A comparison of fitting algorithms for diffusion-weighted MRI data analysis using an intravoxel incoherent motion model

Object

The objective of this study is to propose a modified VARiable PROjection (VARPRO) algorithm specifically tailored for fitting the intravoxel incoherent motion (IVIM) model to diffusion-weighted magnetic resonance imaging (DW-MRI) data from locally advanced rectal cancer (LARC).

Materials and methods

The proposed algorithm is compared with classical non-linear least squares (NLLS) analysis using the Levenberg-Marquardt (LM) algorithm and with two recently proposed algorithms for ‘segmented’ analysis. These latter two comprise two consecutive steps: first, a subset of parameters is estimated using a portion of data; second, the remaining parameters are estimated using the whole data and the previous estimates. The comparison between the algorithms was based on the \(R^2\) goodness-of-fit measure: performance analysis was carried out on real data obtained by DW-MRI on 40 LARC patients.

Results

The performance of the proposed algorithm was higher than that of LM in 64 % of cases; ‘segmented’ methods were poorer than our algorithm in 100 % of cases.

Conclusion

The proposed modified VARPRO algorithm can lead to better fit of the IVIM model to LARC DW-MRI data compared to other techniques.

     

Diffusional kurtosis imaging (DKI) incorporation into an intravoxel incoherent motion (IVIM) MR model to measure cerebral hypoperfusion induced by hyperventilation challenge in healthy subjects

Magnetic Resonance Materials in Physics, Biology and Medicine - The objectives were to investigate the diffusional kurtosis imaging (DKI) incorporation into the intravoxel incoherent motion (IVIM)...     

Rapid measurement of intravoxel incoherent motion (IVIM) derived perfusion fraction for clinical magnetic resonance imaging

Objective

This study aimed to investigate the reliability of intravoxel incoherent motion (IVIM) model derived parameters D and f and their dependence on b value distributions with a rapid three b value acquisition protocol.

Materials and methods

Diffusion models for brain, kidney, and liver were assessed for bias, error, and reproducibility for the estimated IVIM parameters using b values 0 and 1000, and a b value between 200 and 900, at signal-to-noise ratios (SNR) 40, 55, and 80. Relative errors were used to estimate optimal b value distributions for each tissue scenario. Sixteen volunteers underwent brain DW-MRI, for which bias and coefficient of variation were determined in the grey matter.

Results

Bias had a large influence in the estimation of D and f for the low-perfused brain model, particularly at lower b values, with the same trends being confirmed by in vivo imaging. Significant differences were demonstrated in vivo for estimation of D (P = 0.029) and f (P < 0.001) with [300,1000] and [500,1000] distributions. The effect of bias was considerably lower for the high-perfused models. The optimal b value distributions were estimated to be brain_500,1000, kidney_300,1000, and liver_200,1000.

Conclusion

IVIM parameters can be estimated using a rapid DW-MRI protocol, where the optimal b value distribution depends on tissue characteristics and compromise between bias and variability.

     

Deviations from the diffusion tensor model as revealed by contour plot visualization using high angular resolution diffusion-weighted imaging (HARDI)

The standard diffusion tensor model is limited in its ability to provide detailed information about multidirectional fiber architecture in human white matter. Additional directional acquisition of diffusivity properties with high angular resolution diffusion-weighted imaging (HARDI) acquisition schemes may deliver more information on areas with fiber crossings than standard DTI. However, representation of the additional information and the rating and visualization of fiber complexity is challenging. We used projection 2D-plots in combination with a HARDI acquisition scheme of 129 diffusion directions and compared the spherical diffusion variance index (SDI) with the relative anisotropy index (RAI). In normal controls, white matter areas with unidirectional fiber arrangement and areas with more complex fiber composition were identified with this approach. HARDI confirms and can visualize deviations from the tensor-like representation, thereby providing information on fiber structure complexity, which may be of considerable interest for clinical studies.     

Comparison of methods for estimation of the intravoxel incoherent motion (IVIM) diffusion coefficient (<Emphasis Type="Italic">D</Emphasis>) and perfusion fraction (<Emphasis Type="Italic">f</Emphasis>)

Objective

Intravoxel incoherent motion (IVIM) shows great potential in many applications, e.g., tumor tissue characterization. To reduce image-quality demands, various IVIM analysis approaches restricted to the diffusion coefficient (D) and the perfusion fraction (f) are increasingly being employed. In this work, the impact of estimation approach for D and f is studied.

Materials and methods

Four approaches for estimating D and f were studied: segmented IVIM fitting, least-squares fitting of a simplified IVIM model (sIVIM), and Bayesian fitting of the sIVIM model using marginal posterior modes or posterior means. The estimation approaches were evaluated in terms of bias and variability as well as ability for differentiation between tumor and healthy liver tissue using simulated and in vivo data.

Results

All estimation approaches had similar variability and ability for differentiation and negligible bias, except for the Bayesian posterior mean of f, which was substantially biased. Combined use of D and f improved tumor-to-liver tissue differentiation compared with using D or f separately.

Discussion

The similar performance between estimation approaches renders the segmented one preferable due to lower numerical complexity and shorter computational time. Superior tissue differentiation when combining D and f suggests complementary biologically relevant information.

     

Preliminary characterization of an experimental breast cancer cells brain metastasis mouse model by MRI/MRS

PURPOSE: Chemotherapy increases survival in breast cancer patients. Consequently, cerebral metastases have recently become a significant clinical problem, with an incidence of 30-40% among breast carcinoma patients. As this phenomenon cannot be studied longitudinally in humans, models which mimic brain metastasis are needed to investigate its pathogenesis. Such models may later be used in experimental therapeutic approaches. MATERIAL AND METHODS/RESULTS: We report a model in which 69% of the animals (9/13 BALB/c nude mice) developed MR-detectable abnormal masses in the brain parenchyma within a 20 to 62-day time window post intra-carotid injection of 435-Br1 human cells. The masses detected in vivo were either single (7 animals) or multiple (2 animals). Longitudinal MR (MRI/MRS) studies and post-mortem histological data were correlated, revealing a total incidence of experimental brain metastases of 85% in the cases studied (11/13 animals). ADC maps perfectly differentiated edema and/or CSF areas from metastasis. Preliminary MRS data also revealed additional features: decrease in N-acetyl aspartate (NAA) was the first MRS-based marker of metastasis growth in the brain (micrometastasis); choline-containing compounds (Cho) rose and creatine (Cr) levels decreased as these lesions evolved, with mobile lipids and lactate also becoming visible. Furthermore, MRS pattern recognition-based analysis suggested that this approach may help to discriminate different growth stages. CONCLUSIONS: This study paves the way for further in vivo studies oriented towards detection of different tumor progression states and for improving treatment efficiency.     

Rapid microscopic 3D-diffusion tensor imaging fiber-tracking of mouse brain in vivo by super resolution reconstruction: validation on MAP6-KO mouse model

Magnetic Resonance Materials in Physics, Biology and Medicine - Exploring mouse brains by rapid 3D-Diffusion Tensor Imaging (3D-DTI) of high spatial resolution (HSR) is challenging in vivo. Here we...     

Development of partial discharge detection method using an autoregressive model in gas-insulated switchgears (GIS)

Gas-insulated switchgears (GIS) are important equipment in electric-supply stations where advanced techniques for safety and maintenance are required. As a result, a system is being developed that can automatically monitor and diagnose a GIS in service. This article focuses on some problems concerning insulating functions of GIS to discuss a method of signal processing for partial discharge detection and position identification. For sensors, potential detectors were installed on flanges of the GIS. Focusing on the potential fluctuations measured by these detectors, an autoregressive model is designed for potential fluctuation in a state without any partial discharge. This model is called a normal state model. The difference was found between potential fluctuations measured during partial discharge and those of the normal state model. The index of whiteness test method of the residual random process is a useful parameter for representing this difference, and the use of this method allowed detection of partial discharge that could not be found at normal potential levels. These results indicate bright prospects for manufacturing a monitoring system that can detect deterioration in GIS insulation with a high sensitivity in an early stage and that also operates on-line.     

Assessment of neovascular permeability in a pancreatic tumor model using dynamic contrast-enhanced (DCE) MRI with contrast agents of different molecular weights

Object  

We evaluated the relationship of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI)-derived pharmacokinetic parameters and contrast agents with different molecular weights (MW) in a pancreatic tumor mouse model.     

Magnetic resonance imaging detection of multiple ischemic injury produced in an adult rat model of minor stroke followed by mild transient cerebral ischemia

Objectives

To determine whether cumulative brain damage produced adjacent to a minor stroke that is followed by a mild transient ischemia is detectable with MRI and histology, and whether acute or chronic recovery between insults influences this damage.

Materials and methods

A minor photothrombotic (PT) stroke was followed acutely (1–2 days) or chronically (7 days) by a mild transient middle cerebral artery occlusion (tMCAO). MRI was performed after each insult, followed by final histology.

Results

The initial PT produced small hyperintense T₂ and DW infarct lesions and peri-lesion regions of scattered necrosis and modestly increased T₂. Following tMCAO, in a slice and a region adjacent to the PT, a region of T₂ augmentation was observed when recovery between insults was acute but not chronic. Within the PT slice, a modest region of exacerbated T₂ change proximate to the PT was also observed in the chronic group. Corresponding histological changes within regions of augmented T₂ included increased vacuolation and cell death.

Conclusion

Within regions adjacent to an experimental minor stroke, a recurrence of a mild transient cerebral ischemia augmented T₂ above increases produced by tMCAO alone, reflecting increased damage in this region. Exacerbation appeared broader with acute versus chronic recovery between insults.

     

Spatially resolved kinetics of skeletal muscle exercise response and recovery with multiple echo diffusion tensor imaging (MEDITI): a feasibility study

Objectives

We describe measurement of skeletal muscle kinetics with multiple echo diffusion tensor imaging (MEDITI). This approach allows characterization of the microstructural dynamics in healthy and pathologic muscle.

Materials and methods

In a Siemens 3-T Skyra scanner, MEDITI was used to collect dynamic DTI with a combination of rapid diffusion encoding, radial imaging, and compressed sensing reconstruction in a multi-compartment agarose gel rotation phantom and within in vivo calf muscle. An MR-compatible ergometer (Ergospect Trispect) was employed to enable in-scanner plantar flexion exercise. In a HIPAA-compliant study with written informed consent, post-exercise recovery of DTI metrics was quantified in eight volunteers. Exercise response of DTI metrics was compared with that of T2-weighted imaging and characterized by a gamma variate model.

Results

Phantom results show quantification of diffusivities in each compartment over its full dynamic rotation. In vivo calf imaging results indicate larger radial than axial exercise response and recovery in the plantar flexion-challenged gastrocnemius medialis (fractional response: nT2w?=?0.385?±?0.244, nMD?=?0.163?±?0.130, nλ₁?=?0.110?±?0.093, nλ_rad?=?0.303?±?0.185). Diffusion and T2-weighted response magnitudes were correlated (e.g., r?=?0.792, p?=?0.019 for nMD vs. nT2w).

Conclusion

We have demonstrated the feasibility of MEDITI for capturing spatially resolved diffusion tensor data in dynamic systems including post-exercise skeletal muscle recovery following in-scanner plantar flexion.

     

Biexponential and diffusional kurtosis imaging, and generalised diffusion-tensor imaging (GDTI) with rank-4 tensors: a study in a group of healthy subjects

Object Clinical diffusion imaging is based on two assumptions of limited validity: that the radial projections of the diffusion propagator are Gaussian, and that a single directional diffusivity maximum exists in each voxel. The former can be removed using the biexponential and diffusional kurtosis models, the latter using generalised diffusion-tensor imaging. This study provides normative data for these three models. Materials and methods Eighteen healthy subjects were imaged. Maps of the biexponential parameters D _fast, D _slow and f _slow, of D and K from the diffusional kurtosis model, and of diffusivity D′ were obtained. Maps of generalised anisotropy (GA) and scaled entropy(SE) were also generated, for second and fourth rank tensors. Normative values were obtained for 26 regions. Results In grey versus white matter, D _slow and D′ were higher and D _fast, f _slow and K were lower. With respect to maps of D′, anatomical contrast was stronger in maps of D _slow and K. Elevating tensor rank increased SE, generally more significantly than GA, in: anterior limb of internal capsule, corpus callosum, deep frontal and subcortical white matter, along superior longitudinal fasciculus and cingulum. Conclusion The values reported herein can be used for reference in future studies and in clinical settings.     

Sweep frequency response analysis (SFRA) for the assessment of winding displacements and deformation in power transformers

The sweep frequency response analysis (SFRA) is an analysis technique for detecting winding displacement and deformation (among other mechanical and electrical failures) on power and distribution transformers. Nowadays, there is an increasing interest in SFRA method because of its sensibility in detecting mechanical faults without opening the unit. SFRA as a diagnostic technique must integrate both the off-line measurements and the interpretation of the data in order to provide an assessment of the condition of the windings. However, guidelines for the measurement and record interpretation are not available. The evaluation is presently done by experts in the topic through the visual inspection or with the help of statistical parameters such as the correlation coefficient and the standard deviation. However, criteria like the limits of normal variation of the parameters, and the features observed in the records in the presence of a determined type of fault could not to coincide. Although, there are some proposals for making the interpretation more objective, neither of them integrate human expertise along with the different kind of parameters obtained from the evaluation of the records in a diagnostic model. This paper presents a survey on the alternatives in the measurement techniques and interpretation of SFRA measurements, describing some sources of uncertainty in applying this methodology.     

萃取法和FAAS法测定锌中杂质(Ⅰ)

采用二乙基二硫代氨基甲酸钠—甲基异丁基甲酮萃取富集铜、镍、钴、铅等元素,并与大量锌分离,有机相用火焰原子吸收光谱法测定铜、镍、钴、铅。方法可应用于金属锌、锌合金、电池用锌粉和锌饼,测定范围:0.2×10-6～100×10-6的铜;0.3×10-6～100×10-6的镍;0.5×10-6～200×10-6的钴;1×10-6～500×10-6的铅。     

火电机组在孤(小)网中的启动及运行方式研究

通过对火电机组在孤（小）网中运行的常见问题的理论分析和研究,结合火电机组在典型孤（小）网中的运行实践,提出了火电机组在孤（小）网中的启动、正常运行及应对异常工况的运行策略,提高了火电机组抵御和应对事故的能力,对于保证电网安全和火电机组本身安全具有很强的实践价值和参考意义。     

萃取法和FAAS法测定锌中杂质(Ⅱ)

研究了在盐酸介质中，用甲基异丁基甲桐萃取富集铁的条件及盐酸提纯的方法；有机相同火粉原子吸收光谱法（FAAS）测定铁。该方法灵敏度高、选择性好，可应用于金属锌、锌合金、电池用锌粉和锌饼中痕量铁的测定，测定范围：0．0000％－0．01％。     

Growth and electrical properties of Pb(Zr,Ti)O3 thin films by sputtering using an alloy target

Abstract

Pb(Zr_x,Ti_1?x)O₃ thin films were deposited on Pt/SiO₂/Si substrates by the rf magnetron sputtering using an alloy target consisting of Zr-Ti alloy and Pb metal. The dependence of electrical properties on film thickness and sputtering gas pressure was investigated. The dielectric constant and the remanent polarization decreased and the coercive field increased with the decrease of the film thickness. In the dependence of gas pressure, the relative dielectric constant of the film with only a perovskite phase were in the range of 235–280, which were higher than those of the film with only a pyrochlore phase, 20. The asymmetry of hysteresis loops increased with the decrease of the gas pressure.     

分散控制系统网络通信堵塞和人机界面反应迟缓现象分析及其优化调整

对沙角C电厂分散控制系统(DCS)中出现的网络通信堵塞(如节点离线、网络通信缓慢)和操作员站画面反应迟缓的形成原因进行详细分析，并提出分析这类故障现象的方法和合理有效的优化处理策略，对同类DCS网络通信故障分析提供了有效的借鉴。     

On the logic of a Swedish (Karolinska Institute) paper and an analysis of the pooled data of three Nordic epidemiologic studies related to cancer and the magnetic fields of power lines

We show that a logical error is contained in some epidemiologic papers which intend to verify the hypothesis that there is an association between magnetic fields of power lines and childhood leukemia. For instance, the Swedish paper by Feychting and Ahlbom in 1993 presented a leukemia association related to calculated magnetic fields in one‐family houses other than apartments, from the results obtained from stratified analyses according to type of building; this was contradictory to their premise that exposure assessment was calculated from magnetic fields irrespective of type of building. However, the authors stated in conclusion that their hypothesis could be verified, although it needed to be supplemented with other data, as was customary in epidemiologic analysis, in order not to miss any possible causes of cancer. Our views are as follows: Engineers in the field would be unlikely to accept any procedure that needs supplementary data, because it is employed at the author(s)'s will for the intended verification of the hypothesis; it would be natural to suppose that the hypothesis could not be verified. © 2000 Scripta Technica, Electr Eng Jpn, 134(3): 53–59, 2001