Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 2015
Parallel imaging can be used to reduce imaging time and to increase the spatial coverage in hyper... more Parallel imaging can be used to reduce imaging time and to increase the spatial coverage in hyperpolarized gas magnetic resonance imaging of the lung. In this proof-of-concept study, we investigate the effects of parallel imaging on the morphometric measurement of lung microstructure using diffusion magnetic resonance imaging with hyperpolarized (3) He. Fully sampled and under-sampled multi-b diffusion data were acquired from human subjects using an 8-channel (3) He receive coil. A parallel imaging reconstruction technique (generalized autocalibrating partially parallel acquisitions [GRAPPA]) was used to reconstruct under-sampled k-space data. The morphometric results of the generalized autocalibrating partially parallel acquisitions-reconstructed data were compared with the results of fully sampled data for three types of subjects: healthy volunteers, mild, and moderate chronic obstructive pulmonary disease patients. Morphometric measurements varied only slightly at mild accelerati...
DS, Conradi MS. Role of collateral paths in long-range diffusion in lungs. The long-range apparen... more DS, Conradi MS. Role of collateral paths in long-range diffusion in lungs. The long-range apparent diffusion coefficient (LRADC) of 3 He gas in lungs, measured over times of several seconds and distances of 1-3 cm, probes the connections between the airways. Previous work has shown the LRADC to be small in health and substantially elevated in emphysema, reflecting tissue destruction, which is known to create collateral pathways. To better understand what controls LRADC, we report computer simulations and measurements of 3 He gas diffusion in healthy lungs. The lung is generated with a random algorithm using well-defined rules, yielding a three-dimensional set of nodes or junctions, each connected by airways to one parent node and two daughters; airway dimensions are taken from published values. Spin magnetization in the simulated lung is modulated sinusoidally, and the diffusion equation is solved to 1,000 s. The modulated magnetization decays with a time constant corresponding to an LRADC of ϳ0.001 cm 2 /s, which is smaller by a factor of ϳ20 than the values in healthy lungs measured here and previously in vivo and in explanted lungs. It appears that collateral gas pathways, not present in the simulations, are functional in healthy lungs; they provide additional and more direct routes for long-range motion than the canonical airway tree. This is surprising, inasmuch as collateral ventilation is believed to be physiologically insignificant in healthy lungs. We discuss the effect on LRADC of small collateral connections through airway walls and rule out other possible mechanisms. The role of collateral paths is supported by measurements of smaller LRADC in pigs, where collateral ventilation is known to be smaller.
Perfluorinated gases, particularly C2F6, are potentially suitable alternatives to hyperpolarized ... more Perfluorinated gases, particularly C2F6, are potentially suitable alternatives to hyperpolarized noble gases for pulmonary airspace spin density and diffusion MRI. This work focuses mainly on 19F imaging of C2F6 gas in healthy and emphysematous explanted lungs, avoiding regulatory issues of human in vivo measurements. Three-dimensional gradient echo and spin echo spin density images of human lungs can be made in 10 s with adequate signal-to-noise, demonstrating the feasibility for breathing dynamics to be captured during a succession of short breath holds. As expected, the spin echo images have much smaller susceptibility artifacts than the gradient echo images. 19F and 3He images of the same lungs are compared. The apparent diffusion coefficient (ADC) of C2F6 is sensitive to restrictions imposed by the lung microstructure: the average ADC is measured to be 0.018 cm2/s in healthy lungs versus 0.031 cm2/s in emphysematous lungs at a diffusion time Delta=2.2 ms. The low free diffusivity of pure C2F6 (D0=0.033 cm2/s) places it in a regime where the ADC measurement allows the surface-to-volume ratio to be determined in each voxel, a potentially valuable quantitative characterization of regional lung tissue destruction in emphysema.
To present in vivo, human validation of a previously proposed method to measure key pulmonary par... more To present in vivo, human validation of a previously proposed method to measure key pulmonary parameters related to lung microstructure and physiology. Some parameters, such as blood-air barrier thickness, cannot be measured readily by any other noninvasive modality. Healthy volunteers (n = 12) were studied in 1.5T and 3T whole body human scanners using hyperpolarized xenon. Xenon uptake by lung parenchyma and blood was measured using a chemical shift saturation recovery sequence. Both dissolved-xenon peaks at 197 ppm and 217-218 ppm were fitted against a model of xenon exchange (MOXE) as functions of exchange time. Parameters related to lung function and structure can be obtained by fitting to this model. The following results were obtained from xenon uptake (averaged over all healthy volunteers): surface-area-to-volume ratio = 210 ± 50 cm(-1) ; total septal wall thickness = 9.2 ± 6.5 μm; blood-air barrier thickness = 1.0 ± 0.3 μm; hematocrit = 27 ± 4%; pulmonary capillary blood transit time = 1.3 ± 0.3 s, in good agreement with literature values from invasive experiments. More detailed fitting results are listed in the text. The initial in vivo human results demonstrate that our proposed methods can be used to noninvasively determine lung physiology by simultaneous quantification of a few important pulmonary parameters. This method is highly promising to become a versatile screening method for lung diseases.
To characterize the effect of diffusion time on short-range hyperpolarized (3)He magnetic resonan... more To characterize the effect of diffusion time on short-range hyperpolarized (3)He magnetic resonance imaging (MRI) diffusion measurements across a wide range of emphysema severity. (3)He diffusion MRI was performed on 19 lungs or lobes resected from 18 subjects with varying degrees of emphysema using three diffusion times (1.6 msec, 5 msec, and 10 msec) at constant b value. Emphysema severity was quantified as the mean apparent diffusion coefficient (ADC) and as the percentage of pixels with ADC higher than multiple thresholds from 0.30-0.55 cm(2)/sec (ADC index). Quantitative histology (mean linear intercept) was obtained in 10 of the lung specimens from 10 of the subjects. The mean ADCs with diffusion times of 1.6, 5.0, and 10.0 msec were 0.46, 0.40, and 0.37 cm(2)/sec, respectively (P < 0.0001, analysis of variance [ANOVA]). There was no relationship between the ADC magnitude and the effect of diffusion time on ADC values. The mean linear intercept correlated with ADC (r = 0.91-0.94, P < 0.001) and ADC index (r = 0.78-0.92, P < 0.01) at all diffusion times. Decreases in ADC with longer diffusion time were unrelated to emphysema severity. The strong correlations between the ADC at all diffusion times tested and quantitative histology demonstrate that ADC is a robust measure of emphysema.
To evaluate the use of inflation-fixed lung tissue for emphysema quantification with computed tom... more To evaluate the use of inflation-fixed lung tissue for emphysema quantification with computed tomography (CT) and He magnetic resonance (MR) diffusion imaging. Fourteen subjects representing a range of chronic obstructive pulmonary disease severity who underwent complete or lobar lung resection were studied. Computed tomographic measurements of lung attenuation and MR measurements of the hyperpolarized 3He apparent diffusion coefficient (ADC) in resected specimens fixed in inflation with heated formalin vapor were compared with measurements obtained before fixation. The mean (SD) CT emphysema indices were 56% (17%) before and 58% (19%) after fixation (P = 0.77; R = 0.76). Index differences correlated with differences in lung volume (R = 0.47). The mean (SD) 3He ADCs were 0.40 (0.15) cm/s before and 0.39 (0.14) cm/s after fixation (P = 0.03, R = 0.98). The CT emphysema index and the 3He ADC were correlated before (R = 0.89) and after fixation (R = 0.79). Concordance of CT and 3He MR imaging measurements in unfixed and inflation-fixed lungs supports the use of inflation-fixed lungs for quantitative imaging studies in emphysema.
Characterization of the dynamic mechanical behavior of brain tissue is essential for understandin... more Characterization of the dynamic mechanical behavior of brain tissue is essential for understanding and simulating the mechanisms of traumatic brain injury (TBI). Changes in mechanical properties may also reflect changes in the brain due to aging or disease. In this study, we used magnetic resonance elastography (MRE) to measure the viscoelastic properties of ferret brain tissue in vivo. Threedimensional (3D) displacement fields were acquired during wave propagation in the brain induced by harmonic excitation of the skull at 400 Hz, 600 Hz and 800 Hz. Shear waves with wavelengths in the order of millimeters were clearly visible in the displacement field, in strain fields, and in the curl of displacement field (which contains no contributions from longitudinal waves). Viscoelastic parameters (storage and loss moduli) governing dynamic shear deformation were estimated in gray and white matter for these excitation frequencies. To characterize the reproducibility of measurements, two ferrets were studied on three different dates each. Estimated viscoelastic properties of white matter in the ferret brain were generally similar to those of gray matter and consistent between animals and scan dates. In both tissue types G 0 increased from approximately 3 kPa at 400 Hz to 7 kPa at 800 Hz and G 00 increased from approximately 1 kPa at 400 Hz to 2 kPa at 800 Hz. These measurements of shear wave propagation in the ferret brain can be used to both parameterize and validate finite element models of brain biomechanics. assessment of the rheological behavior of human organs using multifrequency MR elastography: a study of brain and liver viscoelasticity. Physics in Medicine and Biology 52, 7281-7294. Kleiven, S., 2002. Finite element modeling of the human head. Ph.D. Dissertation. A new method to measure cortical growth in the developing brain.
Folding of the cerebral cortex is a critical phase of brain development in higher mammals, but th... more Folding of the cerebral cortex is a critical phase of brain development in higher mammals, but the biomechanics of folding remain incompletely understood. During folding, growth of the cortical surface is heterogeneous and anisotropic. We developed and applied a new technique to measure spatial and directional variations in surface growth from longitudinal magnetic resonance imaging (MRI) studies of a single animal or human subject. MRI provides high resolution 3-D image volumes of the brain at different stages of development. Surface representations of the cerebral cortex are obtained by segmentation of these volumes. Estimation of local surface growth between two times requires establishment of a point-to-point correspondence ("registration") between surfaces measured at those times. Here we present a novel approach for the registration of two surfaces in which an energy function is minimized by solving a partial differential equation on a spherical surface. The energy function includes a strain energy term due to distortion and an "error energy" term due to mismatch between surface features. This algorithm, implemented with the finite element method, brings surface features into approximate alignment while minimizing deformation in regions without explicit matching criteria. The method was validated by application to three simulated test cases and applied to characterize growth of the ferret cortex during folding. Cortical surfaces were created from MRI data acquired in vivo at 14, 21 and 28 days of life. Deformation gradient and Lagrangian strain tensors describe the kinematics of growth over this interval. These quantitative results illuminate the spatial, temporal, and directional patterns of growth during cortical folding.
measurements (1, 4) and simulations (3). We would like to emphasize that the length scale L is eq... more measurements (1, 4) and simulations (3). We would like to emphasize that the length scale L is equally important. For the measurements, whether via our method (1) or that of Wang et al. (4), this length scale is the wavelength of magnetization tagging. The simulations (3) of intra-acinar diffusion necessarily involve lengths smaller than the acini themselves (thus L Ͻ 6 mm).
IEEE Transactions on Circuits and Systems for Video Technology, 2000
AbstractA de-interlacing algorithm using adaptive 4-field global/local motion compensated approa... more AbstractA de-interlacing algorithm using adaptive 4-field global/local motion compensated approach is presented. It con-sists of block-based directional edge interpolation, same-parity 4-field motion detection, global/local motion estimation and compensation. The edges are ...
Nuclear spins experience forces in the presence of a magnetic field gradient. The forces cause th... more Nuclear spins experience forces in the presence of a magnetic field gradient. The forces cause the spin-up and spin-down nuclei to move in opposite directions, resulting in a flow of longitudinal magnetization. The effect can generate local longitudinal spin magnetization though it does not involve transitions (flipping) of spins. This phenomenon, spin sorting, competes with real spin-lattice relaxation and is generally not observable when T 1 is short. We present our calculations of the longitudinal magnetization of diffusing spins with long T 1 in magnetic field gradients. We show that the longitudinal spin magnetization due to spin sorting can be dominant at short times in such a system. Experimental confirmation is provided by observation of sorting-induced negative spin magnetization in 3 He gas. We show how this phenomenon can potentially be used to generate nuclear magnetizations larger than thermal equilibrium.
Spatial and temporal variations in cortical growth were studied in the neonatal ferret to illumin... more Spatial and temporal variations in cortical growth were studied in the neonatal ferret to illuminate the mechanisms of folding of the cerebral cortex. Cortical surface representations were created from magnetic resonance images acquired between postnatal day 4 and 35. Global measures of shape (e.g., surface area, normalized curvature, and sulcal depth) were calculated. In 2 ferrets, relative cortical growth was calculated between surfaces created from in vivo images acquired at P14, P21, and P28. The isocortical surface area transitions from a slower (12.7 mm(2)/day per hemisphere) to a higher rate of growth (36.7 mm(2)/day per hemisphere) approximately 13 days after birth, which coincides with the time of transition from neuronal proliferation to cellular morphological differentiation. Relative cortical growth increases as a function of relative geodesic distance from the origin of the transverse neurogenetic gradient and is related to the change in fractional diffusion anisotropy over the same time period. The methods presented here can be applied to study cortical growth during development in other animal models or human infants. Our results provide a quantitative spatial and temporal description of folding in cerebral cortex of the developing ferret brain, which will be important to understand the underlying mechanisms that drive folding.
Rationale and Objectives-MR imaging of the restricted diffusion of laser-polarized 3 He gas provi... more Rationale and Objectives-MR imaging of the restricted diffusion of laser-polarized 3 He gas provides unique insights into the changes in lung microstructure in emphysema.
ABSTRACT Mathematical modeling and computer simulations are widely used for understanding traumat... more ABSTRACT Mathematical modeling and computer simulations are widely used for understanding traumatic brain injury (TBI). However, accurate tissue parameters are needed, especially for the brain in vivo. In this study, we used the ferret as the animal model because it is the smallest mammal with a folded brain and significant white matter tracts. Magnetic resonance elastography (MRE) has proven useful for in vivo measurement of biological tissue properties. Mechanical properties of the ferret brain over a range of frequencies from 400–800 Hz were studied using MRE. Experiment results show both that storage and loss modulus increases with frequency and that dissipative effects in the white matter (characterized by the loss modulus G″) were significant larger than in gray matter.
Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 2015
Parallel imaging can be used to reduce imaging time and to increase the spatial coverage in hyper... more Parallel imaging can be used to reduce imaging time and to increase the spatial coverage in hyperpolarized gas magnetic resonance imaging of the lung. In this proof-of-concept study, we investigate the effects of parallel imaging on the morphometric measurement of lung microstructure using diffusion magnetic resonance imaging with hyperpolarized (3) He. Fully sampled and under-sampled multi-b diffusion data were acquired from human subjects using an 8-channel (3) He receive coil. A parallel imaging reconstruction technique (generalized autocalibrating partially parallel acquisitions [GRAPPA]) was used to reconstruct under-sampled k-space data. The morphometric results of the generalized autocalibrating partially parallel acquisitions-reconstructed data were compared with the results of fully sampled data for three types of subjects: healthy volunteers, mild, and moderate chronic obstructive pulmonary disease patients. Morphometric measurements varied only slightly at mild accelerati...
DS, Conradi MS. Role of collateral paths in long-range diffusion in lungs. The long-range apparen... more DS, Conradi MS. Role of collateral paths in long-range diffusion in lungs. The long-range apparent diffusion coefficient (LRADC) of 3 He gas in lungs, measured over times of several seconds and distances of 1-3 cm, probes the connections between the airways. Previous work has shown the LRADC to be small in health and substantially elevated in emphysema, reflecting tissue destruction, which is known to create collateral pathways. To better understand what controls LRADC, we report computer simulations and measurements of 3 He gas diffusion in healthy lungs. The lung is generated with a random algorithm using well-defined rules, yielding a three-dimensional set of nodes or junctions, each connected by airways to one parent node and two daughters; airway dimensions are taken from published values. Spin magnetization in the simulated lung is modulated sinusoidally, and the diffusion equation is solved to 1,000 s. The modulated magnetization decays with a time constant corresponding to an LRADC of ϳ0.001 cm 2 /s, which is smaller by a factor of ϳ20 than the values in healthy lungs measured here and previously in vivo and in explanted lungs. It appears that collateral gas pathways, not present in the simulations, are functional in healthy lungs; they provide additional and more direct routes for long-range motion than the canonical airway tree. This is surprising, inasmuch as collateral ventilation is believed to be physiologically insignificant in healthy lungs. We discuss the effect on LRADC of small collateral connections through airway walls and rule out other possible mechanisms. The role of collateral paths is supported by measurements of smaller LRADC in pigs, where collateral ventilation is known to be smaller.
Perfluorinated gases, particularly C2F6, are potentially suitable alternatives to hyperpolarized ... more Perfluorinated gases, particularly C2F6, are potentially suitable alternatives to hyperpolarized noble gases for pulmonary airspace spin density and diffusion MRI. This work focuses mainly on 19F imaging of C2F6 gas in healthy and emphysematous explanted lungs, avoiding regulatory issues of human in vivo measurements. Three-dimensional gradient echo and spin echo spin density images of human lungs can be made in 10 s with adequate signal-to-noise, demonstrating the feasibility for breathing dynamics to be captured during a succession of short breath holds. As expected, the spin echo images have much smaller susceptibility artifacts than the gradient echo images. 19F and 3He images of the same lungs are compared. The apparent diffusion coefficient (ADC) of C2F6 is sensitive to restrictions imposed by the lung microstructure: the average ADC is measured to be 0.018 cm2/s in healthy lungs versus 0.031 cm2/s in emphysematous lungs at a diffusion time Delta=2.2 ms. The low free diffusivity of pure C2F6 (D0=0.033 cm2/s) places it in a regime where the ADC measurement allows the surface-to-volume ratio to be determined in each voxel, a potentially valuable quantitative characterization of regional lung tissue destruction in emphysema.
To present in vivo, human validation of a previously proposed method to measure key pulmonary par... more To present in vivo, human validation of a previously proposed method to measure key pulmonary parameters related to lung microstructure and physiology. Some parameters, such as blood-air barrier thickness, cannot be measured readily by any other noninvasive modality. Healthy volunteers (n = 12) were studied in 1.5T and 3T whole body human scanners using hyperpolarized xenon. Xenon uptake by lung parenchyma and blood was measured using a chemical shift saturation recovery sequence. Both dissolved-xenon peaks at 197 ppm and 217-218 ppm were fitted against a model of xenon exchange (MOXE) as functions of exchange time. Parameters related to lung function and structure can be obtained by fitting to this model. The following results were obtained from xenon uptake (averaged over all healthy volunteers): surface-area-to-volume ratio = 210 ± 50 cm(-1) ; total septal wall thickness = 9.2 ± 6.5 μm; blood-air barrier thickness = 1.0 ± 0.3 μm; hematocrit = 27 ± 4%; pulmonary capillary blood transit time = 1.3 ± 0.3 s, in good agreement with literature values from invasive experiments. More detailed fitting results are listed in the text. The initial in vivo human results demonstrate that our proposed methods can be used to noninvasively determine lung physiology by simultaneous quantification of a few important pulmonary parameters. This method is highly promising to become a versatile screening method for lung diseases.
To characterize the effect of diffusion time on short-range hyperpolarized (3)He magnetic resonan... more To characterize the effect of diffusion time on short-range hyperpolarized (3)He magnetic resonance imaging (MRI) diffusion measurements across a wide range of emphysema severity. (3)He diffusion MRI was performed on 19 lungs or lobes resected from 18 subjects with varying degrees of emphysema using three diffusion times (1.6 msec, 5 msec, and 10 msec) at constant b value. Emphysema severity was quantified as the mean apparent diffusion coefficient (ADC) and as the percentage of pixels with ADC higher than multiple thresholds from 0.30-0.55 cm(2)/sec (ADC index). Quantitative histology (mean linear intercept) was obtained in 10 of the lung specimens from 10 of the subjects. The mean ADCs with diffusion times of 1.6, 5.0, and 10.0 msec were 0.46, 0.40, and 0.37 cm(2)/sec, respectively (P < 0.0001, analysis of variance [ANOVA]). There was no relationship between the ADC magnitude and the effect of diffusion time on ADC values. The mean linear intercept correlated with ADC (r = 0.91-0.94, P < 0.001) and ADC index (r = 0.78-0.92, P < 0.01) at all diffusion times. Decreases in ADC with longer diffusion time were unrelated to emphysema severity. The strong correlations between the ADC at all diffusion times tested and quantitative histology demonstrate that ADC is a robust measure of emphysema.
To evaluate the use of inflation-fixed lung tissue for emphysema quantification with computed tom... more To evaluate the use of inflation-fixed lung tissue for emphysema quantification with computed tomography (CT) and He magnetic resonance (MR) diffusion imaging. Fourteen subjects representing a range of chronic obstructive pulmonary disease severity who underwent complete or lobar lung resection were studied. Computed tomographic measurements of lung attenuation and MR measurements of the hyperpolarized 3He apparent diffusion coefficient (ADC) in resected specimens fixed in inflation with heated formalin vapor were compared with measurements obtained before fixation. The mean (SD) CT emphysema indices were 56% (17%) before and 58% (19%) after fixation (P = 0.77; R = 0.76). Index differences correlated with differences in lung volume (R = 0.47). The mean (SD) 3He ADCs were 0.40 (0.15) cm/s before and 0.39 (0.14) cm/s after fixation (P = 0.03, R = 0.98). The CT emphysema index and the 3He ADC were correlated before (R = 0.89) and after fixation (R = 0.79). Concordance of CT and 3He MR imaging measurements in unfixed and inflation-fixed lungs supports the use of inflation-fixed lungs for quantitative imaging studies in emphysema.
Characterization of the dynamic mechanical behavior of brain tissue is essential for understandin... more Characterization of the dynamic mechanical behavior of brain tissue is essential for understanding and simulating the mechanisms of traumatic brain injury (TBI). Changes in mechanical properties may also reflect changes in the brain due to aging or disease. In this study, we used magnetic resonance elastography (MRE) to measure the viscoelastic properties of ferret brain tissue in vivo. Threedimensional (3D) displacement fields were acquired during wave propagation in the brain induced by harmonic excitation of the skull at 400 Hz, 600 Hz and 800 Hz. Shear waves with wavelengths in the order of millimeters were clearly visible in the displacement field, in strain fields, and in the curl of displacement field (which contains no contributions from longitudinal waves). Viscoelastic parameters (storage and loss moduli) governing dynamic shear deformation were estimated in gray and white matter for these excitation frequencies. To characterize the reproducibility of measurements, two ferrets were studied on three different dates each. Estimated viscoelastic properties of white matter in the ferret brain were generally similar to those of gray matter and consistent between animals and scan dates. In both tissue types G 0 increased from approximately 3 kPa at 400 Hz to 7 kPa at 800 Hz and G 00 increased from approximately 1 kPa at 400 Hz to 2 kPa at 800 Hz. These measurements of shear wave propagation in the ferret brain can be used to both parameterize and validate finite element models of brain biomechanics. assessment of the rheological behavior of human organs using multifrequency MR elastography: a study of brain and liver viscoelasticity. Physics in Medicine and Biology 52, 7281-7294. Kleiven, S., 2002. Finite element modeling of the human head. Ph.D. Dissertation. A new method to measure cortical growth in the developing brain.
Folding of the cerebral cortex is a critical phase of brain development in higher mammals, but th... more Folding of the cerebral cortex is a critical phase of brain development in higher mammals, but the biomechanics of folding remain incompletely understood. During folding, growth of the cortical surface is heterogeneous and anisotropic. We developed and applied a new technique to measure spatial and directional variations in surface growth from longitudinal magnetic resonance imaging (MRI) studies of a single animal or human subject. MRI provides high resolution 3-D image volumes of the brain at different stages of development. Surface representations of the cerebral cortex are obtained by segmentation of these volumes. Estimation of local surface growth between two times requires establishment of a point-to-point correspondence ("registration") between surfaces measured at those times. Here we present a novel approach for the registration of two surfaces in which an energy function is minimized by solving a partial differential equation on a spherical surface. The energy function includes a strain energy term due to distortion and an "error energy" term due to mismatch between surface features. This algorithm, implemented with the finite element method, brings surface features into approximate alignment while minimizing deformation in regions without explicit matching criteria. The method was validated by application to three simulated test cases and applied to characterize growth of the ferret cortex during folding. Cortical surfaces were created from MRI data acquired in vivo at 14, 21 and 28 days of life. Deformation gradient and Lagrangian strain tensors describe the kinematics of growth over this interval. These quantitative results illuminate the spatial, temporal, and directional patterns of growth during cortical folding.
measurements (1, 4) and simulations (3). We would like to emphasize that the length scale L is eq... more measurements (1, 4) and simulations (3). We would like to emphasize that the length scale L is equally important. For the measurements, whether via our method (1) or that of Wang et al. (4), this length scale is the wavelength of magnetization tagging. The simulations (3) of intra-acinar diffusion necessarily involve lengths smaller than the acini themselves (thus L Ͻ 6 mm).
IEEE Transactions on Circuits and Systems for Video Technology, 2000
AbstractA de-interlacing algorithm using adaptive 4-field global/local motion compensated approa... more AbstractA de-interlacing algorithm using adaptive 4-field global/local motion compensated approach is presented. It con-sists of block-based directional edge interpolation, same-parity 4-field motion detection, global/local motion estimation and compensation. The edges are ...
Nuclear spins experience forces in the presence of a magnetic field gradient. The forces cause th... more Nuclear spins experience forces in the presence of a magnetic field gradient. The forces cause the spin-up and spin-down nuclei to move in opposite directions, resulting in a flow of longitudinal magnetization. The effect can generate local longitudinal spin magnetization though it does not involve transitions (flipping) of spins. This phenomenon, spin sorting, competes with real spin-lattice relaxation and is generally not observable when T 1 is short. We present our calculations of the longitudinal magnetization of diffusing spins with long T 1 in magnetic field gradients. We show that the longitudinal spin magnetization due to spin sorting can be dominant at short times in such a system. Experimental confirmation is provided by observation of sorting-induced negative spin magnetization in 3 He gas. We show how this phenomenon can potentially be used to generate nuclear magnetizations larger than thermal equilibrium.
Spatial and temporal variations in cortical growth were studied in the neonatal ferret to illumin... more Spatial and temporal variations in cortical growth were studied in the neonatal ferret to illuminate the mechanisms of folding of the cerebral cortex. Cortical surface representations were created from magnetic resonance images acquired between postnatal day 4 and 35. Global measures of shape (e.g., surface area, normalized curvature, and sulcal depth) were calculated. In 2 ferrets, relative cortical growth was calculated between surfaces created from in vivo images acquired at P14, P21, and P28. The isocortical surface area transitions from a slower (12.7 mm(2)/day per hemisphere) to a higher rate of growth (36.7 mm(2)/day per hemisphere) approximately 13 days after birth, which coincides with the time of transition from neuronal proliferation to cellular morphological differentiation. Relative cortical growth increases as a function of relative geodesic distance from the origin of the transverse neurogenetic gradient and is related to the change in fractional diffusion anisotropy over the same time period. The methods presented here can be applied to study cortical growth during development in other animal models or human infants. Our results provide a quantitative spatial and temporal description of folding in cerebral cortex of the developing ferret brain, which will be important to understand the underlying mechanisms that drive folding.
Rationale and Objectives-MR imaging of the restricted diffusion of laser-polarized 3 He gas provi... more Rationale and Objectives-MR imaging of the restricted diffusion of laser-polarized 3 He gas provides unique insights into the changes in lung microstructure in emphysema.
ABSTRACT Mathematical modeling and computer simulations are widely used for understanding traumat... more ABSTRACT Mathematical modeling and computer simulations are widely used for understanding traumatic brain injury (TBI). However, accurate tissue parameters are needed, especially for the brain in vivo. In this study, we used the ferret as the animal model because it is the smallest mammal with a folded brain and significant white matter tracts. Magnetic resonance elastography (MRE) has proven useful for in vivo measurement of biological tissue properties. Mechanical properties of the ferret brain over a range of frequencies from 400–800 Hz were studied using MRE. Experiment results show both that storage and loss modulus increases with frequency and that dissipative effects in the white matter (characterized by the loss modulus G″) were significant larger than in gray matter.
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Papers by Yulin Chang