Biomedical subjects

J Lian

Publications and source records attributed to J Lian.

At least 19 recordsLinked to original sources

PubMed2006

Effect of structure and thermodynamic stability on the response of lanthanide stannate pyrochlores to ion beam irradiation.

The lanthanide stannates, Ln2Sn2O7, Ln=La-Lu and Y, have the isometric pyrochlore structure, A2B2O7, and their structural properties have been refined by Rietveld analysis of powder neutron and synchrotron X-ray diffraction data. In this study, the enthalpies of formation of selected stannate pyrochlores, Ln=La, Nd, Sm, Eu, Dy, and Yb, were measured by high-temperature oxide melt solution calorimetry. Their radiation response was determined by 1 MeV Kr2+ ion irradiation combined with in situ TEM observation over the temperature range of 25 to 1000 K. The enthalpy of formation from binary oxides of stannate pyrochlores became more endothermic (from -145 to -40 kJ/mol) as the size of the lanthanide in the A-site decreases. A more exothermic trend of the enthalpy of formation was observed in stannate pyrochlores with larger lanthanide ions, particularly La, possibly as a result of increased covalency in the Sn-O bond. In contrast to lanthanide titanate pyrochlores, Ln2Ti2O7, that are generally susceptible to radiation-induced amorphization and zirconate pyrochlores, Ln2Zr2O7, that are generally resistant to radiation-induced amorphization, the lanthanide stannate pyrochlores show a much greater variation in their response to ion irradiation. La, Nd, and Gd stannates experience the radiation-induced transformation to the aperiodic state, and the critical amorphization temperatures are approximately 960, 700, and 350 K, respectively. Y and Er stannate pyrochlores cannot be amorphized by ion beam irradiation, even at 25 K, and instead disorder to a defect fluorite structure. Comparison of the calorimetric and ion irradiation data for titanate, zirconate, and stannate pyrochlores reveals a strong correlation among subtle changes in crystal structure with changing composition, the energetics of the disordering process, and the temperature above which the material can no longer be amorphized. In summary, as the structure approaches the ideal, ordered pyrochlore structure, radiation-induced amorphization is more easily attained. This is consistent with an increasingly exothermic trend in the enthalpies of formation of pyrochlores from the oxides, that is, the greater the thermochemical stability of the pyrochlore structure, the more likely it will be amorphized upon radiation damage rather than recover to a disordered fluorite structure.

Journal Article↗

PubMed2004

Mapping of the prostate in endorectal coil-based MRI/MRSI and CT: a deformable registration and validation study.

The endorectal coil is being increasingly used in magnetic resonance imaging (MRI) and MR spectroscopic imaging (MRSI) to obtain anatomic and metabolic images of the prostate with high signal-to-noise ratio (SNR). In practice, however, the use of endorectal probe inevitably distorts the prostate and other soft tissue organs, making the analysis and the use of the acquired image data in treatment planning difficult. The purpose of this work is to develop a deformable image registration algorithm to map the MRI/MRSI information obtained using an endorectal probe onto CT images and to verify the accuracy of the registration by phantom and patient studies. A mapping procedure involved using a thin plate spline (TPS) transformation was implemented to establish voxel-to-voxel correspondence between a reference image and a floating image with deformation. An elastic phantom with a number of implanted fiducial markers was designed for the validation of the quality of the registration. Radiographic images of the phantom were obtained before and after a series of intentionally introduced distortions. After mapping the distorted phantom to the original one, the displacements of the implanted markers were measured with respect to their ideal positions and the mean error was calculated. In patient studies, CT images of three prostate patients were acquired, followed by 3 Tesla (3 T) MR images with a rigid endorectal coil. Registration quality was estimated by the centroid position displacement and image coincidence index (CI). Phantom and patient studies show that TPS-based registration has achieved significantly higher accuracy than the previously reported method based on a rigid-body transformation and scaling. The technique should be useful to map the MR spectroscopic dataset acquired with ER probe onto the treatment planning CT dataset to guide radiotherapy planning.

Algorithms↗

PubMed2004

Cortical potential imaging of brain electrical activity by means of parametric projection filter.

OBJECTIVES: The objective of this study was to explore suitable spatial filters for inverse estimation of cortical potentials from the scalp electroencephalogram. The effect of incorporating noise covariance into inverse procedures was examined by computer simulations and tested in human experiment. METHODS: The parametric projection filter, which allows inverse estimation with the presence of information on the noise, was applied to an inhomogeneous three-concentric-sphere model under various noise conditions in order to estimate the cortical potentials from the scalp potentials. The method for determining the optimum regularization parameter, which can be applied for parametric inverse techniques, is also discussed. RESULTS: Human visual evoked potential experiment was carried out to examine the performance of the proposed restoration method. The parametric projection filter gave more localized inverse solution of cortical potential distribution than the truncated SVD and Tikhonov regularization. CONCLUSION: The present simulation results suggest that incorporation of information on the noise covariance allows better estimation of cortical potentials, than inverse solutions without knowledge about the noise covariance, when the correlation between the signal and noise is low.

Algorithms↗

PubMed2002

X-ray photoelectron spectroscopy study of disordering in Gd2(Ti1-xZrx)2O7 pyrochlores.

The dramatic increases in ionic conductivity in Gd2(Ti1-xZrx)2O7 solid solution are related to disordering on the cation and anion lattices. Disordering in Gd2(Ti1-xZrx)2O7 was characterized using x-ray photoelectron spectroscopy (XPS). As Zr substitutes for Ti in Gd2Ti2O7 to form Gd2(Ti1-xZrx)2O7 (0.25 < x < or =0.75), the corresponding O 1s XPS spectrum merges into a single symmetric peak. This confirms that the cation antisite disorder occurs simultaneously with anion disorder. Furthermore, the O 1s XPS spectrum of Gd2Zr2O7 experimentally suggests the formation of a split vacancy.

Journal Article↗

PubMed2002

Boundary element method-based cortical potential imaging of somatosensory evoked potentials using subjects' magnetic resonance images.

A boundary element method-based cortical potential imaging technique has been developed to directly link the scalp potentials with the cortical potentials with the aid of magnetic resonance images of the subjects. First, computer simulations were conducted to evaluate the new approach in a concentric three-sphere inhomogeneous head model. Second, the corresponding cortical potentials were estimated from the patients' preoperative scalp somatosensory evoked potentials (SEPs) based on the boundary element models constructed from subjects' magnetic resonance images and compared to the postoperative direct cortical potential recordings in the same patients. Simulation results demonstrated that the cortical potentials can be estimated from the scalp potentials using different scalp electrode configurations and are robust against measurement noise. The cortical imaging analysis of the preoperative scalp SEPs recorded from patients using the present approach showed high consistency in spatial pattern with the postoperative direct cortical potential recordings. Quantitative comparison between the estimated and the directly recorded subdural grid potentials resulted in reasonably high correlation coefficients in cases studied. Amplitude difference between the estimated and the recorded potentials was also observed as indexed by the relative error, and the possible underlying reasons are discussed. The present numerical and experimental results validate the boundary element method-based cortical potential imaging approach and demonstrate the feasibility of the new approach in noninvasive high-resolution imaging of brain electric activities from scalp potential measurement and magnetic resonance images.

Algorithms↗

PubMed2002

Body surface Laplacian mapping of atrial depolarization in healthy human subjects.

In the present study, we report body surface Laplacian mapping of atrial depolarization under sinus rhythm in 8 healthy male subjects. For each subject, 95 unipolar disk electrodes with inter-electrode distance of 2 cm were used to record simultaneously potential ECGs over the anterior chest. The Laplacian ECG was then estimated during the P wave using a novel spline Laplacian technique. The body surface potential map (BSPM) and body surface Laplacian map (BSLM) at different time instants or time intervals of the P wave were constructed and compared. The present results showed that the BSPMs during the P wave were characterized by the rotation of a pair of positive/negative potential distribution from right to left around the anterior torso. On the other hand, the corresponding BSLMs revealed more spatial details, including two positive activities (denoted as P1 and P2, appeared in all 8 subjects), and three negative activities (denoted as N1, N2, and N3, appeared in 7, 7, and 4 subjects, respectively). The separation of these activities and their evolving patterns were also compared and confirmed by computer simulation using a realistic geometry heart-torso model. The above findings may be directly related to the underlying activation sequence during atrial depolarization in healthy subjects, suggesting the potential clinical applications of the Laplacian ECG technique.

Adult↗

PubMed2002

High-resolution spatio-temporal functional neuroimaging of brain activity.

The past decades have shown extraordinary progress in our ability to noninvasively image the functions of the human brain. Of particular interest is the recent trend in combining information from electrophysiological and magnetic resonance imaging, which we termed eMRI, to achieve high-resolution functional neuroimaging in both space and time domains. In this article, we review the recent progress in high-resolution functional neuroimaging, in particular the multimodal integration of electroencephalography (EEG) and magnetic resonance imaging (MRI). The state-of-the-art EEG inverse solutions based on different brain electric source models and various approaches to integrate the information from MRI are reviewed. The remaining challenges, future trends, and potential applications of the high-resolution functional neuroimaging research are discussed.

Action Potentials↗

PubMed2001

Propagation of non-synaptic epileptiform activity across a lesion in rat hippocampal slices.

1. Spontaneous non-synaptic epileptiform activity was induced by bathing rat hippocampal slices in low-Ca(2+) solution. Extracellular recordings from electrodes placed on both sides of a complete cut showed that non-synaptic activity was synchronized across the lesion. 2. Ion-selective electrode recordings showed that each event was accompanied by a transient increase in extracellular potassium that diffused across the lesion. The synchrony was destroyed when a thin film was inserted into the lesion site. 3. Local pressure ejection of KCl evoked an event that subsequently propagated across the lesion. 4. After a complete lesion was made, afterdischarges evoked on one half of a slice were not detected on the other half. 5. Voltage-sensitive dye imaging methods showed that epileptic activity propagated across the mechanical lesion without significant attenuation or additional delays. The velocity of the activity was consistent with that of the slow diffusion of a potassium wave. 6. Since field effects were significantly attenuated across the lesion and all gap junctions and cell processes across the lesion would be cut, these data show that extracellular diffusion, most probably potassium, is sufficient to synchronize populations of neurons and propagate slow frequency epileptiform activity.

Animals↗

PubMed2001

Nanoscale manipulation of pyrochlore: new nanocomposite ionic conductors.

The ionic conductivity of isometric pyrochlore, ideally A2B2O (7), is extremely sensitive to disordering of A- and B-site cations and oxygen anion vacancies. We report the first use of ion beam irradiation-induced disordering in Gd 2Ti 2O (7) to produce a strain-free, buried, disordered defect-fluorite layer approximately 12 nm thick within an ordered pyrochlore matrix. This approach provides a new means of creating nanoscale, mixed ionic-electronic conductors in pyrochlore ceramics, such as those required for solid-state electrochemical cells.

Journal Article↗

PubMed2001

Disruption of NF-kappa B signaling and chemokine gene activation by retroviral mediated expression of IKK gamma/NEMO mutants.

Phosphorylation of I kappa Bs--the cytoplasmic inhibitors of the NF-kappa B transcription factors--is the key event which triggers activation of the NF-kappa B cascade. Signal-mediated phosphorylation of I kappa B alpha is mediated by a multiprotein complex, the I kappa B kinase (IKK) complex, which is composed of at least three identified subunits. Two of these polypeptides, IKK alpha and IKK beta, also known as IKK1 and IKK2, are the catalytic subunits of the kinase complex and phosphorylate I kappa B alpha and I kappa B beta. The third component, NEMO/IKK gamma, does not exhibit kinase activity, but rather constitutes a regulatory subunit. In the present study, C-terminal truncated forms of IKK gamma--Delta C-IKK gamma 306 and Delta C-IKK gamma 261--were stably expressed in the myeloid cell line U937 by retroviral-mediated gene transfer. Overexpression of Delta C-IKK gamma resulted in a reduction in IKK kinase activity in vitro, a subsequent decrease in NF-kappa B DNA binding activity, and inhibition of chemokine gene induction in response to TNFalpha stimulation or paramyxovirus infection. This study demonstrates the efficacy of Delta C-IKK gamma as a repressor of IKK signaling and NF-kappa B activation and suggests a potential gene therapy approach to limit chronic inflammation due to chemokine hyperactivation.

Blotting, Western↗

PubMed2001

Positive Lyapunov exponents calculated from time series of strange nonchaotic attractors.

Time-series methods for estimating Lyapunov exponents may give a positive exponent when they are applied to the time series of strange nonchaotic systems. Strange nonchaotic systems are characterized by expanding and contracting regions in phase space that result in repeatedly expanding or contracting trajectories. Using time-series methods, the maximum time-series Lyapunov exponent is calculated as an average of the locally most expanding exponents that characterize the divergence of nearby trajectories following a reconstructed attractor over time. A positive exponent is reported by time-series methods for trajectories in an expanding region. While in a converging region, the most expanding dynamics are related to the quasiperiodic driving force. Statistically, a zero exponent related to the quasiperiodic force is obtained through time-series methods within converging regions. As a result, the calculated maximum Lyapunov exponent is positive.

Journal Article↗

PubMed2001

Whole genome sequencing of meticillin-resistant Staphylococcus aureus.

BACKGROUND: Staphylococcus aureus is one of the major causes of community-acquired and hospital-acquired infections. It produces numerous toxins including superantigens that cause unique disease entities such as toxic-shock syndrome and staphylococcal scarlet fever, and has acquired resistance to practically all antibiotics. Whole genome analysis is a necessary step towards future development of countermeasures against this organism. METHODS: Whole genome sequences of two related S aureus strains (N315 and Mu50) were determined by shot-gun random sequencing. N315 is a meticillin-resistant S aureus (MRSA) strain isolated in 1982, and Mu50 is an MRSA strain with vancomycin resistance isolated in 1997. The open reading frames were identified by use of GAMBLER and GLIMMER programs, and annotation of each was done with a BLAST homology search, motif analysis, and protein localisation prediction. FINDINGS: The Staphylococcus genome was composed of a complex mixture of genes, many of which seem to have been acquired by lateral gene transfer. Most of the antibiotic resistance genes were carried either by plasmids or by mobile genetic elements including a unique resistance island. Three classes of new pathogenicity islands were identified in the genome: a toxic-shock-syndrome toxin island family, exotoxin islands, and enterotoxin islands. In the latter two pathogenicity islands, clusters of exotoxin and enterotoxin genes were found closely linked with other gene clusters encoding putative pathogenic factors. The analysis also identified 70 candidates for new virulence factors. INTERPRETATION: The remarkable ability of S aureus to acquire useful genes from various organisms was revealed through the observation of genome complexity and evidence of lateral gene transfer. Repeated duplication of genes encoding superantigens explains why S aureus is capable of infecting humans of diverse genetic backgrounds, eliciting severe immune reactions. Investigation of many newly identified gene products, including the 70 putative virulence factors, will greatly improve our understanding of the biology of staphylococci and the processes of infectious diseases caused by S aureus.

Animals↗

PubMed2001

Suppression of epileptiform activity by high frequency sinusoidal fields in rat hippocampal slices.

1. Sinusoidal high frequency (20-50 Hz) electric fields induced across rat hippocampal slices were found to suppress zero-Ca2+, low-Ca2+, picrotoxin, and high-K+ epileptiform activity for the duration of the stimulus and for up to several minutes following the stimulus. 2. Suppression of spontaneous activity by high frequency stimulation was found to be frequency (< 500 Hz) but not orientation or waveform dependent. 3. Potassium-sensitive microelectrodes showed that block of epileptiform activity was always coincident with a stimulus-induced rise in extracellular potassium concentration during stimulation. Post-stimulus inhibition was always associated with a decrease in extracellular potassium activity below baseline levels. 4. Intracellular recordings and optical imaging with voltage-sensitive dyes showed that during suppression neurons were depolarized yet did not fire action potentials. 5. Direct injection of sinusoidal current into individual pyramidal cells did not result in a tonic depolarization. Injection of large direct current (DC) depolarized neurons and suppressed action potential generation. 6. These findings suggest that high frequency stimulation suppresses epileptiform activity by inducing potassium efflux and depolarization block.

Animals↗

PubMed2001

Nonlinear dynamic properties of low calcium-induced epileptiform activity.

The analysis of the dynamic properties of epileptiform activity in vitro has led to a better understanding of the time course of neural synchronization and seizure states. Nonlinear analysis is thus potentially useful for the prediction of seizure onset. We have used nonlinear analysis methods to investigate the development of activity in the low calcium model of epilepsy in brain slices. This model is particularly interesting since neurons synchronize in the absence of synaptic transmission. The dynamic properties calculated from extracellular recordings of activity were used to analyze the transition to synchronous firing and their relation to neuronal excitability. The global embedding dimension, local dimension and the Lyapunov exponent were calculated from time segments corresponding to the onset, transition and fully developed stages of activity. The analysis was repeated for recordings made in the presence of various levels of DC electric fields to modulate neuronal excitability. The global and local dimensions did not change once activity was first initiated, even in the presence of the electric field. The maximum Lyapunov exponents increased during the onset of activity but decreased when the applied hyperpolarizing electric field was large enough to partially suppress the activity. These findings establish a relationship between neuronal excitability and the maximum Lyapunov exponent, and suggest that the Lyapunov exponent may be used to distinguish between various states of the neural network and might be important in seizure prediction and control.

Action Potentials↗

PubMed2001

A cortical potential imaging analysis of the P300 and novelty P3 components.

The P300 and Novelty P3 are positive components of the event related brain potential (ERP) with a latency of at least 300 ms, which are manifestations of brain activity evoked by deviant events. Spencer et al. [1999, 2001] demonstrated that these are two distinct components, both of which may be elicited, with different amplitudes, by both rare and novel events. However, the locations of the intracranial sources of the components remain unknown. We describe the application of cortical potential imaging (CPI) analysis to the data described by Spencer et al. [1999]. The ERPs were recorded from 15 healthy subjects presented with auditory oddball sequences. Cortical potential maps (CPMs) were reconstructed from the scalp potential maps (SPMs) corresponding to the P300 and Novelty P3 components by deblurring the smoothing effect of the head volume conductor. The reconstructed CPMs, derived from the SPMs by means of the CPI, showed localized areas of activity distributed in both the frontal and parietal lobes; the parietal region was active throughout the period of the late positivities. The reconstructed CPMs associated with novel events showed prominent activity at the frontal lobe (Novelty P3) followed by progressively pronounced parietal lobe activity (P300), and these two components can be well separated by the CPMs. These analyses show how the CPI can be used to relate the scalp electrical recordings to the underlying brain activity.

Brain Mapping↗

PubMed2001

High-resolution EEG: a new realistic geometry spline Laplacian estimation technique.

BACKGROUND: A new realistic geometry (RG) spline Laplacian estimation technique has been developed for high-resolution EEG imaging. METHODS: Estimation of the parameters associated with the spline Laplacian is formulated by seeking the general inverse of a transfer matrix. The number of spline parameters, which need to be determined through regularization, is reduced to one in the present approach, thus enabling easy implementation of the RG spline Laplacian estimator. RESULTS: Computer simulation studies have been conducted to test the feasibility of the new approach in a 3-concentric-sphere head model. The new technique has also been applied to human visual evoked potential data with a RG head model. CONCLUSIONS: The present numerical and experimental results demonstrate the feasibility of the new approach and indicate that the RG spline Laplacian can be estimated easily from the surface potentials and the scalp geometry.

Acoustic Stimulation↗

PubMed2001

A minimal product method and its application to cortical imaging.

In order to reduce the spatial blurring effect due to the head volume conductor, cortical imaging technique (CIT) can be used to reconstruct the cortical potential distribution from the scalp potential measurement with enhanced spatial resolution. To overcome the ill-posed nature of the inverse problem, Tikhonov regularization (TIK) and truncated Singular Value Decomposition (TSVD) are commonly used by choosing the appropriate regularization parameter and truncation parameter, respectively. We have developed a minimal product method (MINP) to determine the regularization and truncation parameters. The present computer simulation and experimental results indicate that the MINP can be easily implemented in both TIK and TSVD with satisfactory performance, and suggest the potential applications of the MINP method in determining the corner of the L-curve.

Algorithms↗

PubMed2001

A study of equivalent source techniques for high-resolution EEG imaging.

High-resolution EEG imaging has been an important topic in recent EEG research, and much work has been done on the two equivalent source imaging techniques: the equivalent distributed dipole-layer source imaging technique (EST) and the equivalent multipole source imaging technique (SAT). In this paper we first develop a forward density formula for a spherical equivalent distributed dipole layer of an arbitrary dipole in a three-concentric-sphere head model. It is clarified using the derived forward formula that the equivalent dipole-layer source and equivalent multipole source are interrelated in theory. Finally, simulation comparisons are conducted, the results of which suggest that EST has a higher spatial resolution than SAT when both of them are implemented by a truncated singular value decomposition algorithm. This is due to the different singularities of the inversion equations involved in the two techniques. An empirical VEP data study also shows that EST is better than SAT in providing higher spatial resolution EEG imaging.

Cerebral Cortex↗