincidence timing resolution is 82.1 ns FWHM and coincidence efficiency was measured to be 1.04 x 10(-3) % from two silicon detectors with annihilation photons of (18)F source Image data were acquired and reconstructed using conventional 2-D filtered-back projection (FBP) and a maximum likelihood expectation maximization (ML-EM) method. Image resolution of approximately 1.45 mm FWHM is obtained from 1-D profile of 1.1 mm diameter (18)F line source image. Even better resolution can be obtained with smaller detector element sizes. While many challenges remain in scaling up the instrument to useful efficiency including densely packed detectors and significantly improved timing resolution, performance of the test setup in terms of easily achieving submillimeter resolution is compelling.

Performance evaluation of a very high resolution small animal PET imager using silicon scatter detectors.

A very high resolution positron emission tomography (PET) scanner for small animal imaging based on the idea of inserting a ring of high-granularity solid-state detectors into a conventional PET scanner is under investigation. A particularly interesting configuration of this concept, which takes the form of a degenerate Compton camera, is shown capable of providing sub-millimeter resolution with good sensitivity. We present a Compton PET system and estimate its performance using a proof-of-concept prototype. A prototype single-slice imaging instrument was constructed with two silicon detectors 1 mm thick, each having 512 1.4 mm x 1.4 mm pads arranged in a 32 x 16 array. The silicon detectors were located edgewise on opposite sides and flanked by two non-position sensitive BGO detectors. The scanner performance was measured for its sensitivity, energy, timing, spatial resolution and resolution uniformity. Using the experimental scanner, energy resolution for the silicon detectors is 1%. However, system energy resolution is dominated by the 23% FWHM BGO resolution. Timing resolution for silicon is 82.1 ns FWHM due to time-walk in trigger devices. Using the scattered photons, time resolution between the BGO detectors is 19.4 ns FWHM. Image resolution of 980 microm FWHM at the center of the field-of-view (FOV) is obtained from a 1D profile of a 0.254 mm diameter (18)F line source image reconstructed using the conventional 2D filtered back-projection (FBP). The 0.4 mm gap between two line sources is resolved in the image reconstructed with both FBP and the maximum likelihood expectation maximization (ML-EM) algorithm. The experimental instrument demonstrates sub-millimeter resolution. A prototype having sensitivity high enough for initial small animal images can be used for in vivo studies of small animal models of metabolism, molecular mechanism and the development of new radiotracers.