从随机行走模型到磁共振扩散权重成像

目的:利用布朗运动的随机行走模型导出磁共振扩散权重成像(DWI)公式,以验证随机行走模型对磁共振扩散权重成像的适用性。方法:从布朗运动的随机行走模型出发,分析存在梯度磁场情况下质子的扩散行为,计算梯度磁场对扩散运动中质子进动相位相散的积累,从而用统计方法算出在梯度磁场和扩散两个因素作用下导致的信号衰减,最后结合扩散成像序列导出磁共振扩散权重成像的信号强度公式。结果:证明了扩散运动中质子进动相位相散的积累符合高斯分布,导出了扩散所致信号衰减与梯度磁场及施加时间的关系,并进一步导出了脉冲梯度自旋回波序列(PGSE)的扩散权重成像公式——Stejskal-Tanner方程。结论:在磁共振扩散成像中人体水分子的扩散运动可视为随机行走。通过施加运动探测梯度(MPG)磁场,可将组织中水分子的扩散能力反映到图像亮度上。虽然Stejskal和Tanner也曾利用存在扩散和梯度磁场情况下的布洛赫方程导出了Stejskal-Tanner方程,但本文直接从随机行走模型出发导出该方程,为磁共振扩散权重成像的微观基础提供一个新的视角。     

一种新的磁共振扩散成像实验参数控制方法

扩散敏感梯度磁度的方向及强度是磁共振扩散成像实验的重要参数，但这二个参数不能由用户通过设备自带的软件设定。本文介绍一种新的方法，通过修改MRI扫描机内部的数据文件，用户可以方便与精确地设定扩散加权成像DWI及扩散张景成像DTI的实验参数，而且可以为MRI扫描机增加新的功能。     

医用MRI系统快速获取梯度回波脉冲序列开发

目的:探讨在i_Open 0.36T医用磁共振成像(MRI)系统上开发快速获取梯度回波脉冲序列(RAGE)的可行性与实现方法。方法:pascal语言编辑源程序,可调参数控制激发层数,相位编码时序,激发脉冲相位循环,数据采集次数,各段梯度脉冲开关时序等,编排k空间数据记录方式,满足快速傅立叶变换重建需要。编译源程序,调试图像。结果:在256×256采集矩阵的情况下,单层面成像时间最小值4s,所获图像分辨率、信噪比等指标能满足临床诊断需要。结论:利用该方法设计快速获取梯度回波脉冲序列是可行的。     

恒定梯度场磁共振成像脉冲序列研究

本研究提出了恒定梯度场下磁共振快速成像的方法。这种成像方法对磁体场强的要求不高 ( 15 0 0高斯 ) ,在相对较低的场强下具有成像速度快、对梯度场功放要求低、全开放设计等特点。本研究从磁体设计、射频脉冲波形、扩散效应几个方面进行了讨论 ,并提出了几种快速成像脉冲序列。最后给出了用计算机模拟的初步成像结果。     

健康儿童颈髓磁共振扩散张量成像

目的 应用扩散张量成像技术研究儿童颈髓发育规律。方法 使用单次激发自旋回波平面回波序列对90例健康儿童行颈髓扩散张量成像。在颈髓节段分别测量其表观扩散系数值（ADC）、各向异性分数值（FA）、纤维束平均长度（Ltract）以及纤维束体积（Vtract）。结果 各组的ADC值、FA值、Ltract及Vtract分别如下：0.9747±0.2777、0.8493±0.2236、0.8210±0.1432、0.9198±0.1444、0.9048±0.1676;0.4117±0.0391、0.4712±0.0199、0.4944±0.0439、0.5608±0.0443、0.6169±0.0551;25.61±8.63、24.66±7.14、27.03±7.23、34.93±10.99、37.63±10.22;3.07±1.49、3.00±1.52、3.81±1.33、5.41±2.35、6.64±2.84。各年龄组的FA值、Ltract和Vtract不完全相同（P<0.001）,而ADC值各年龄组的均值差异无统计学意义（F=1.758, P=0.145）。在组间的两两比较中：1、2组间FA值差异具有统计学意义;3、4组间FA值、Ltract和Vtract差异均具有统计学意义;4、5组间FA值差异具有统计学意义。FA值、Ltract和Vtract与年龄呈正相关。
结论 儿童颈髓发育具有阶段性,且具有阶段性特征;磁共振扩散张量技术可用于观测儿童颈髓并评价其发育。     

快速自旋回波脉冲序列的设计与实现

快速自旋回波序列可减少磁共振成像的数据扫描时间,提高成像速度,是国内外MR扫描仪必备的快速序列之一。本研究对快速自旋回波序列的设计、编程和实现方法进行了研究,对快速自旋回波序列与自旋回波序列的成像结果进行了比较和分析。该方法为国内MR扫描仪的自主研发提供了重要参考。     

自旋回波序列参数TR的选择对低场磁共振测温准确性的影响

目的在肿瘤热疗过程中,磁共振成像技术可以进行热疗组织的温度监测,而基于纵向弛豫时间(longitudinal relaxation time,T1)加权成像的测温方法的准确性受到自旋回波序列(spin-echo sequence,SE)参数的影响。为给临床磁共振测温提供参考并提高测温的准确性,本文将探讨自旋回波序列参数重复时间(TR)的选取对基于纵向弛豫时间加权成像的测温方法的测温准确性的影响。方法基于T1加权成像测温的原理,通过改变SE序列的参数重复时间(TR),在0.4 T磁共振设备下对不同温度的猪肝组织进行扫描,找到T1加权图像信号强度值与温度的拟合关系,计算不同TR值下的理论温度,将其与数字温度计测量的参考温度进行比较,以此量化参数TR对测温准确性的影响。结果参数TR的设置会一定程度上影响测温准确性。在本实验条件下,当参数TR为400 ms(回波时间TE=18ms)测温的准确性最高,利用该SE序列测得的猪肝组织温度与实际温度最为接近,测温误差基本在±1℃左右。结论核磁测温过程中,SE序列参数TR值的恰当选择,对提升测温准确性具有明显的作用,本文的研究对临床热疗中的核磁测温具有一定参考和指导价值。     

扩散张量MR成像研究

本文一方面综述了8年来国际上扩散张量磁共振成像(DTI)的研究进展情况。包括总结了到目前为止所使用的4种提高DTI成像精度的方法，并指出精度的提高依赖于成像中脉冲序列的优化、实验方法的完善和后处理算法两个方面。文中比较了目前研究扩散张量的两种基本模型：扩散张量模型和q-空间模型。指出这两种模型侧重应用于不同的场合。前者侧重于研究器官或宏观组织中的扩散，而后者侧重于研究小到细胞尺度(μm量级)的扩散行为。两者在应用研究方面是互补的，所要求的实验条件是有差别的。另一方面结合文献对扩散张量模型的实验条件进行了理论分析。认为b因子取得过高并不合理。并用DTI实验数据和结果进行了初步验证。为了改进扩散张量模型本文探讨了考虑多指数衰减的设想。文中综述了DTI的导出量和一些实验结果，在此基础上分析了设计和优化各向异性指标(DAI)的原则。对于DTI的重要导出结果神经纤维柬成像(fiber tractography)重点分析了造成其成像精度不高的主要因素，指出了改进纤维束传导方向甄选算法和寻求纤维束方向场的几何性质是两种可能的解决办法。     

磁共振扩散张量成像扩散椭球面积的简化计算方法

目的:提出一种可以精确计算磁共振扩散张量成像扩散椭球表面积的方法。方法:根据理想扩散椭球是旋转椭球及扩散张量矩阵均正定,可将三轴椭球校正为旋转椭球,进而由A.M.Legendre公式推出计算扩散椭球表面积的简化公式。分别利用A.M.Legendre、Knud Thomsen及本文提出的方法计算四种扩散椭球的表面积以评价本文方法的可行性。结果:对于旋转椭球,本文所提方法的计算结果与A.M.Legendre方法的结果一致。结论:本文所提方法计算量较小,可以精确地计算扩散椭球表面积。     

扩散加权成像在超急性、急性脑梗死中的诊断价值

目的通过与T2加权成像(T2-weighted imaging,T2WI)、T1加权成像(T1-weighted imaging,T1WI)进行比较,探讨磁共振扩散加权成像(diffusion-weighted imaging,DWI)及表观扩散系数(apparentdiffusion coeffecient,ADC)在超急性、急性脑梗死诊断中的应用价值。材料与方法应用单次激发SE-EPI(echoplanar imaging)序列进行DWI扫描和常规MR扫描,对临床40例脑梗死患者进行检查,观察病灶在DWI上的信号特点并测定病灶的ADC值。结果40例病例中超急性期(0~6h)脑梗死6例、急性期(7~24h)脑梗死34例均在DWI上呈高信号,在ADC图上呈低信号,平均ADC值为(0.689±1.04)×10-3mm2/s。结论DWI及ADC对急性、超急性脑梗死的诊断较常规MR更早、更准确,尤其是对超急性脑诊断,而且能准确反应出缺血半暗带的相应病理生理变化。     

Diffusion of hyperpolarized 13C‐metabolites in tumor cell spheroids using real‐time NMR spectroscopy

The detection of tumors noninvasively, the characterization of their progression by defined markers and the monitoring of response to treatment are goals of medical imaging techniques. In this article, a method which measures the apparent diffusion coefficients (ADCs) of metabolites using hyperpolarized ¹³C diffusion‐weighted spectroscopy is presented. A pulse sequence based on the pulsed gradient spin echo (PGSE) was developed that encodes both kinetics and diffusion information. In experiments with MCF‐7 human breast cancer cells, we detected an ADC of intracellularly produced lactate of 1.06 ± 0.15 µm²/ms, which is about one‐half of the value measured with pyruvate in extracellular culture medium. When monitoring tumor cell spheroids during progressive membrane permeabilization with Triton X‐100, the ratio of lactate ADC to pyruvate ADC increases as the fraction of dead cells increases. Therefore, ¹³C ADC detection can yield sensitive information on changes in membrane permeability and subsequent cell death. Our results suggest that both metabolic label exchange and ¹³C ADCs can be acquired simultaneously, and may potentially serve as noninvasive biomarkers for pathological changes in tumor cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Oscillating gradient diffusion kurtosis imaging of normal and injured mouse brains

Recent advances in diffusion MRI employ multiple diffusion encoding schemes with varying diffusion direction, weighting, and diffusion time to investigate specific microstructural properties in biological tissues. In this study, we examined time‐dependent diffusion kurtosis contrast in adult mouse brains and in neonatal mouse brains after hypoxic–ischemic (HI) injury. In vivo diffusion kurtosis maps were acquired with a short diffusion time using an oscillating gradient spin echo (OGSE) sequence at 100 Hz and with a relatively long diffusion time (20 ms) using a pulsed gradient spin echo (PGSE) sequence. In the adult mouse brain, we found that the cortex and hippocampus showed larger differences between OGSE kurtosis and PGSE kurtosis than major white matter tracts. In neonatal mouse brains with unilateral HI injury, the OGSE kurtosis map overall provided stronger edema contrast than the PGSE kurtosis map, and the differences between OGSE and PGSE kurtosis measurements in the edema region reflected heterogeneity of injury. This is the first in vivo study that has demonstrated multi‐direction OGSE kurtosis contrasts in the mouse brain. Comparing PGSE and OGSE kurtosis measures may provide additional information on microstructural changes after ischemic stroke.     

The apparent dependence of the diffusion coefficient of N-acetylaspartate upon magnetic field strength: evidence of an interaction with NMR methodology

An inverse relationship between applied magnetic field strength and the apparent diffusion coefficient (ADC) of several important brain metabolites including N-acetyl-l-aspartate (NAA), choline and creatine, measured in vivo using proton magnetic resonance spectroscopy (MRS), has been reported. In this investigation, using phantom studies of NAA at magnetic field strengths of 3 and 7 T, these observations have been verified under controlled MRS conditions in vitro, and the ADC of NAA has been found to vary inversely with magnetic field strength, decreasing at a rate of 2.5%/T at 20 degrees C. We have also assessed whether the effect is a function of a systemic bias in methodology, or if the effect is actually on the rate of molecular diffusion. This was done using an MRS-independent method for measurement of molecular diffusion in NAA phantoms at 0, 0.025 and 7 T applied magnetic field strengths. As a result, it has been demonstrated that the observed apparent magnetic field dependence of the ADC of NAA is a consequence of the NMR measurement and is apparently not a real effect on molecular diffusion.     

Validation of surface‐to‐volume ratio measurements derived from oscillating gradient spin echo on a clinical scanner using anisotropic fiber phantoms

A diffusion measurement in the short‐time surface‐to‐volume ratio (S/V) limit (Mitra et al., Phys Rev Lett. 1992;68:3555) can disentangle the free diffusion coefficient from geometric restrictions to diffusion. Biophysical parameters, such as the S/V of tissue membranes, can be used to estimate microscopic length scales non‐invasively. However, due to gradient strength limitations on clinical MRI scanners, pulsed gradient spin echo (PGSE) measurements are impractical for probing the S/V limit. To achieve this limit on clinical systems, an oscillating gradient spin echo (OGSE) sequence was developed. Two phantoms containing 10 fiber bundles, each consisting of impermeable aligned fibers with different packing densities, were constructed to achieve a range of S/V values. The frequency‐dependent diffusion coefficient, D(ω), was measured in each fiber bundle using OGSE with different gradient waveforms (cosine, stretched cosine, and trapezoidal), while D(t) was measured from PGSE and stimulated‐echo measurements. The S/V values derived from the universal high‐frequency behavior of D(ω) were compared against those derived from quantitative proton density measurements using single spin echo (SE) with varying echo times, and from magnetic resonance fingerprinting (MRF). S/V estimates derived from different OGSE waveforms were similar and demonstrated excellent correlation with both SE‐ and MRF‐derived S/V measures (ρ ≥ 0.99). Furthermore, there was a smoother transition between OGSE frequency f and PGSE diffusion time when using , rather than the commonly used t_eff = 1/(4f), validating the specific frequency/diffusion time conversion for this regime. Our well‐characterized fiber phantom can be used for the calibration of OGSE and diffusion modeling techniques, as the S/V ratio can be measured independently using other MR modalities. Moreover, our calibration experiment offers an exciting perspective of mapping tissue S/V on clinical systems.     

High angular resolution diffusion imaging with stimulated echoes: compensation and correction in experiment design and analysis

Stimulated echo acquisition mode (STEAM) diffusion MRI can be advantageous over pulsed‐gradient spin‐echo (PGSE) for diffusion times that are long compared with T₂. It therefore has potential for biomedical diffusion imaging applications at 7T and above where T₂ is short. However, gradient pulses other than the diffusion gradients in the STEAM sequence contribute much greater diffusion weighting than in PGSE and lead to a disrupted experimental design. Here, we introduce a simple compensation to the STEAM acquisition that avoids the orientational bias and disrupted experiment design that these gradient pulses can otherwise produce. The compensation is simple to implement by adjusting the gradient vectors in the diffusion pulses of the STEAM sequence, so that the net effective gradient vector including contributions from diffusion and other gradient pulses is as the experiment intends. High angular resolution diffusion imaging (HARDI) data were acquired with and without the proposed compensation. The data were processed to derive standard diffusion tensor imaging (DTI) maps, which highlight the need for the compensation. Ignoring the other gradient pulses, a bias in DTI parameters from STEAM acquisition is found, due both to confounds in the analysis and the experiment design. Retrospectively correcting the analysis with a calculation of the full B matrix can partly correct for these confounds, but an acquisition that is compensated as proposed is needed to remove the effect entirely. © 2014 The Authors. NMR in Biomedicine published by John Wiley & Sons, Ltd.     

Reproducibility of Apparent Diffusion Coefficient Measurements in Malignant Breast Masses

This study aimed to evaluate the reproducibility of apparent diffusion coefficient (ADC) measurements in malignant breast masses, and to determine the influence of mammographic parenchymal density on this reproducibility. Sixty-six patients with magnetic resonance findings of the mass were included. Two breast radiologists measured the ADC of the malignant breast mass and the same area on the contralateral normal breast in each patient twice. The effects of mammographic parenchymal density, histology, and lesion size on reproducibility were also assessed. There was no significant difference in the mean ADC between repeated measurements in malignant breast masses and normal breast tissue. The overall reproducibility of ADC measurements was good in both. The 95% limits of agreement for repeated ADCs were approximately 30.2%-33.4% of the mean. ADC measurements in malignant breast masses were highly reproducible irrespective of mass size, histologic subtype, or coexistence of microcalcifications; however, the measurements tended to be less reproducible in malignant breast masses with extremely dense parenchymal backgrounds. ADC measurements in malignant breast masses are highly reproducible; however, mammographic parenchymal density can potentially influence this reproducibility.     

Wavelet-Based Diffusion Approach for DTI Image Restoration

The Rician noise introduced into the diffusion tensor images （DTIs） can bring serious impacts on tensor calculation and fiber tracking. To decrease the effects of the Rician noise, we propose to consider the wavelet-based diffusion method to denoise multiehannel typed diffusion weighted （DW） images. The presented smoothing strategy, which utilizes anisotropic nonlinear diffusion in wavelet domain, successfully removes noise while preserving both texture and edges. To evaluate quantitatively the efficiency of the presented method in accounting for the Rician noise introduced into the DW images, the peak-to-peak signal-to-noise ratio （PSNR） and signal-to-mean squared error ratio （SMSE） metrics are adopted. Based on the synthetic and real data, we calculated the ap- parent diffusion coefficient （ADC） and tracked the fibers. We made comparisons between the presented model, the wave shrinkage and regularized nonlinear diffusion smoothing method. All the experiment results prove quantitatively and visually the better performance of the presented filter.