James Walter Leblanc

Evaluation of a prototype small-animal PET detector with depth-of-interaction encoding

We report on the evaluation of a prototype small-animal positron emission tomography (PET) detector with depth-of-interaction (DOI) encoding. The detector consisted of an 8/spl times/8 array of scintillator crystals that were read out on the top and bottom by position sensitive avalanche photodiodes (PSAPD). Each scintillator crystal had dimensions of 1.65 mm/spl times/1.65 mm/spl times/22.00 mm, and the array had a pitch of 1.75 mm. The 14/spl times/4 mm PSAPDs were coupled to compact, discrete transimpedance preamplifiers. The DOI response was measured by illuminating the detector module from the side with an electronically collimated fan beam of 511 keV gamma rays. The signals from the corner contacts of the PSAPDs were used to identify the crystal of interaction, and the ratio of the total signals from the two PSAPDs was used to calculate the depth z at which the interaction took place. By stepping the crystal array in the z direction (perpendicular to the fan beam), we were able to determine the DOI resolution for each individual crystal. For measurements made at 10/spl deg/C, the average DOI resolution was better than 3 mm. Energy resolution and timing resolution (versus a fast plastic scintillator coupled to a photomultiplier tube) were made under the same operating conditions. The average energy resolution across the array was 16.6% at 511 keV, and the average timing resolution was 3.9 ns. Importantly, the detector performance was maintained all the way out to the crystals at the edges of the PSAPDs.

Multi-source inverse geometry CT: a new system concept for x-ray computed tomography

Third-generation CT architectures are approaching fundamental limits. Spatial resolution is limited by the focal spot size and the detector cell size. Temporal resolution is limited by mechanical constraints on gantry rotation speed, and alternative geometries such as electron-beam CT and two-tube-two-detector CT come with severe tradeoffs in terms of image quality, dose-efficiency and complexity. Image noise is fundamentally linked to patient dose, and dose-efficiency is limited by finite detector efficiency and by limited spatio-temporal control over the X-ray flux. Finally, volumetric coverage is limited by detector size, scattered radiation, conebeam artifacts, Heel effect, and helical over-scan. We propose a new concept, multi-source inverse geometry CT, which allows CT to break through several of the above limitations. The proposed architecture has several advantages compared to third-generation CT: the detector is small and can have a high detection efficiency, the optical spot size is more consistent throughout the field-of-view, scatter is minimized even when eliminating the anti-scatter grid, the X-ray flux from each source can be modulated independently to achieve an optimal noise-dose tradeoff, and the geometry offers unlimited coverage without cone-beam artifacts. In this work we demonstrate the advantages of multi-source inverse geometry CT using computer simulations.

Evaluation of a position sensitive avalanche photodiode for PET

A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm /spl times/ 1.5 mm /spl times/ 15 mm. The assembled 7 /spl times/ 7 array, including intercrystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm /spl times/ 14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.

A novel PET detector block with three-dimensional hit position encoding

A three-dimensional position encoding PET detector block that employs a monolithic scintillator is investigated in simulation. The block consists of an unsegmented scintillator crystal that is coupled on up to six sides by non-position-sensitive photosensors. In some incarnations envisioned, position sensitivity could be employed. The hit position is encoded via the relative signals on the photosensors. Results of simulations of a monolithic block read out on three, four, and five sides by single channel photosensors are described.

An electronically-collimated gamma camera with a parallel plate collimator for Tc-99m imaging

We present a new idea to apply a parallel plate collimator to an electronically-collimated gamma camera, to reduce the complexity of image reconstruction. Back projected images obtained experimentally and using Monte Carlo simulations are shown. Using the collimator, backprojected images of single low-energy gamma-ray emitters can be obtained with better spatial resolution and larger signal-to-noise ratio than those obtained without the collimator. The results show that this method is useful in obtaining Tc-99m images.

Inverse geometry CT: The next-generation CT architecture?

We present a new system architecture for X-ray computed tomography (CT). A multi-source inverse-geometry CT scanner is composed of a large distributed X-ray source with an array of discrete electron emitters and focal spots, and a high frame-rate flat-panel X-ray detector. In this work we study the advantages and the challenges of this new architecture. We predict potential breakthroughs in volumetric coverage, dose efficiency, and spatial resolution. We also present experimental results obtained with a universal benchtop system.

Temporal response of CZT detectors under intense irradiation

The temporal response of CZT detectors is measured under different X-ray flux, spectra, and detector bias conditions. A comprehensive model has been developed to investigate the detector response under these conditions. The calculations have been compared with our measured results. Reasonable qualitative agreement is shown between the model and measurement results. This model provides a powerful tool to understand the detector temporal response, photocurrent dependence on the irradiation intensity, bias voltage, and defect characteristics. Understanding the detector response from a microscopic level can provide a guide to improve material properties and detector device design.

Modeling the x-ray energy characteristics of DQE for full-field digital mammography

The modulation transfer function and detective quantum efficiency are modeled for a Full Field Digital Mammography detector constructed with a CsI scintillator deposited on an amorphous silicon active matrix array. The model is evaluated against experimental measurements using different exposure levels, x-ray tube voltages, target composition and beam filtrations as well as varying thicknesses and compositions of filtration materials placed in the path between the tube and detector. Available x-ray tube emission spectrum models were evaluated by comparison against the measured transmission through aluminum. The observed variation of DQE at zero spatial frequency among different target/filter conditions, acrylic filtration thicknesses and kVp is well characterized by a x-ray model. This variation is largely accounted for by just two effects -- the attenuation of x-rays through the detector enclosure and the stopping power of x-rays in the CsI layer. Additional considerations such as the Lubberts effect were included in the analysis in order to match the measured DQE(k) as a function of spatial frequency, k. The pixel aperture and light channeling through the scintillator shape the MTF which acts favorably to avoid aliasing due to digital sampling.

Depth of interaction effect on timing resolution in PET block detectors

We have investigated the effect of the depth of interaction (DOI) on the coincidence timing distribution of a PET block detector excited with 511 keV photons. Measurements were performed with a detector consisting of a quad photomultiplier tube (PMT) optically coupled to a 6times6 array of mixed lutetium silicate (MLS) scintillator crystals with different surface treatments. The PET detector was side-illuminated with an electronically collimated beam and the coincidence timing distribution was recorded at different DOI as the detector block was stepped through the beam. The shift of the timing distribution peak measured the variation of the average propagation time of the scintillation photons within the block. From top to bottom (30 mm distance), the average delay ranged from 120 ps for polished crystals up to 350 ps for crystals with all sides roughened. The results of this study allowed modeling the effect of DOI delay on timing performance for front-end irradiation with 511 keV photons, assuming a range of different values for the intrinsic timing resolution of the detector. The relative contribution from DOI effects to the total time coincidence resolution was found to be significant only for detectors with an intrinsic time resolution less than 250 ps FWHM

A new surface parameterization for modeling thin layers of reflector material in the DETECT2000 optical modeling program

A new surface parameterization has been implemented in DETECT2000 that allows for the efficient representation of a thin film of reflector material with non-negligible transmission. Material is usually modeled in DETECT by specifying the optical attenuation and scatter lengths. For a material such as Teflon, commonly used as a reflector material for nuclear medicine scintillators, the scatter length can be very short. Modeling the propagation of photons in a material with a very short scattering length can be computationally intensive as there can be many scatter interactions before the photon either exits the material or is absorbed. In a detector design that makes generous use of Teflon, most of the computations can be spent simply transporting photons in the Teflon. To address this issue, a parameterized surface has been implemented which is specified not by the bulk optical scatter and attenuation lengths, but rather by a reflection and transmission coefficient. This allows computationally efficient modeling of thin film reflectors where surface properties and transmission are relevant.