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At least 631 records · Page 35Linked to original sources
PubMed2004

Frameless stereotactic navigation in transsphenoidal surgery: comparison with fluoroscopy.

Surgical navigation systems (frameless stereotaxy) have been used in addition to or instead of fluoroscopy during transsphenoidal surgery. This study compares the intraoperative localization by an optical tracking system (Elekta Viewscope) with fluoroscopy. Viewscope and fluoroscope sagittal images were compared by the establishment of a Cartesian coordinate system based on anatomical landmarks and by the spatial localization of surgically relevant points for 20 patients. The Viewscope was found to have a total deviation of 3.0 +/- 0.6 mm (mean +/- SD) compared to fluoroscopy (p < 0.01). Much of the error resulted from the registration process, which according to the Viewscope software had an expected error of 3.1 +/- 0.8 mm for this series of patients, and from the probe-to-system correlation (error of 1.0 +/- 0.3 mm). Although frameless stereotactic systems give the surgeon useful trajectory data with three-dimensional visualizations, they remain somewhat inaccurate. The multiplanar abilities of the Viewscope provide an additional but not mandatory advantage to the simplicity and accuracy of fluoroscopy during this type of surgery.

Adolescent↗
PubMed2003

Frequency-doubling technology perimetry and optical defocus.

PURPOSE: The frequency-doubling technology (FDT) perimeter has no provision for introducing corrective lenses, save for the patient's spectacles, and so patients are sometimes tested in the presence of moderate levels of defocus. The effect of defocus on frequency-doubling (FD) sensitivity was determined for both the commercially available instrument and for smaller targets that may be useful in spatially localizing visual field defects. In addition, whether stimulus artifacts may be detectable in the presence of defocus was assessed. METHODS: Detection and resolution thresholds for FD stimuli, along with detection thresholds for spatially uniform flickering stimuli, were measured in normal observers in the presence of up to +6 D of defocus. In addition, the effect of defocus on the summary indices (mean deviation [MD], and pattern standard deviation [PSD]) of both the commercial FDT perimeter and a customized perimeter with smaller, higher spatial frequency targets was determined. RESULTS: Thresholds for conventional FD targets (0.25 cyc/deg, 25 Hz) increased 0.1 log unit with +6 D defocus, whereas thresholds for smaller, higher spatial frequency (0.5 cyc/deg) targets increased 0.4 log unit. Despite the presence of a luminance artifact in the FD stimuli, detection, and resolution thresholds remained coincident at maximum defocus, suggesting that the artifact was not detectable. MD was significantly affected by defocus for the customized FD perimeter only, and PSD was not altered for either the conventional or customized test. CONCLUSIONS: In normal observers, optical defocus has little effect on FDT perimetry sensitivity and does not make low spatial frequency artifacts detectable. Improving the spatial resolution of FDT perimetry decreases its robustness to defocus.

Adult↗
PubMed1980

Differential electrical excitation of the auditory nerve.

The multichannel cochlear prosthesis requires an electrode stimulus configuration which produces a stimulus field spatially localized to each electrode. In this paper, a three-dimensional discrete resistance model of the cochlea was developed which exhibits electrical response properties similar to those observed during electrical stimulation of the cochlea. The model results suggest that the spatial attenuation of current within the cochlea varies greatly in magnitude, depending on the stimulus configuration. In addition, the model suggests that the spatial attenuation of current in both the auditory nerve fiber endings in the organ of Corti and in the myelinated fibers within the cochlear ground paths is different from the voltage attenuation in the scalar fluids. Therefore the efficacy with which a particular stimulus configuration differentially excites local terminal auditory nerve fiber populations cannot be deduced from scalar voltage measurements which have previously been recorded in the literature. Consequently physiological experiments were performed in the cat to measure the current distributions in the terminal nerve fiber region for monopolar and bipolar stimulation of the scala tympani, and also for stimulation between the scala tympani and the scala vestibuli. The mean length constants measured in the basal turn for these stimuli were found to be 12, 3, and 7.5 mm, respectively.

Animals↗
PubMed2001

The effects of slice-selective excitation/refocusing in localized spectral editing with gradient-selected double-quantum coherence transfer.

Spectral editing using gradient-selected double-quantum filtering (DQF) with PRESS localization has been used for selective observation of metabolites in vivo. In previous studies using localized DQF sequences, it is generally assumed that the slice-selective pulses used in the sequence have no roles in coherence transfer, and do not interfere with DQF. To validate this assumption, the effects of slice-selective excitation/refocusing on DQF were investigated in DQF lactate editing sequences combined with PRESS localization. Contrary to the previous assumption, the results show that, due to chemical shift displacement artifact and J coupling, slice selection in DQF does interfere with coherence transfer, affecting both the accuracy of spatial localization and the detection sensitivity adversely. In the case of lactate editing, the effects of this interference can be accounted for simply by adjusting the strength of the slice-selection gradients and by using narrowband slice-selective refocusing pulses.

Calibration↗
PubMed1987

Comparison of perceived spatial frequency between the fovea and the periphery.

A grating viewed in the periphery usually appears to be of higher frequency than the same grating viewed at the fovea, for frequencies below the Nyquist frequency of the periphery. Systematic shifts in perceived frequency between fovea and periphery were found under several experimental conditions: spatially localized or extended sine-wave patterns, test stimuli presented in the superior or the inferior visual field, and stimuli of high or low mean luminance at several different contrasts. A multiple-spatial-frequency channels model can qualitatively account for our results, if it is assumed that each channel has a receptive-field size that increases as a function of eccentricity but has a perceived frequency label that does not change as a function of eccentricity.

Fovea Centralis↗
PubMed1990

Localization of visually evoked cortical activity using magnetic resonance imaging and computerized tomography.

Evoked scalp potentials, computerized tomography, and magnetic resonance imaging were used together to localize cortical activity evoked by visual stimuli in humans. The temporal resolution of evoked potential measurements is sufficient to track the flow of cortical activity which evolves in epochs of a few tens of msec. Spatial localization was enhanced by deconvolving scalp potential fields with a Laplacian operator. Markers glue to the scalp served to unify the three geometric reference frames into a single computer graphics database.

Brain Mapping↗
PubMed2004

Developmental and hormonal regulated gene expression of fibroblast growth factor 2 (FGF-2) and its receptors in porcine endometrium.

This study examined the mRNA levels of the fibroblast growth factor 2 (FGF-2) and two of its receptors, FGFR1IIIc and FGFR2IIIc, at days 12 and 20 of the ovarian cycle (DC 12 and DC 20), days 1 and 12 of pregnancy (DP 1 and DP 12) as well as the influence of progesterone (P) and estradiolbenzoate (EB) on their expression in the endometrium of ovariectomized (ovx) gilts by real-time PCR. Proteins of FGF-2 and FGFR1 were immunolocalized. FGF-2 and FGFR2IIIc mRNAs were always found with a 5- to 30-fold higher absolute concentration compared to FGFR1IIIc. The latter transcript significantly declined between DP 1 and DP 12, whereas FGF-2 and FGFR2IIIc showed no significant changes at that time. FGF-2 transcription was greater at DC 20 than at DC 12, but significantly most transcripts were found in ovx gilts. EB induced a significant suppression of FGF-2 mRNA, an effect which was antagonized by P and even prevented by P+EB. FGFR1IIIc mRNA was significantly increased at DC 20, that of FGFR2IIIc at DC 12 displaying a 10 times higher absolute mRNA amount. Suppression of FGFR1IIIc mRNA by P was abolished by EB while P+EB attenuated this effect. FGFR2IIIc transcripts were equally restrained by P or EB while a combination of both slightly reduced such declines. Localization of FGF-2 and FGFR1 proteins in stromal, glandular and vascular compartments was effected by sex steroids. Both proteins were strongly expressed at DP 12 but not at DP 1. Summarized, differential temporal and spatial localization of FGF-2 and FGFR1 after response to sex steroids support a complex regulation of this ligand receptor system important for proliferation and differentiation of uterine cells including angiogenic processes. While FGFR1IIIc is presumed to be promoted by estradiol FGFR2IIIc appears to be dominated by progesterone implicating different biological importance for a functional endometrium.

Animals↗
PubMed2006

CORSICA: correction of structured noise in fMRI by automatic identification of ICA components.

When applied to functional magnetic resonance imaging (fMRI) data, spatial independent component analysis (sICA), a data-driven technique that addresses the blind source separation problem, seems able to extract components specifically related to physiological noise and brain movements. These components should be removed from the data to achieve structured noise reduction and improve any subsequent detection and analysis of signal fluctuations related to neural activity. We propose a new automatic method called CORSICA (CORrection of Structured noise using spatial Independent Component Analysis) to identify the components related to physiological noise, using prior information on the spatial localization of the main physiological fluctuations in fMRI data. As opposed to existing spectral priors, which may be subject to aliasing effects for long-TR data sets (typically acquired with TR >1 s), such spatial priors can be applied to fMRI data, regardless of the TR of the acquisitions. By comparing the proposed automatic selection to a manual selection performed visually by a human operator, we first show that CORSICA is able to identify the noise-related components for long-TR data with a high sensitivity and a specificity of 1. On short-TR data sets, we validate that the proposed method of noise reduction allows a substantial improvement of the signal-to-noise ratio evaluated at the cardiac and respiratory frequencies, even in the gray matter, while preserving the main fluctuations related to neural activity.

Biometry↗
PubMed2003

Modeling the topological organization of cellular processes.

The cell as a dynamical system presents the characteristics of having a dynamical structure. That is, the exact phase space of the system cannot be fixed before the evolution and integrative cell models must state the evolution of the structure jointly with the evolution of the cell state. This kind of dynamical systems is very challenging to model and simulate. New programming concepts must be developed to ease their modeling and simulation. In this context, the goal of the MGS project is to develop an experimental programming language dedicated to the simulation of this kind of systems. MGS proposes a unified view on several computational mechanisms (CHAM, Lindenmayer systems, Paun systems, cellular automata) enabling the specification of spatially localized computations on heterogeneous entities. The evolution of a dynamical structure is handled through the concept of transformation which relies on the topological organization of the system components. An example based on the modeling of spatially distributed biochemical networks is used to illustrate how these notions can be used to model the spatial and temporal organization of intracellular processes.

Algorithms↗
PubMed2005

Spatial and spectral correlations in MALDI mass spectrometry images by clustering and multivariate analysis.

Matrix-assisted laser desorption/ionization mass spectrometry imaging is a technique for direct analysis of tissue sections without the use of molecular tags or contrast agents. The combination of spatial and mass resolution results in large and complex data sets that require powerful and efficient analysis and interpretation tools. Conventional images, derived from a specific analyte mass, do not identify the spatially localized correlations between analytes that are latent in the data. A new approach to find and visualize these correlations is presented. Clustering methods are used to classify pixels by spectral similarity, facilitating definition of distinct spatial regions. Principal component and discriminant analyses are combined to comprehensively identify changes in the mass spectra between regions. Images are generated by projecting the spectra of each pixel on the discriminant spectra; contrast is then a function of multiple correlated peaks.

Algorithms↗
PubMed2004

Performance measures for video object segmentation and tracking.

We propose measures to evaluate quantitatively the performance of video object segmentation and tracking methods without ground-truth (GT) segmentation maps. The proposed measures are based on spatial differences of color and motion along the boundary of the estimated video object plane and temporal differences between the color histogram of the current object plane and its predecessors. They can be used to localize (spatially and/or temporally) regions where segmentation results are good or bad; and/or they can be combined to yield a single numerical measure to indicate the goodness of the boundary segmentation and tracking results over a sequence. The validity of the proposed performance measures without GT have been demonstrated by canonical correlation analysis with another set of measures with GT on a set of sequences (where GT information is available). Experimental results are presented to evaluate the segmentation maps obtained from various sequences using different segmentation approaches.

Algorithms↗
PubMed2001

Treatment of frequency-dependent admittance boundary conditions in transient acoustic finite/infinite-element models.

The paper addresses the handling of frequency-dependent, local admittance boundary conditions in acoustic transient finite/infinite-element models. The proposed approach avoids the evaluation of a convolution integral along the related boundary. Based on a similar technique developed in an aeroacoustic/finite difference context, the spatially local boundary condition is rewritten in a discrete form that involves normal accelerations and pressure time derivatives at the current time step and few steps before. The incorporation of such a discrete (in time) boundary condition in a finite/infinite-element context is addressed. The infinite-element scheme selected for that purpose relies on the conjugated Astley-Leis formulation. Implementation aspects cover the handling of frequency-dependent boundary conditions along both finite- and infinite-element edges. Numerical examples (waveguide, single source in a half-space bounded by an impedance plane, diffraction by an acoustically treated screen) are presented in order to demonstrate the efficiency of the proposed approach.

Journal Article↗
PubMed2006

On the use of ZIP codes and ZIP code tabulation areas (ZCTAs) for the spatial analysis of epidemiological data.

BACKGROUND: While the use of spatially referenced data for the analysis of epidemiological data is growing, issues associated with selecting the appropriate geographic unit of analysis are also emerging. A particularly problematic unit is the ZIP code. Lacking standardization and highly dynamic in structure, the use of ZIP codes and ZIP code tabulation areas (ZCTA) for the spatial analysis of disease present a unique challenge to researchers. Problems associated with these units for detecting spatial patterns of disease are explored. RESULTS: A brief review of ZIP codes and their spatial representation is conducted. Though frequently represented as polygons to facilitate analysis, ZIP codes are actually defined at a narrower spatial resolution reflecting the street addresses they serve. This research shows that their generalization as continuous regions is an imposed structure that can have serious implications in the interpretation of research results. ZIP codes areas and Census defined ZCTAs, two commonly used polygonal representations of ZIP code address ranges, are examined in an effort to identify the spatial statistical sensitivities that emerge given differences in how these representations are defined. Here, comparative analysis focuses on the detection of patterns of prostate cancer in New York State. Of particular interest for studies utilizing local, spatial statistical tests, is that differences in the topological structures of ZIP code areas and ZCTAs give rise to different spatial patterns of disease. These differences are related to the different methodologies used in the generalization of ZIP code information. Given the difficulty associated with generating ZIP code boundaries, both ZIP code areas and ZCTAs contain numerous representational errors which can have a significant impact on spatial analysis. While the use of ZIP code polygons for spatial analysis is relatively straightforward, ZCTA representations contain additional topological features (e.g. lakes and rivers) and contain fragmented polygons that can hinder spatial analysis. CONCLUSION: Caution must be exercised when using spatially referenced data, particularly that which is attributed to ZIP codes and ZCTAs, for epidemiological analysis. Researchers should be cognizant of representational errors associated with both geographies and their resulting spatial mismatch, especially when comparing the results obtained using different topological representations. While ZCTAs can be problematic, topological corrections are easily implemented in a geographic information system to remedy erroneous aggregation effects.

Bias↗
PubMed1985

Relationship between spatial-frequency and orientation tuning of striate-cortex cells.

If striate cells had the receptive-field (RF) shapes classically attributed to them, their preferred spatial frequencies would vary considerably with orientation. Other models of RF shape would predict a greater independence between orientation and spatial-frequency tuning. We have examined this by recording the responses of cat striate-cortex cells to a wide range of different spatial-frequency and orientation combinations. In almost all cells studied, peak orientation did not consistently vary with spatial frequency, but the majority of cells showed some change in peak spatial-frequency tuning with orientation. The amount of change in peak spatial frequency tended to be greater for cells that were narrowly tuned for orientation. However, cells narrowly (and also very broadly) tuned for spatial frequency tended to show considerable independence of spatial-frequency and orientation tuning, and in all but a few cells the degree of change was less than predicted by the classic RF model. Such cells were found to fire only to patterns whose local spatial spectra fell within a compact, restricted, roughly circular two-dimensional spatial-frequency region. We conclude that the two-dimensional RF shape of striate cells more closely approximates that predicted by a two-dimensional Gabor model or by a Gaussian-derivative model than it does the classic shape based on the output of geniculate cells with aligned RF's.

Animals↗
PubMed2006

NMR-based localization of ions involved in salting out of hen egg white lysozyme.

NaCl-induced aggregation of hen egg white lysozyme (HEWL) was monitored by NMR spectroscopy. Small, but significant, changes induced by salt addition in TOCSY spectra were attributed to the effect of local reorganization of protein backbone upon ion binding. Salt-induced variations in HN and H alpha chemical shifts were mapped on the HEWL 3D structure which allowed the construction of a scheme of the spatial localization of potential ion binding sites. It was found that in a 0.5 M NaCl solution six chloride anions and at least one sodium cation are bound to preferred sites on the HEWL surface.

Animals↗
PubMed2006

Characterizing polygonality in biological structures.

Several systems involve spatial arrangements of elements such as molecules or cells, the characterization of which bears important implications to biological and physical investigations. Traditional approaches to quantify spatial order and regularity have relied on nearest neighbor distances or the number of sides of cells. The current work shows that enhanced performance can be achieved by considering angular regularity. Voronoi tessellations are obtained for each basic element and the angular regularity is then estimated from the differences between the angles defined by adjacent cells and a reference angle. In case this angle is 60 degrees, the measurement quantifies the hexagonality of the system. Other reference angles can be considered in order to quantify other types of spatial symmetries. The performance of the angular regularity is compared with other measurements including the conformity ratio (based on nearest neighbor distances) and the number of sides of the cells, indicating its improved sensitivity and discrimination power. The performance evaluation included synthetic (progressively perturbed hexagonal lattices) and real data (retinal mosaics). The good performance of the hexagonality measurements are illustrated also with respect to the problem of quantifying local spatial order in structures involving regions with different organizations as well as systems of points characterized by gradients of local order.

Journal Article↗
PubMed1999

A localized double-quantum filter for in vivo detection of taurine.

Noninvasive detection of taurine, an important amino acid involved in numerous physiological processes, by in vivo (1)H magnetic resonance (MR) spectroscopy is complicated by severe overlap of the taurine resonances with those of a number of other metabolites. Unambiguous differentiation of the taurine resonances requires spectral editing. In this study, the development of a localized spectral editing technique based on double-quantum filtering optimized for in vivo detection of taurine is described. The sequence recovers the taurine signal while substantially eliminating overlapping resonances and provides excellent three-dimensional spatial localization. The performance of the sequence is demonstrated both in phantoms and in rat brain in vivo. Magn Reson Med 42:454-460, 1999.

Animals↗
PubMedDate not supplied

In vivo 1H-[13C]-NMR spectroscopy of cerebral metabolism.

13C NMR spectroscopy in combination with the infusion of (13)C-labeled precursors is currently the only technique that is capable of quantitatively studying energy metabolism, neurotransmission and other metabolic pathways non-invasively in vivo. (1)H-[(13)C]-NMR spectroscopy is a high-sensitivity alternative to direct (13)C NMR spectroscopy. The development of improved NMR methods for water suppression, spatial localization, broadband decoupling, shimming and signal quantification, together with the availability of high magnetic field strengths, has made (1)H-[(13)C]-NMR spectroscopy the method of choice for the detection of metabolism at a high spatial and/or temporal resolution. (1)H-[(13)C]-NMR spectroscopy can now be used to discriminate glutamatergic (excitatory) and GABAergic (inhibitory) neuronal activity. The improved sensitivity allows the detection of metabolism in different tissues (e.g. gray and white matter) and potentially even in smaller structures, like cortical layers. Finally, (1)H-[(13)C]-NMR spectroscopy allows the detection of energy metabolism and neurotransmission during functional activation, thereby further strengthening our understanding of the neurochemical basis of brain function.

Algorithms↗