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ABSTRACT: BACKGROUND: Partial volume effects (PVEs) in PET imaging result in incorrect regional activity estimates due to both spill-out and spill-in from activity in neighboring regions. It is important to compensate for both effects to achieve accurate quantification. In this study, an image-based partial volume compensation (PVC) method was developed and validated for cardiac PET. METHODS AND RESULTS: The method uses volume-of-interest (VOI) maps segmented from contrast-enhanced CTA images to compensate for both spill-in and spill-out in each VOI. The PVC method was validated with simulation studies and also applied to images of dog cardiac perfusion PET data. The PV effects resulting from cardiac motion and myocardial uptake defects were investigated and the efficacy of the proposed PVC method in compensating for these effects was evaluated. RESULTS: Results indicate that the magnitude and the direction of PVEs in cardiac imaging change over time. This affects the accuracy of activity distributions estimates obtained during dynamic studies. The defect regions have different PVEs as compared to the normal myocardium. Cardiac motion contributes around 10% to the PVEs. PVC effectively removed both spill-in and spill-out in cardiac imaging. CONCLUSIONS: PVC improved left ventricular wall uniformity and quantitative accuracy. The best strategy for PVC was to compensate for the PVEs in each cardiac phase independently and treat severe uptake defects as independent regions from the normal myocardium.
Journal of Nuclear Cardiology 11/2012; · 2.67 Impact Factor
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ABSTRACT: Hydrogen abstraction reaction of fenofibric acid (FA) in acetonitrile and isopropyl alcohol solvents was studied by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy experiments. The singlet excite state ((1)FA) (nπ*) with a maximum transient absorption at 352 nm observed in the fs-TA experiments undergoes efficient intersystem crossing (ISC) to convert into a nπ* triplet state FA ((3)FA) that exhibits two transient absorption bands at 345 and 542 nm. The nπ* (3)FA species does not decay obviously within 3000 ps. In the ns-TR(3) experiments, the nπ* (3)FA is also observed and completely decays by 120 ns. Compared with the triplet states of benzophenone (BP) and ketoprofen (KP), the nπ* (3)FA species seems to have a much higher hydrogen abstraction reactivity so that (3)FA decays fast and generates a FA ketyl radical like species. In isopropyl alcohol solvent, the nπ* (3)FA exhibits similar reactivity and promptly abstracts a hydrogen from the strong hydrogen donor isopropyl alcohol solvent to generate a ketyl radical intermediate. With the decay of the FA ketyl radical, no light absorption transient (LAT) intermediate is observed in isopropyl alcohol solvent although such a LAT species was observed after similar experiments for BP and KP. Comparison of the ns-TR(3) spectra for the species of interest with results from density functional theory calculations were used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the ns-TR(3) spectra. This comparison provides insight into the structure and hydrogen abstraction reactivity of the triplet states of BP derivatives.
The Journal of Physical Chemistry A 11/2011; 115(49):14168-74. · 2.95 Impact Factor
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Jiadan Xue,
Shubham Vyas, Yong Du,
Hoi Ling Luk,
Yung Ping Chuang,
Tracy Yuen Sze But,
Patrick H Toy,
Jin Wang,
Arthur H Winter,
David Lee Phillips,
Christopher M Hadad,
Matthew S Platz
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ABSTRACT: A time-resolved resonance Raman (TR(3)) and computational investigation of the photochemistry of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide in acetonitrile is presented. Photolysis of 4-acetamidophenyl azide appears to initially produce singlet 4-acetamidophenylnitrene which undergoes fast intersystem crossing (ISC) to form triplet 4-acetamidophenylnitrene. The latter species formally produces 4,4'-bisacetamidoazobenzene. RI-CC2/TZVP and TD-B3LYP/TZVP calculations predict the formation of the singlet nitrene from the photogenerated S(1) surface of the azide excited state. The triplet 4-acetamidophenylnitrene and 4,4'-bisacetamidoazobenzene species are both clearly observed on the nanosecond to microsecond time-scale in TR(3) experiments. In contrast, only one species can be observed in analogous TR(3) experiments after photolysis of 4-N-methylacetamidophenyl azide in acetonitrile, and this species is tentatively assigned to the compound resulting from dimerization of a 1,2-didehydroazepine. The different photochemical reaction outcomes for the photolysis of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide molecules indicate that the 4-acetamido group has a substantial influence on the ISC rate of the corresponding substituted singlet phenylnitrene, but the 4-N-methylacetamido group does not. CASSCF analyses predict that both singlet nitrenes have open-shell electronic configurations and concluded that the dissimilarity in the photochemistry is probably due to differential geometrical distortions between the states. We briefly discuss the probable implications of this intriguing substitution effect on the photochemistry of phenyl azides and the chemistry of the related nitrenes.
The Journal of Physical Chemistry A 06/2011; 115(26):7521-30. · 2.95 Impact Factor
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ABSTRACT: The radionuclide 131I has found widespread use in targeted radionuclide therapy (TRT), partly due to the fact that it emits photons that can be imaged to perform treatment planning or posttherapy dose verification as well as beta rays that are suitable for therapy. In both the treatment planning and dose verification applications, it is necessary to estimate the activity distribution in organs or tumors at several time points. In vivo estimates of the 131I activity distribution at each time point can be obtained from quantitative single-photon emission computed tomography (QSPECT) images and organ activity estimates can be obtained either from QSPECT images or quantification of planar projection data. However, in addition to the photon used for imaging, 131I decay results in emission of a number of other higher-energy photons with significant abundances. These higher-energy photons can scatter in the body, collimator, or detector and be counted in the 364 keV photopeak energy window, resulting in reduced image contrast and degraded quantitative accuracy; these photons are referred to as downscatter. The goal of this study was to develop and evaluate a model-based downscatter compensation method specifically designed for the compensation of high-energy photons emitted by 131I and detected in the imaging energy window.
In the evaluation study, we used a Monte Carlo simulation (MCS) code that had previously been validated for other radionuclides. Thus, in preparation for the evaluation study, we first validated the code for 131I imaging simulation by comparison with experimental data. Next, we assessed the accuracy of the downscatter model by comparing downscatter estimates with MCS results. Finally, we combined the downscatter model with iterative reconstruction-based compensation for attenuation (A) and scatter (S) and the full (D) collimator-detector response of the 364 keV photons to form a comprehensive compensation method. We evaluated this combined method in terms of quantitative accuracy using the realistic 3D NCAT phantom and an activity distribution obtained from patient studies. We compared the accuracy of organ activity estimates in images reconstructed with and without addition of downscatter compensation from projections with and without downscatter contamination.
We observed that the proposed method provided substantial improvements in accuracy compared to no downscatter compensation and had accuracies comparable to reconstructions from projections without downscatter contamination.
The results demonstrate that the proposed model-based downscatter compensation method is effective and may have a role in quantitative 131I imaging.
Medical Physics 06/2011; 38(6):3193-204. · 2.83 Impact Factor
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ABSTRACT: Our goal was to evaluate a multi-pinhole (MPH) collimator which allows changing configurations for mouse imaging and rat imaging.
The collimator length can be adjusted from 5 cm for rat imaging to a maximum of 8 cm for mouse imaging. Projections of mouse- and rat-size phantoms were simulated with collimator length of 8 cm, and the rat-size phantom was additionally simulated with collimator length of 5 cm. Bias and noise were assessed in the reconstructed images. Three physical phantoms were used to evaluate the axial sampling and resolutions for one-, four-, and five-pinhole single photon emission computed tomography (SPECT). Images of three different-sized rodents were also acquired.
Simulations showed that for rat imaging, shorter collimator length provided an improved bias-noise trade-off compared to that of longer collimator length. Axial distortions were significantly reduced for MPH compared to single pinhole imaging. The smallest rods visible for mouse imaging and rat imaging were 1 and 1.6 mm, respectively, and their corresponding absolute sensitivities were 3.47% and 2.02% at the center field-of-view for 5-pinhole imaging. The count ratios were 1:3.78:4.42, respectively, for one-, four-, and five-pinhole for same acquisition time. Good image quality was observed in real animal studies.
This collimator allows flexible single pinhole and MPH SPECT imaging for rodents, achieving high resolution and detection efficiency with minimal image artifacts.
Molecular imaging and biology: MIB: the official publication of the Academy of Molecular Imaging 02/2011; 14(1):60-9. · 2.47 Impact Factor
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ABSTRACT: A combined femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopic investigation of the photoreaction of 2-benzoylpyridine (2-BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs-TA results showed that the npi* triplet 2-BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns-TR(3) spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground-state conformation of 2-BPy in acidic aqueous solvent is the pyridine nitrogen-protonated 2-BPy cation (2-BPy-NH(+)) rather than the neutral form of 2-BPy. After laser photolysis, the singlet excited state (S(1)) of 2-BPy-NH(+) is generated and evolves through excited-state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T(1)) of the carbonyl oxygen-protonated 2-BPy cation (2-BPy-OH(+)) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen-bonding of the solvent on the reactions.
Chemistry 06/2010; 16(23):6961-72. · 5.93 Impact Factor
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ABSTRACT: Ketoprofen is an important photosensitive drug molecule that has received much attention for the study of its photochemistry in different solvents. In this paper, nanosecond time-resolved resonance Raman spectroscopy was utilized to investigate the photochemistry of ketoprofen in aqueous solutions with varying water concentrations. The rate constants and reaction mechanism of ketoprofen are strongly dependent on the concentration of the solvent. In neat acetonitrile and solvents with low concentrations of water (like water-acetonitrile <or= 1 : 1, v/v), ketoprofen exhibits mostly benzophenone-like photochemistry to generate a triplet state which in turn produces a ketyl radical-like species by a hydrogen abstraction reaction. However, in solvents with very high concentrations of water (such as water-acetonitrile >or= 9 : 1, v/v), the triplet state ketoprofen is observed first and then undergoes a prompt decarboxylation process to form a triplet protonated biradical carbanion species. For solvents with moderate higher water concentrations (such as between 50% and 90% water by volume), the hydrogen abstraction and decarboxylation processes are two competitive pathways with different rate constants. The triplet state of ketoprofen will simultaneously produce a ketyl radical species and a triplet protonated biradical carbanion species with the amount of each species dependent on the water concentration.
Physical Chemistry Chemical Physics 05/2010; 12(18):4800-8. · 3.57 Impact Factor
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ABSTRACT: A nanosecond time-resolved resonance Raman investigation of the photoreactions of 3-benzoylpyridine (3-BPy) in different pH aqueous solutions is reported. In neutral, basic, and pH = 5 aqueous solution conditions, the photoreduction reaction from the triplet 3-BPy species is observed to produce the corresponding 3-phenyl pyridyl ketyl radical that was also observed in a 2-propanol solvent. Under moderate acidic conditions (at pH = 3 for example), most of the 3-BPy triplet state species goes through two protonation steps at the nitrogen atom and the carbonyl oxygen atom after UV laser photolysis and then forms a short-lived hydration intermediate via a hydration reaction at the ortho position in the benzene ring. This new species is tentatively assigned to the o-3[3-BPyH+.H2O] hydration species. In acidic aqueous solutions with a pH < or = 1, the protonated triplet states of 3-BPy cations at the nitrogen atom are generated from photoexcitation of the protonated ground state and are subsequently further protonated at the carbonyl oxygen atom to form a 3-BPy-dication triplet state. This dication intermediate reacts with water molecules at the ortho position of the benzene ring to produce the o-3[3-BPyH+.H2O] hydration species. The mechanisms of photoreduction observed for 3-BPy in different pH aqueous solutions were investigated using density functional theory calculations, and these results were used to help assign the intermediates observed in the experiments. The structures and properties of these species are briefly discussed, and an overall photoreaction mechanism is proposed based on the results from the time-resolved resonance Raman experiments and the density functional theory calculations.
The Journal of Physical Chemistry A 10/2009; 113(44):12215-24. · 2.95 Impact Factor
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ABSTRACT: In this work, we developed and validated a Monte Carlo simulation (MCS) tool for investigation and evaluation of multi-pinhole (MPH) SPECT imaging.
This tool was based on a combination of the SimSET and MCNP codes. Photon attenuation and scatter in the object, as well as penetration and scatter through the collimator detector, are modeled in this tool. It allows accurate and efficient simulation of MPH SPECT with focused pinhole apertures and user-specified photon energy, aperture material, and imaging geometry. The MCS method was validated by comparing the point response function (PRF), detection efficiency (DE), and image profiles obtained from point sources and phantom experiments. A prototype single-pinhole collimator and focused four- and five-pinhole collimators fitted on a small animal imager were used for the experimental validations. We have also compared computational speed among various simulation tools for MPH SPECT, including SimSET-MCNP, MCNP, SimSET-GATE, and GATE for simulating projections of a hot sphere phantom.
We found good agreement between the MCS and experimental results for PRF, DE, and image profiles, indicating the validity of the simulation method. The relative computational speeds for SimSET-MCNP, MCNP, SimSET-GATE, and GATE are 1: 2.73: 3.54: 7.34, respectively, for 120-view simulations. We also demonstrated the application of this MCS tool in small animal imaging by generating a set of low-noise MPH projection data of a 3D digital mouse whole body phantom.
The new method is useful for studying MPH collimator designs, data acquisition protocols, image reconstructions, and compensation techniques. It also has great potential to be applied for modeling the collimator-detector response with penetration and scatter effects for MPH in the quantitative reconstruction method.
Molecular imaging and biology: MIB: the official publication of the Academy of Molecular Imaging 09/2009; 12(3):295-304. · 2.47 Impact Factor
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ABSTRACT: Ketoprofen is known to induce photosensitivity due to its specific structure and electronic features, and this limits its use in medical applications. In this Article, the photochemistry of (S)-ketoprofen has been investigated by time-resolved resonance Raman spectroscopy to gain additional information so as to better elucidate the possible photochemical reaction mechanism of ketoprofen in different solvents. In nonaqueous solvents like neat acetonitrile and isopropyl alcohol, and 1:1 acetonitrile:water and 1:1 acetonitrile:acidic water aqueous solvents, (S)-ketoprofen exhibits benzophenone-like photochemistry to produce a triplet state, which in turn produces a ketyl radical-like species that then undergoes a cross-coupling reaction with either a dimethyl radical (which is generated by hydrogen abstraction of isopropyl alcohol) or a water molecule, respectively, at the para-position to form a transient species that has a lifetime up to the microsecond time scale. However, photolysis of (S)-ketoprofen in a 1:1 acetonitrile:alkaline water solution and 3:7 acetonitrile:phosphate buffered solution appears to undergo a prompt decarboxylation reaction. Only one species was observed in the nanosecond time-resolved resonance Raman experiments under these conditions, and this species was tentatively assigned to be a triplet protonated biradical carbanion.
The Journal of Physical Chemistry B 08/2009; 113(30):10530-9. · 3.70 Impact Factor
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ABSTRACT: A model-based method has been previously developed to estimate and compensate for the crosstalk and downscatter contamination in simultaneous 123I/99mTc dual-isotope SPECT imaging. In this method, photon scatter in the object is modeled using the effective source scatter estimate technique. Photon interactions with the collimator-detector are estimated using precalculated Monte Carlo simulated point response functions. Two different approaches, simultaneous and alternating model-based compensations, have been proposed for iterative reconstruction-based crosstalk and downscatter contamination compensation. In this work, both model-based approaches were evaluated in the context of quantitative accuracy when imaging the dopaminergic system using both Monte Carlo simulated and experimentally acquired data. Results indicate that mddel-based estimates of the crosstalk and downscatter contamination in both energy windows were in good agreement with the truth for the simulated data. The effects of the contamination reduced image contrast and overestimated absolute activity in all structures by up to 66%. Compensation using both model-based approaches improved image contrast. Errors in absolute activity quantitation were also reduced to less than +/-5% for most brain structures. The accuracy of striatal specific binding potentials, calculated as the ratio of activity in various striatal structures to the background, was also greatly improved after model-based compensation. In conclusion, model-based compensation of simultaneously acquired images of 99mTc and 123I labeled brain imaging agents provided image quality and quantitative accuracy that were comparable to the image without crosstalk. Both proposed compensation approaches can potentially be applied clinically, but when reconstruction time is a limiting factor, the alternating model-based compensation may be preferable.
Medical Physics 07/2009; 36(6):2021-33. · 2.83 Impact Factor
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Bin He,
Richard L Wahl,
George Sgouros, Yong Du,
Heather Jacene,
Wayne R Kasecamp,
Ian Flinn,
Richard J Hammes,
Jay Bianco,
Brad Kahl,
Eric C Frey
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ABSTRACT: The estimation of organ residence time is essential for high-dose myeloablative regimens in radioimmunotherapy (RIT). Frequently, this estimation is based on a series of simple planar scans and planar processing. The authors previously performed a simulation study which demonstrated that the accuracy of this methodology is limited compared to a hybrid planar/SPECT residence time estimation method. In this work the authors applied this hybrid method to data from a clinical trial of high-dose myeloablative yttrium-90 ibritumomab tiuxetan therapy. Image data acquired from 18 patients were comprised of planar scans at five time points ranging from 1 to 144 h postinjection and abdominal and thoracic SPECT/CT scans obtained at 24 h postinjection. The simple planar processing method used in this work was based on the geometric mean method with energy window based scatter compensation. No explicit background subtraction nor object or source thickness corrections were performed. The SPECT projections were reconstructed using iterative reconstruction with compensations for attenuation, scatter, and full collimator-detector response. Large differences were observed when residence times were estimated using the simple planar method compared to the hybrid method. The differences were not constant but varied in magnitude and sign. For the dose-limiting organ (liver), the average difference was -18% and variation in the difference was 19%, similar to the differences observed in a previously reported simulation study. The authors also looked at the relationship between the weight of the patient and the liver residence time and found that there was no meaningful correlation for either method. This indicates that weight would not be an adequate proxy for an experimental estimate of residence time when choosing the activity to administer for therapy. The authors conclude that methods such as the simple planar method used here are inadequate for RIT treatment planning. More sophisticated methods, such as the hybrid SPECT/planar method investigated here, are likely to be better predictors of organ dose and, as a result, organ toxicities.
Medical Physics 06/2009; 36(5):1595-601. · 2.83 Impact Factor
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ABSTRACT: A time-resolved resonance Raman study of the photoreactions of benzophenone (BP) in neutral, alkaline, and acidic aqueous solutions is reported. Under neutral and alkaline conditions, a hydrogen abstraction reaction is observed to form the corresponding diphenyl ketyl (DPK) radical with different reaction rates while under acidic condition the protonation of triplet benzophenone occurs simultaneously, which is followed by a faster hydration reaction to produce the corresponding photohydration short-lived intermediates that were observed during experiments and that were tentatively assigned to the m-(3)BP x H(2)O and the o-(1)BP x H(2)O species. The dynamics, structures, and properties of these species are briefly discussed.
The Journal of Physical Chemistry A 05/2009; 113(14):3344-52. · 2.95 Impact Factor
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ABSTRACT: Density functional theory computations and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy experiments were done to study the photocyclization of 2-pyridyl phenyl ketone in acid solution. The most probable cyclization pathway was assigned to the reaction RX1, which had the lowest free-energy barrier. The factors that determine the free-energy barriers among the different reactions examined were discussed. Solvent effects on these reactions were also explored by calculations that included a polarizable continuum model for the bulk solvent effect.
The Journal of Physical Chemistry A 02/2009; 113(10):1999-2003. · 2.95 Impact Factor
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ABSTRACT: Estimating organ residence times is an essential part of patient-specific dosimetry for radioimmunotherapy (RIT). Quantitative imaging methods for RIT are often evaluated using a single physical or simulated phantom but are intended to be applied clinically where there is variability in patient anatomy, biodistribution, and biokinetics. To provide a more relevant evaluation, the authors have thus developed a population of phantoms with realistic variations in these factors and applied it to the evaluation of quantitative imaging methods both to find the best method and to demonstrate the effects of these variations. Using whole body scans and SPECT/CT images, organ shapes and time-activity curves of 111In ibritumomab tiuxetan were measured in dosimetrically important organs in seven patients undergoing a high dose therapy regimen. Based on these measurements, we created a 3D NURBS-based cardiac-torso (NCAT)-based phantom population. SPECT and planar data at realistic count levels were then simulated using previously validated Monte Carlo simulation tools. The projections from the population were used to evaluate the accuracy and variation in accuracy of residence time estimation methods that used a time series of SPECT and planar scans, Quantitative SPECT (QSPECT) reconstruction methods were used that compensated for attenuation, scatter, and the collimator-detector response. Planar images were processed with a conventional (CPlanar) method that used geometric mean attenuation and triple-energy window scatter compensation and a quantitative planar (QPlanar) processing method that used model-based compensation for image degrading effects. Residence times were estimated from activity estimates made at each of five time points. The authors also evaluated hybrid methods that used CPlanar or QPlanar time-activity curves rescaled to the activity estimated from a single QSPECT image. The methods were evaluated in terms of mean relative error and standard deviation of the relative error in the residence time estimates taken over the phantom population. The mean errors in the residence time estimates over all the organs were < 9.9% (pure QSPECT), < 13.2% (pure QPLanar), < 7.2% (hybrid QPlanar/QSPECT), < 19.2% (hybrid CPlanar/QSPECT), and 7%-159% (pure CPlanar). The standard deviations of the errors for all the organs over all the phantoms were < 9.9%, < 11.9%, < 10.8%, < 22.0%, and < 107.9% for the same methods, respectively. The processing methods differed both in terms of their average accuracy and the variation of the accuracy over the population of phantoms, thus demonstrating the importance of using a phantom population in evaluating quantitative imaging methods. Hybrid CPlanar/QSPECT provided improved accuracy compared to pure CPlanar and required the addition of only a single SPECT acquisition. The QPlanar or hybrid QPlanar/QSPECT methods had mean errors and standard deviations of errors that approached those of pure QSPECT while providing simplified image acquisition protocols, and thus may be more clinically practical.
Medical Physics 02/2009; 36(2):612-9. · 2.83 Impact Factor
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ABSTRACT: A time-resolved resonance Raman (TR(3)) and density functional theory (DFT) study of the reaction of the 4-biphenylnitrenium ion with 4-biphenyl azide in a mixed aqueous solution is reported. The reaction of the 4-biphenylnitrenium ion with its unphotolyzed precursor 4-biphenyl azide in a mixed aqueous solution generates a 4,4'-azobisbiphenyl stable product via an intermediate species. With the aid of DFT calculations for likely transient species, this intermediate was tentatively assigned to a 4,4'-azobisbiphenyl cation. The DFT calculations predict this reaction can take place via two pathways that compete with one another to produce the trans and cis 4,4'-azobisbiphenyl product. The observation of the 4,4'-azobisbiphenyl cation intermediate demonstrates that the reaction of the arylnitrenium ion with its aryl azide to produce a stable azo product occurs via a stepwise mechanism.
The Journal of Physical Chemistry A 12/2008; 112(46):11582-9. · 2.95 Impact Factor
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ABSTRACT: Estimating the residence times in tumor and normal organs is an essential part of treatment planning for radioimmunotherapy (RIT). This estimation is usually done using a conjugate view whole body scan time series and planar processing. This method has logistical and cost advantages compared to 3-D imaging methods such as Single photon emission computed tomography (SPECT), but, because it does not provide information about the 3-D distribution of activity, it is difficult to fully compensate for effects such as attenuation and background and overlapping activity. Incomplete compensation for these effects reduces the accuracy of the residence time estimates. In this work we compare residence times estimates obtained using planar methods to those from methods based on quantitative SPECT (QSPECT) reconstructions. We have previously developed QSPECT methods that provide compensation for attenuation, scatter, collimator-detector response, and partial volume effects. In this study we compared the use of residence time estimation methods using QSPECT to planar methods. The evaluation was done using the realistic NCAT phantom with organ time activities that model <sup>111</sup>In ibritumomab tiuxetan. Projection data were obtained using Monte Carlo simulations (MCS) that realistically model the image formation process including penetration and scatter in the collimator-detector system. These projection data were used to evaluate the accuracy of residence time estimation using a time series of QSPECT studies, a single QSPECT study combined with planar scans and the planar scans alone. The errors in the residence time estimates were 3.8%, 15%, and 2%-107% for the QSPECT, hybrid planar/QSPECT, and planar methods, respectively. The quantitative accuracy was worst for pure planar processing and best for pure QSPECT processing. Hybrid planar/QSPECT methods, where a single QSPECT study was combined with a series of planar scans, provided a large and statistically significant impro-
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vement in quantitative accuracy for most organs compared to the planar scans alone, even without sophisticated attention to background subtraction or thickness corrections in planar processing. These results indicate that hybrid planar/QSPECT methods are generally superior to pure planar methods and may be an acceptable alternative to performing a time series of QSPECT studies.
IEEE Transactions on Medical Imaging 05/2008; · 3.64 Impact Factor
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ABSTRACT: A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic investigation of the intermolecular hydrogen-abstraction reaction of the triplet state of 4-benzoylpyridine (4-BPy) in 2-propanol solvent is reported. The TR3 results reveal a rapid hydrogen abstraction (<10 ns) by the 4-BPy triplet state (nπ*) with the 2-propanol solvent, leading to formation of a 4-BPy ketyl radical and an associated dimethyl ketyl radical partner from the solvent. The recombination of these two radical species occurs with a time constant about 200 ns to produce a para-N-LAT (light absorbing transient). The structure, major spectral features, and identification of the ketyl radical and the para-N-LAT coupling complex have been determined and confirmed by comparison of the TR3 results with results from density functional theory (DFT) calculations. A reaction pathway for the photolysis of 4-BPy in 2-propanol deduced from the TR3 results is also presented. The electron-withdrawing effect of the heterocyclic nitrogen for 4-BPy on the triplet state makes it have a significantly higher chemical reactivity for the hydrogen abstraction with 2-propanol compared to the previously reported corresponding benzophenone triplet reaction under similar reaction conditions. In addition, the 4-BPy ketyl radical reacts with the dimethyl ketyl radical to attach at the para-N atom position of the pyridine ring to form a cross-coupling product such as 2-[4-(hydroxy-phenyl-methylene)-4h-pyridin-1-yl]-propan-2-ol instead of attacking at the para-C atom position as was observed for the corresponding benzophenone reaction reported in an earlier study. Copyright © 2008 John Wiley & Sons, Ltd.
Journal of Raman Spectroscopy 03/2008; 39(4):503 - 514. · 3.09 Impact Factor
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ABSTRACT: Time-resolved resonance Raman (TR3) studies of the photochemistry of phenyl azide, 3-hyroxyphenyl azide, 3-methoxyphenyl azide and 3-nitrophenyl azide in acetonitrile:water solutions is reported. After photolysis of these four aryl azides in room temperature solutions, only one species was observed in the TR3 spectra for each azide, respectively at the probe wavelengths employed in the TR3 experiments. The species observed after photolysis of 3-nitrophenyl azide was assigned to 3,3'-dinitroazobenzene, an azo compound formed from the dimerization reaction of triplet 3-nitrophenylnitrene. In contrast, the species observed after photolysis of phenyl azide, 3-hydroxyphenyl azide and 3-methoxyphenyl azide were tentatively assigned to intermediates formed from the dimerization of didehydroazepines that are produced from the ring expansion reaction of the respective singlet arylnitrene. To our knowledge, this is the first time-resolved vibrational spectroscopic observation of the dimerization reaction of didehydroazepines in solution. In addition, these are the first resonance Raman spectra reported for dimers formed from didehydroazepines. We briefly discuss the structures, properties and chemical reactivity of the dimer species observed in the TR3 spectra and possible implications for the photochemistry of aryl azides.
The Journal of Physical Chemistry A 03/2008; 112(7):1502-10. · 2.95 Impact Factor
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IEEE Trans. Med. Imaging. 01/2008; 27:521-530.
