The diffusion tensor (DT) is a three-dimensional (3D) model of diffusivity in biological tissues.... more The diffusion tensor (DT) is a three-dimensional (3D) model of diffusivity in biological tissues. In white matter (WM), the major eigenvector, which is the direction of greatest diffusivity, is generally assumed to align with the direction of the fiber bundles. The distribution of major eigenvectors in WM has been investigated using color-based maps and WM tractography (WMT). However, anatomical patterns in the medium and minor eigenvector directions have largely been ignored in DTI studies of the human brain. In this study, the patterns of medium and minor eigenvectors in the brain were investigated using both color-based maps and WMT. Specific WM structures, such as the corona radiata, internal and external capsules, sagittal stratum, cingulum, and superior longitudinal fasciculus, demonstrated coherent patterns in the medium and minor eigenvector directions. These patterns were consistent across subjects. The orthogonal or axial diffusion asymmetry may be explained by merging, diverging, or crossing fiber geometries. The effects of orthogonal diffusion asymmetry on WMT were also investigated. This study shows that WM axial asymmetry causes anisotropic dispersion patterns in the estimated tract trajectories. The medium and minor eigenvector patterns may be useful for elucidating the local dispersion distributions of WM tracts.
IEEE International Symposium on Biomedical Imaging, 2006
White matter tractography is a noninvasive method for estimating the white matter connectivity pa... more White matter tractography is a noninvasive method for estimating the white matter connectivity pathways using diffusion tensor imaging. Experimental noise may induce errors in the measured fiber directions and affect the accuracy and precision of the estimated trajectories. Both model-based (parametric) and model-free (non-parametric) probabilistic tractography methods have been proposed to account for the uncertainty in the fiber direction estimation.
Abnormality in the "fear circuitry" has been known as a major neural characteristic of ... more Abnormality in the "fear circuitry" has been known as a major neural characteristic of posttraumatic stress disorder (PTSD). Recent studies also revealed decreased functional connectivity in the default mode network in PTSD. The present study aims to investigate, in war-zone-related PTSD, the spontaneous activity and functional connectivity of the amygdala and the precuneus, which are two representative brain regions of the two networks, respectively. Two groups of 52 male US Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF) veterans (PTSD vs. controls), well matched on age and ethnicity, were clinically assessed and then studied in a resting state functional magnetic resonance imaging (fMRI) procedure. Functional connectivity analysis was conducted on the resting state fMRI data with the amygdala and precuneus as seeds. Compared with controls, veterans with PTSD had lower functional connectivity in the default mode network, as well as lower amygdala-front...
We review the normal anatomy of the white matter (WM) tracts as they appear on directional diffus... more We review the normal anatomy of the white matter (WM) tracts as they appear on directional diffusion tensor imaging (DTI) color maps, which will almost certainly be available to the general radiologist as part of a commercial DTI software package in the near future. Anatomic drawings and gross dissection pho- tographs are correlated with the directional DTI color maps to
Neurotherapeutics : the journal of the American Society for Experimental NeuroTherapeutics, 2007
Diffusion tensor imaging (DTI) is a promising method for characterizing microstructural changes o... more Diffusion tensor imaging (DTI) is a promising method for characterizing microstructural changes or differences with neuropathology and treatment. The diffusion tensor may be used to characterize the magnitude, the degree of anisotropy, and the orientation of directional diffusion. This review addresses the biological mechanisms, acquisition, and analysis of DTI measurements. The relationships between DTI measures and white matter pathologic features (e.g., ischemia, myelination, axonal damage, inflammation, and edema) are summarized. Applications of DTI to tissue characterization in neurotherapeutic applications are reviewed. The interpretations of common DTI measures (mean diffusivity, MD; fractional anisotropy, FA; radial diffusivity, D(r); and axial diffusivity, D(a)) are discussed. In particular, FA is highly sensitive to microstructural changes, but not very specific to the type of changes (e.g., radial or axial). To maximize the specificity and better characterize the tissue m...
Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2009
We present a novel parametric encoding scheme for efficiently recording white matter fiber bundle... more We present a novel parametric encoding scheme for efficiently recording white matter fiber bundle information obtained from diffusion tensor imaging. The coordinates of fiber tracts are parameterized using a cosine series expansion. For an arbitrary tract, a 19 degree expansion is found to be sufficient to reconstruct the tract with an average error of about 0.26 mm. Then each tract is fully parameterized with 60 parameters, which results in a substantial data reduction. Unlike traditional splines, the proposed method does not have internal knots and explicitly represents the tract as a linear combination of basis functions. This simplicity in the representation enables us to design statistical models, register tracts and perform subsequent analysis in a more streamlined mathematical framework. As an illustration, we apply the proposed method in characterizing abnormal tracts that pass through the splenium of the corpus callosum in autistic subjects.
We present a novel unified framework for explicitly param- eterizing white fiber tracts. The coor... more We present a novel unified framework for explicitly param- eterizing white fiber tracts. The coordinates of tracts are parameterized using a Fourier series expansion. For an arbitrary tract, a 19 degree co- sine expansion is found to be sufficient to reconstruct the tract with an error of about 0.26 mm. By adding specific periodic constraints to open tracts, we can avoid using the sine basis. Then each tract is fully parameterized with 60 parameters, which results in a substantial data reduction. Unlike available spline models, the proposed method does not have internal knots and explicitly represents the tract as a linear com- bination of basis functions. This simplicity in the representation enables us to design statistical models, register tracts and segment tracts in a unified Hilbert space formulation.
The diffusion tensor (DT) is a three-dimensional (3D) model of diffusivity in biological tissues.... more The diffusion tensor (DT) is a three-dimensional (3D) model of diffusivity in biological tissues. In white matter (WM), the major eigenvector, which is the direction of greatest diffusivity, is generally assumed to align with the direction of the fiber bundles. The distribution of major eigenvectors in WM has been investigated using color-based maps and WM tractography (WMT). However, anatomical patterns in the medium and minor eigenvector directions have largely been ignored in DTI studies of the human brain. In this study, the patterns of medium and minor eigenvectors in the brain were investigated using both color-based maps and WMT. Specific WM structures, such as the corona radiata, internal and external capsules, sagittal stratum, cingulum, and superior longitudinal fasciculus, demonstrated coherent patterns in the medium and minor eigenvector directions. These patterns were consistent across subjects. The orthogonal or axial diffusion asymmetry may be explained by merging, diverging, or crossing fiber geometries. The effects of orthogonal diffusion asymmetry on WMT were also investigated. This study shows that WM axial asymmetry causes anisotropic dispersion patterns in the estimated tract trajectories. The medium and minor eigenvector patterns may be useful for elucidating the local dispersion distributions of WM tracts.
IEEE International Symposium on Biomedical Imaging, 2006
White matter tractography is a noninvasive method for estimating the white matter connectivity pa... more White matter tractography is a noninvasive method for estimating the white matter connectivity pathways using diffusion tensor imaging. Experimental noise may induce errors in the measured fiber directions and affect the accuracy and precision of the estimated trajectories. Both model-based (parametric) and model-free (non-parametric) probabilistic tractography methods have been proposed to account for the uncertainty in the fiber direction estimation.
Abnormality in the "fear circuitry" has been known as a major neural characteristic of ... more Abnormality in the "fear circuitry" has been known as a major neural characteristic of posttraumatic stress disorder (PTSD). Recent studies also revealed decreased functional connectivity in the default mode network in PTSD. The present study aims to investigate, in war-zone-related PTSD, the spontaneous activity and functional connectivity of the amygdala and the precuneus, which are two representative brain regions of the two networks, respectively. Two groups of 52 male US Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF) veterans (PTSD vs. controls), well matched on age and ethnicity, were clinically assessed and then studied in a resting state functional magnetic resonance imaging (fMRI) procedure. Functional connectivity analysis was conducted on the resting state fMRI data with the amygdala and precuneus as seeds. Compared with controls, veterans with PTSD had lower functional connectivity in the default mode network, as well as lower amygdala-front...
We review the normal anatomy of the white matter (WM) tracts as they appear on directional diffus... more We review the normal anatomy of the white matter (WM) tracts as they appear on directional diffusion tensor imaging (DTI) color maps, which will almost certainly be available to the general radiologist as part of a commercial DTI software package in the near future. Anatomic drawings and gross dissection pho- tographs are correlated with the directional DTI color maps to
Neurotherapeutics : the journal of the American Society for Experimental NeuroTherapeutics, 2007
Diffusion tensor imaging (DTI) is a promising method for characterizing microstructural changes o... more Diffusion tensor imaging (DTI) is a promising method for characterizing microstructural changes or differences with neuropathology and treatment. The diffusion tensor may be used to characterize the magnitude, the degree of anisotropy, and the orientation of directional diffusion. This review addresses the biological mechanisms, acquisition, and analysis of DTI measurements. The relationships between DTI measures and white matter pathologic features (e.g., ischemia, myelination, axonal damage, inflammation, and edema) are summarized. Applications of DTI to tissue characterization in neurotherapeutic applications are reviewed. The interpretations of common DTI measures (mean diffusivity, MD; fractional anisotropy, FA; radial diffusivity, D(r); and axial diffusivity, D(a)) are discussed. In particular, FA is highly sensitive to microstructural changes, but not very specific to the type of changes (e.g., radial or axial). To maximize the specificity and better characterize the tissue m...
Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2009
We present a novel parametric encoding scheme for efficiently recording white matter fiber bundle... more We present a novel parametric encoding scheme for efficiently recording white matter fiber bundle information obtained from diffusion tensor imaging. The coordinates of fiber tracts are parameterized using a cosine series expansion. For an arbitrary tract, a 19 degree expansion is found to be sufficient to reconstruct the tract with an average error of about 0.26 mm. Then each tract is fully parameterized with 60 parameters, which results in a substantial data reduction. Unlike traditional splines, the proposed method does not have internal knots and explicitly represents the tract as a linear combination of basis functions. This simplicity in the representation enables us to design statistical models, register tracts and perform subsequent analysis in a more streamlined mathematical framework. As an illustration, we apply the proposed method in characterizing abnormal tracts that pass through the splenium of the corpus callosum in autistic subjects.
We present a novel unified framework for explicitly param- eterizing white fiber tracts. The coor... more We present a novel unified framework for explicitly param- eterizing white fiber tracts. The coordinates of tracts are parameterized using a Fourier series expansion. For an arbitrary tract, a 19 degree co- sine expansion is found to be sufficient to reconstruct the tract with an error of about 0.26 mm. By adding specific periodic constraints to open tracts, we can avoid using the sine basis. Then each tract is fully parameterized with 60 parameters, which results in a substantial data reduction. Unlike available spline models, the proposed method does not have internal knots and explicitly represents the tract as a linear com- bination of basis functions. This simplicity in the representation enables us to design statistical models, register tracts and segment tracts in a unified Hilbert space formulation.
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Papers by Mariana Lazar