Robustness of fat quantification using chemical shift imaging

The purpose of this study was to investigate the effect of parameter changes that can potentially lead to unreliable measurements in fat quantification. Chemical shift imaging was performed using spoiled gradient echo sequences with systematic variations in the following: two-dimensional/three-dimensional sequence, number of echoes, delta echo time, fractional echo factor, slice thickness, repetition time, flip angle, bandwidth, matrix size, flow compensation and field strength. Results indicated no significant (or significant but small) changes in fat fraction with parameter. The significant changes can be attributed to the known effects of T1 bias and two forms of noise bias.     

不同网格扭曲率下压力修正全隐算法——IDEAL求解性能研究

2008年,本文作者和陶文铨等提出了一种用于速度和压力耦合求解的高效稳定压力修正全隐算法IDEAL,该算法通过在每个迭代层次上对压力方程进行两次内迭代计算,完全克服了SIMPLE算法的两个假设,充分满足了速度和压力之间的耦合,从而大大提高了计算的收敛性和健壮性.为了进一步实现IDEAL算法的推广应用,本文基于三维倾斜方腔顶盖驱动流动,研究了IDEAL算法在不同网格扭曲率下的求解特性.研究发现,在不同网格扭曲率下,IDEAL算法的健壮性和收敛性均优于SIMPLE算法,特别在高网格扭曲率情况下,IDEAL算法求解性能更加优于SIMPLE算法.在不同网格扭曲率下,IDEAL算法健壮性保持不变,几乎可以在任意速度亚松弛因子下获得收敛的解,同时IDEAL算法最短计算耗时较SIMPLE算法减少了56%~89%,验证了IDEAL算法的优越性.     

不同网格扭曲率下压力修正全隐算法——IDEAL求解性能研究

2008年,本文作者和陶文铨等提出了一种用于速度和压力耦合求解的高效稳定压力修正全隐算法IDEAL,该算法通过在每个迭代层次上对压力方程进行两次内迭代计算,完全克服了SIMPLE算法的两个假设,充分满足了速度和压力之间的耦合,从而大大提高了计算的收敛性和健壮性。为了进一步实现IDEAL算法的推广应用,本文基于三维倾斜方腔顶盖驱动流动,研究了IDEAL算法在不同网格扭曲率下的求解特性。研究发现,在不同网格扭曲率下,IDEAL算法的健壮性和收敛性均优于SIMPLE算法,特别在高网格扭曲率情况下,IDEAL算法求解性能更加优于SIMPLE算法。在不同网格扭曲率下,IDEAL算法健壮性保持不变,几乎可以在任意速度亚松弛因子下获得收敛的解,同时IDEAL算法最短计算耗时较SIMPLE算法减少了56%~89%,验证了IDEAL算法的优越性。     

Quantification of low fat contents: a comparison of MR imaging and spectroscopy methods at 1.5 and 3 T

Magnetic resonance spectroscopy (MRS) has long been considered the golden standard for non-invasive measurement of tissue fat content. With improved techniques for fat/water separation, imaging has become an alternative to MRS for fat quantification. Several imaging models have been proposed, but their performance relative to MRS at very low fat contents is yet not fully established. In this work, imaging and spectroscopy were compared at 1.5 T and 3 T in phantoms with 0-3% fat fraction (FF). We propose a multispectral model with individual a priori R₂ relaxation rates for water and fat, and a common unknown R₂′ relaxation. Magnitude and complex image reconstructions were also compared. Best accuracy was obtained with the imaging method at 1.5 T. At 3 T, the FFs were underestimated due to larger fat-water phase shifts. Agreement between measured and true FF was excellent for the imaging method at 1.5 T (imaging: FF_meas= 0.98 FF_true− 0.01%, spectroscopy: FF_meas= 0.77 FF_true+ 0.08%), and fair at 3 T (imaging: FF_meas= 0.91 FF_true− 0.19%, spectroscopy: FF_meas= 0.79 FF_true+ 0.02%). The imaging method was able to quantify FFs down to approx. 0.5%. We conclude that the suggested imaging model is capable of fat quantification with accuracy and precision similar to or better than spectroscopy and offers an improvement vs. a model with a common R₂* relaxation only.     

Performance analysis of IDEAL algorithm for three‐dimensional incompressible fluid flow and heat transfer problems

Recently, an efficient segregated algorithm for incompressible fluid flow and heat transfer problems, called inner doubly iterative efficient algorithm for linked equations (IDEAL), has been proposed by the present authors. In the algorithm there exist inner doubly iterative processes for pressure equation at each iteration level, which almost completely overcome two approximations in SIMPLE algorithm. Thus, the coupling between velocity and pressure is fully guaranteed, greatly enhancing the convergence rate and stability of solution process. However, validations have only been conducted for two‐dimensional cases. In the present paper the performance of the IDEAL algorithm for three‐dimensional incompressible fluid flow and heat transfer problems is analyzed and a systemic comparison is made between the algorithm and three other most widely used algorithms (SIMPLER, SIMPLEC and PISO). By the comparison of five application examples, it is found that the IDEAL algorithm is the most robust and the most efficient one among the four algorithms compared. For the five three‐dimensional cases studied, when each algorithm works at its own optimal under‐relaxation factor, the IDEAL algorithm can reduce the computation time by 12.9–52.7% over SIMPLER algorithm, by 45.3–73.4% over SIMPLEC algorithm and by 10.7–53.1% over PISO algorithm. Copyright © 2009 John Wiley & Sons, Ltd.     

求解流动与传热问题的一种高效稳定的分离式算法-IDEAL

本文提出了一种求解流动与传热问题的高效稳定的分离式算法-IDEAL(Inner Doubly-iterative EfficientAlgorithm for Linked-equations).在IDEAL算法中每个迭代层次上对压力方程进行两次内迭代计算,第一次内迭代过程用于克服SIMPLE算法的第一个假设,第二次内迭代过程用于克服SIMPLE算法的第二个假设.这样在每个迭代层次上充分满足了速度和压力之间的耦合,从而大大提高了计算的收敛速度和计算过程的稳定性.本文通过2个三维不可压缩流动和传热的算例对IDEAL算法与其它三个被广泛使用的算法(SIMPLER、SIMPLEC和PISO)进行了比较.通过分析比较得出IDEAL算法在收敛性和健壮性上均优于SIMPLER、SIMPLEC和PISO算法.在这2个算例中IDEAL算法几乎可以在任意的松弛因子下获得收敛的解,并且IDEAL算法所需最短计算时间较SIMPLER算法减少12.9%～52.6%;较SIMPLEC算法减少48.3%～79.1%;较PISO算法减少10.7%～46.5%.     

Development of new finite volume schemes on unstructured triangular grid for simulating the gas–liquid two‐phase flow

This paper presents a coupled finite volume inner doubly iterative efficient algorithm for linked equations (IDEAL) with level set method to simulate the incompressible gas–liquid two‐phase flows with moving interfaces on unstructured triangular grid. The finite volume IDEAL method on a collocated grid is employed to solve the incompressible two‐phase Navier–Stokes equations, and the level set method is used to capture the moving interfaces. For the sake of mass conservation, an effective second‐order accurate finite volume scheme is developed to solve the level set equation on triangular grid, which can be implemented much easier than the classical high‐order level set solvers. In this scheme, the value of level set function on the boundary of control volume is approximated using a linear combination of a high‐order Larangian interpolation and a second‐order upwind interpolation. By the rotating slotted disk and stretching and shrinking of a circular fluid element benchmark cases, the mass conservation and accuracy of the new scheme is verified. Then the coupled method is applied to two‐phase flows, including a 2D bubble rising problem and a 2D dam breaking problem. The computational results agree well with those reported in literatures and experimental data. Copyright © 2015 John Wiley & Sons, Ltd.     

基于复值和模值的肝脏磁共振水脂分离组合算法

质子密度脂肪分数（proton density fat fraction,PDFF）是用于评估肝脏脂肪的重要定量指标.为了获得更为精确的肝脏PDFF,本文通过分离R₂^*估计,提出了一种改进的基于磁共振成像（magnetic resonance imaging,MRI）脂肪定量方法.通过在3.0 T MRI扫描仪器上对肝脏脂肪含量不同的人体被试进行MRI扫描,进一步验证了该方法的可行性和准确性.研究结果显示,本文方法得到的肝脏脂肪分数和磁共振波谱（magnetic resonance spectroscopy,MRS）分析得到的脂肪含量高度一致,表明该方法对临床脂肪肝的检测具有重要的参考指导价值.