Biomedical subjects

Hanbing Lu

Publications and source records attributed to Hanbing Lu.

5 recordsLinked to original sources

PubMed2005

Temporal evolution of the CBV-fMRI signal to rat whisker stimulation of variable duration and intensity: a linearity analysis.

Cerebral blood volume functional magnetic resonance imaging (CBV-fMRI) experiments employing iron oxide contrast agent were conducted in rat whisker barrel cortex at 3 Tesla. Stimuli of constant frequency (12 Hz) but variable duration and intensity were delivered separately using a homemade whisker stimulator. The temporal behavior of CBV-fMRI signals was investigated using a linearity analysis method. Both superposition and scaling tests show that CBV-fMRI responses are generally consistent with a linear time-invariant (LTI) model. In particular, the response to a stimulus of higher intensity can be linearly scaled from that of lower intensity. However, significant deviations exist when the stimulus duration is manipulated, particularly in the trailing edge of the CBV-fMRI response curves.

Animals↗

PubMed2004

Momentum-weighted conjugate gradient descent algorithm for gradient coil optimization.

MRI gradient coil design is a type of nonlinear constrained optimization. A practical problem in transverse gradient coil design using the conjugate gradient descent (CGD) method is that wire elements move at different rates along orthogonal directions (r, phi, z), and tend to cross, breaking the constraints. A momentum-weighted conjugate gradient descent (MW-CGD) method is presented to overcome this problem. This method takes advantage of the efficiency of the CGD method combined with momentum weighting, which is also an intrinsic property of the Levenberg-Marquardt algorithm, to adjust step sizes along the three orthogonal directions. A water-cooled, 12.8 cm inner diameter, three axis torque-balanced gradient coil for rat imaging was developed based on this method, with an efficiency of 2.13, 2.08, and 4.12 mT.m(-1).A(-1) along X, Y, and Z, respectively. Experimental data demonstrate that this method can improve efficiency by 40% and field uniformity by 27%. This method has also been applied to the design of a gradient coil for the human brain, employing remote current return paths. The benefits of this design include improved gradient field uniformity and efficiency, with a shorter length than gradient coil designs using coaxial return paths.

Algorithms↗

PubMed2004

Spatial correlations of laminar BOLD and CBV responses to rat whisker stimulation with neuronal activity localized by Fos expression.

The spatial relationship between a measured fMRI signal and its underlying neuronal activity remains unclear. One obstacle is the localization of neuronal activity; another is the spatial resolution of fMRI. In the present study, high-resolution BOLD and CBV fMRI experiments (voxel size: 156 x 156 x 2000 microm3) were conducted in the rat whisker barrel cortex at 3 T; neuronal activity across cortical layers was mapped using the Fos expression technique. Results show that BOLD response is weighted by blood volume and that pixels with high BOLD response can be located at the cortical surface or in deep layers, depending on local vasculature. In contrast to BOLD response, the pixels with high CBV response were consistently clustered in the deep cortical layers. Percentage-CBV change in cortical layers IV-V was 7.3 +/- 1.5%, which was significantly higher than in layers I-III (4.1 +/- 0.9%) and VI (4.3 +/- 0.7%) (mean +/- SEM). The laminar distribution of CBV response correlates well with neuronal activity localized by Fos expression. We conclude that neuronal activity can be inferred from CBV fMRI data with high spatial accuracy. The data indicate that both intracolumn functional connectivity and neurovascular coupling can be studied using CBV fMRI.

Animals↗

PubMed2003

Characterization of continuously distributed cortical water diffusion rates with a stretched-exponential model.

Experience with diffusion-weighted imaging (DWI) shows that signal attenuation is consistent with a multicompartmental theory of water diffusion in the brain. The source of this so-called nonexponential behavior is a topic of debate, because the cerebral cortex contains considerable microscopic heterogeneity and is therefore difficult to model. To account for this heterogeneity and understand its implications for current models of diffusion, a stretched-exponential function was developed to describe diffusion-related signal decay as a continuous distribution of sources decaying at different rates, with no assumptions made about the number of participating sources. DWI experiments were performed using a spin-echo diffusion-weighted pulse sequence with b-values of 500-6500 s/mm(2) in six rats. Signal attenuation curves were fit to a stretched-exponential function, and 20% of the voxels were better fit to the stretched-exponential model than to a biexponential model, even though the latter model had one more adjustable parameter. Based on the calculated intravoxel heterogeneity measure, the cerebral cortex contains considerable heterogeneity in diffusion. The use of a distributed diffusion coefficient (DDC) is suggested to measure mean intravoxel diffusion rates in the presence of such heterogeneity.

Animals↗

PubMed2003

Multishot partial-k-space EPI for high-resolution fMRI demonstrated in a rat whisker barrel stimulation model at 3T.

A multishot partial-k-space EPI technique is presented and validated by fMRI at high spatial resolution. High-resolution phase maps corrected by phase-encoded reference scans have less off-resonance effects. Phantom studies demonstrate that this method can substantially improve partial-k-space EPI image formation. BOLD fMRI at submillimeter spatial resolution (156 x 156 x 2000 microm(3), 0.049 microl) was achieved in a rat whisker barrel stimulation model using this technique. The study included eight rats, five of which were administered an intravascular contrast agent (monocrystalline iron oxide nanocolloid (MION)) after the BOLD experiments. In two rats the highest BOLD responses were in the deep layers (IV-VI), and in six rats the highest responses were on the surface and in the deep cortical layers. Most of the pixels that exhibited high BOLD responses had high blood volume weightings. The benefits of this technique are expected to increase for high-resolution fMRI at higher magnetic fields, where T(2) (*) is shorter.

Animals↗