A layered single-side readout depth of interaction time-of-flight-PET detector.

We are exploring a scintillator-based PET detector with potential of high sensitivity, depth of interaction (DOI) capability, and timing resolution, with single-side readout. Our design combines two previous concepts: (1) multiple scintillator arrays stacked with relative offset, yielding inherent DOI information, but good timing performance has not been demonstrated with conventional light sharing readout. (2) Single crystal array with one-to-one coupling to the photodetector, showing superior timing performance compared to its light sharing counterparts, but lacks DOI. The combination, where the first layer of a staggered design is coupled one-to-one to a photodetector array, may provide both DOI and timing resolution and this concept is here evaluated through light transport simulations. Results show that: (1) unpolished crystal pixels in the staggered configuration yield better performance across all metrics compared to polished pixels, regardless of readout scheme. (2) One-to-one readout of the first layer allows for accurate DOI extraction using a single threshold. The number of multi pixel photon counter (MPPC) pixels with signal amplitudes exceeding the threshold corresponds to the interaction layer. This approach was not possible with conventional light sharing readout. (3) With a threshold of 2 optical photons, the layered approach with one-to-one coupled first layer improves timing close to the MPPC compared to the conventional one-to-one coupling non-DOI detector, due to effectively reduced crystal thickness. Single detector timing resolution values of 91, 127, 151 and 164 ps were observed per layer in the 4-layer design, to be compared to 148 ps for the single array with one-to-one coupling. (4) For the layered design with light sharing readout, timing improves with increased MPPC pixel size due to higher signal per channel. In conclusion, the combination of straightforward DOI determination, good timing performance, and relatively simple design makes the proposed concept promising for DOI-Time-of-Flight PET detectors.

Exploring light confinement in laser-processed LYSO:Ce for photon counting CT application.

With the goal of developing a low-cost scintillator-based photon counting detector (PCD) with high dose efficiency suitable for CT, the light transport characteristics in LYSO:Ce detectors containing laser induced optical barriers (LIOB) are simulated. Light confinement and light collection efficiencies (LCE) are studied for a variety of optical barrier patterns and properties (refractive index (RI) and barrier/crystal interface roughness). Up to 80% confinement is achievable with a simple pixel pattern with one barrier wall separating each pixel coupled one-to-one to a photodetector (PD) pixel. Confinement is heavily dependent on barrier properties, and rough interfaces and higher RI results in increased cross-talk. Three approaches to enhance performance beyond the basic pattern are explored: (1) Multiple barrier walls separating each crystal pixel. (2) Introduction of long and short range confinement by having multiple crystal pixels per PD pixel. (3) Combination of LIOB and laser ablation (LA). (1) Is effective for rough interfaces where confinement can be increased by up to 24% for double compared to single walls. (2) Results in high confinement in the pixel centered on the PD pixel, but lower confinement closer to the PD edge. This feature may be explored to achieve spatial resolution beyond the PD pixel size using light sharing based positioning algorithms. (3) Can increase confinement for smooth interfaces using a smooth ablation in the bottom part of the crystal. A general trend across all configurations is a trade-off between light confinement and LCE. The LCE attainable is found comparable to that for mechanically pixelated arrays. While the confinement achievable with LIOB is always lower compared to a mechanically pixelated array, the former may offer a high level of flexibility in terms of detector design. This, in combination with the possibility to fabricate sub-mm pixels in a cost-effective manner, makes LIOB a promising technology for scintillator-based PCDs.

Reservoir Weed Hosts for Turnip mosaic virus in Iran.

During the summer of 2003, weed samples of Rapistrum rugosum and Sisymbrium loeselii showing severe mosaic, malformation, and stunting were collected from cauliflower fields in Tehran Province of Iran. Using double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) with specific polyclonal antibodies, the samples were tested for the presence of Beet western yellows virus, Cauliflower mosaic virus, Radish mosaic virus, Turnip crinkle virus, Turnip mosaic virus (TuMV) (DSMZ, Braunschweig, Germany), Cucumber mosaic virus, and Tobacco mosaic virus (Sanofi Diagnostics Pasteur, Marnes-La-Coquette, France). Leaf extracts were used for mechanical inoculation and they produced chlorotic local lesions on Chenopodium amaranticolor, necrotic lesions on leaves and shoot apex necrosis on Nicotiana glutinosa, leaf deformation, mosaic, and stunting on Petunia hybrida, and severe mosaic, distortion, and stunting on Brassica rapa. These symptoms were similar to those that were described previously for TuMV (4). ELISA results showed that the original leaf samples and inoculated indicator plants reacted positively with TuMV antibodies, but not with antibodies for any of the other viruses listed above. Also, reverse transcription-polymerase chain reaction of total RNA extracted from the collected leaf samples using the universal primers for potyviruses (3) resulted in the amplification of two fragments of the expected sizes, approximately 700 and 1,700 bp. TuMV, a member of the genus Potyvirus in the family Potyviridae, is transmitted by aphids in a nonpersistent manner (4). This virus is geographically widespread with a wide host range that can infect 318 species in 156 genera of 43 plant families including, Brassicaceae, Chenopodiaceae, Asteraceae, Cucurbitaceae, and Solanaceae (2,4). R. rugosum and S. loeselii, two annual or biennial plants in the Brassicaceae family, were common and widely distributed in the fields surveyed. The presence of TuMV-infected weed hosts in cauliflower fields may impact disease management strategies. TuMV was first observed on stock plants (Matthiola sp.) in Iran (1). To our knowledge, this is the first report of natural occurrence of TuMV on weed hosts in Iran. References: (1) M. Bahar et al. Iran. J. Plant Pathol. 21:11, 1985. (2) J. R. Edwardson and R. G. Christie. The potyvirus group. Fla. Agric. Exp. Stn. Monogr. Ser. No. 16, 1991. (3) A. Gibbs and A. Mackenzie. J. Virol. Methods 63:9, 1997. (4) J. A. Tomlinson. Turnip mosaic virus. No. 8 in: Descriptions of Plant Viruses. CMI/AAB, Surrey, England, 1970.