Soil Buffering Capacity Can Be Used To Optimize Biostimulation of Psychrotrophic Hydrocarbon Remediation.

Effective bioremediation of hydrocarbons requires innovative approaches to minimize phosphate precipitation in soils of different buffering capacities. Understanding the mechanisms underlying sustained stimulation of bacterial activity remains a key challenge for optimizing bioremediation-particularly in northern regions. Positron emission tomography (PET) can trace microbial activity within the naturally occurring soil structure of intact soils. Here, we use PET to test two hypotheses: (1) optimizing phosphate bioavailability in soil will outperform a generic biostimulatory solution in promoting hydrocarbon remediation and (2) oligotrophic biostimulation will be more effective than eutrophic approaches. In so doing, we highlight the key bacterial taxa that underlie aerobic and anaerobic hydrocarbon degradation in subarctic soils. In particular, we showed that (i) optimized phosphate bioavailability outperformed generic biostimulatory solutions in promoting hydrocarbon degradation, (ii) oligotrophic biostimulation is more effective than eutrophic approaches, and (iii) optimized biostimulatory solutions stimulated specific soil regions and bacterial consortia. The knowledge gleaned from this study will be crucial in developing field-scale biodegradation treatments for sustained stimulation of bacterial activity in northern regions.

Gaussian position-weighted center of gravity algorithm for multiplexed readout.

Readout signal multiplexing is a commonly used method to reduce the electronics cost in positron emission tomography (PET) systems, and the calculation of the scintillation coordinates typically is done by using a center of gravity (CoG) technique due to its simplicity and ease of implementation. This comes with a drawback, since CoG has a non-linear response at the periphery of the detector due to the lack of weights beyond the detector. Detectors with multiplexed readout that are based on finely segmented scintillators and coarsely segmented photosensors are known to suffer from the so-called edge effect where a pile-up of the reconstructed coordinates from the edge crystals is observed. This may lead to incorrect assignment of the events to crystal pixels and result in the formation of erroneous lines of response causing a degradation of spatial resolution and reduction of image contrast. To overcome the abovementioned limitations in gamma-ray detectors with multiplexed photosensor readout, we propose to use a modified Gaussian position-weighted center of gravity (PW-CoG) technique for the calculation of gamma-ray interaction position. Here, the proposed method is applied to PET detectors with 24 [Formula: see text] 24 LYSO crystals read out by 8 [Formula: see text] 8 SiPM array with 64:16 row/column multiplexing. Furthermore, we compared the modified Gaussian PW-CoG and truncated center of gravity coordinate reconstruction methods. It was observed that both algorithms resolve peaks corresponding to events registered in the crystal pixels on the periphery of the crystal array. However peak-to-valley ratios and crystal resolvability metrics for the PW-CoG algorithm are generally greater.

Development of a novel high quantum efficiency MV x-ray detector for image-guided radiotherapy: A feasibility study.

PURPOSE: To develop a new scintillating fiber-based electronic portal imaging device (EPID) with a high quantum efficiency (QE) while preserving an adequate spatial resolution. METHODS: Two prototypes were built: one with a single pixel readout and the other with an active matrix flat-panel imager (AMFPI) for readout. The energy conversion layer of both prototypes was made of scintillating fiber layers interleaved with corrugated lead sheets to form a honeycomb pattern. The scintillating fibers have a diameter of 1 mm and the distance between the centers of neighboring fibers on the same layer is 1.35 mm. The layers have 1.22 mm spacing between them. The energy conversion layer has a thickness of 2 cm. The modulation transfer function (MTF), antiscatter properties and sensitivity of the detector with a single pixel readout were measured using a 6-MV beam on a LINAC machine. In addition, a Monte Carlo simulation was conducted to calculate the zero-frequency detective quantum efficiency (DQE(0)) of the proposed detector with an active matrix flat-panel imager for readout. RESULTS: The DQE(0) of the proposed detector can be 11.5%, which is about an order of magnitude higher than that of current EPIDs. The frequency of 50% modulation ( f 50 ) of the measured MTF is 0.2 mm - 1 at 6 MV, which is comparable to that of video-based EPIDs. The scatter to primary ratio (SPR) measured with the detector at 10 cm air gap and 20 × 20 cm 2 field size is approximately 30% lower than that of ionization chamber-based detectors with a comparable QE. The detector noise which includes the x-ray quantum noise and absorption noise is much larger than the electronic noise per pixel of the flat-panel imager at a dose of less than two LINAC pulses. Thus, the proposed detector is quantum noise limited down to very low doses (∼a couple of radiation pulses of the LINAC). A proof-of-concept image has been obtained using a 6-MV beam. CONCLUSIONS: This work indicates that by using scintillating fibers and lead layers it is possible to increase the thickness of the detecting materials, and therefore the QE or the DQE(0) of the detector, while maintaining an adequate spatial resolution for MV x-ray imaging. Due to the use of lead as the spacing material, the new detector also has antiscatter property, which will help improve the signal-to-noise ratio of the images. Further investigation to optimize the design of the detector and achieve a better combination of DQE and spatial resolution is warranted.