Narrow Energy Window Research Articles

The Effect of FEW Scatter Correction Method in In-111 Imaging

Introduction: In nuclear medicine imaging, detection of the scattered photons along with the primary photons is one of the major problems that can lead to a decrease in ddiagnostic accuracy. Therefore, use of a suitable scatter correction method can help to improve of the image quality and increase of diagnostic accuracy. The aim of this study is evaluation of five energy window (FEW) method for scatter correction of In-111 imaging that can be used for evaluation of brain tumor. Materials and Methods: For evaluation of the scatter correction method, six cold (without activity) spheres with diameters 3.2, 2.6, 2, 1.6, 1.3 and 1 cm placed in the water-filled cylindrical phantom is simulated by SIMIND Monte Carlo simulation program (version 6.1). The scatter correction is performed by setting five energy window in In-111 energy spectrum: two 20% photopeak energy window centered on E1=171 keV and E2=245keV, and three narrow energy windows, including two 8% energy window centered on lower- and upper-energy limits of the first photopeak energy window, and a 8% energy window centered on lower-energy limit of the second photopeak energy window. To evaluate the performance of the scatter correction method, the image contrasts and signal-to-noise ratios were calculated for six cold spheres. Results: The results obtained in this study are shown that the relative increase of the image contrast of the cold sphere 1, 2, 3, 4, 5 and 6 is about 31.3, 34.16, 26.73, 4.50, 2.48 and 5.76%, respectively. The improvement in the image contrast of sphere 6 is larger than that of sphere 5 that shows the nonuniform performance of this scatter correction method. The relative noise of background for the uncorrected and the corrected images is 0.031 and 0.0878, respectively, that shows the scatter correction by the FEW method results in the increase of the image noise significantly. . Conclusion: The results of this study show that the FEW scatter correction method improves the image contrasts of six cold spheres nonuniformly with a significant increase in the level of noise

Non-Local Low-Rank Cube-Based Tensor Factorization for Spectral CT Reconstruction.

Spectral computed tomography (CT) reconstructs material-dependent attenuation images from the projections of multiple narrow energy windows, which is meaningful for material identification and decomposition. Unfortunately, the multi-energy projection datasets usually have lower signal-noise ratios (SNR). Very recently, a spatial-spectral cube matching frame (SSCMF) was proposed to explore the non-local spatial-spectral similarities for spectral CT. This method constructs a group by clustering up a series of non-local spatial-spectral cubes. The small size of spatial patches for such a group makes the SSCMF fail to fully encode the sparsity and low-rank properties. The hard-thresholding and collaboration filtering in the SSCMF also cause difficulty in recovering the image features and spatial edges. While all the steps are operated on 4-D group, the huge computational cost and memory load might not be affordable in practice. To avoid the above limitations and further improve the image quality, we first formulate a non-local cube-based tensor instead of group to encode the sparsity and low-rank properties. Then, as a new regularizer, the Kronecker-basis-representation tensor factorization is employed into a basic spectral CT reconstruction model to enhance the capability of image feature extraction and spatial edge preservation, generating a non-local low-rank cube-based tensor factorization (NLCTF) method. Finally, the split-Bregman method is adopted to solve the NLCTF model. Both numerical simulations and preclinical mouse studies are performed to validate and evaluate the NLCTF algorithm. The results show that the NLCTF method outperforms the other state-of-the-art competing algorithms.

Spatial-spectral cube matching frame for spectral CT reconstruction

Spectral computed tomography (CT) reconstructs the same scanned object from projections of multiple narrow energy windows, and it can be used for material identification and decomposition. However, the multi-energy projection dataset has a lower signal-noise-ratio (SNR), resulting in poor reconstructed image quality. To address this thorny problem, we develop a spectral CT reconstruction method, namely spatial-spectral cube matching frame (SSCMF). This method is inspired by the following three facts: (i) human body usually consists of two or three basic materials implying that the reconstructed spectral images have a strong sparsity; (ii) the same basic material component in a single channel image has similar intensity and structures in local regions. Different material components within the same energy channel share similar structural information; (iii) multi-energy projection datasets are collected from the subject by using different narrow energy windows, which means images reconstructed from different energy-channels share similar structures. To explore those information, we first establish a tensor cube matching frame (CMF) for a BM4D denoising procedure. Then, as a new regularizer, the CMF is introduced into a basic spectral CT reconstruction model, generating the SSCMF method. Because the SSCMF model contains an L0-norm minimization of 4D transform coefficients, an effective strategy is employed for optimization. Both numerical simulations and realistic preclinical mouse studies are performed. The results show that the SSCMF method outperforms the state-of-the-art algorithms, including the simultaneous algebraic reconstruction technique, total variation minimization, total variation plus low rank, and tensor dictionary learning.

Pressure-induced organic topological nodal-line semimetal in the three-dimensional molecular crystal Pd(dddt)2

The nodal-line semimetal represents a class of topological materials characterized with highest band degeneracy. It is usually found in inorganic materials of high crystal symmetry or a minimum symmetry of inversion aided with accidental band degeneracy [Phys. Rev. Lett. 118, 176402 (2017)]. Based on first-principles band structure, Wannier charge center, and topological surface state calculations, here we predict a pressure-induced topological nodal-line semimetal in the absence of spin-orbit coupling (SOC) in the synthesized single-component 3D molecular crystal $\mathrm{Pd}{(\mathrm{dddt})}_{2}$. We show a $\mathrm{\ensuremath{\Gamma}}$-centered single nodal line undulating within a narrow energy window across the Fermi level. This intriguing nodal line is generated by pressure-induced accidental band degeneracy, without protection from any crystal symmetry. When SOC is included, the fourfold degenerated nodal line is gapped and $\mathrm{Pd}{(\mathrm{dddt})}_{2}$ becomes a strong 3D topological metal with an ${Z}_{2}$ index of (1;000). However, the tiny SOC gap makes it still possible to detect the nodal-line properties experimentally. Our findings afford an attractive route for designing and realizing topological states in 3D molecular crystals, as they are weakly bonded through van der Waals forces with a low crystal symmetry so that their electronic structures can be easily tuned by pressure.

Comprehensive Experimental and Computational Spectroscopic Study of Hexacyanoferrate Complexes in Water: From Infrared to X-ray Wavelengths.

We present a joint experimental and computational study of the hexacyanoferrate aqueous complexes at equilibrium in the 250 meV to 7.15 keV regime. The experiments and the computations include the vibrational spectroscopy of the cyanide ligands, the valence electronic absorption spectra, and Fe 1s core hole spectra using element-specific-resonant X-ray absorption and emission techniques. Density functional theory-based quantum mechanics/molecular mechanics molecular dynamics simulations are performed to generate explicit solute-solvent configurations, which serve as inputs for the spectroscopy calculations of the experiments spanning the IR to X-ray wavelengths. The spectroscopy simulations are performed at the same level of theory across this large energy window, which allows for a systematic comparison of the effects of explicit solute-solvent interactions in the vibrational, valence electronic, and core-level spectra of hexacyanoferrate complexes in water. Although the spectroscopy of hexacyanoferrate complexes in solution has been the subject of several studies, most of the previous works have focused on a narrow energy window and have not accounted for explicit solute-solvent interactions in their spectroscopy simulations. In this work, we focus our analysis on identifying how the local solvation environment around the hexacyanoferrate complexes influences the intensity and line shape of specific spectroscopic features in the UV/vis, X-ray absorption, and valence-to-core X-ray emission spectra. The identification of these features and their relationship to solute-solvent interactions is important because hexacyanoferrate complexes serve as model systems for understanding the photochemistry and photophysics of a large class of Fe(II) and Fe(III) complexes in solution.

Selective state spectroscopy and multifractality in disordered Bose-Einstein condensates: a numerical study

We propose to apply a modified version of the excitation scheme introduced by Volchkov et al. on bosons experiencing hyperfine state dependent disorder to address the critical state at the mobility edge of the Anderson localization transition, and to observe its intriguing multifractal structure. An optimally designed, spatially focused external radio frequency pulse can be applied to generate transitions to eigenstates in a narrow energy window close to the mobility edge, where critical scaling and multifractality emerge. Alternatively, two-photon laser scanning microscopy is proposed to address individual localized states even close to the transition. The projected image of the cloud is shown to inherit multifractality and to display universal density correlations. Interactions – unavoidably present – are taken into account by solving the Gross-Pitaevskii equations, and their destructive effect on the spectral resolution and the multifractal spectrum is analyzed. Time of flight images of the excited states are predicted to show interference fringes in the localized phase, while they allow one to map equal energy surfaces deep in the metallic phase.

Crystal Delay and Time Walk Correction Methods for Coincidence Resolving Time Improvements of a Digital-Silicon-Photomultiplier-Based PET/MRI Insert

Our group has built the MRI compatible PET insert Hyperion IID, which is based on digital silicon photomultipliers [digital photon counters (DPCs)]. We characterized its performance for two different scintillator configurations already in previous publications. For this paper, we reused the raw detector data that were used in previous publications and stored for offline analysis. We reprocessed these data in order to improve the timing performance of the PET scanner. Different crystal delay and time walk calibration and correction methods are evaluated with respect to the coincidence resolving time (CRT) of the scanner. For all applied correction methods, we report CRTs both for a preclinical and a clinical scintillator configuration for all DPC trigger schemes and several energy windows. The newly obtained results were compared with the previously published values, and the additional benefit of using a time walk correction was evaluated. Previously published CRTs could be improved using a refined version of the original crystal delay calibration method without applying a walk correction by 6.3%–10.9% for a wide energy window ranging from 250–625 keV, and we could show an additional improvement of up to 2.6% by adding a walk correction. Using trigger scheme 1 and a very narrow energy window around the photopeak (500–520 keV), we could reach CRT values on system level of 208/240 ps for the clinical/preclinical scintillator configuration.

Quasi-monochromatic imaging in x-ray CT via spectral deconvolution using photon-counting detectors

Photon-counting detectors can obtain the spectral information from an incident x-ray spectrum, although the detected counts may differ from the incident counts due to the detector response. If uncorrected or uncompensated, the response will lead to distortion in CT reconstruction. With the intention of reducing the distortion and exploring the potential of photon-counting detectors, a novel reconstruction strategy with spectral deconvolution, which attempts to set itself apart from traditional material decomposition frameworks, is proposed in this paper. It applies deconvolution to the energy window counts using a calibrated detector response and then uses the post-deconvolution photon counts to reconstruct images in multi-energy windows. The output has a quantitative meaning as a quasi-monochromatic attenuation coefficient, because a relatively narrow energy window width is selected. The deconvolution settings and results are carefully discussed in the numerical simulation. An experimental study is then carried out to verify the effectiveness and robustness. The results show that the reconstructed attenuation coefficients after deconvolution fit the standard reference data very well in most of the energy windows, which implies the feasibility of this quasi-monochromatic imaging method.

Alignment and absolute wavelength calibration of imaging Bragg spectrometers.

In the present and the next generation of fusion devices, imaging Bragg spectrometers are key diagnostics to measure plasma parameters in the hot core, especially ion temperature and plasma rotation. The latter quantities are routinely obtained using the Doppler-width and -shift of the emitted spectral lines, respectively. Line shift measurements require absolute accuracies Δλ/λ of about 10 ppm, where λ-is the observed wavelength. For ITER and the present fusion devices, spectral lines of He-and H-like argon, iron, and krypton as well as Ne-like tungsten are foreseen for the measurements. For these lines, Kα lines can be found, some in higher order, which fit into the narrow energy window of the spectrometers. For arbitrary wavelength settings, Kα lines are also used to measure the miscut of the spherical crystals; afterwards the spectrometers can be set according to the geometrical imaging properties using coordinate measurement machines. For the spectrometers measuring Lyα lines of H-like ions, fluorescence targets can provide in situ localized calibration lines on the spectra. The fluorescence targets are used best in transmission and are excited by the thermal x-ray radiation of the plasma. An analytic theory of fluorescence is worked out.

Plasma-graphene interaction and its effects on nanoscale patterning

Scalable and precise nanopatterning of graphene is an essential step for graphene-based device fabrication. Hydrogen-plasma reactions have been shown to narrow graphene only from the edges, or to selectively produce circular or hexagonal holes in the basal plane of graphene, but the underlying plasma-graphene chemistry is unknown. Here, we study the hydrogen-plasma etching of monolayer graphene supported on $\mathrm{Si}{\mathrm{O}}_{2}$ substrates across the range of plasma ion energies using scale-bridging molecular dynamics (MD) simulations based on reactive force-field potential. Our results uncover distinct etching mechanisms, operative within narrow ion energy windows, which fully explain the differing plasma-graphene reactions observed experimentally. Specific ion energy ranges are demonstrated for stable isotropic ($\ensuremath{\sim}2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) versus anisotropic hole growth ($\ensuremath{\sim}20--30\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) within the basal plane of graphene, as well as for pure edge etching of graphene ($\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$). Understanding the complex plasma-graphene chemistry opens up a means for controlled patterning of graphene nanostructures.

PET performance and MRI compatibility evaluation of a digital, ToF-capable PET/MRI insert equipped with clinical scintillators

We evaluate the MR compatibility of the Hyperion-IID positron emission tomography (PET) insert, which allows simultaneous operation in a clinical magnetic resonance imaging (MRI) scanner. In contrast to previous investigations, this work aims at the evaluation of a clinical crystal configuration. An imaging-capable demonstrator with an axial field-of-view of 32 mm and a crystal-to-crystal spacing of 217.6 mm was equipped with LYSO scintillators with a pitch of 4 mm which were read out in a one-to-one coupling scheme by sensor tiles composed of digital silicon photomultipliers from Philips Digital Photon Counting (DPC 3200-22). The PET performance degradation (energy resolution and coincidence resolution time (CRT)) was evaluated during simultaneous operation of the MRI scanner. We used clinically motivated imaging sequences as well as synthetic gradient stress test sequences. Without activity of the MRI scanner, we measured for trigger scheme 1 (first photon trigger) an energy resolution of 11.4% and a CRT of 213 ps for a narrow energy (NE) window using five 22Na point-like sources. When applying the synthetic gradient sequences, we found worst-case relative degradations of the energy resolution by 5.1% and of the CRT by 33.9%. After identifying the origin of the degradations and implementing a fix to the read-out hardware, the same evaluation revealed no degradation of the PET performance anymore even when the most demanding gradient stress tests were applied. The PET performance of the insert was initially evaluated using the point sources, a high-activity phantom and hot-rod phantoms in order to assess the spatial resolution. Trigger schemes 2–4 delivered an energy resolution of 11.4% as well and CRTs of 279 ps, 333 ps and 557 ps for the NE window, respectively. An isocenter sensitivity of 0.41% using the NE window and 0.71% with a wide energy window was measured. Using a hot-rod phantom, a spatial resolution in the order of 2 mm was demonstrated and the benefit of time-of-flight PET was shown with a larger rabbit-sized phantom. In conclusion, the Hyperion architecture is an interesting platform for clinically driven hybrid PET/MRI systems.

XPS study of the effects of long-term Ar+ ion and Ar cluster sputtering on the chemical degradation of hydrozincite and iron oxide

Monoatomic ion guns mounted on X-ray photoelectron spectrometers are frequently used for depth profiling to determine the depth distribution of various chemical compounds, or for surface cleaning. Sputtering with single ions may cause severe damage to some materials. Hence, in this study the influence of different sputter parameters on the degradation kinetics was examined. For comparison, the potential of Ar cluster sputtering was tested with the same materials, namely hydrozincite and FeO – two representatives of corrosion products that are susceptible to degradation. Chemical damage could only be minimized by cooling or cluster sputtering within a narrow cluster energy window.

ToF Performance Evaluation of PET Modules With Digital Silicon Photomultiplier Technology During MR Operation

In 2012, we presented the Hyperion-II $^{\rm D}$ preclinical PET insert which uses Philips Digital Photon Counting’s digital SiPMs and is designed to be operated in a 3-T MRI. In this work we use the same platform equipped with scintillators having dimensions closer to a clinical application. This allows an investigation of the time of flight (ToF) performance of the platform and its behavior during simultaneous MR operation. We employ LYSO crystal arrays of $4\times 4 \times 10~\hbox{mm}^{3}$ coupled to $4 \times 4$ PDPC DPC 3200-22 sensors (DPC) resulting in a one-to-one coupling of crystals to read-out channels. Six sensor stacks are mounted onto a singles processing unit in a $2 \times 3$ arrangement. Two modules are mounted horizontally facing each other on a gantry with a crystal-to-crystal spacing of 217.6 mm (gantry position). A second arrangement places the modules at the maximum distance of approximately 410 mm inside the MR bore (maximum distance position) which brings each module close to the gradient system. The DPCs are cooled down to approximately $5 - {10^ \circ }{\rm C}$ under operation. We disable 20% of the worst cells and use an overvoltage of ${V_{{\rm ov}}} = 2.0~\hbox{V}$ and 2.5 V. To obtain the best time stamps, we use the trigger scheme 1 (first photon trigger), a narrow energy window of $511 \pm 50~\hbox{keV}$ and a minimum required light fraction of the main pixel of more than 65% to reject intercrystal scatter. By using a $^{22}\hbox{Na}$ point source in the isocenter of the modules, the coincidence resolution time (CRT) of the two modules is evaluated inside the MRI system without MR activity and while using highly demanding gradient sequences. Inside the ${{\rm B}_0}$ field without any MR activity at an overvoltage of ${V_{{\rm ov}}} = 2.0~\hbox{V}$ , the energy resolution is 11.45% (FWHM) and the CRT is 250 ps (FWHM). At an overvoltage of ${V_{{\rm ov}}} = 2.5~\hbox{V}$ , the energy resolution is 11.15% (FWHM) and the CRT is 240 ps (FWHM). During a heavy $z$ -gradient sequence (EPI factor: 49, gradient strength: 30 mT/m, slew rate: 192.3 mT/m/ms, TE/TR: 12/25 ms and switching duty cycle: 67%) at the gantry position and an overvoltage of ${V_{{\rm ov}}} = 2.0~\hbox{V}$ , the energy resolution is degraded relatively by 4.1% and the CRT by 25%. Using the same sequence but at the maximum distance position and an overvoltage of ${V_{{\rm ov}}} = 2.5~\hbox{V}$ , we measure a degradation of the energy resolution of 9.2% and a 52% degradation of the CRT. The Hyperion-II $^{\rm D}$ platform proofs to deliver good timing performance and energy resolution inside the MRI system even under highly demanding gradient sequences.

Performance assessment of a NaI(Tl) gamma counter for PET applications with methods for improved quantitative accuracy and greater standardization.

BackgroundAlthough NaI(Tl) gamma counters play an important role in many quantitative positron emission tomography (PET) protocols, their calibration for positron-emitting samples has not been standardized across imaging sites. In this study, we characterized the operational range of a gamma counter specifically for positron-emitting radionuclides, and we assessed the role of traceable 68Ge/68Ga sources for standardizing system calibration.MethodsA NaI(Tl) gamma counter was characterized with respect to count rate performance, adequacy of detector shielding, system stability, and sample volume effects using positron-emitting radionuclides (409- to 613-keV energy window). System efficiency was measured using 18F and compared with corresponding data obtained using a long-lived 68Ge/68Ga source that was implicitly traceable to a national standard.ResultsOne percent count loss was measured at 450 × 103 counts per minute. Penetration of the detector shielding by 511-keV photons gave rise to a negligible background count rate. System stability tests showed a coefficient of variation of 0.13% over 100 days. For a sample volume of 4 mL, the efficiencies relative to those at 0.1 mL were 0.96, 0.94, 0.91, 0.78, and 0.72 for 11C, 18F, 125I, 99mTc, and 51Cr, respectively. The efficiency of a traceable 68Ge/68Ga source was 30.1% ± 0.07% and was found to be in close agreement with the efficiency for 18F after consideration of the different positron fractions.ConclusionsLong-lived 68Ge/68Ga reference sources, implicitly traceable to a national metrology institute, can aid standardization of gamma counter calibration for 18F. A characteristic feature of positron emitters meant that accurate calibration could be maintained over a wide range of sample volumes by using a narrow energy window centered on the 511-keV peak.

ToF performance evaluation of a PET insert with Digital Silicon Photomultiplier technology during MR operation.

2012, our group presented the Hyperion IID platform which uses PDPC’s digitial SiPMs (DPC). In this work we use the same platform equipped with scintillator dimensions closer to a clinical application. This allows an investigation of the time of flight (ToF) performance of the platform and its behavior during simultaneous MR operation. We employ LYSO crystal arrays of 4×4×10 mm³ coupled to 4×4 PDPC DPC 3200-22 sensors allowing a one-to-one coupling of crystals to readout channels. Six sensor stacks are mounted onto a singles processing unit in a 2×3 arrangement. Two units are mounted on a gantry with a diameter of 216 mm. The DPCs are cooled down to approximately 5-10 °C under operation. We disable 20% of the worst cells and use an overvoltage (OV) of 2.0V and 2.5V. To obtain the best time stamps we use the first photon trigger and employ data quality cuts to filter out crystal scatter events. A narrow energy window of 511±50 keV is used and a minimal light fraction of the main pixel of more than 65% is requested. Using a Na22 point source in the isocenter of the modules the coincidence resolution time (CRT) of the two modules is evaluated outside the MR and inside the MR using different MR sequences. Gradient stress tests with switching z-gradients are performed. Inside the B0 field at 2.0V overvoltage the energy resolution is 11.45% (FWHM) and the CRT is 250ps (FWHM). At 2.5V overvoltage, the energy resolution is 11.15% (FWHM) and the CRT is 240ps (FWHM). During the heavy z-gradient sequence the energy resolution is degraded by 4.1% at 2V and 9.2% at 2.5V. The degradation of the CRT is 25% at 2V and 52% at 2.5V OV. During standard TSE and EPI sequences the CRT and energy resolution is not influenced. The Hyperion IID platform proofs to deliver good timing performance outside and inside an MR. The CRT is not influenced (<0.2%) using normal MR imaging sequences.

Optimization of the Energy Window for PETbox4, a Preclinical PET Tomograph With a Small Inner Diameter.

Small animal positron emission tomography (PET) systems are often designed by employing close geometry configurations. Due to the different characteristics caused by geometrical factors, these tomographs require data acquisition protocols that differ from those optimized for conventional large diameter ring systems. In this work we optimized the energy window for data acquisitions with PETbox4, a 50 mm detector separation (box-like geometry) pre-clinical PET scanner, using the Geant4 Application for Tomographic Emission (GATE). The fractions of different types of events were estimated using a voxelized phantom including a mouse as well as its supporting chamber, mimicking a realistic mouse imaging environment. Separate code was developed to extract additional information about the gamma interactions for more accurate event type classification. Three types of detector backscatter events were identified in addition to the trues, phantom scatters and randoms. The energy window was optimized based on the noise equivalent count rate (NECR) and scatter fraction (SF) with lower-level discriminators (LLD) corresponding to energies from 150 keV to 450 keV. The results were validated based on the calculated image uniformity, spillover ratio (SOR) and recovery coefficient (RC) from physical measurements using the National Electrical Manufacturers Association (NEMA) NU-4 image quality phantom. These results indicate that when PETbox4 is operated with a more narrow energy window (350-650 keV), detector backscatter rejection is unnecessary. For the NEMA NU-4 image quality phantom, the SOR for the water chamber decreases by about 45% from 15.1% to 8.3%, and the SOR for the air chamber decreases by 31% from 12.0% to 8.3% at the LLDs of 150 and 350 keV, without obvious change in uniformity, further supporting the simulation based optimization. The optimization described in this work is not limited to PETbox4, but also applicable or helpful to other small inner diameter geometry scanners.

Spin-dependent electronic conduction along zigzag graphene nanoribbons bearing adsorbed Ni and Fe nanostructures

Using SMEAGOL, an ab initio computational method that combines the non-equilibrium Green's function formalism with density-functional theory, we calculated spin-specific electronic conduction in systems consisting of single Fen and Nin nanostructures (n = 1−4) adsorbed on a hydrogen-passivated zigzag graphene nanoribbon. For each cluster we considered both ferromagnetically and antiferromagnetically coupled ribbon edges (Ferro-F and Ferro-A systems, respectively). Adstructures located laterally on Ferro-A ribbons caused significant transmittance loss at energies 0.6–0.25 eV below the Fermi level for one spin and 0.2–0.4 eV above the Fermi level for the other, allowing the potential use of these systems in transistors to create a moderately spin-polarized current of one or the other sign depending on the gate voltage. Ni3 and Ni4 clusters located at the centre of Ferro-F ribbons exhibited a strong spin-filtering effect in a narrow energy window around the Fermi level.

Reduction method for intrinsic random coincidence events from 176Lu in low activity PET imaging

For clinical studies, the effects of the intrinsic radioactivity of lutetium-based scintillators such as LSO used in PET imaging can be ignored within a narrow energy window. However, the intrinsic radioactivity becomes problematic when used in low-count-rate situations such as gene expression imaging or in-beam PET imaging. Time-of-flight (TOF) measurement capability promises not only to improve PET image quality, but also to reduce intrinsic random coincidences. On the other hand, we have developed a new reduction method for intrinsic random coincidence events based on multiple-coincidence information. Without the energy window, an intrinsic random coincidence is detected simultaneously with an intrinsic true coincidence as a multiple coincidence. The multiple-coincidence events can serve as a guide to identification of the intrinsic coincidences. After rejection of multiple-coincidence events detected with a wide energy window, data obtained included a few intrinsic random and many intrinsic true coincidence events. We analyzed the effect of intrinsic radioactivity and used Monte Carlo simulation to test both the TOF-based method and the developed multiple-coincidence-based (MC-based) method for a whole-body LSO-PET scanner. Using the TOF- and MC-based reduction methods separately, we could reduce the intrinsic random coincidence rates by 77 and 30 %, respectively. Also, the intrinsic random coincidence rate could be reduced by 84 % when the TOF+MC reduction methods were applied. The developed MC-based method showed reduced number of the intrinsic random coincidence events, but the reduction performance was limited compared to that of the TOF-based reduction method.

Optimal Sunlight Harvesting in Photovoltaics and Photosynthesis

Materials employed to harvest sunlight are commonly recognized to be at a premium when their optical absorption peaks in the visible, extends to the infrared, is panchromatic, and is matched to the solar spectrum. By contrast, natural photosynthetic absorbers such as chlorophylls and carotenoids display absorption spectra with narrow peaks for yet-unknown evolutionary reasons. Beyond such general observations, a rigorous treatment of sunlight harvesting optimization is still lacking. In this work, we provide a quantitative analysis of optimal solar energy harvesting in materials. We derive optimal absorption spectra as a function of absorber thickness, elucidate the concept of solar-matched absorption and its applicability limits, and define a procedure to rank photovoltaic materials for sunlight harvesting. In addition, we suggest a possible explanation for why absorption in plant photosynthetic pigments occurs in narrow energy windows.

Plasma enhanced growth of single walled carbon nanotubes at low temperature: A reactive molecular dynamics simulation

Low-temperature growth of carbon nanotubes (CNTs) has been claimed to provide a route towards chiral-selective growth, enabling a host of applications. In this contribution, we employ reactive molecular dynamics simulations to demonstrate how plasma-based deposition allows such low-temperature growth. We first show how ion bombardment during the growth affects the carbon dissolution and precipitation process. We then continue to demonstrate how a narrow ion energy window allows CNT growth at 500K. Finally, we also show how CNTs in contrast cannot be grown in thermal CVD at this low temperature, but only at high temperature, in agreement with experimental data.