基于磁共振扩散加权波谱技术的水分子扩散测量方法

目的 在常规医用磁共振机上利用扩散加权波谱技术测量水分子的扩散系数.方法 在受激回波采集模式序列中第二个π/2射频脉冲之前及第三个π/2射频脉冲之后加入两个强扩散梯度磁场,在3.0特斯拉医用磁共振机上运行此脉冲序列扫描磁共振波谱模体,扩散梯度磁场强度固定,通过改变两扩散梯度磁场间隔得到不同的扩散权重,扫描过程中不抑制水信号.结果 在重扩散权重条件下得到的波谱中水信号信噪比依然较高,扩散权重对水分子表观扩散系数的影响不显著(P＞0.05).结论 在常规医用磁共振机上利用扩散加权波谱技术测量水分子扩散系数是可行的,而且结果不受扩散权重的影响.     

不同数量梯度磁场方向对正常脑白质纤维束扩散张量成像的比较定量研究

目的比较扩散张量成像中施加不同数量的梯度磁场方向对扩散的各向异性（FA）和表观扩散系数（ADC）参数的影响. 资料与方法对24名正常人进行3.0 T磁共振扩散张量成像扫描,分别施加6个和25个方向的扩散敏感梯度磁场.分别测得2个梯度方向所获得FA图和ADC图的内囊前、后肢及胼胝体膝部、压部白质的FA与ADC值,进行比较. 结果在其他扫描参数不变的情况下（b=0.1000 s/mm^2）,6个方向与25个方向,相应部位的FA和ADC值的两组数据无显著差异. 结论磁共振扫描选择扩散张量成像扫描参数时,在不影响各定量值测量的情况下,应尽量减少扫描时间,6个方向是扩散张量成像的较好选择.     

扩散张量成像和示踪图评估异体移植肾功能障碍——初步研究

目的评价MR扩散张量成像(DTI)作为无创性的诊断方法,检测急性及慢性移植肾功能障碍和器官微结构改变的能力。方法在1.5TMR设备上对15例异体移植肾功能障碍病人和14名健康志愿者行脂肪抑制平面回波DTI序列(扩散敏感梯度场施加在6个方向上,b值为0、600s/mm2)成     

扩散张量成像导出量对扩散时间依赖关系的实验研究

目的研究扩散张量导出量与扩散时间的关系。方法保持扩散敏感梯度磁场强度不变,使用8个不同的扩散时间对11名被试者进行扩散张量成像扫描得到脑部的各向异性与各向同性信息,计算出各个感兴趣区的平均扩散率与各向异性分数后进行比较。结果不同扩散时间对应的平均扩散率有显著性差异（P〈0．05）,而各向异性分数无显著性差异（P〉0．05）。结论扩散时间对扩散张量成像导出量中的各向异性分数无影响,对平均扩散率有影响。原因是细胞内外水分子扩散性质不同。     

苯丙酮尿症脑部病变扩散加权成像

目的 使用磁共振扩散加权成像(DWI)技术研究苯丙酮尿症(phenylketonuria,PKU)患者脑白质内病灶的扩散特点,并观察表观扩散系数(apparent diffusion coefficient,ADC)的变化.资料与方法 对8例临床确诊的PKU患儿及性别、年龄相匹配的8名健康儿童(对照组)行T1WI、T2WI及DWI,测量侧脑室后角T2WI异常信号区及放射冠正常白质的ADC值和对照组相应部位的ADC值.结果 在T2WI上,所有未经治疗的PKU患儿均表现为非占位性、斑片状、条带状高信号,在X、Y、Z 3个方向同时施加扩散梯度的跟踪ADC图(trace APC map)上显示更清晰.患儿大脑白质病灶X、Y、Z 3个方向的ADC值明显低于对照组相应部位脑白质的ADC值,两者差异存在统计学意义(P＜0.05).平均ADC值比对照组相应部位减少23%,侧脑室后角扩散各向异性消失(F=0.195,P=0.825).结论 未经治疗的PKU白质异常在DWIav上显示更清晰,DWI可提供PKU患儿脑白质病灶的病理信息.     

乳腺磁共振扩散加权成像的应用

目的：探讨乳腺磁共振扩散加权成像（DWI）检查的可行性,并重点探讨影响DWI图像质量的技术参数。方法：使用GE1．5T磁共振扫描仪及阵列线圈对32例乳腺疾病患者行常规SE序列扫描,其中19例为良性肿瘤,5例炎性病变,6例恶性肿瘤．均经手术及病理证实;另硅胶置入2例。使用体线圈行DWI序列扫描,采用全方位扩散梯度及5个b值扫描。DWI总的扫描时间40s。结果：在DWI序列扫描中,良性和恶性肿瘤均为高信号,计算ADC值可鉴别良性和恶性肿瘤．通过各种扫描参数的合理匹配,可使图像质量的信噪比达到最佳,并减少图像的几何变形。结论：DWI对于检查乳腺病变是一种快速可行并行之有效的技术。     

联合多种MRI序列评估弥漫性轴索损伤

目的:探讨弥漫性轴索损伤(DAD在MR扩散加权成像,T2加权液体衰减翻转恢复序列(T2- FLAIR)及增强梯度回波T2*加权血管成像序列(ESWAN)上的影像学特征及各序列的诊断价值,合理规划DAI患者的MRI检查方案.方法:10例DAI患者在伤后两周内行DWI、T2-FLAIR及ESWAN检查,DWI为3个垂直方向...     

非中风类脑疾病的扩散加权成像

扩散加权成像提供了一种不同于传统MRI技术的影像对比。这种技术对检出急性缺血性中风和鉴别急性中风与其他表现为突发神经功能障碍的病变特别敏感。扩散加权成像同时也可提供其它脑部病变如肿瘤、颅内感染、颅脑外伤及脱髓鞘病变的必要信息。在大多数脑部MR成像时，扩散加权MR成像应作为一种必要的序列。     

磁共振扩散成像诊断颅内表皮样囊肿

目的讨论磁共振扩散成像诊断颅内表皮样囊肿的有用性和特异性。方法分析15例经手术证实的颅内表皮样囊肿磁共振扩散成像的表现，也比较其他MRI序列，包括T1、T2WI、质子密度加权像和快速水抑制反转恢复(fast fluid attenuated inversion recovery FLAIR)序列。结果磁共振扩散成像显示所有肿瘤(15个瘤灶)呈明亮的高信号，肿瘤与周围脑组织和脑脊液形成鲜明的对比；对手术后小的残存肿瘤(4个瘤灶)和复发肿瘤(3个瘤灶)的检出也极其敏感。结论磁共振扩散成像对表皮样囊肿有高度的敏感性和特异性，对手术前的定性诊断和手术后肿瘤的残存以及肿瘤复发的判断起着重要的作用。     

磁共振扩散加权成像在评估鼻咽癌放射治疗中的应用

目的 探讨磁共振扩散加权成像(DW-MRI)在鼻咽癌放射治疗中的应用价值.方法 经穿刺活检病理确诊鼻咽癌病例15例,在放射治疗前及放射治疗剂量达10、20 Gy及40 Gy时行磁共振检查(包括常规MRI及DW-MRI),观察肿瘤原发灶及颈部转移淋巴结大小、DWI信号及ADC值的变化.结果 放疗前DWI序列显示肿瘤原发灶最大层面面积要明显小于增强T1WI脂肪抑制序列(P<0.05);肿瘤原发灶及转移淋巴结在放疗后T2WI信号不均匀增高,其ADC值在放疗后逐渐升高,放疗前及放疗剂量达40 Gy时ADC值分别为(0.690±0.072)×10-3 mm2/s和(0.813±0.091)×10-3 mm2/s及(1.167±0.057)×10-3 mm2/s和(1.802±0.173)×10-3 mm2/s,且放疗前后各组间相互差异均有统计学意义(P<0.05);放疗前转移淋巴结最大层面面积为(2.58±0.64) cm2,放疗剂量达10 Gy时为(1.07±0.25) cm2,缩小率为58.5%,而肿瘤原发灶缩小率为2%.结论 与常规磁共振比较,扩散加权成像能更早、更特异、更敏感地反映鼻咽癌放疗后的生物学变化,对其疗效的监测具有重要价值.     

一种用于估算磁共振成像中水分子扩散系数的非迭代算法

目的 提出一种更加可靠和高效的算法,用于计算磁共振扩散加权成像(DWI)中人体内水分子的扩散系数.方法 根据扩散加权成像中磁共振信号的双指数衰减规律,我们提出了双线性拟合算法以替代现在广泛使用的Levenberg-Marquardt迭代算法.使用两次线性拟合分别计算快慢扩散系数,整个计算过程中不必估算初始值.结果 双线性拟合算法计算所用时间远少于Levenberg-Marquardt算法,计算结果合理.而Levenberg-Marquardt算法的结果中有大量额外解.结论 与Levenberg-Marquardt算法相比,双线性拟合算法在分析磁共振扩散加权成像数据方面不仅算法可靠而且计算效率高.     

0.5T磁共振机弥散加权像诊断急性脑梗塞

目的：分析０．５Ｔ磁共振机的弥散加权像（ＤＷＩ）对急性脑梗塞诊断的临床价值。材料与方法：计算５０例健康志愿者正常脑组织各不同部位的表观弥散系数（ＡＤＣ）值,并对急性脑梗塞发作后３～１２小时的１０名患者进行ＤＷＩ及常规Ｔ_１ＷＩ、Ｔ_２ＷＩ、ＦＬＡＩＲ及ＭＲＡ检查。结果：测得正常人额、顶、枕叶脑白质、半卵圆中心、基底节、脑干、小脑半球、脑脊液部位的ＡＤＣ平均值。对于临床患者,ＤＷＩ可明确显示急性期脑梗塞病灶,常规Ｔ_１ＷＩ、Ｔ_２ＷＩ、ＦＬＡＩＲ均不能显示或显示不清。结论：以ＤＷＩ为主,结合Ｔ_１ＷＩ、Ｔ_２ＷＩ、ＦＬＡＩＲ及ＭＲＡ序列能非常准确、可靠的诊断急性脑梗塞。     

Efficient fat suppression by slice‐selection gradient reversal in twice‐refocused diffusion encoding

Most diffusion imaging sequences rely on single‐shot echo‐planar imaging (EPI) for spatial encoding since it is the fastest acquisition available. However, it is sensitive to chemical‐shift artifacts due to the low bandwidth in the phase‐encoding direction, making fat suppression necessary. Often, spectral‐selective RF pulses followed by gradient spoiling are used to selectively saturate the fat signal. This lengthens the acquisition time and increases the specific absorption rate (SAR). However, in pulse sequences that contain two slice‐selective 180° refocusing pulses, the slice‐selection gradient reversal (SSGR) method of fat suppression can be implemented; i.e., using slice‐selection gradients of opposing polarity for the two refocusing pulses. We combined this method with the twice‐refocused spin‐echo sequence for diffusion encoding and tested its performance in both phantoms and in vivo. Unwanted fat signal was entirely suppressed with this method without affecting the water signal intensity or the slice profile. Magn Reson Med 60:1256–1260, 2008. © 2008 Wiley‐Liss, Inc.     

Diffusion weighted vertical gradient and spin echo

In this work, diffusion weighting and parallel imaging is combined with a vertical gradient and spin echo data readout. This sequence was implemented and evaluated on healthy volunteers using a 1.5 and a 3 T whole‐body MR system. As the vertical gradient and spin echo trajectory enables a higher k‐space velocity in the phase‐encoding direction than single‐shot echo planar imaging, the geometrical distortions are reduced. When combined with parallel imaging such as generalized autocalibrating partially parallel acquisition, the geometric distortions are reduced even further, while also keeping the minimum echo time reasonably low. However, this combination of a diffusion preparation and multiple refocusing pulses during the vertical gradient and spin echo readout, generally violates the Carr–Purcell–Meiboom–Gill condition, which leads to interferences between echo pathways. To suppress the stimulated echo pathway, refocusing pulses with a sharper slice profiles and an odd/even crusher variation scheme were implemented and evaluated. Being a single‐shot acquisition technique, the reconstructed images are robust to rigid‐body head motion and spatially varying brain motion, both of which are common sources of artifacts in diffusion MRI. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

A new diffusion SSFP imaging technique

In this paper a new diffusion sensitive steady-state free precession (SSFP) pulse sequence with a reduced sensitivity to physiological brain motion is presented. The signal attenuation due to diffusion in this SSFP sequence is derived theoretically and confirmed experimentally with a phantom. It is shown that for brain tissue this signal attenuation is approximately independent of T1 and T2, but depends only on the pulse sequence used, i.e., the timing and the size of the RF and the gradient pulses. On this basis the diffusion constant can be calculated for any region in the image. Diffusion sensitive images of the brain obtained with our pulse sequence are presented and shown to be superior over an image obtained with a “conventional” diffusion sensitive SSFP sequence.     

Diffusion imaging with prospective motion correction and reacquisition

A major source of artifacts in diffusion‐weighted imaging is subject motion. Slow bulk subject motion causes misalignment of data when more than one average or diffusion gradient direction is acquired. Fast bulk subject motion can cause signal dropout artifacts in diffusion‐weighted images and results in erroneous derived maps, e.g., fractional anisotropy maps. To address both types of artifacts, a fully automatic method is presented that combines prospective motion correction with a reacquisition scheme. Motion correction is based on the prospective acquisition correction method modified to work with diffusion‐weighted data. The images to reacquire are determined automatically during the acquisition from the imaging data, i.e., no extra reference scan, navigators, or external devices are necessary. The number of reacquired images, i.e., the additional scan duration can be adjusted freely. Diffusion‐weighted prospective acquisition correction corrects slow bulk motion well and reduces misalignment artifacts like image blurring. Mean absolute residual values for translation and rotation were <0.6 mm and 0.5°. Reacquisition of images affected by signal dropout artifacts results in diffusion maps and fiber tracking free of artifacts. The presented method allows the reduction of two types of common motion related artifacts at the cost of slightly increased acquisition time. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Diffusion-weighted whole-body MRI with background body signal suppression: technical improvements at 3.0 T

Purpose:

To improve image quality of diffusion‐weighted body magnetic resonance imaging (MRI) with background body signal suppression (DWIBS) at 3.0 T.

Materials and Methods:

In 30 patients and eight volunteers, a diffusion‐weighted spin‐echo echo‐planar imaging sequence with short TI inversion recovery (STIR) fat suppression was applied and repeated using slice‐selective gradient reversal (SSGR) and/or dual‐source parallel radiofrequency (RF) transmission (TX). The quality of diffusion‐weighted images and gray scale inverted maximum intensity projections (MIP) were visually assessed by intraindividual comparison with respect to the level of fat suppression and signal homogeneity. Moreover, the contrast between lesions/lymph nodes and background (C_lb) was analyzed in the MIP reconstructions.

Results:

By combining STIR with SSGR, fat suppression was significantly improved (P < 0.001) and C_lb was increased two times. The use of TX allowed the reduction of acquisition time and improved image quality with regard to signal homogeneity (P < 0.001) and fat suppression (P = 0.005).

Conclusion:

DWIBS at 3.0 T can be improved by using SSGR and TX. J. Magn. Reson. Imaging 2012;456‐461. © 2011 Wiley Periodicals, Inc.     

Differences in apparent diffusion coefficients of brain metabolites between grey and white matter in the human brain measured at 7 T

Diffusion weighted spectroscopy can provide microstructural information that is specific to compartmental geometry. So far, in human brain, apparent diffusion coefficients (ADCs) of only the metabolites N-acetyl aspartate, creatine (tCr) and choline (tCho) have been assessed. High field MR at 7 T allows the collection and analysis of diffusion weighted spectroscopy data of additional metabolites of interest such as glutamate (Glu), N-acetyl aspartyl glutamate, and glutamine (Gln), which are of interest due to their different compartmentalization and role in brain physiology. In this study, we performed (1)H diffusion weighted spectroscopy at 7 T using a diffusion-weighted PRESS sequence in parietal white matter (n = 6) and occipital grey matter (n = 7). Data were analyzed using the LCmodel. ADCs could reliably be obtained of N-acetyl aspartate, tCr, tCho, Glu, Gln in grey and white matter, and N-acetyl aspartyl glutamate in white matter. Significant differences in ADC values were observed between grey and white matter for all metabolites. ADCs in grey matter were consistently lower than in white matter. These differences can probably be attributed to different compartmentalization as well as to the differential impact of diffusion time on ADC of different molecules under conditions of restricted diffusion.     

If J doesn't evolve,it won't J‐resolve: J‐PRESS with bandwidth‐limited refocusing pulses

There is increasing interest in the J‐PRESS technique, an in vivo implementation of two‐dimensional J‐spectroscopy combined with PRESS localization, for high‐field spectroscopy studies of the human brain. The experiment is designed to resolve scalar couplings in the second, indirectly detected dimension, but will only do so if the slice‐selective refocusing pulses in the PRESS sequence affect all coupled spins equally. At high magnet field strengths, due to limited RF pulse bandwidth, PRESS‐based localization results in spatially dependent evolution of coupling. In some regions of the localized volume, coupling evolves during the PRESS echo time, while in other regions it may be partially or fully refocused. This study investigates the impact of this effect on the appearance of the J‐PRESS spectrum for coupled spins, focusing on two commonly observed metabolites, lactate and N‐acetyl aspartate, showing that such behavior results in additional peaks in the J‐resolved spectrum (termed J‐refocused peaks). It is also demonstrated that increasing the bandwidth of refocusing pulses significantly reduces the size of such signals. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.