Relevance of the skewness index in DTI exploration of multiple sclerosis

Object  To this day, no parameter can really monitor the progression of multiple sclerosis (MS). In this study, an index the skewness (S) derived from parameters calculated in diffusion tensor imaging (DTI) has been tested on MS patients for its ability to monitor the disease course. Materials and methods  Eighteen patients underwent two examinations within 3 months consisting of a clinical evaluation (EDSS) and DTI acquisitions on a 1.5 T imager. Tensor was calculated thanks to“home-made” software. Mean diffusivity (MD) and fractional anisotropy (FA) histograms were described for normal-appearing white matter (NAWM) and gray matter (GM) of patients with S and also with usually indices peak position (pp) and peak height (ph) for the whole group of patients and for two separate groups according to their clinical status (EDSS  ≤  3 and EDSS  > 3 at month 0). Results  Although no significant clinical evolution is observed over 3 months, S in GM showed a significant shift for both MD/FA histograms towards abnormal values for the whole group of patients (p = 0.02/p = 0.04) and for the group with EDSS  ≤  3 (p = 0.04/p = 0.007), while ph and pp do not. Conclusion  S in GM could be an alternative marker to monitor the disease course before the repercussion on the clinical score.     

3D diffusion tensor imaging with 2D navigated turbo spin echo

A Cartesian two-dimensional navigator with variable orientation for online motion correction is introduced. It corrects for all possible zeroth- and first-order phase errors due to rigid-body motion of a subject during the diffusion-weighting preparation. The technique is developed for the application of three-dimensional (3D) imaging sequences, which offer the opportunity of high-resolution diffusion-weighted imaging, or diffusion tensor imaging (DTI) with isotropic voxel resolution. The navigator was applied to a displaced 3D turbo spin-echo sequence with an ECG-gated diffusion preparation to avoid phase errors due to gross brain pulsation. Online and offline corrected in vivo images acquired with this sequence are compared to investigate the advantages of online correction. Also eigenvector maps of the diffusion tensor are presented with an isotropic resolution of 1 mm³, which indicate that this new navigator technique is a promising approach for high-resolution DTI.     

The role of tissue microstructure and water exchange in biophysical modelling of diffusion in white matter

Biophysical models that describe the outcome of white matter diffusion MRI experiments have various degrees of complexity. While the simplest models assume equal-sized and parallel axons, more elaborate ones may include distributions of axon diameters and axonal orientation dispersions. These microstructural features can be inferred from diffusion-weighted signal attenuation curves by solving an inverse problem, validated in several Monte Carlo simulation studies. Model development has been paralleled by microscopy studies of the microstructure of excised and fixed nerves, confirming that axon diameter estimates from diffusion measurements agree with those from microscopy. However, results obtained in vivo are less conclusive. For example, the amount of slowly diffusing water is lower than expected, and the diffusion-encoded signal is apparently insensitive to diffusion time variations, contrary to what may be expected. Recent understandings of the resolution limit in diffusion MRI, the rate of water exchange, and the presence of microscopic axonal undulation and axonal orientation dispersions may, however, explain such apparent contradictions. Knowledge of the effects of biophysical mechanisms on water diffusion in tissue can be used to predict the outcome of diffusion tensor imaging (DTI) and of diffusion kurtosis imaging (DKI) studies. Alterations of DTI or DKI parameters found in studies of pathologies such as ischemic stroke can thus be compared with those predicted by modelling. Observations in agreement with the predictions strengthen the credibility of biophysical models; those in disagreement could provide clues of how to improve them. DKI is particularly suited for this purpose; it is performed using higher b-values than DTI, and thus carries more information about the tissue microstructure. The purpose of this review is to provide an update on the current understanding of how various properties of the tissue microstructure and the rate of water exchange between microenvironments are reflected in diffusion MRI measurements. We focus on the use of biophysical models for extracting tissue-specific parameters from data obtained with single PGSE sequences on clinical MRI scanners, but results obtained with animal MRI scanners are also considered. While modelling of white matter is the central theme, experiments on model systems that highlight important aspects of the biophysical models are also reviewed.     

Association of lumbar artery narrowing, degenerative changes in disc and endplate and apparent diffusion in disc on postcontrast enhancement of lumbar intervertebral disc

Objective  A decreased supply of nutrition to the intervertebral disc can lead to disc degeneration. Nutrient supply can be simulated in vivo by measuring gadolinium enhancement of the disc. We aimed to study the changes associated with disc degeneration that may have effect on the nutrition of the disc, i.e. lumbar artery narrowing, Modic changes, endplate defects, and apparent diffusion coefficient (ADC) in nucleus pulposus. Patients and methods  Twenty male volunteers underwent a lumbar spine examination at 1.5 T for anatomical imaging, diffusion weighted imaging, magnetic resonance angiography, and for T1 relaxation time quantification of contrast enhancement of intervertebral disc. Results  Enhancement of the disc increased with degeneration. Disc space narrowing associated strongly with the enhancement (Pearson’s correlation coefficient 0.46, P < 0.001). The enhancement rate in discs adjacent to Modic type 2 changes was 24%, adjacent to type 1/2 changes 58%, and 13% in the absence of Modic changes. Discs adjacent to endplate defects enhanced 32% compared to 10% of normal endplates. Lumbar artery narrowing or ADC in the disc were not associated with the enhancement. Conclusion  Increased enhancement of a degenerated disc is associated mostly with disc space narrowing and with the presence of degenerative endplate changes and endplate defects.     

3D MRI of explanted sheep hearts with submillimeter isotropic spatial resolution: comparison between diffusion tensor and structure tensor imaging

Magnetic Resonance Materials in Physics, Biology and Medicine - The aim of the study is to&nbsp;compare structure tensor imaging (STI) with diffusion tensor imaging (DTI) of the sheep heart...     

Water diffusion in rat brain in vivo as detected at very largeb values is multicompartmental

The diffusion-weighted signal attenuation of water in rat brain was measured with pulsed-field gradient nuclear magnetic resonance methods in a single voxel under in vivo and global ischemic conditions. The diffusion-attenuated water signal was observed in vivo atb values of 300 ms/μm² (strength of diffusion weighting) and diffusion times up to 400 ms. A series of constant diffusion time (CT) experiments with varied gradient directions and diffusion times revealed a multiexponential decay with apparent diffusion coefficients (ADC) covering two orders of magnitude from 1 to 0.01 μm²/ms. In a four-exponential fit, the observed changes during global ischemia could be fully explained by changes in the relative volume fractions only with unchanged ADCs. An anisotropy of the ADC, detected at smallb values, was not observed for the ADC at largeb values, but for the concomitant volume fractions. An inverse Laplace Transform of the CT curves, performed with CONTIN, resulted in continuously distributed diffusion coefficients, for which the term ‘diffusogram’ is proposed. This approach was more appropriate than a discrete exponential model with four to six components, being related to the morphology of brain tissue and its cell size distribution. On the basis of an analytical, quantitative model, it is suggested that the measured ADC at smallb values reflects mainly properties of the restricting boundaries, i.e. the relative volume fractions and the extracellular tortuosity, while the intrinsic intracellular diffusion constant and the exchange time are predicted to have minor influence.     

Diffusion-weighted imaging of the spine using radialk-space trajectories

Introduction Diffusion-weighted MR imaging (DWI) of the spine requires robust imaging methods, that are insensitive to susceptibility effects caused by the transition from bone to soft tissue and motion artifacts due to breathing, swallowing, and cardiac motion. The purpose of this study was to develop a robust imaging method suitable for DWI of the spine. Methods and subjects A radialk-space spin echo sequence has been implemented, which is sell-navigating because each acquisition line passes through the origin ofk-space. Influence of cardiac motion and associated flow of cerebrospinal fluid is minimized by cardiac gating with a finger photoplethysmograph. The sequence has been tested on a 1.5T system. Diffusion-weighted images of six normal volunteers were acquired in the sagittal plane with 4b values between 50 and 500 s mm⁻². Because of the symmetries of the cord, diffusion measurements in the head-foot (HF) or left-right (LR) directions were sufficient to measure the dominant effects of anisotropy. Results The apparent diffusion coefficients (ADCs) measured, respectively, in the LR and HF directions were (0.699 ± 0.050) × 10⁻³ and (1.805 ± 0.086) × 10⁻³ mm² s⁻¹ in the spinal cord. (1.588 ± 0.082) × 10⁻³ and (1.528 ± 0.052) × 10⁻³ mm² s⁻¹ in the intervertebral disks, and (0.346 ± 0.047) × 10⁻³ and (0.306 ± 0.035) × 10⁻³ mm² s⁻¹ in the vertebrae of the cervicothoraeic spine. Conclusion Diffusion-weighted spin echo sequences with radial trajectories ink-space provide a means of achieving robust, high quality diffusion-weighted imaging and measuring ADCs in the spine. The application of the diffusion-weighting gradients in different directions allows diffusion anisotropy to be measured.     

Assessment of diffusion tensor imaging indices in calf muscles following postural change from standing to supine position

Object

To investigate whether postural change from erect to recumbent position affects calf muscle water diffusivity.

Materials and methods

Ten healthy adults (27.2 ± 4.9 years, 3 females) were imaged at baseline (following assumption of recumbent position), and after 34 min (session 2) and 64 min (session 3) of laying supine within a 3T MRI scanner. Diffusion tensor imaging (DTI) eigenvalues, fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were evaluated in five calf muscles (anterior and posterior tibialis and triceps surae) during each of the three imaging sessions.

Results

Significant decreases were observed in all of the eigenvalues and ADC in each of the muscles with postural change. These reductions ranged from 3.2 to 6.7 % and 3.4 to 7.5 % for the various DTI metrics, following 34 and 64 min of supine rest, respectively (P < 0.05). No significant differences were noted in ADC or eigenvalues between the second and third imaging sessions for any muscle. FA did not change significantly with postural manipulation in any muscle compartment.

Conclusion

Diffusion tensor imaging indices were altered with postural change. As differences were not apparent between the latter two imaging sessions, we suggest that a short supine resting period (~34 min) is sufficient for muscle diffusivity to stabilize prior to quantitative MR imaging in healthy young adults.     

Image restoration using total variation and anisotropic diffusion equation

This paper proposes a new model for the image restoration which combines the total variation minimization with the “pure” anisotropic diffusion equation of Alvarez and Morel. According to the introduction of new diffusion term, this model can not only remove noise but also enhance edges and keep their locality. And it can also keep textures and large-scale fine features that are not characterized by edges. Due to these favorable characteristics, the processed images turn much clearer and smoother, meanwhile, their significant details are kept, which results in appealing vision. __________ Translated from Journal of Xidian Universiy, 2006, 33(5): 759–762 [译自: 西安电子科技大学学报 (自学科学版)]     

Compression algorithm for electrocardiograms based on sparse decomposition

Sparse decomposition is a new theory in signal processing, with the advantage in that the base (dictionary) used in this theory is over-complete, and can reflect the nature of a signal. Thus, the sparse decomposition of signal can obtain sparse representation, which is very important in data compression. The algorithm of compression based on sparse decomposition is investigated. By training on and learning electrocardiogram (ECG) data in the MITBIH Arrhythmia Database, we constructed an overcomplete dictionary of ECGs. Since the atoms in this dictionary are in accord with the character of ECGs, it is possible that an extensive ECG datum is reconstructed by a few nonzero coefficients and atoms. The proposed compression algorithm can adjust compression ratio according to practical request, and the distortion is low (when the compression ratio is 20:1, the standard error is 5.11%). The experiments prove the feasibility of the proposed compression algorithm. __________ Translated from Chinese Journal of Biomedical Engineering, 2008, 27 (1): 13–17 [译自: 中国生物医学工程学报]     

White and grey matter development in utero assessed using motion-corrected diffusion tensor imaging and its comparison to ex utero measures

Objective

Fetal brain diffusion tensor imaging (DTI) offers quantitative analysis of the developing brain. The objective was to 1) quantify DTI measures across gestation in a cohort of fetuses without brain abnormalities using full retrospective correction for fetal head motion 2) compare results obtained in utero to those in preterm infants.

Materials and methods

Motion-corrected DTI analysis was performed on data sets obtained at 1.5T from 32 fetuses scanned between 21.29 and 37.57 (median 31.86) weeks. Results were compared to 32 preterm infants scanned at 3T between 27.43 and 37.14 (median 33.07) weeks. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were quantified by region of interest measurements and tractography was performed.

Results

Fetal DTI was successful in 84% of fetuses for whom there was sufficient data for DTI estimation, and at least one tract could be obtained in 25 cases. Fetal FA values increased and ADC values decreased with age at scan (PLIC FA: p = 0.001; R² = 0.469; slope = 0.011; splenium FA: p < 0.001; R² = 0.597; slope = 0.019; thalamus ADC: p = 0.001; R² = 0.420; slope = − 0.023); similar trends were found in preterm infants.

Conclusion

This study demonstrates that stable DTI is feasible on fetuses and provides evidence for normative values of diffusion properties that are consistent with aged matched preterm infants.

     

In vivo comparison of atherosclerotic plaque progression with vessel wall strain and blood flow velocity in apoE−/− mice with MR microscopy at 17.6 T

Object  At present, in vivo plaque characterization in mice by MRI is typically limited to the visualization of vascular lesions with no accompanying analysis of vessel wall function. The aim of this study was to analyze the influence of atherosclerotic plaque development on the morphological and mechanical characteristics of the aortic vessel wall in a pre-clinical murine model of atherosclerosis. Materials and methods  Groups of apolipoprotein E-deficient (apoE^−/−) and C57BL/6J control mice fed a high-fat diet were monitored over a 12-week time period by high-field MRI. Multi-Slice-Multi-Spin-Echo and Phase-Contrast MRI sequences were employed to track changes to aortic vessel wall area, blood flow velocity and distensibility. Results  After 6- and 12-weeks, significant changes in vessel wall area and circumferential strain were detected in the apoE^−/− mice relative to the control animals. Blood flow velocity and intravascular lumen remained unchanged in both groups, findings that are in agreement with the theory of positive remodeling of the ascending aorta during plaque progression. Conclusion  This study has demonstrated the application of high-field MRI for characterizing the temporal progression of morphological and mechanical changes to murine aortic vasculature associated with atherosclerotic lesion development.     

Chemical solution deposition of copper thin films and integration into a multilayer capacitor structure

Metallization layers with thicknesses well below a micron are needed for future generation multilayer ceramic devices such as capacitors and integrated passive components. In many cases, the limiting thickness for the electrode is governed by dewetting of the metals from the oxide surface. Therefore, thin, stable metallization layers with low electrical resistivities that can survive high processing temperatures are of interest for these applications. For this purpose, Cu films prepared from 2-methoxyethanol-based solutions were developed using adhesion promoters such as Ti, Zn, and Zr. The solutions were spun onto BaTiO₃/SiO₂/Si or SiO₂/Si substrates, pyrolyzed, and annealed in a reducing ambient. The microstructure of films prepared in this way was found to be uniform and continuous at thicknesses as low as 80 nm. Cu films modified with 15 mol% Zr had electrical resistivities of about 16–17 μΩ-cm after 500°C annealing and 5–6 μΩ-cm after annealing at 900°C in a reducing ambient.     

Security analysis of image encryption based on twodimensional chaotic maps and improved algorithm

The article proposes a new algorithm to improve the security of image encryption based on twodimensional chaotic maps. Chaotic maps are often used in encrypting images. However, the encryption has periodicity, no diffusion, and at the same time, the real keys space of encryption are fewer than the theoretical keys space, which consequently results in potential security problems. Thus, this article puts forward several ways to solve the problems including adding diffusion mechanism, changing the design of keys and developing a composite encryption system. It designs an algorithm for the version B of the discretized baker map, which is one of the most prevalent chaotic maps, based on which a new image encryption is proposed to avoid the above problems. The simulation results show that the new encryption algorithm is valid and the result can be applied to other two-dimensional chaotic maps, such as the cat map. __________ Translated from Journal of Harbin Institute of Technology, 2007, 39(9): 1411–1414 [译自: 哈尔滨工业大学学报]     

The holographic reconstructing algorithm and its error analysis about phase-shifting phase measurement

Phase-shifting measurement and its error estimation method were studied according to the holographic principle. A function of synchronous superposition of object complex amplitude reconstructed from N-step phase-shifting through one integral period (N-step phase-shifting function for short) was proposed. In N-step phase-shifting measurement, the interferograms are seen as a series of in-line holograms and the reference beam is an ideal parallel-plane wave. So the N-step phase-shifting function can be obtained by multiplying the interferogram by the original reference wave. In ideal conditions, the proposed method is a kind of synchronous superposition algorithm in which the complex amplitude is separated, measured and superposed. When error exists in measurement, the result of the N-step phase-shifting function is the optimal expected value of the least-squares fitting method. In the above method, the N+1-step phase-shifting function can be obtained from the N-step phase-shifting function. It shows that the N-step phase-shifting function can be separated into two parts: the ideal N-step phase-shifting function and its errors. The phase-shifting errors in N-steps phase-shifting phase measurement can be treated the same as the relative errors of amplitude and intensity under the understanding of the N+1-step phase-shifting function. The difficulties of the error estimation in phase-shifting phase measurement were restricted by this error estimation method. Meanwhile, the maximum error estimation method of phase-shifting phase measurement and its formula were proposed. Translated from Acta Optica Sinica, 2006, 26(9): 1 367–1 371 [译自: 光学学报]     

Natural Minimal Set of State Equations of Linear Power Grids

For linear RLC power grids, a minimal set of state equations is derived that is suited to impressed voltage sources and contains only physically existing quantities (currents and node voltages) as state variables. In principle, other kinds of sources are also possible, e.g. impressed currents or mixtures of impressed voltages and currents. The described algorithm needs the eigenvalues of the system – in particular their number. Such methods of calculating the eigenvalues have already been developed; the stability of the results has been proved for grids with more than 500 nodes. Three-phase modeled grids are briefly treated too.     

Absorption of electromagnetic wave by inhomogeneous, unmagnetized plasma

In this article, a novel and normalized Z-transform finite-difference time-domain (ZTFDTD) method is presented. This method uses a more general form of Maxwell’s equations using the E, H, D fields. The iterative model of D-E-H-D can be obtained by using the Z-transform resulted frequency-dependent formula between D and E. The advantages of the ZTFDTD consist in that the discrete equations are simple, the results are precise, easy to program and capable of dealing with the key technologies of finite-difference time-domain (FDTD), such as absorbing boundary conditions (uniaxial anisotropic perfectly matched layer, UPML) and near-to-far-field transformation. The ZTFDTD method is then used to simulate the interaction of electromagnetic wave with plasma. Using a simplified two-dimensional model, the stealth effect of inhomogeneous, unmagnetized plasma is studied both in different electron densities of plasma, different electromagnetic wave frequencies and different plasma collision frequencies. The numerical results indicate that plasma stealth is effective in theory and a reasonable selection with the plasma parameters that can greatly enhance the effectiveness of plasma stealth. __________ Translated from Journal of Huazhong University of Science and Technology (Nature Science Edition), 2006, 35(6): 57–59 [译自: 华中科技大学学报(自然科学版)]     

Magnetic resonance imaging at 9.4 T: the Maastricht journey

The 9.4 T scanner in Maastricht is a whole-body magnet with head gradients and parallel RF transmit capability. At the time of the design, it was conceptualized to be one of the best fMRI scanners in the world, but it has also been used for anatomical and diffusion imaging. 9.4 T offers increases in sensitivity and contrast, but the technical ultra-high field (UHF) challenges, such as field inhomogeneities and constraints set by RF power deposition, are exacerbated compared to 7 T. This article reviews some of the 9.4 T work done in Maastricht. Functional imaging experiments included blood oxygenation level-dependent (BOLD) and blood-volume weighted (VASO) fMRI using different readouts. BOLD benefits from shorter T₂* at 9.4 T while VASO from longer T₁. We show examples of both ex vivo and in vivo anatomical imaging. For many applications, pTx and optimized coils are essential to harness the full potential of 9.4 T. Our experience shows that, while considerable effort was required compared to our 7 T scanner, we could obtain high-quality anatomical and functional data, which illustrates the potential of MR acquisitions at even higher field strengths. The practical challenges of working with a relatively unique system are also discussed.

     

MR monitoring of tumour thermal therapy

Thermal therapy of tumour including hyperthermia and thermal ablation by heat or cold delivery requires on line monitoring. Due to its temperature sensitivity, Magnetic Resonance Imaging (MRI) allows thermal mapping at the time of the treatment. The different techniques of MR temperature monitoring based on water proton resonance frequency (PRF), longitudinal relaxation time Tl, diffusion coefficient and MR Spectroscopic Imaging (MRSI) are reviewed and debated. The PRF method appears the most widely used and the most efficient at high magnetic field in spite of important drawbacks. The Tl method is the easiest method of visualisation of qualitative temperature distribution and quantitative measurement seems possible in the tissue surrounding the tumour up to a temperature of 45–65°C. Despite its high temperature sensitivity, application of the diffusion method in vivo is restricted due to its high motion sensitivity. The recent MRSI technique seems very promising provided acquisition times can be reduced. Results from the literature indicate that MR temperature monitoring in vivo can be achieved in vivo with a precision of about 3°C in 13 s for a voxel of 16 mm³ (1.5 × 1.5 × 7 mm) in 1.5 T scanners.     

PASADENA hyperpolarization of 13C biomolecules: equipment design and installation

Object  The PASADENA method has achieved hyperpolarization of 16–20% (exceeding 40,000-fold signal enhancement at 4.7 T), in liquid samples of biological molecules relevant to in vivo MRI and MRS. However, there exists no commercial apparatus to perform this experiment conveniently and reproducibly on the routine basis necessary for translation of PASADENA to questions of biomedical importance. The present paper describes equipment designed for rapid production of six to eight liquid samples per hour with high reproducibility of hyperpolarization. Materials and methods  Drawing on an earlier, but unpublished, prototype, we provide diagrams of a delivery circuit, a laminar-flow reaction chamber within a low field NMR contained in a compact, movable housing. Assembly instructions are provided from which a computer driven, semi-automated PASADENA polarizer can be constructed. Results  Together with an available parahydrogen generator, the polarizer, which can be operated by a single investigator, completes one cycle of hyperpolarization each 52 s. Evidence of efficacy is presented. In contrast to competing, commercially available devices for dynamic nuclear polarization which characteristically require 90 min per cycle, PASADENA provides a low-cost alternative for high throughput. Conclusions  This equipment is suited to investigators who have an established small animal NMR and wish to explore the potential of heteronuclear (¹³ C and ¹⁵ N) MRI, MRS, which harnesses the enormous sensitivity gain offered by hyperpolarization.