CISS effect: Magnetocurrent-voltage characteristics with Coulomb interactions. II.

One of the manifestations of chirality-induced spin selectivity is the appearance of a magnetocurrent. Magnetocurrent is defined as the difference between the charge currents at finite bias in a two terminal device for opposite magnetizations of one of the leads. In experiments on chiral molecules assembled in monolayers the magnetocurrent is dominantly odd in bias voltage, while theory often yields an even one. From theory it is known that the spin-orbit coupling and chirality of the molecule can only generate a finite magnetocurrent in the presence of interactions, either of the electrons with vibrational modes or among themselves, through the Coulomb interaction. Here we analytically show that the magnetocurrent in bipartite-chiral structures mediated through Coulomb interactions is exactly even in the wide band limit and exactly odd for semi-infinite leads due to the bipartite lattice symmetry of the Green's function. Our numerical results confirm these analytical findings.

Transport gap renormalization at a metal-molecule interface using DFT-NEGF and spin unrestricted calculations.

A method is presented for predicting one-particle energies for a molecule in a junction with one metal electrode, using density functional theory methods. In contrast to previous studies, in which restricted spin configurations were analyzed, we take spin polarization into account. Furthermore, in addition to junctions in which the molecule is weakly coupled, our method is also capable of describing junctions in which the molecule is chemisorbed to the metal contact. We implemented a fully self-consistent scissor operator to correct the highest occupied molecular orbital-lowest unoccupied molecular orbital gap in transport calculations for single molecule junctions. We present results for various systems and compare our results with those obtained by other groups.

Electronic components embedded in a single graphene nanoribbon.

The use of graphene in electronic devices requires a band gap, which can be achieved by creating nanostructures such as graphene nanoribbons. A wide variety of atomically precise graphene nanoribbons can be prepared through on-surface synthesis, bringing the concept of graphene nanoribbon electronics closer to reality. For future applications it is beneficial to integrate contacts and more functionality directly into single ribbons by using heterostructures. Here, we use the on-surface synthesis approach to fabricate a metal-semiconductor junction and a tunnel barrier in a single graphene nanoribbon consisting of 5- and 7-atom wide segments. We characterize the atomic scale geometry and electronic structure by combined atomic force microscopy, scanning tunneling microscopy, and conductance measurements complemented by density functional theory and transport calculations. These junctions are relevant for developing contacts in all-graphene nanoribbon devices and creating diodes and transistors, and act as a first step toward complete electronic devices built into a single graphene nanoribbon.Adding functional electronic components to graphene nanoribbons requires precise control over their atomic structure. Here, the authors use a bottom-up approach to build a metal-semiconductor junction and a tunnel barrier directly into a single graphene nanoribbon, an exciting development for graphene-based electronic devices.

3D quantitative breast ultrasound analysis for differentiating fibroadenomas and carcinomas smaller than 1cm.

PURPOSE: In (3D) ultrasound, accurate discrimination of small solid masses is difficult, resulting in a high frequency of biopsies for benign lesions. In this study, we investigate whether 3D quantitative breast ultrasound (3DQBUS) analysis can be used for improving non-invasive discrimination between benign and malignant lesions. METHODS AND MATERIALS: 3D US studies of 112 biopsied solid breast lesions (size <1cm), were included (34 fibroadenomas and 78 invasive ductal carcinomas). The lesions were manually delineated and, based on sonographic criteria used by radiologists, 3 regions of interest were defined in 3D for analysis: ROI (ellipsoid covering the inside of the lesion), PER (peritumoural surrounding: 0.5mm around the lesion), and POS (posterior-tumoural acoustic phenomena: region below the lesion with the same size as delineated for the lesion). After automatic gain correction (AGC), the mean and standard deviation of the echo level within the regions were calculated. For the ROI and POS also the residual attenuation coefficient was estimated in decibel per cm [dB/cm]. The resulting eight features were used for classification of the lesions by a logistic regression analysis. The classification accuracy was evaluated by leave-one-out cross-validation. Receiver operating characteristic (ROC) curves were constructed to assess the performance of the classification. All lesions were delineated by two readers and results were compared to assess the effect of the manual delineation. RESULTS: The area under the ROC curve was 0.86 for both readers. At 100% sensitivity, a specificity of 26% and 50% was achieved for reader 1 and 2, respectively. Inter-reader variability in lesion delineation was marginal and did not affect the accuracy of the technique. The area under the ROC curve of 0.86 was reached for the second reader when the results of the first reader were used as training set yielding a sensitivity of 100% and a specificity of 40%. Consequently, 3DQBUS would have achieved a 40% reduction in biopsies for benign lesions for reader 2, without a decrease in sensitivity. CONCLUSION: This study shows that 3DQBUS is a promising technique to classify suspicious breast lesions as benign, potentially preventing unnecessary biopsies.

Image effects in transport at metal-molecule interfaces.

We present a method for incorporating image-charge effects into the description of charge transport through molecular devices. A simple model allows us to calculate the adjustment of the transport levels, due to the polarization of the electrodes as charge is added to and removed from the molecule. For this, we use the charge distributions of the molecule between two metal electrodes in several charge states, rather than in gas phase, as obtained from a density-functional theory-based transport code. This enables us to efficiently model level shifts and gap renormalization caused by image-charge effects, which are essential for understanding molecular transport experiments. We apply the method to benzene di-amine molecules and compare our results with the standard approach based on gas phase charges. Finally, we give a detailed account of the application of our approach to porphyrin-derivative devices recently studied experimentally by Perrin et al. [Nat. Nanotechnol. 8, 282 (2013)], which demonstrates the importance of accounting for image-charge effects when modeling transport through molecular junctions.

Adiabatic and non-adiabatic charge pumping in a single-level molecular motor.

We propose a design for realizing quantum charge pump based on a recent proposal for a molecular motor (Seldenthuis J S et al 2010 ACS Nano 4 6681). Our design is based on the presence of a moiety with a permanent dipole moment which can rotate, thereby modulating the couplings to metallic contacts at both ends of the molecule. Using the non-equilibrium Keldysh Green's function formalism (NEGF), we show that our design indeed generates a pump current. In the non-interacting pump, the variation of frequency from adiabatic to non-adiabatic regime, can be used to control the direction as well as the amplitude of the average current. The effect of Coulomb interaction is considered within the first- and the second- order perturbation. The numerical implementation of the scheme is quite demanding, and we develop an analytical approximation to obtain a speed-up giving results within a reasonable time. We find that the amplitude of the average pumped current can be controlled by both the driving frequency and the Coulomb interaction. The direction of of pumped current is shown to be determined by the phase difference between left and right anchoring groups.

Interference enhanced thermoelectricity in quinoid type structures.

Quantum interference (QI) effects in molecular junctions may be used to obtain large thermoelectric responses. We study the electrical conductance G and the thermoelectric response of a series of molecules featuring a quinoid core using density functional theory, as well as a semi-empirical interacting model Hamiltonian describing the π-system of the molecule which we treat in the GW approximation. Molecules with a quinoid type structure are shown to have two distinct destructive QI features close to the frontier orbital energies. These manifest themselves as two dips in the transmission, that remain separated, even when either electron donating or withdrawing side groups are added. We find that the position of the dips in the transmission and the frontier molecular levels can be chemically controlled by varying the electron donating or withdrawing character of the side groups as well as the conjugation length inside the molecule. This feature results in a very high thermoelectric power factor S(2)G and figure of merit ZT, where S is the Seebeck coefficient, making quinoid type molecules potential candidates for efficient thermoelectric devices.

Applicability of the wide-band limit in DFT-based molecular transport calculations.

Transport properties of molecular junctions are notoriously expensive to calculate with ab initio methods, primarily due to the semi-infinite electrodes. This has led to the introduction of different approximation schemes for the electrodes. For the most popular metals used in experiments, such as gold, the wide-band limit (WBL) is a particularly efficient choice. In this paper, we investigate the performance of different WBL schemes relative to more sophisticated approaches including the fully self-consistent non-equilibrium Green's function method. We find reasonably good agreement between all schemes for systems in which the molecule (and not the metal-molecule interface) dominates the transport properties. Moreover, our implementation of the WBL requires negligible computational effort compared to the ground-state density-functional theory calculation of a molecular junction. We also present a new approximate but efficient scheme for calculating transport with a finite bias. Provided the voltage drop occurs primarily inside the molecule, this method provides results in reasonable agreement with fully self-consistent calculations.

Abnormal two-dimensional strain echocardiography findings in children with congenital valvar aortic stenosis.

PURPOSE: Congenital valvar aortic stenosis (VAS) causes a pressure overload to the left ventricle. In the clinical setting, the severity of stenosis is graded by the pressure drop over the stenotic valve (pressure gradient). This parameter is dependent on the hemodynamic status and does not provide information regarding myocardial performance. This study was undertaken to reveal the potential of two-dimensional strain echocardiography (2DSTE) for the detection of myocardial functional changes due to congenital VAS in children. MATERIALS AND METHODS: A total of 86 patients (aged from birth to 18 years) with various degrees of isolated congenital VAS were enrolled in this study. None of the patients had undergone any form of surgical or balloon intervention. 139 healthy children served as a control group. Two-dimensional cine-loop recordings of apical 4-chamber, mid-cavity short-axis and basal short-axis views were digitally stored for off-line analysis. Longitudinal, circumferential and radial peak systolic strain and strain rate values were determined as well as the time to peak systolic strain (T2P). Two-way analysis of variance was performed to assess the relationship between VAS severity and 2DSTE parameters. RESULTS: In all patients conventional echocardiographic findings did not indicate systolic left ventricular dysfunction. All strain parameters of the control group were significantly different from those of VAS patients. There was a statistically significant, inverse relationship between global peak systolic strain parameters in all three directions and the degree of VAS (p < 0.05). Local peak systolic strain (rate) in the interventricular septum was most affected. T 2P increased significantly with VAS severity (p < 0.05). The decline in LV longitudinal systolic performance preceded that in other directions. CONCLUSION: 2DSTE detects alterations in myocardial function in children diagnosed with congenital VAS, whose conventional echocardiographic findings did not indicate ventricular systolic dysfunction.

In situ transmission electron microscopy imaging of grain growth in a platinum nanobridge induced by electric current annealing.

We report an in situ transmission electron microscopy (TEM) imaging of grain growth in a Pt nanobridge induced by a high electric current density. The change in morphology at the nanoscale was recorded in real time together with the electrical characterization of the Pt nanobridge. We find a drop in the differential resistance as the voltage across the bridge is increased; TEM inspection shows that this coincides with thermally induced grain growth, indicating that a reduction of grain boundary scattering is the cause of the resistance decrease.

Fluids density functional theory studies of supramolecular polymers at a hard surface.

We have applied a fluids density functional theory based on that of Yu and Wu [J. Chem. Phys. 116, 7094 (2002)] to treat reversible supramolecular polymers near a hard surface. This approach combines a hard-sphere fluids density functional theory with the first-order thermodynamic perturbation theory of Wertheim. The supramolecular polymers are represented in the theory by hard-spheres with two associating sites. We explore the effects of the bonding scheme, monomer concentration, and association energy upon the equilibrium chain sizes and the depletion lengths. This study is performed on simple systems containing two-site monomers and binary mixtures of two-site monomers combined with end stopper monomers which have only a single association site. Our model has correct behavior in the dilute and overlap regimes and the bulk results can be easily connected to simpler random-flight models. We find that there is a nonmonotonic behavior of the depletion length of the polymers as a function of concentration and that this depletion length can be controlled through the concentration of end stoppers. These results are applicable to the study of colloidal dispersions in supramolecular polymer solutions.

Noninvasive detection of hepatic lipidosis in dairy cows with calibrated ultrasonographic image analysis.

The aim was to test the accuracy of calibrated digital analysis of ultrasonographic hepatic images for diagnosing fatty liver in dairy cows. Digital analysis was performed by means of a novel method, computer-aided ultrasound diagnosis (CAUS), previously published by the authors. This method implies a set of pre- and postprocessing steps to normalize and correct the transcutaneous ultrasonographic images. Transcutaneous hepatic ultrasonography was performed before surgical correction on 151 German Holstein dairy cows (mean +/- standard error of the means; body weight: 571+/-7 kg; age: 4.9+/-0.2 yr; DIM: 35+/-5) with left-sided abomasal displacement. Concentration of triacylglycerol (TAG) was biochemically determined in liver samples collected via biopsy and values were considered the gold standard to which ultrasound estimates were compared. According to histopathologic examination of biopsies, none of the cows suffered from hepatic disorders other than hepatic lipidosis. Hepatic TAG concentrations ranged from 4.6 to 292.4 mg/g of liver fresh weight (FW). High correlations were found between the hepatic TAG and mean echo level (r=0.59) and residual attenuation (ResAtt; r=0.80) obtained in ultrasonographic imaging. High correlation existed between ResAtt and mean echo level (r=0.76). The 151 studied cows were split randomly into a training set of 76 cows and a test set of 75 cows. Based on the data from the training set, ResAtt was statistically selected by means of stepwise multiple regression analysis for hepatic TAG prediction (R(2)=0.69). Then, using the predicted TAG data of the test set, receiver operating characteristic analysis was performed to summarize the accuracy and predictive potential of the differentiation between various measured hepatic TAG values, based on TAG predicted from the regression formula. The area under the curve values of the receiver operating characteristic based on the regression equation were 0.94 (<50 vs. >or=50mg of TAG/g of FW), 0.83 (<100 vs. >or=100mg of TAG/g of FW), and 0.97 (<50 vs. >or=100mg of TAG/g of FW). The CAUS methodology and software for digitally analyzing liver ultrasonographic images is considered feasible for noninvasive screening of fatty liver in dairy herd health programs. Using the single parameter linear regression equation might be ideal for practical applications.

Cardiac biplane strain imaging: initial in vivo experience.

In this study, first we propose a biplane strain imaging method using a commercial ultrasound system, yielding estimation of the strain in three orthogonal directions. Secondly, an animal model of a child's heart was introduced that is suitable to simulate congenital heart disease and was used to test the method in vivo. The proposed approach can serve as a framework to monitor the development of cardiac hypertrophy and fibrosis. A 2D strain estimation technique using radio frequency (RF) ultrasound data was applied. Biplane image acquisition was performed at a relatively low frame rate (<100 Hz) using a commercial platform with an RF interface. For testing the method in vivo, biplane image sequences of the heart were recorded during the cardiac cycle in four dogs with an aortic stenosis. Initial results reveal the feasibility of measuring large radial, circumferential and longitudinal cumulative strain (up to 70%) at a frame rate of 100 Hz. Mean radial strain curves of a manually segmented region-of-interest in the infero-lateral wall show excellent correlation between the measured strain curves acquired in two perpendicular planes. Furthermore, the results show the feasibility and reproducibility of assessing radial, circumferential and longitudinal strains simultaneously. In this preliminary study, three beagles developed an elevated pressure gradient over the aortic valve (Deltap: 100-200 mmHg) and myocardial hypertrophy. One dog did not develop any sign of hypertrophy (Deltap = 20 mmHg). Initial strain (rate) results showed that the maximum strain (rate) decreased with increasing valvular stenosis (-50%), which is in accordance with previous studies. Histological findings corroborated these results and showed an increase in fibrotic tissue for the hearts with larger pressure gradients (100, 200 mmHg), as well as lower strain and strain rate values.

In vivo validation of cardiac output assessment in non-standard 3D echocardiographic images.

Automatic segmentation of the endocardial surface in three-dimensional (3D) echocardiographic images is an important tool to assess left ventricular (LV) geometry and cardiac output (CO). The presence of speckle noise as well as the nonisotropic characteristics of the myocardium impose strong demands on the segmentation algorithm. In the analysis of normal heart geometries of standardized (apical) views, it is advantageous to incorporate a priori knowledge about the shape and appearance of the heart. In contrast, when analyzing abnormal heart geometries, for example in children with congenital malformations, this a priori knowledge about the shape and anatomy of the LV might induce erroneous segmentation results. This study describes a fully automated segmentation method for the analysis of non-standard echocardiographic images, without making strong assumptions on the shape and appearance of the heart. The method was validated in vivo in a piglet model. Real-time 3D echocardiographic image sequences of five piglets were acquired in radiofrequency (rf) format. These ECG-gated full volume images were acquired intra-operatively in a non-standard view. Cardiac blood flow was measured simultaneously by an ultrasound transit time flow probe positioned around the common pulmonary artery. Three-dimensional adaptive filtering using the characteristics of speckle was performed on the demodulated rf data to reduce the influence of speckle noise and to optimize the distinction between blood and myocardium. A gradient-based 3D deformable simplex mesh was then used to segment the endocardial surface. A gradient and a speed force were included as external forces of the model. To balance data fitting and mesh regularity, one fixed set of weighting parameters of internal, gradient and speed forces was used for all data sets. End-diastolic and end-systolic volumes were computed from the segmented endocardial surface. The cardiac output derived from this automatic segmentation was validated quantitatively by comparing it with the CO values measured from the volume flow in the pulmonary artery. Relative bias varied between 0 and -17%, where the nominal accuracy of the flow meter is in the order of 10%. Assuming the CO measurements from the flow probe as a gold standard, excellent correlation (r = 0.99) was observed with the CO estimates obtained from image segmentation.

New reference values for echocardiographic dimensions of healthy Dutch children.

AIM: To renew the echocardiographic reference values of heart dimensions in healthy children. METHODS AND RESULTS: Group 1 consisted of 587 children, of which 361 boys and 226 girls, age from birth to 18 years, body weight over 2500 g, who visited the Pediatric Cardiology outclinic during the period January 2000 till March 2004. All included children were diagnosed as normal, or as having innocent heart murmur. The second group was taken from an earlier study and comprised 160 children (77 boys and 83 girls). The echocardiographic measures were taken from conventional M-mode recording of the left ventricle (LV) parasternal long axis view. End diastolic septal (IVS) and LV posterior wall thickness (LVPW) and end diastolic as well as end systolic LV intracavity dimensions were retrospectively analyzed. The regression lines from all measured sizes are significantly different from those collected in the early eighties. Especially the thickness of the IVS is smaller. The regression lines are independent of gender. CONCLUSIONS: New reference values have been found which should replace the presently used ones. There is no difference between boys and girls. Why the muscular wall thicknesses are thinner than found 20 years ago needs to be further explored.

Monitoring cerebral perfusion using near-infrared spectroscopy and laser Doppler flowmetry.

This paper describes the simultaneous use of two, noninvasive, near-infrared techniques near-infrared spectroscopy (NIRS) and a continuous wave NIR laser Doppler flow system (LDF) to measure changes in the blood oxygenation, blood concentration and blood flow velocity in the brain. A piglet was used as animal model. A controlled change in the arterial CO2 pressure (PaCO2) was applied for achieving changes in the listed cerebrovascular parameters. The time courses of blood concentration parameters (NIRS) and RMS blood flow velocity (LDF) were found to correspond closely with those of carotid blood flow and arterial carbon dioxide pressure (PaCO2). This result shows the additional value of LDF when combined with NIRS, preferably in one instrument. Development of pulsed LDF for regional blood flow measurement is indicated.

Assessment of local changes of cerebral perfusion and blood concentration by ultrasound harmonic B-mode contrast measurement in piglet.

This study tested the hypothesis that changes in the blood concentration, and possibly in the perfusion, of different areas in the brain can be assessed by the use of ultrasound contrast agent (CA) and (linear) echo densitometry. The experiments were performed with piglets (n=3) under general anesthesia and artificial ventilation. Ultrasound CA was administered through a femoral vein as a short bolus. First passage wash-in curve was measured from image gray level during continuous low level (mechanical index<0.2) ultrasound imaging. This curve was obtained from 1-cm2 areas of the cortex (surface), the brain stem (inner) and the left carotid artery (vessel). Cerebral hemoglobin concentration changes were measured with near-infrared spectroscopy (NIRS). This approach enabled a cross-validation of these techniques. The measurements were repeated under conditions of normocapnia, mild hypercapnia and deep hypercapnia. Several physiologic signals, as well as the carotid blood flow, were measured simultaneously and related to gray level by linear regression analysis. The most significant results found were a high R2-statistic of the regression of the percentage change of the peak of the surface and inner wash-in curves with the arterial carbon dioxide pressure (R2=0.63 and R2=0.70, respectively), the blood pH (R2=0.79 and R2=0.81), the carotid flow (R2=0.75 and R2=0.72) and the partial arterial oxygen pressure (R2=0.47 and R2=0.55). Finally, a high correlation of peak gray level with total hemoglobin concentration change, independently measured by NIRS, was found (R2=0.69). In conclusion, these experiments show a reasonable intersubject variability of various relative measures derived from gray level ultrasound wash-in curves. High sensitivity to physiologic changes related to hypercapnia was observed for the peak contrast of wash-in curves. For up-slope and area-under-the-curve (first passage) this was lower but still highly significant. The gray-level ultrasound measures are highly correlated to changes in regional hemoglobin concentration in brain tissue assessed by NIRS.

Experimental verification of conditions for near infrared spectroscopy (NIRS).

OBJECTIVES: in vitro assessment of the reproducibility and the optimal separation and position of the optodes in continuous wave (CW-) NIRS measurement of local inhomogeneities in absorption and/or scattering. METHODS: a CW- NIRS system (OXYMON) was used with laser diodes at wavelengths of 767 nm, 845 nm, 905 nm, 945 nm and 975 nm. For practical considerations (dimensions of neonatal head) the measurements were performed on a cylindrical tissue-equivalent phantom (70 mm diameter of base material with mua = 0.01 mm(-1) (800 nm) and mu's = 1.00 mm(-1) (800 nm)), containing rods with 10 x absorption, or 10 x scattering, and 5 x both Monte Carlo simulations were carried out of a cylinder with transport scattering coefficient mu's = 0.525 mm(-1) and absorption coefficient mua = 0.075 mm(-1) and two optode positions. RESULTS: reproducibility of repeated measurements (n = 10) was +/- 0.005 OD. Maximum OD in case of absorbing rod, and of absorbing + scattering rod was measured with optodes separated by 90 degrees and rod position angle symmetrically (45 degrees ) in between. Minimum OD for these rods was obtained with optodes at 150 degrees angle and rod position at 240 degrees (i.e. relative to transmitting optode position at 0 degrees ). A second maximum OD was obtained at an optode angle 180 degrees and rod position at 180 degrees. Maximum OD (i.e. attenuation) for the scattering rod was at optode separation angle of 90 degrees and rod at 0 degrees. Minimum OD for this case was obtained with optode angle of 180 degrees and rod positions around 80 degrees and 280 degrees. Maximum OD changes by absorbing rod were in the order of +0.12 OD and -0.04 OD, respectively. Simulations at an optode separation angle of 90 degrees showed a spatial sensitivity path enclosing the rod position at maximum absorption found experimentally. CONCLUSIONS: when considering the phantom as a realistic geometrical model for the neonatal head, it can be concluded that the optode position at 90 degrees angle would be optimal for detecting an inhomogeneity at 15 mm depth, i.e. the location of the periventricular white matter. Since the rods are relatively strongly different from the base material the question remains to be answered whether local ischemia, which might lead to irreversible brain damage, can be detected by CW-NIRS

Performance testing of medical echo/Doppler equipment.

This paper describes the methods applied in a software package developed by the authors for use in a performance testing protocol for medical ultrasound equipment. The history of performance testing of medical ultrasound equipment is briefly reviewed. This paper is confined to the testing of performance of usage aspects, i.e. imaging performance and Doppler velocity estimation performance. Simple test objects are used which have a long life expectancy. The tests performed both in fundamental and in (tissue) harmonic modes when applicable are spatial resolution, contrast sensitivity, and clutter. The concept of a computational observer is used to define the lesion signal-to-noise ratio and the tissue-to-clutter ratio. Further imaging performance features are penetration depth, slice thickness and geometric conformity of display. Pulsed Doppler velocity measurement features tested are: sensitivity, depth and 3D size of the sample volume, velocity measurement, channel separation. The whole performance measurement protocol as well as the quantitative measurements in the digitized images are implemented in software, together with the graphs and data obtained from the measurements.

Performance testing of medical ultrasound equipment: fundamental vs. harmonic mode.

Assessment of the performance of medical ultrasound equipment is generally based on the image quality in fundamental mode. Recent development of the so-called tissue harmonic imaging (THI) mode induces the need for assessment of differences in the quality of imaging in THI vs. fundamental imaging mode. Quality features to be tested are sensitivity (penetration depth), spatial resolution, contrast resolution, lesion signal-to-noise ratio, and tissue-to-clutter ratio (TCR). These features are explained and examples are shown. The main conclusion from a comparison of the results for the two imaging modes might be that when using THI improvement of TCR, in particular in the near field, is obtained at the expense of a loss in axial resolution. Furthermore, lesion detection is not significantly improved.