Biomedical subjects

Itamar Ronen

Publications and source records attributed to Itamar Ronen.

9 recordsLinked to original sources

PubMed2006

Spatial resolution dependence of DTI tractography in human occipito-callosal region.

Diffusion tensor imaging (DTI) and fiber tracking have been used to measure the fiber structural connectivity in humans in a non-invasive manner. However, low sensitivity is a principal limitation of these methods, causing a large number of possibly missing fiber tracts (FTs). Here we studied how the spatial resolution affects the sensitivity of the fiber tracing by rescaling data to different resolutions. Our data suggest that the spatial resolution can change the degree of the asymmetric cross-callosal connections in the lower visual field (loVF) compared to the upper visual field (upVF). Among connections from loVF, a larger voxel size resulted in a smaller number of FTs that was not commensurate to the number of seed points, while the number of connections from upVF was not significantly affected by variation in seeding point numbers. We conclude from our study that the spatial resolution of the acquired data will have to be taken into consideration in interpreting DTI fiber tracking data. Our results further suggest that the acquisition resolution of around 2 mm iso-voxel in the conventional DTI scheme can reconstruct the asymmetric upper and lower white matter structure in occipito-callosal region.

Adult↗

PubMed2006

Investigation of multicomponent diffusion in cat brain using a combined MTC-DWI approach.

In this study, multiple-component water diffusion in the cat brain is investigated using an approach that combines diffusion-weighted images using multiple b values with magnetization transfer contrast (MTC). The MTC allows filter of signal originating from water molecules that rapidly exchange with binding sites on large macromolecular structures, and in brain white matter, it is assumed that a significant portion of the MTC is due to the interaction of water with the extraaxonal myelin sheath. Henceforth, multicomponent analysis of diffusion curves with and without MTC may shed light on the contribution of the extraaxonal water to the diffusion signal and on the relationship between diffusion components and tissue compartments in the brain. When a biexponential model was applied to the data, the volume fractions of the two diffusion components changed significantly in white matter with the application of the MTC. These changes are then discussed in the frame of tissue components and the possible interaction with the myelin sheath.

Animals↗

PubMed2006

Anatomical correlates of the functional organization in the human occipitotemporal cortex.

The connectivity between functionally distinct areas in the human brain is unknown because of the limitations posed by current postmortem anatomical labeling techniques. Diffusion tensor imaging (DTI) has previously been used to define large white matter tracts based on well-known anatomical landmarks in the living human brain. In the present study, we used DTI coupled with functional magnetic resonance imaging (fMRI) to assess neuronal connections between human striate and functionally defined extrastriate ventral cortical areas. Functional areas were identified with conventional fMRI mapping procedures and then used as seeding points in a DTI analysis to ascertain connectivity patterns between cortical areas, thus yielding the pattern of connections between human occipitoventral visual areas in vivo.

Adult↗

PubMed2005

Analysis of the distribution of diffusion coefficients in cat brain at 9.4 T using the inverse Laplace transformation.

In this work, the usefulness of the inverse Laplace transformation (ILT) in the characterization of diffusion processes in the brain has been investigated. The method has been implemented on both phantom and in vivo cat brain data acquired at high resolution at 9.4 T. The results were compared with monoexponential and biexponential analyses of the same data. It is shown that in the case of diffusion restricted by white matter axonal tracts, the resulting diffusograms are in good agreement with the biexponential model. In gray matter, however, the non-monoexponential decay does not lead to a bimodal distribution in the ILT, even though the data can be fitted to a biexponential. This finding suggests the possibility of a distribution of diffusion coefficients rather than a discrete biexponential behavior. It is shown that this distribution is sensitive, for example, to experimental parameters such as the diffusion time. Thus, the ILT offers the possibility of implementing a unique tool for the analysis of heterogeneous diffusion, that is, the analysis of the diffusion coefficient distribution, which has the yet unexplored potential of being a valuable parameter in the characterization of tissue structure.

Animals↗

PubMed2005

How does DWI correlate with white matter structures?

Diffusion-weighted MRI (DWI) is widely used to characterize brain white matter (WM), particularly through the use of diffusion tensor imaging (DTI). In this study the spatial characteristics of DWI in WM of cat visual cortex were investigated at 9.4T at very high resolution. It is shown that the spatial extent of the WM tract as measured from the DWI images depends highly on the b-value. In particular, when the diffusion gradient is applied perpendicular to the main direction of the fiber tract, the estimated thickness of the tract at the commonly used b-value of 1000 s/mm2 exceeds by 50% the thickness as it appears on a T2-weighted image. Only at b-values greater than 6000 s/mm2 does the thickness of the tract approach the thickness characterized by the T2-weighted image and that observed on histological slices of the same area. Further analysis of these results indicates that the choice of b-value of 1000 s/mm2 may not be optimal for the demarcation of anisotropic WM structures. DWI at high b-value may contain spatial information that is more specific to WM tracts.

Animals↗

PubMed2004

Spatial relationship between neuronal activity and BOLD functional MRI.

Despite the ubiquitous use of functional magnetic resonance imaging (fMRI), the extent to which the magnitude and spatial scale of the fMRI signal correlates with neuronal activity is poorly understood. In this study, we directly compared single and multiunit neuronal activity with blood oxygenation level-dependent (BOLD) fMRI responses across a large area of the cat area 18. Our data suggest that at the scale of several millimeters, the BOLD contrast correlates linearly with the underlying neuronal activity. At the level of individual electrode recording sites, however, the correlation between the two signals varied substantially. We conclude from our study that T(2)*-based positive BOLD signals are a robust predictor for neuronal activity only at supra-millimeter spatial scales.

Animals↗

PubMed2003

Retinotopic mapping in cat visual cortex using high-field functional magnetic resonance imaging.

In the field of neuroscience, there has always been a need for imaging techniques that provide high-resolution, large field-of-view measurements of neural activity. Functional MRI has this capability, but the link between the blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal and neural activity is indirect. High magnetic field strengths (>3 T) improve the strength and specificity of the BOLD signal, but there are additional concerns about imaging artifacts at high field. We have tested the capabilities of ultra high field fMRI in the anesthetized juvenile cat, demonstrating rapid, non-invasive retinotopic mapping of early visual areas. Maps of topographic organization and measured cortical magnification factors are in good agreement with electrophysiological studies. Measurement precision was estimated at 1 mm. This mapping, performed with an MRI scanner at ultra high field (9.4 T), demonstrates the capabilities of high-resolution functional mapping of the visual system at ultra high field.

Animals↗

PubMed2003

Conventional DTI vs. slow and fast diffusion tensors in cat visual cortex.

Diffusion tensor imaging (DTI) uses water diffusion anisotropy in axonal fibers to provide a tool for analyzing and tracking those fibers in brain white matter. In the present work, multidirectional diffusion MRI data were collected from a cat brain and decomposed into slow and fast diffusion tensors and directly compared with conventional DTI data from the same imaging slice. The fractional anisotropy of the slow diffusing component (D(slow)) was significantly higher than the anisotropy measured by conventional DTI while reflecting a similar directionality and appeared to account for most of the anisotropy observed in gray matter, where the fiber density is notoriously low. Preliminary results of fiber tracking based on the slow diffusion component are shown. Fibers generated based on the slow diffusion component appear to follow the vertical fibers in gray matter. D(slow)TI may provide a way for increasing the sensitivity to anisotropic structures in cortical gray matter.

Animals↗

PubMed2003

In vivo mapping of functional domains and axonal connectivity in cat visual cortex using magnetic resonance imaging.

Noninvasive cognitive neuroimaging studies based on functional magnetic resonance imaging (fMRI) are of ever-increasing importance for basic and clinical neurosciences. The explanatory power of fMRI could be greatly expanded, however, if the pattern of the neuronal circuitry underlying functional activation could be made visible in an equally noninvasive manner. In this study, blood oxygenation level-dependent (BOLD)-based fMRI and diffusion tensor imaging (DTI) were performed in the same cat visual cortex, and the foci of fMRI activation utilized as seeding points for 3D DTI fiber reconstruction algorithms, thus providing the map of the axonal circuitry underlying visual information processing. The methods developed in this study will lay the foundation for in vivo neuroanatomy and the ability for noninvasive longitudinal studies of brain development.

Algorithms↗