Yuji Shen

Axial and Radial Diffusivity in Preterm Infants Who Have Diffuse White Matter Changes on Magnetic Resonance Imaging at Term-Equivalent Age

Objective. Diffuse excessive high signal intensity (DEHSI) is observed in the majority of preterm infants at term-equivalent age on conventional MRI, and diffusion-weighted imaging has shown that apparent diffusion coefficient values are elevated in the white matter (WM) in DEHSI. Our aim was to obtain diffusion tensor imaging on preterm infants at term-equivalent age and term control infants to test the hypothesis that radial diffusivity was significantly different in the WM in preterm infants with DEHSI compared with both preterm infants with normal-appearing WM on conventional MRI and term control infants. Methods. Diffusion tensor imaging was obtained on 38 preterm infants at term-equivalent age and 8 term control infants. Values for axial (λ1) and radial [(λ2 + λ3)/2] diffusivity were calculated in regions of interest positioned in the central WM at the level of the centrum semiovale, frontal WM, posterior periventricular WM, occipital WM, anterior and posterior portions of the posterior limb of the internal capsule, and the genu and splenium of the corpus callosum. Results. Radial diffusivity was elevated significantly in the posterior portion of the posterior limb of the internal capsule and the splenium of the corpus callosum, and both axial and radial diffusivity were elevated significantly in the WM at the level of the centrum semiovale, the frontal WM, the periventricular WM, and the occipital WM in preterm infants with DEHSI compared with preterm infants with normal-appearing WM and term control infants. There was no significant difference between term control infants and preterm infants with normal-appearing WM in any region studied. Conclusions. These findings suggest that DEHSI represents an oligodendrocyte and/or axonal abnormality that is widespread throughout the cerebral WM.

Correction of High-Order Eddy Current Induced Geometric Distortion in Diffusion-Weighted Echo-Planar Images

Diffusion‐weighted images acquired with the echo‐planar imaging technique are highly sensitive to eddy current induced geometric distortions that vary with the magnitude and direction of the diffusion sensitizing gradients. Such distortions cause misalignment of images acquired with different diffusion strengths and orientations. This in turn can result in errors when calculating maps of the apparent diffusion coefficient and diffusion tensor. Previous correction methods either require separate calibration data or only deal with low‐order errors. In this study, we demonstrate a method that can correct for higher‐order errors. The method relies on collecting pairs of images with diffusion sensitizing gradients reversed. This paired data are first corrected for shifts and linear distortion and then combined to cancel higher‐order errors. All acquired data contribute to the final results. The method has been tested by simulation, on phantoms, on adult volunteers, and on neonatal brain examinations. Magn Reson Med 52:1184–1189, 2004.

Enhanced Blocking Expanding Ring Search in Mobile Ad Hoc Networks

We introduce BERS*, an enhanced Blocking Expanding Ring Search (BERS) protocol for route discovery in mobile ad hoc networks (MANETs). BERS is an energy efficient alternative that was developed recently based on the Expanding Ring Search (ERS). ERS is widely applied in reactive routing protocols such as DSR and AODV. BERS* is a faster and more energy-time efficient version of BERS. It reduces the route discovery latency of BERS by nearly half while maintainning a similar level of energy saving. Our results show that, among the three protocol schemes (BERS*, BERS and ERS), BERS* incurs the least search latency when the hop number of the route nodes is greater than 3, and has achieved the best performance in terms of energy-time efficiency when the hop number of the route nodes is greater than 7. We have also discovered the conditions that allow collective optimisation of BERS* and ERS.

Magnetic resonance imaging of the mean venous vessel size in the human brain using transient hyperoxia.

Vessel size imaging is an emerging magnetic resonance imaging (MRI) technique which has been demonstrated to provide clinically relevant information about microvascular morphology. While previous studies of vessel size in humans relied on MRI contrast agents or hypercapnia-induced changes in blood oxygenation, the technique described here uses transient hyperoxia to alter the venous blood oxygenation. The experimental paradigm consisted of two 3-minute intervals of breathing 100% O(2) interleaved with three 2-minute intervals of breathing room air. Parametric maps of the mean venous vessel radius were calculated from changes in the blood oxygenation level dependent (BOLD) contrast which were measured using a combined spin-echo (SE) and gradient echo (GE) echo-planar imaging (EPI) sequence. The corresponding mean values in grey and white matter were r=6.5±0.3 μm and r=6.2±0.3 μm (n=6). While the hypercapnia technique requires a specialised gas mixture containing a low concentration of CO(2) (typically 5-6%), the hyperoxia technique presented here uses the inhalation of medical oxygen (100% O(2)) which is routinely available in a clinical environment. Furthermore, 100% O(2) is generally better tolerated than low doses of CO(2) which makes this technique particularly suitable for applications in critically ill patients.

Quantification of venous vessel size in human brain in response to hypercapnia and hyperoxia using magnetic resonance imaging

Hypercapnia and hyperoxia give rise to vasodilation and vasoconstriction, respectively. This study investigates the influence of hypercapnia and hyperoxia on venous vessel size in the human brain. Venous vessel radii were measured in response to hypercapnia and hyperoxia. The venous vessel radii were determined by calculation of the changes in R2* and R2 that are induced by breathing 6% CO2 or pure oxygen. The experimental paradigm consisted of two 3‐min intervals of inhaling 6% CO2 or 100% O2 interleaved with three 2‐min intervals of breathing air. Hypercapnic and hyperoxic experiments were performed on eight subjects on a 3T scanner. Parametric maps of mean venous vessel radius were calculated from the changes in R2* and R2, which were measured by simultaneous acquisition of gradient‐echo and spin‐echo signals. The mean venous vessel radii in hypercapnia were 7.3 ± 0.3 μm in gray matter and 6.6 ± 0.5 μm in white matter. The corresponding vessel radii in hyperoxia were 5.6 ± 0.2 μm in gray matter and 5.4 ± 0.2 μm in white matter. These results show that the venous vessel radius was larger in hypercapnia than that in hyperoxia in both gray matter and white matter (P < 0.005), which agrees with the hypothesis that hypercapnia causes vasodilation and hyperoxia induces vasoconstriction. Magn Reson Med, 2013.

A functional magnetic resonance imaging technique based on nulling extravascular gray matter signal

A new functional magnetic resonance imaging (fMRI) technique is proposed based on nulling the extravascular gray matter (GM) signal, using a spatially nonselective inversion pulse. The remaining MR signal provides cerebral blood volume (CBV) information from brain activation. A theoretical framework is provided to characterize the sources of GM-nulled (GMN) fMRI signal, effects of partial voluming of cerebrospinal fluid (CSF) and white matter, and behaviors of GMN fMRI signal during brain activation. Visual stimulation paradigm was used to explore the GMN fMRI signal behavior in the human brain at 3T. It is shown that the GMN fMRI signal increases by 7.2% ± 1.5%, which is two to three times more than that obtained with vascular space occupancy (VASO)-dependent fMRI (−3.2% ± 0.2%) or blood oxygenation level-dependent (BOLD) fMRI (2.9% ± 0.7%), using a TR of 3,000 ms and a resolution of 2 × 2 × 5 mm3. Under these conditions the fMRI signal-to-noise ratio (SNRfMRI) for BOLD, GMN, and VASO images was 4.97 ± 0.76, 4.56 ± 0.86, and 2.43 ± 1.06, respectively. Our study shows that both signal intensity and activation volume in GMN fMRI depend on spatial resolution because of partial voluming from CSF. It is shown that GMN fMRI is a convenient tool to assess CBV changes associated with brain activation.

Improving time-efficiency in blocking expanding ring search for mobile ad hoc networks

We propose a new strategy for reducing the amount of latency and energy consumption in Blocking Expanding Ring Search (BERS) and enhanced Blocking Expanding Ring Search (BERS*) for mobile ad hoc networks (MANETs). BERS and BERS* are respectively energy and energy-time efficient route discovery protocols for MANETs as compared to conventional Expanding Ring Search (ERS). In this study, we identify unnecessary waiting time caused by a STOP/END instruction in BERS/BERS* and explore the potential of further improvement of their time efficiency. This leads to tBERS and tBERS*, the improved BERS and BERS* respectively. In tBERS/tBERS*, a route node may also issue the STOP/END instruction to terminate flooding. We implement this idea in algorithms, conduct analysis, and achieve further latency reduction in both tBERS and tBERS* as well as the energy saving in tBERS*.

Analytical Studies of Energy–Time Efficiency of Blocking Expanding Ring Search

This paper investigates the energy–time efficiency of the Blocking Expanding Ring Search algorithms (BERS) using an extended graph model. BERS is an energy efficient alternative that was developed recently based on the Expanding Ring Search (ERS). ERS is widely applied in reactive routing protocols for mobile ad hoc networks. Most studies in this area focus on minimising either energy consumption or search latency, but few look into the strong correlation between the energy saving and the increased latency incurred. We analyse, in this study, the concurrency mechanism of BERS and have developed BERS*, an enhanced scheme based on BERS. Our results show that, among the three schemes (BERS*, BERS and ERS), BERS* incurs the least latency when the hop number of the route nodes is greater than 3, and has achieved the best performance in terms of energy–time efficiency when the hop number of the route nodes is greater than 7. We have also discovered the conditions that allow collective optimisation of BERS* and ERS.

Measuring Energy-Time Efficiency of Protocol Performance in Mobile Ad Hoc Networks

This paper introduces two new metrics for assessment of mobile ad hoc network performance in terms of energy-time efficiency. The combined effect of both energy and time consumption is considered and represented in mathematical terms. The measures have demonstrated a number of advantages over the conventional ones in which the energy and time were often considered separately. The proposed new metrics are simple, generic and flexible. As an application, we have compared the energy-time efficiency of Blocking Expanding Ring Search (BERS) and Expanding Ring Search (ERS), two similar Time to Live (TTL)-based expanding ring search algorithms using our new metrics. The results show that the new metrics can be applied efficiently in assessment of different protocols.

Energy-time efficiency of two routing strategies with chase packets in expanding ring search

We investigate the energy-time efficiency of two reactive route discovery protocols (BERS and DTC) with chase packets in the expanding ring search (ERS). BERS introduces deliberately waiting time in broadcasts of RREQs. DTC uses a threshold to switch between two different channel speeds to allow the chase packets to catch up with fulfilled RREQs. Taking an analytical approach, we gain the insight into these protocols and explore the trade-off nature of the energy and time efficiency. An optimal threshold for DTC is identified, and the performance of DTC under this optimal condition is used to compare with the performance of BERS in the light of ERS. Our results have practical values and implications as well as theoretical interests. We found that BERS has achieved the best overall performance in terms of energy-time efficiency, while DTC achieved the least overall performance despite the reduction of latency.