Single Linear Relationship Research Articles

Technical Note: Relation between dual-energy subtraction of CT images for electron density calibration and virtual monochromatic imaging.

For accurate tissue inhomogeneity correction in radiotherapy treatment planning, the author previously proposed a simple conversion of the energy-subtracted computed tomography (CT) number to an electron density (ΔHU-ρe conversion), which provides a single linear relationship between ΔHU and ρe over a wide ρe range. The purpose of the present study was to reveal the relation between the ΔHU image for ρe calibration and a virtually monochromatic CT image by performing numerical analyses based on the basis material decomposition in dual-energy CT. The author determined the weighting factor, α0, of the ΔHU-ρe conversion through numerical analyses of the International Commission on Radiation Units and Measurements Report-46 human body tissues using their attenuation coefficients and given ρe values. Another weighting factor, α(E), for synthesizing a virtual monochromatic CT image from high- and low-kV CT images, was also calculated in the energy range of 0.03 < E < 5 MeV, assuming that cortical bone and water were the basis materials. The mass attenuation coefficients for these materials were obtained using the xcom photon cross sections database. The effective x-ray energies used to calculate the attenuation were chosen to imitate a dual-source CT scanner operated at 80-140 and 100-140 kV/Sn. The determined α0 values were 0.455 for 80-140 kV/Sn and 0.743 for 100-140 kV/Sn. These values coincided almost perfectly with the respective maximal points of the calculated α(E) curves located at approximately 1 MeV, in which the photon-matter interaction in human body tissues is exclusively the incoherent (Compton) scattering. The ΔHU image could be regarded substantially as a CT image acquired with monoenergetic 1-MeV photons, which provides a linear relationship between CT numbers and electron densities.

Initial implementation of the conversion from the energy-subtracted CT number to electron density in tissue inhomogeneity corrections: an anthropomorphic phantom study of radiotherapy treatment planning.

To achieve accurate tissue inhomogeneity corrections in radiotherapy treatment planning, the authors had previously proposed a novel conversion of the energy-subtracted computed tomography (CT) number to an electron density (ΔHU-ρ(e) conversion), which provides a single linear relationship between ΔHU and ρ(e) over a wide range of ρ(e). The purpose of this study is to present an initial implementation of the ΔHU-ρ(e) conversion method for a treatment planning system (TPS). In this paper, two example radiotherapy plans are used to evaluate the reliability of dose calculations in the ΔHU-ρ(e) conversion method. CT images were acquired using a clinical dual-source CT (DSCT) scanner operated in the dual-energy mode with two tube potential pairs and an additional tin (Sn) filter for the high-kV tube (80-140 kV/Sn and 100-140 kV/Sn). Single-energy CT using the same DSCT scanner was also performed at 120 kV to compare the ΔHU-ρ(e) conversion method with a conventional conversion from a CT number to ρ(e) (Hounsfield units, HU-ρ(e) conversion). Lookup tables for ρ(e) calibration were obtained from the CT image acquisitions for tissue substitutes in an electron density phantom (EDP). To investigate the beam-hardening effect on dosimetric uncertainties, two EDPs with different sizes (a body EDP and a head EDP) were used for the ρ(e) calibration. Each acquired lookup table was applied to two radiotherapy plans designed using the XiO TPS with the superposition algorithm for an anthropomorphic phantom. The first radiotherapy plan was for an oral cavity tumor and the second was for a lung tumor. In both treatment plans, the performance of the ΔHU-ρ(e) conversion was superior to that of the conventional HU-ρ(e) conversion in terms of the reliability of dose calculations. Especially, for the oral tumor plan, which dealt with dentition and bony structures, treatment planning with the HU-ρ(e) conversion exhibited apparent discrepancies between the dose distributions and dose-volume histograms (DVHs) of the body-EDP and head-EDP calibrations. In contrast, the dose distributions and DVHs of the body-EDP and head-EDP calibrations coincided with each other almost perfectly in the ΔHU-ρ(e) conversion for 100-140 kV/Sn. The difference between the V100's (the mean planning target volume receiving 100% of the prescribed dose; a DVH parameter) of the body-EDP and head-EDP calibrations could be reduced to less than 1% using the ΔHU-ρ(e) conversion, but exceeded 11% for the HU-ρ(e) conversion. The ΔHU-ρ(e) conversion can be implemented for currently available TPS's without any modifications or extensions. The ΔHU-ρ(e) conversion appears to be a promising method for providing an accurate and reliable inhomogeneity correction in treatment planning for any ill-conditioned scans that include (i) the use of a calibration EDP that is nonequivalent to the patient's body tissues, (ii) a mismatch between the size of the patient and the calibration EDP, or (iii) a large quantity of high-density and high-atomic-number tissue structures.

English

A study was carried out to compare the ballistic performance of high purity alumina and zirconia-toughened alumina (ZTA) using depth of penetration (DoP) test configuration against 7.62 mm armour piercing (AP) ammunition. The effect of tile thickness on the differential efficiency factor (DEF) was studied for tile thickness in the range of 3 mm to 6 mm for alumina tiles and 3 mm to 5 mm for ZTA tiles. The DEF is found to increase as tile thickness increases. An analysis on the failed shots showed that the residual shot weight does not follow a single linear relationship with ceramic tile thickness unlike the residual DoP for all thicknesses of tiles. Post-ballistic analysis on ceramic powder for particle size distribution was carried out and the results are presented. Defence Science Journal, Vol. 64, No. 5, September 2014, pp.477-483, DOI:http://dx.doi.org/10.14429/dsj.64.6745

Technical Note: exploring the limit for the conversion of energy-subtracted CT number to electron density for high-atomic-number materials.

For accurate tissue inhomogeneity correction in radiotherapy treatment planning, the authors had previously proposed a novel conversion of the energy-subtracted CT number to an electron density (ΔHU-ρe conversion), which provides a single linear relationship between ΔHU and ρe over a wide ρe range. The purpose of this study is to address the limitations of the conversion method with respect to atomic number (Z) by elucidating the role of partial photon interactions in the ΔHU-ρe conversion process. The authors performed numerical analyses of the ΔHU-ρe conversion for 105 human body tissues, as listed in ICRU Report 46, and elementary substances with Z = 1-40. Total and partial attenuation coefficients for these materials were calculated using the XCOM photon cross section database. The effective x-ray energies used to calculate the attenuation were chosen to imitate a dual-source CT scanner operated at 80-140 kV/Sn under well-calibrated and poorly calibrated conditions. The accuracy of the resultant calibrated electron density,[Formula: see text], for the ICRU-46 body tissues fully satisfied the IPEM-81 tolerance levels in radiotherapy treatment planning. If a criterion of [Formula: see text]ρe - 1 is assumed to be within ± 2%, the predicted upper limit of Z applicable for the ΔHU-ρe conversion under the well-calibrated condition is Z = 27. In the case of the poorly calibrated condition, the upper limit of Z is approximately 16. The deviation from the ΔHU-ρe linearity for higher Z substances is mainly caused by the anomalous variation in the photoelectric-absorption component. Compensation among the three partial components of the photon interactions provides for sufficient linearity of the ΔHU-ρe conversion to be applicable for most human tissues even for poorly conditioned scans in which there exists a large variation of effective x-ray energies owing to beam-hardening effects arising from the mismatch between the sizes of the object and the calibration phantom.

Retrieval of Wheat Growth Parameters With Radar Vegetation Indices

The radar vegetation index (RVI) has low sensitivity to changes in environmental conditions and has the potential as a tool to monitor vegetation growth. In this letter, we expand on previous research by investigating the radar response over a wheat canopy. RVI was computed using observations made with a ground-based multifrequency polarimetric scatterometer system over an entire wheat growth cycle. We analyzed the temporal variations of backscattering coefficients for L-, C-, and X-bands; RVI; vegetation water content (VWC); and fresh weight. We found that the L-band RVI was highly correlated with both VWC (r = 0.98) and fresh weight (r = 0.98). Based upon these analyses, linear equations were developed for estimation of VWC (root-mean-square error (RMSE = 0.126 kg m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> )) and fresh weight (RMSE = 0.12 kg m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ). In addition, the results of the wheat study were combined with previous investigations with other crops (e.g., rice and soybean). We found that a single linear relationship between L-band RVI and VWC can be used for all crop types (RMSE = 0.47 kg m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ). These results clearly demonstrate the potential of RVI as a robust method for characterizing vegetation canopies. VWC is a key input requirement for retrieving soil moisture from microwave remote sensing observations. The results of this investigation will be useful for the Soil Moisture Active and Passive mission (2014), which is designed to measure global soil moisture.

Potential of antibody&amp;ndash;drug conjugates and novel therapeutics in breast cancer management

Progress in the treatment of cancer over the past decade has been slow. Targeting a mutated gene of an individual patient tumor, tumor-guided agents, and the first draft of the human genome sequence have created an overenthusiasm to achieve personalized medicine. However, we now know that this effort is misleading. Extreme interpatient and intratumor heterogeneity, scarce knowledge in how genome-wide mutational landscape and epigenetic changes affect transcriptional processes, gene expression, signaling transduction networks and cell regulation, and clinical assessment of temporary efficacy of targeted drugs explain the limitations of these currently available agents. Trastuzumab and a few other monoclonal antibodies or small-molecule tyrosine kinase inhibitors (TKIs) represent an exception to this rule. By blocking ligand-binding receptor in patients with human epidermal growth factor receptor 2 (HER2) amplification and overexpression, trastuzumab added to chemotherapy in HER2-positive patients has been proven to provide significant overall survival benefit in both metastatic and adjuvant settings. Lapatinib, a small-molecule dual inhibitor (TKI) of both HER2 and EGFR (epidermal growth factor receptor) pathways, has an antitumor activity translated into progression-free survival benefit in HER2-positive metastatic patients previously treated with a taxane, an anthracycline, and trastuzumab. Despite these advances, ~25% of patients with HER2-positive breast cancer experience recurrence in the adjuvant setting, while in the metastatic setting, median survival time is 25 months. In this review, we discuss the safety, efficacy, and limitations of the trastuzumab emtansine (T-DM1) conjugate in the treatment of HER2-positive metastatic breast cancer. We also highlight Phase III randomized trials, currently underway, using either the T-DM1 conjugate or various combinations of monoclonal antibodies and TKIs. Moreover, in contrast with all these agents developed on the basis of “central dogma” of simplified reductionist transcription and single gene–phenotype linear relationship, we summarize the emerging, amazing era of next-generation, transcriptional circuitry and intracellular signaling network-based drugs guided by the latest advances in genome science and dynamics of network biology.

Exploring Seasonality in Escherichia coli and Fecal Coliform Ratios in Urban Watersheds

AbstractAs regulators in the United States develop plans to mitigate microbial pollution via the design of total maximum daily load (TMDL) plans, historical data sets are often used to establish existing conditions for watersheds. Due to the revision of guidance recommending Escherichia coli (E. coli) as the preferred target indicator species in surface-water quality-monitoring programs, historical fecal-coliform concentration data are often used to estimate E. coli concentrations and loadings. Data from Raleigh and Durham, North Carolina, suggest the ratio of E. coli to fecal coliform (EC/FC) varies significantly by season throughout the year (p<0.05). These seasonal differences are not accounted for in most translator equations, which assume a single linear relationship between E. coli and fecal coliform regardless of season. These observations imply that seasonal translator equations may more accurately reflect the relationship between fecal coliforms and E. coli, and would thus result in the developme...

Regional ion channel gene expression heterogeneity and ventricular fibrillation dynamics in human hearts.

RationaleStructural differences between ventricular regions may not be the sole determinant of local ventricular fibrillation (VF) dynamics and molecular remodeling may play a role.ObjectivesTo define regional ion channel expression in myopathic hearts compared to normal hearts, and correlate expression to regional VF dynamics.Methods and ResultsHigh throughput real-time RT-PCR was used to quantify the expression patterns of 84 ion-channel, calcium cycling, connexin and related gene transcripts from sites in the LV, septum, and RV in 8 patients undergoing transplantation. An additional eight non-diseased donor human hearts served as controls. To relate local ion channel expression change to VF dynamics localized VF mapping was performed on the explanted myopathic hearts right adjacent to sampled regions. Compared to non-diseased ventricles, significant differences (p<0.05) were identified in the expression of 23 genes in the myopathic LV and 32 genes in the myopathic RV. Within the myopathic hearts significant regional (LV vs septum vs RV) expression differences were observed for 13 subunits: Nav1.1, Cx43, Ca3.1, Cavα2δ2, Cavβ2, HCN2, Na/K ATPase-1, CASQ1, CASQ2, RYR2, Kir2.3, Kir3.4, SUR2 (p<0.05). In a subset of genes we demonstrated differences in protein expression between control and myopathic hearts, which were concordant with the mRNA expression profiles for these genes. Variability in the expression of Cx43, hERG, Na+/K+ ATPase ß1 and Kir2.1 correlated to variability in local VF dynamics (p<0.001). To better understand the contribution of multiple ion channel changes on VF frequency, simulations of a human myocyte model were conducted. These simulations demonstrated the complex nature by which VF dynamics are regulated when multi-channel changes are occurring simultaneously, compared to known linear relationships.ConclusionsIon channel expression profile in myopathic human hearts is significantly altered compared to normal hearts. Multi-channel ion changes influence VF dynamic in a complex manner not predicted by known single channel linear relationships.

Using DNA to test the utility of pellet-group counts as an index of deer counts

Despite widespread use of fecal pellet-group counts as an index of ungulate density, techniques used to convert pellet-group numbers to ungulate numbers rarely are based on counts of known individuals, seldom evaluated across spatial and temporal scales, and precision is infrequently quantified. Using DNA from fecal pellets to identify individual deer, we evaluated the relationship between pellet-group count and count of Sitka black-tailed deer (Odocoileus hemionus sitkensis) during a 3-year study (2006–2008) in 3 watersheds in southeast Alaska, USA. We surveyed 141,054 m2 of transect, counted 10,569 pellet groups, and identified 737 unique deer. We used a multilevel mixed-effects generalized linear model to analyze expected deer count as a function of pellet-group count. Pellet-group count was a significant predictor of DNA-based index of deer count, but that relationship varied by transect, watershed, and year, indicating that extrapolation of a single linear relationship across space and time was not possible. More importantly, most of the variation in our models was residual and unexplained. Assuming that our DNA-based results were a more accurate and precise metric of true deer count, we do not support the use of pellet-group count to index deer count in southeast Alaska unless confounding factors are accounted for at fine spatial (e.g., habitat patch) scales. Because of the difficulty in routinely evaluating the influence of confounding variables in remote and unmanaged landscapes, we suggest that wildlife programs in these environments consider alternatives, such as DNA-based methods, for monitoring trends in ungulate populations. © 2013 The Wildlife Society.

Conversion of the energy-subtracted CT number to electron density based on a single linear relationship: an experimental verification using a clinical dual-source CT scanner

In radiotherapy treatment planning, the conversion of the computed tomography (CT) number to electron density is one of the main processes that determine the accuracy of patient dose calculations. However, in general, the CT number and electron density of tissues cannot be interrelated using a simple one-to-one correspondence. This study aims to experimentally verify the clinical feasibility of an existing novel conversion method proposed by the author of this note, which converts the energy-subtracted CT number (ΔHU) to the relative electron density (ρe) via a single linear relationship by using a dual-energy CT (DECT). The ΔHU–ρe conversion was performed using a clinical second-generation dual-source CT scanner operated in the dual-energy mode with tube potentials of 80 kV and 140 kV with and without an additional tin filter. The ΔHU–ρe calibration line was obtained from the DECT image acquisition for tissue substitutes in an electron density phantom. In addition, the effect of object size on ΔHU–ρe conversion was also experimentally investigated. The plot of the measured ΔHU versus nominal ρe values exhibited a single linear relationship over a wide ρe range from 0.00 (air) to 2.35 (aluminum). The ΔHU–ρe conversion performed with the tin filter yielded a lower dose and more reliable ρe values that were less affected by the object-size variation when compared to the corresponding values obtained for the case without the tin filter.

Feasibility and accuracy of relative electron density determined by virtual monochromatic CT value subtraction at two different energies using the gemstone spectral imaging

BackgroundRecent work by Saito (2012) has demonstrated a simple conversion from energy-subtracted computed tomography (CT) values (ΔHU) obtained using dual-energy CT to relative electron density (RED) via a single linear relationship. The purpose of this study was to investigate the feasibility of this method to obtain RED from virtual monochromatic CT images obtained by the gemstone spectral imaging (GSI) mode with fast-kVp switching.MethodsA tissue characterization phantom with 13 inserts made of different materials was scanned using the GSI mode on a Discovery CT750 HD. Four sets of virtual monochromatic CT images (60, 77, 100 and 140 keV) were obtained from a single GSI acquisition. When we define Δ HU in terms of the weighting factor for the subtraction α, Δ HU ≡ (1 + α)H - αL (H and L represent the CT values for high and low energy respectively), the relationship between Δ HU and RED is approximated as a linear function, a × Δ HU/1000 + b (a, b = unity). We evaluated the agreement between the determined and nominal RED. We also have investigated reproducibility over short and long time periods.ResultsFor the 13 insert materials, the RED determined by monochromatic CT images agreed with the nominal values within 1.1% and the coefficient of determination for this calculation formula was greater than 0.999. The observed reproducibility (1 standard deviation) of calculation error was within 0.5% for all materials.ConclusionsThese findings indicate that virtual monochromatic CT scans at two different energies using GSI mode can provide an accurate method for estimating RED.

The nonlinear effects of conscientiousness on overall job performance and performance dimensions in the Chinese context

The relationship between conscientiousness and job performance has been found to be nonlinear in the West, which challenges conceptually and empirically the traditional assumption of the single linear relationship. In this research, we examined the nonlinear effects of conscientiousness on both overall job performance and performance dimensions (i.e., task performance, adaptive performance and contextual performance) in the Chinese context. The results of our two studies supported some evidence for the nonlinear effect of conscientiousness on overall job performance. In addition, it was found that conscientiousness has different (linear or nonlinear) effects on performance dimensions. These findings suggest that the nonlinear effects of conscientiousness on job performance deserve further investigation, and a distinction should be made with regard to job performance in personnel evaluation. Results are discussed in terms of the significance of considering the nonlinear relationship between conscientiousness and performance criteria.

Potential of dual‐energy subtraction for converting CT numbers to electron density based on a single linear relationship

The conversion of the computed tomography (CT) number to electron density is one of the main processes that determine the accuracy of patient dose calculations in radiotherapy treatment planning. However, the CT number and electron density of tissues cannot be generally interrelated via a simple one-to-one correspondence because the CT number depends on the effective atomic number as well as the electron density. The purpose of this study is to present a simple conversion from the energy-subtracted CT number (ΔHU) by means of dual-energy CT (DECT) to the relative electron density (ρ(e)) via a single linear relationship. The ΔHU-ρ(e) conversion method was demonstrated by performing analytical DECT image simulations that were intended to imitate a second-generation dual-source CT (DSCT) scanner with an additional tin filtration for the high-kV tube. The ΔHU-ρ(e) calibration line was obtained from the image simulation with a 33 cm-diameter electron density calibration phantom equipped with 16 inserts including polytetrafluoroethylene, polyvinyl chloride, and aluminum; the elemental compositions of these three inserts were quite different to those of body tissues. The ΔHU-ρ(e) conversion method was also applied to previously published experimental CT data, which were measured using two different CT scanners, to validate the clinical feasibility of the present approach. In addition, the effect of object size on ρ(e)-calibrated images was investigated by image simulations using a 25 cm-diameter virtual phantom for two different filtrations: with and without the tin filter for the high-kV tube. The simulated ΔHU-ρ(e) plot exhibited a predictable linear relationship over a wide range of ρ(e) from 0.00 (air) to 2.35 (aluminum). Resultant values of the coefficient of determination, slope, and intercept of the linear function fitted to the data were close to those of the ideal case. The maximum difference between the ideal and simulated ρ(e) values was -0.7%. The satisfactory linearity of ΔHU-ρ(e) was also confirmed from analyses of the experimental CT data. In the experimental cases, the maximum difference between the nominal and simulated ρ(e) values was found to be 2.5% after two outliers were excluded. When compared with the case without the tin filter, the ΔHU-ρ(e) conversion performed with the tin filter yielded a lower dose and more reliable ρ(e) values that were less affected by the object-size variation. The ΔHU-ρ(e) calibration line with a simple one-to-one correspondence would facilitate the construction of a well-calibrated ρ(e) image from acquired dual-kV images, and currently, second generation DSCT may be a feasible modality for the clinical use of the ΔHU-ρ(e) conversion method.

A linear solvation energy relationship model of organic chemical partitioning to particulate organic carbon in soils and sediments

Predicting the association of contaminants with particulate organic matter in the environment is critical in determining the fate and bioavailability of chemicals. A ubiquitous measure of contaminant association with soil and sediment particulate organic matter is the organic carbon partition coefficient K(OC) . Chemical class-specific models relating the K(OC) to the octanol-water partition coefficient K(OW) have been used to predict the partitioning to organic carbon in the water column and sediment for nonpolar hydrophobic pollutants and some polar pollutants. A single linear solvation energy relationship (LSER) is proposed as a simpler and chemically based alternative for predicting K(OC) for a more diverse set of compounds. A chemically diverse set of K(OC) data is used to obtain a more robust and more universally representative model of organic carbon partitioning than previously available LSER models. The resulting model has a root mean square error (RMSE) of prediction for log K(OC) of RMSE = 0.48 for the fitted data set and RMSE = 0.55 for an independent data set. An analysis of LSER coefficients highlights the relative importance of hydrogen bonding interactions.

Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg3Bi2 intermetallic compound, the reduction in the volume fraction of the less stable Mg2Sn phase, and the dissolution of Bi in the remaining Mg2Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol−1. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol−1. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.

Patterns of baseline autonomic nerve activity and the development of pacing-induced sustained atrial fibrillation

Whether autonomic nerve activity is important in the development of pacing-induced sustained atrial fibrillation (AF) is unclear. The purpose of this study was to test the hypothesis that patterns of baseline autonomic nerve activity are important in the development of pacing-induced sustained AF. Radiotransmitters were implanted in 12 ambulatory dogs to record left stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA). Sustained (>48 hours) AF was induced with intermittent rapid atrial pacing. At baseline (before pacing), 1-minute integrated nerve activity between SGNA and VNA demonstrated either a single linear relationship with excellent correlation (group 1, N = 3, r = 0.816 ± 0.105) or nonlinear relationships with poor correlation (group 2, N = 9, r = 0.316 ± 0.162, P <.05 vs group 1). Group 1 dogs had higher VNA (97.0 ± 11.5 mV-s) compared to group 2 (33.4 ± 21.7 mV-s, P <.001). Group 1 dogs had more frequent sympathovagal co-activation episodes than did group 2 (50 ± 19 per day vs 15 ± 6 per day, P <.05) and more paroxysmal atrial tachycardia (PAT; 5 ± 1 per day vs 2 ± 1 per day, P <.05) at baseline. Sustained AF occurred after 16 ± 4 days (range 13-20 days) of pacing in group 1 and after 46 ± 18 days (range 23-72 days) of pacing in group 2 (P <.05). In the week before development of sustained AF, VNA of group 2 dogs was significantly increased compared to baseline (P <.05). Ambulatory dogs with good linear sympathovagal correlation and higher vagal tone at baseline have more PAT episodes at baseline and faster induction of sustained AF by rapid pacing. Rapid atrial pacing increased the VNA of the remaining dogs before induction of sustained AF.

Comparing Linear Free Energy Relationships for Organic Chemicals in Soils: Effects of Soil and Solute Properties

Sorption isotherm data were determined for a set of 8 aromatic organic compounds with varying physical chemical properties in three soils with organic matter of differing quantity and composition. The primary goals of this study were to test single and multiparameter linear free energy relationships on their ability to predict the observed sorption behavior on different types of natural sorbents across a range of solutes and concentrations and to relate the accuracy of the predictions to sorbent and solute structural descriptors. Organic carbon normalized sorption coefficients (K(OC)) predicted using both single and multiparameter LFERs were in good agreement with experimental data obtained at the highest tested aqueous concentrations (average deviation less than 0.1 log units over all solutes and sorbents), but deviations were more substantial (0.59-0.65 log units) at the lowest tested concentrations. For chlorinated benzenes there was a significant correlation between experiment-prediction discrepancies and the aromatic content of the soil organic matter measured by (13)C NMR, and the magnitude of the effect was similar to that observed previously for dissolved organic matter.

Solubility and dissimilatory reduction kinetics of iron(III) oxyhydroxides: A linear free energy relationship

Rates of reduction of Fe(III) oxyhydroxides by the bacterium Shewanella putrefaciens were measured as a function of the bacterial density and the Fe(III) substrate concentration. The results show that an earlier reported positive correlation between the solubility products ( ∗K so) and the maximum cell-specific reduction rates ( v max) of predominantly poorly crystalline Fe(III) oxyhydroxides also applies to insoluble and crystalline Fe(III) oxyhydroxides. The mineral solubilities were measured by a dialysis bag technique under acidic conditions (pH 1 up to 2.5) at 25 °C. Initial iron reduction rates by S. putrefaciens were determined in the presence of excess lactate as electron donor. In all cases, the microbial reduction rate exhibited saturation behavior with respect to the Fe(III) oxyhydroxide concentration. On a double logarithmic scale, the maximum rates v max and the solubility products defined a single linear free energy relationship (LFER) for all the Fe(III) oxyhydroxides considered. The solubility provided a better predictor of v max than the specific surface area of the mineral phase. A rate limitation by the electron transfer between an iron reductase and a Fe(III) center, or by the subsequent desorption of Fe 2+ from the iron oxide mineral surface, are both consistent with the observed LFER.

Estimation of Molecular Acidity via Electrostatic Potential at the Nucleus and Valence Natural Atomic Orbitals

An effective approach of estimating molecular pK(a) values from simple density functional calculations is proposed in this work. Both the molecular electrostatic potential (MEP) at the nucleus of the acidic atom and the sum of valence natural atomic orbitals are employed for three categories of compounds, amines and anilines, carbonyl acids and alcohols, and sulfonic acids and thiols. A strong correlation between experimental pK(a) values and each of these two quantities for each of the three categories has been discovered. Moreover, if the MEP is subtracted by the isolated atomic MEP for each category of compounds, we observe a single unique linear relationship between the resultant MEP difference and experimental pK(a) data of amines, anilines, carbonyl acids, alcohols, sulfonic acids, thiols, and their substituents. These results can generally be utilized to simultaneously estimate pK(a) values at multiple sites with a single calculation for either relatively small molecules in drug design or amino acids in proteins and macromolecules.

Exploring neuro-vascular and neuro-metabolic coupling in rat somatosensory cortex

The existence of a coupling between changes in neuronal activity, cerebral blood flow and blood oxygenation is well known. The explicit relationship between these systems, however, is complex and remains a subject of intense research. Here, we use direct electrophysiological recordings to predict blood flow and oxygenation changes measured with optical methods during parametric stimulation applied to the somatosensory cortex in rat brain. Using a multimodal model of the cerebral functional unit, we estimate a neuro-vascular and a neuro-metabolic transfer function relating the experimentally measured neural responses with the inputs to a vascular model predicting hemodynamic and blood oxygenation changes. We show that our model can accurately predict experimentally measured parametric hemodynamic evoked responses by using a single linear transfer function relationship with a reduced number of state parameters to relate the level of neural activity to evoked cerebral blood flow and oxygen metabolism changes. At the same time, we characterize the metabolic and vascular neural response functions and interpret their physiological significance.