腹部不同b值弥散加权成像：单、双射频源图像质量及ADC值对比研究
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摘要
研究目的
3.0T高场磁共振(magnetic resonance, MR)下,前瞻性、个体内对比研究腹部不同b值弥散加权成像(diffusion-weighted imaging, DWI)时,图像质量与表观弥散系数(apparent diffusion coefficient, ADC)测量值在传统单射频(radiofrequency, RF)源与并行双射频源两组图像上有无差异。
研究方法
本研究经签署知情同意书后,共纳入18例健康男性志愿者(平均年龄,27.3岁；年龄范围,24-33岁),所有研究对象均无既往腹部相关疾病病史。2011年7月至8月期间,18名志愿者先后在飞利浦公司3.0T并行双射频源临床MR扫描仪(Achieva3.0TTX; Philips Healthcare)进行腹部DWI扫描。该MR仪可进行单、双射频源切换,因此检查者无需重新定位即可进行前后两次扫描,这一优势使得个体内对比研究得以实现。
由Philips Healthcare两名工程师协助设计扫描序列各个参数。DWI序列扫描之前,分别在单、双射频源下进行B1mapping成像,所得B1map用于定量评价射频场(B1)的均匀性。扫描参数如下：重复时间／回波时间(repetition time/echo time, TR/TE)=517/3.3ms,视野(field of view, FOV)=326mm×278mm,矩阵(matrix size)=64×58,体素(voxel size)=5mm×5mm×6mm,双翻转角度(dual flip angle)=60°和120°,带宽(bandwidth)=4293.7Hz,单层扫描层厚为6mm.单次屏气10s内可完成扫描。
腹部DWI扫描包括一系列不同的b值组：0/100,0/500,0/800和0/100/800s/mm2。每一b值组均重复两次：一次于双射频源下扫描,一次于单射频源下扫描。扫描均采用呼吸触发技术,其他参数如下：TR/TE=2400/40ms, FOV=326mmx278mm, matrix size=108×89, voxel size=3mm×3mm×6mm, bandwidth=9.9pixel/43.8Hz,半采集因子(halfscan)=0.698,回波链长(EPI factor)=39,信号采集数(NSA)=4,加速因子(sense factor)=2.5,射频激励角度(RF excitation degrees)=90°,层厚/层间距(section thickness/intersection spacing)=6mm/1mm,层数(number of sections)=24,压脂方式为频谱空间预置饱和翻转回复(spectral presaturation with inversion recovery, SPIR)。
采集每一组b值的DWI图像,传输至工作站应用标准软件(Diffusion Calculation, Philips Medical Systems)计算ADC图。此外,筛选0/100/800b值组内高b值组100/800图像计算ADC图。因此,每一志愿者共计算10组ADC图(5组不同b值,单、双射频源2种扫描模式)。数据分析分两部分进行：
(1)图像分析
主观图像质量评分：两名放射学医师(分别具有10年和4年腹部放射学研究经验)独立、随机评价每一b值(0,100,500,800s/mm2)的单射频源和双射频源两组DWI图像。两组评价时间间隔为1周。两名医师对图像采集所使用的b值、单或双射频源均未知晓。图像评分采用4分制：1代表较差图像质量,明显驻波伪影；2代表中等图像质量,中度驻波伪影；3代表较好图像质量,轻度驻波伪影；4代表很好图像质量,无驻波伪影。
信号噪声比测量：DWI图像上,分别于肝右后叶、肝左外叶、胰腺尾部、脾4个解剖部位设置兴趣区(region-of-interest, ROI),测量各解剖部位在每一b值(0,100,500,800s/mm2)下,单射频源和双射频源两组DWI图像的信号噪声比(Signal to noise ratio, SNR)。因并行采集成像技术的使用,无法用传统的方法于空气中测量噪声,故参照文献将每一器官测得信号的标准差作为噪声。为避免不同组织差异、各种伪影对噪声测量的干扰,ROI应尽量避开血管、胆管等其他组织结构,设置于信号均匀度较好的区域。使用工作站上的复制黏贴功能确保单射频源与双射频源图像上ROI位置的一致性。
B1场的均匀性测量：B1map上,分别于肝右后叶、肝左外叶、胰腺尾部、脾4个解剖部位设置ROI,测量射频脉冲激励后各解剖部位分别在单射频源和双射频源B1map上的实际翻转角度,并计算实际翻转角度的变异度(coefficient of variation, CV,即标准差/均数)。比较各解剖部位实际翻转角度分布的一致性(即信号分布的均匀度)在单、双射频源间有无差异,从而评价射频场的均匀性有无改善。
(2)腹部器官ADC值测量
分别于肝右后叶、肝左外叶、胰腺尾部、脾4个解剖部位设置ROI(3个椭圆形、无重叠ROI/解剖部位),测量每一组b值(0/100,0/500,0/800,0/100/800,100/800s/mm2)的单射频源和双射频源两组图像中各解剖部位的ADC值。每一志愿者共测得120个数据：3个ROI/解剖部位,4个解剖部位,5组不同b值,单、双射频源2种扫描模式。
所有统计学分析均使用SPSS17.0软件(SPSS, Chicago, Ill)。主观图像质量评分、各解剖部位SNR、各解剖部位B1map实际翻转角度及其变异度在单射频源和双射频源两组图像上的比较,均采用一系列配对Wilcoxon秩和检验。图像质量评分者间结果一致性,采用Kappa检验评价：Kappa值≤0.4时提示有一致性但较差,0.41-0.75和>0.75时分别提示一致性良好、很好。每一解剖部位,不同b值组时单、双射频源两组图像ADC测量值比较,采用对象内因子(within-subject factor)重复测量方差分析(repeated measure analysis of variance, ANOVA)。检验水准为P<0.05。
结果
(1)图像分析
每一b值下,并行双射频源主观图像质量评分均显著高于单射频源图像质量评分(每一组比较,P值均<0.0001)。两位评分者的结果间存在较好或很好的一致性(Kappa值范围,0.75～1.00)。与同b值单射频源图像比较,双射频源b值为0图像组完全消除了驻波伪影(两位评分者的记分均为4.00),b值为100图像组与高b值图像组(b-500,b-800)驻波伪影显著减轻。
每一b值下,并行双射频源图像各解剖部位的SNR均高于单射频源图像SNR。其中,仅肝左外叶两者SNR测量达到了统计学差异(P<0.001)。所有解剖部位中,脾并行双射频源扫描时SNR最高,肝左外叶单射频源扫描SNR最低。
B1场的均匀性测量显示,并行双射频源各解剖部位实际翻转角度均显著高于单射频源图像(P<0.0001),且前者各解剖部位实际翻转角度的变异度均显著低于后者(P<0.01),即较单射频源比,并行双射频源各解剖部位实际翻转角度分布更为一致、信号分布更为均匀。较单射频源比,并行双射频源不同解剖部位间的信号分布及不同研究对象间的信号分布也更趋于一致。
(2)腹部器官ADC测量值比较
主效应分析结果显示：对象内因子射频源主效应无统计学意义(P=0.074),即双射频源下,所有解剖部位在所有b值组时测得的ADC值合并后,与单射频源图像测得的值比较无统计学差异。对象内因子b值组与解剖部位的主效应均有统计学意义(P值均<0.0001),即不同b值组间ADC值、不同解剖部位间ADC值均有统计学差异。
二次交互效应分析结果显示：射频源与b值组二次交互效应有统计学意义,b值组为0/100时双射频源测得ADC值显著低于单射频源(P<0.0001),b值组为0/100/800、100/800时,双射频源测得ADC值显著高于单射频源(P值分别为0.007,0.031),b值组为0/500、0/800时,单、双射频源测得ADC值无统计学差异(P值分别为0.437,0.236)。射频源与解剖部位二次交互效应也有统计学意义,各个解剖部位中,仅脾双射频源测得ADC值显著低于单射频源(P<0.0001),余解剖部位单、双射频源测得ADC值均无统计学差异。
三次交互效应分析结果显示：射频源与解剖部位、b值组的三次交互效应有统计学意义,于是行简单效应分析：b值组为0/800(P<0.001)、100/800(P=0.001)、0/100/800(P=0.001)时,肝左外叶并行双射频源图像测得ADC值显著高于传统单射频源测值,而b值组为0/100时,肝左外叶并行双射频源图像测得ADC值显著低于传统单射频源测值(P<0.001)；b值组为0/100(P<0.001)、0/500(P=0.047)、0/800(P=0.012)时,脾并行双射频源图像测得ADC值显著低于传统单射频源测值；任意一组b值下,肝右后叶、胰腺尾部并行双射频源图像测得ADC值与传统单射频源测值比均无统计学差异。
射频源与b值二次交互效应分析并结合简单效应分析结果显示：b值组为0/500时,除脾有略微显著的统计学差异外,余各解剖部位ADC测量值在单、双射频源上均无显著差异。
结论
1、与传统单射频源比,并行双射频源技术显著提高了腹部不同b值DWI图像质量与射频场的均匀性；
2、与传统单射频源比,并行双射频源技术提高不同b值DWI图像上腹部器官的SNR,其中,肝左外叶的SNR差异达到统计学意义；
3、高b值组或低b值组时,肝左外叶和脾单、双射频源两组图像测得ADC值均有显著差异；
4、b值组为0/500时,腹部各器官单、双射频源两组图像测得ADC值差异最小
Objective
At3.0T higher-fields magnetic resonance (MR), to prospectively and intraindividually compare abdomen diffusion-weighted imaging (DWI) obtained with different b values between dual-source parallel radiofrequency (RF) transmission and conventional single-source RF transmission in terms of image quality and measured apparent diffusion coefficient (ADC).
Materials and Methods
The written informed consent was obtained from each participating volunteer and a total of eighteen healthy male volunteers (mean age,27.3years; age range,24-33years) with no prior history or findings related to abdomen disease were enrolled in our study. From July,2011to August, eighteen healthy male volunteers sequentially underwent DWI of the abdomen at a commercially available clinical3.0-T MR imaging system (Achieva3.0T TX; Philips Healthcare) equipped with a dual-source parallel RF transmission system. The system used in our study allows one to perform both dual-source and conventional single-source excitation with the quadrature body coil and thus enables an intraindividual comparative study design without repositioning.
Two engineers (Philips Healthcare) assisted in the design of the sequence and imaging protocol. Before DWI was started, Bi mapping was performed by using dual-source RF transmission and conventional single-source RF transmission, respectively. This was done for quantitative assessment of RF fields (B1) homogeneity on B1map. Acquisition parameters were as follows:repetition time/echo time (TR/TE) =517/3.3ms, field of view (FOV)=326×278mm, matrix size=64×58, voxel size (mm)=5×5×6, dual flip angle=60°and120°, bandwidth=4293.7Hz, a single section and a section thickness of6mm. The acquisition time was10s during a single breath hold.
The DW imaging protocol included a series of different b values:0/100,0/500,0/800and0/100/800s/mm2. Each b value was performed twice:one by using dual-source parallel RF transmission (with two independent RF sources), another by using conventional single-source RF transmission (with a single RF source). Respiratory triggering technique was used and the other sequence parameters were as follows:TR/TE=2400/40ms, FOV (mm)=326×278, matrix size=108×89, voxel size (mm)=3×3×6, water-fat shift (bandwidth)=9.9pixel/43.8Hz, halfscan=0.698, Echo train length (EPI factor)=39, number of acquired signals=4, sense factor=2.5, RF excitation (degrees)=90°, section thickness/intersection spacing (mm)=6/1, number of sections=24, spectral presaturation with inversion recovery (SPIR) for fat suppression.
ADC map of each DWI series was calculated on a workstation with standard software (Diffusion Calculation, Philips Medical Systems). In addition, higher b value100/800included in0/100/800DW images was selected for ADC map calculation to separate perfusion and true diffusion effects. Therefore, a total of ten ADC map calculations were made for each volunteer (five different b values, dual-source parallel RF transmission and single-source RF transmission). Data were analyzed as the following two steps:
(1) Image Analysis
Subjective Image Quality. Two radiologists (10and4years experience in body radiology, respectively) independently evaluated both the dual-source parallel RF transmission DWI images and the conventional single-source RF transmission DWI images in a randomized order during two separate reading sessions. The two sessions were separated1week. The observers were fully blinded to RF transmission and b value. Images obtained with the b-values of0,100,500, and800s/mm2were evaluated on a4-point scale (1=poor image quality, major standing-wave artifacts;2=fair image quality, moderate standing-wave artifacts;3=good image quality, mild standing-wave artifacts;4=excellent image quality, no standing-wave artifacts). Signal to Noise Ratio:Signal to noise ratio (SNR) was calculated on both the dual-source parallel RF transmission DWI images and the conventional single-source RF transmission DWI images at b values of0,100,500, and800s/mm2for posterior right hepatic lobe, lateral left hepatic lobe, spleen and pancreatic tail. Parallel imaging prohibits the use of a conventional method of measuring noise in which the region-of-interest (ROI) is placed in the air. Thus, as recommended in the literature, we used Standard Deviation (SD) of normal organ signal intensity as an estimate of local noise. In order to avoiding the interference from tissues with different properties and various artifacts, large blood vessels and biliary ducts should be avoided and regions with good homogeneity should be chosen when place ROI. Using copy and paste function to ensure the ROI concordance between dual-source and single-source RF transmission images.
Quantitative Assessment of B1Homogeneity:The actual flip angles excitated by90°RF pulse were measured and the coefficient of variation (CV, equal to standard deviation divided by mean) of the actual flip angles was calculated on dual-source parallel RF transmission Bi map and the conventional single-source RF transmission B1map for the posterior right hepatic lobe, lateral left hepatic lobe, pancreatic tail and spleen. The consistency of the actual flip angles distribution (uniformity of local signal intensity distribution) was compared between the dual-source parallel RF transmission and the conventional single-source RF transmission to assess the homogeneity of B1field.
(2) ADC Measurements of Abdominal Organs
ROI measurements were obtained in the posterior right hepatic lobe, lateral left hepatic lobe, pancreatic tail and spleen. For each anatomic location studied,3nonoverlapping elliptic ROIs were placed on the images. ADC measurements were obtained from both the dual-source parallel RF transmission images and the conventional single-source RF transmission images at each b value (0/100,0/500,0/800,0/100/800,100/800s/mm2). Therefore, a total of120data points per volunteer were collected:3ROIs per anatomic location,4anatomic locations,5different b values, dual-source parallel RF transmission and the conventional single-source RF transmission.
All statistical analyses were performed with SPSS, version17.0(SPSS, Chicago, Ill). A series of paired Wilcoxon tests were used to compare the subjective image quality, SNR of each anatomic location, the actual flip angles and CV of each anatomic location on B1map between dual-source parallel RF transmission images and the conventional single-source RF transmission images. In order to assess interobserver agreement, we calculated the kappa statistic for two observers. Kappa values of0.4or less were considered to indicate positive but poor agreement, while those of0.41-0.75and greater than0.75indicated good and excellent agreements, respectively. At each b value, the measured ADC values of each anatomic location were compared between dual-source parallel RF transmission and conventional single-source RF transmission by using the within-subject factor repeated measure analysis of variance (ANOVA). P-values less than0.05were considered statistically significant.
Results
(1) Image analysis
At each of the selected b values, subjective image quality within the abdomen was statistically better at dual-source parallel RF transmission imaging than at conventional single-source RF transmission imaging (P<0.0001for all comparisons). There was a good to excellent agreement between observers for image quality assessment (Kappa values from0.75to1.00). Comparing with the conventional single-source RF transmission, dual-source parallel RF transmission prevent the standing-wave artifacts at b-0DW images (the scores of both the observers were4.00) and significantly reduced the artifacts at b-100,500and800DW images.
At each of the selected b-values (0,100,500and800s/mm2), there was a trend toward better SNR at dual-source parallel RF transmission imaging than at conventional single-source RF transmission imaging for all the organs, despite only the lateral left hepatic lobe achieved the significant difference (P<0.001). Among the various anatomic locations, spleen has the highest SNR at dual-source parallel RF transmission imaging, while the lateral left hepatic lobe has the lowest SNR at conventional single-source RF transmission imaging.
Quantitative assessment of B1homogeneity showed that the actual flip angles of each anatomic location on dual-source parallel RF transmission Bi map were significantly higher than on the conventional single-source RF transmission B1map. CV of the actual flip angles of each anatomic location on dual-source parallel RF transmission B1map was significantly lower than on the conventional single-source RF transmission B1map, which suggested that the actual flip angle distribution (local signal intensity distribution) of each anatomic location is more consistent on dual-source parallel RF transmission images than on conventional single-source RF transmission images. Additionally, the local signal intensity distribution between different anatomic locations and between different subjects tends to be more consistent on the dual-source parallel RF transmission imaging, comparing with the conventional single-source RF transmission imaging.
(2) ADC Measurements of Abdominal Organs
The main effects analysis showed:there was no significant (P=0.074) main effect of RF transmission on the measured ADC values. It means when considering all b values and all anatomic locations together, the measured ADC values showed no significant difference between the dual-source parallel RF transmission imaging and the conventional single-source RF transmission imaging. The main effects of the other two within-subject factors, b values and anatomic locations, were significant (P<0.0001). It means the measured ADC values between different b values and between different anatomic locations achieved significant difference.
The two-way interaction effects analysis showed:RF transmission and b values obtained the significant two-way interaction effects when considering all anatomic locations together. At b value of0/100, measured ADC values on dual-source parallel RF transmission images were significantly lower than on the conventional single-source RF transmission images (P<0.0001). At b value of0/100/800and100/800, measured ADC values on dual-source parallel RF transmission images were significantly higher than on the conventional single-source RF transmission images (P value was0.007,0.031, respectively). At b value of0/500and0/800, measured ADC values between dual-source parallel RF transmission images and conventional single-source RF transmission images showed no significant difference (P value was0.437,0.236, respectively). Additionally, RF transmission and anatomic locations also obtained the significant two-way interaction effects when considering all b values together. Among all of the anatomic locations, only measured ADC values of spleen on dual-source parallel RF transmission images were significantly lower than on conventional single-source RF transmission images (P<0.0001). Measured ADC values of other anatomic locations showed no significant difference between dual-source parallel RF transmission images and conventional single-source RF transmission images.
The three-way interaction effects analysis showed:RF transmission, b values and anatomic locations obtained the significant three-way interaction effects. Thus, the simple effect was analyzed and the results were as follow:for lateral left hepatic lobe, the measured ADC values of the dual-source parallel RF transmission images were significantly higher than the conventional single-source RF transmission image at b=0/800(P<0.001),100/800(P=0.001),0/100/800(P=0.001) and lower than the conventional single-source RF transmission image at b=0/100(P<0.001); for spleen, the measured ADC values of the dual-source parallel RF transmission images were significantly lower than the conventional single-source RF transmission image at b=0/100(P<0.001),0/500(P=0.047),0/800(P=0.012); for posterior right hepatic lobe and pancreatic tail, there was no significant difference in the measured ADC values between the dual-source parallel RF transmission images and the conventional single-source RF transmission images at any b value.
Combining with the simple effect analysis and the two-way interaction effect analysis between RF transmission and b value, it showed that there was no significant difference on ADC measurements between the parallel and the conventional RF transmission images for all organs at b=0/500, except a slight statistically significant differences for spleen (P=0.047).
Conclusion
1. Comparing with the conventional single-source RF transmission, the dual-source parallel RF transmission significantly improved the image quality and the homogeneity of the RF field.
2. Comparing with the conventional single-source RF transmission, the dual-source parallel RF transmission improved the SNR for all organs at each b-value, of which only the lateral left hepatic lobe achieved the significant difference.
3. At either lower or higher b value, the measured ADC values of lateral left hepatic lobe and spleen between the dual-source parallel RF transmission and the conventional single-source RF transmission were significantly different.
4. At b value of0/500, the minimum difference on ADC measurements between the dual-source parallel RF transmission images and the conventional single-source RF transmission images was achieved for all abdominal organs.
3.0T高场磁共振(magnetic resonance, MR)下,前瞻性、个体内对比研究腹部不同b值弥散加权成像(diffusion-weighted imaging, DWI)时,图像质量与表观弥散系数(apparent diffusion coefficient, ADC)测量值在传统单射频(radiofrequency, RF)源与并行双射频源两组图像上有无差异。
研究方法
本研究经签署知情同意书后,共纳入18例健康男性志愿者(平均年龄,27.3岁；年龄范围,24-33岁),所有研究对象均无既往腹部相关疾病病史。2011年7月至8月期间,18名志愿者先后在飞利浦公司3.0T并行双射频源临床MR扫描仪(Achieva3.0TTX; Philips Healthcare)进行腹部DWI扫描。该MR仪可进行单、双射频源切换,因此检查者无需重新定位即可进行前后两次扫描,这一优势使得个体内对比研究得以实现。
由Philips Healthcare两名工程师协助设计扫描序列各个参数。DWI序列扫描之前,分别在单、双射频源下进行B1mapping成像,所得B1map用于定量评价射频场(B1)的均匀性。扫描参数如下：重复时间／回波时间(repetition time/echo time, TR/TE)=517/3.3ms,视野(field of view, FOV)=326mm×278mm,矩阵(matrix size)=64×58,体素(voxel size)=5mm×5mm×6mm,双翻转角度(dual flip angle)=60°和120°,带宽(bandwidth)=4293.7Hz,单层扫描层厚为6mm.单次屏气10s内可完成扫描。
腹部DWI扫描包括一系列不同的b值组：0/100,0/500,0/800和0/100/800s/mm2。每一b值组均重复两次：一次于双射频源下扫描,一次于单射频源下扫描。扫描均采用呼吸触发技术,其他参数如下：TR/TE=2400/40ms, FOV=326mmx278mm, matrix size=108×89, voxel size=3mm×3mm×6mm, bandwidth=9.9pixel/43.8Hz,半采集因子(halfscan)=0.698,回波链长(EPI factor)=39,信号采集数(NSA)=4,加速因子(sense factor)=2.5,射频激励角度(RF excitation degrees)=90°,层厚/层间距(section thickness/intersection spacing)=6mm/1mm,层数(number of sections)=24,压脂方式为频谱空间预置饱和翻转回复(spectral presaturation with inversion recovery, SPIR)。
采集每一组b值的DWI图像,传输至工作站应用标准软件(Diffusion Calculation, Philips Medical Systems)计算ADC图。此外,筛选0/100/800b值组内高b值组100/800图像计算ADC图。因此,每一志愿者共计算10组ADC图(5组不同b值,单、双射频源2种扫描模式)。数据分析分两部分进行：
(1)图像分析
主观图像质量评分：两名放射学医师(分别具有10年和4年腹部放射学研究经验)独立、随机评价每一b值(0,100,500,800s/mm2)的单射频源和双射频源两组DWI图像。两组评价时间间隔为1周。两名医师对图像采集所使用的b值、单或双射频源均未知晓。图像评分采用4分制：1代表较差图像质量,明显驻波伪影；2代表中等图像质量,中度驻波伪影；3代表较好图像质量,轻度驻波伪影；4代表很好图像质量,无驻波伪影。
信号噪声比测量：DWI图像上,分别于肝右后叶、肝左外叶、胰腺尾部、脾4个解剖部位设置兴趣区(region-of-interest, ROI),测量各解剖部位在每一b值(0,100,500,800s/mm2)下,单射频源和双射频源两组DWI图像的信号噪声比(Signal to noise ratio, SNR)。因并行采集成像技术的使用,无法用传统的方法于空气中测量噪声,故参照文献将每一器官测得信号的标准差作为噪声。为避免不同组织差异、各种伪影对噪声测量的干扰,ROI应尽量避开血管、胆管等其他组织结构,设置于信号均匀度较好的区域。使用工作站上的复制黏贴功能确保单射频源与双射频源图像上ROI位置的一致性。
B1场的均匀性测量：B1map上,分别于肝右后叶、肝左外叶、胰腺尾部、脾4个解剖部位设置ROI,测量射频脉冲激励后各解剖部位分别在单射频源和双射频源B1map上的实际翻转角度,并计算实际翻转角度的变异度(coefficient of variation, CV,即标准差/均数)。比较各解剖部位实际翻转角度分布的一致性(即信号分布的均匀度)在单、双射频源间有无差异,从而评价射频场的均匀性有无改善。
(2)腹部器官ADC值测量
分别于肝右后叶、肝左外叶、胰腺尾部、脾4个解剖部位设置ROI(3个椭圆形、无重叠ROI/解剖部位),测量每一组b值(0/100,0/500,0/800,0/100/800,100/800s/mm2)的单射频源和双射频源两组图像中各解剖部位的ADC值。每一志愿者共测得120个数据：3个ROI/解剖部位,4个解剖部位,5组不同b值,单、双射频源2种扫描模式。
所有统计学分析均使用SPSS17.0软件(SPSS, Chicago, Ill)。主观图像质量评分、各解剖部位SNR、各解剖部位B1map实际翻转角度及其变异度在单射频源和双射频源两组图像上的比较,均采用一系列配对Wilcoxon秩和检验。图像质量评分者间结果一致性,采用Kappa检验评价：Kappa值≤0.4时提示有一致性但较差,0.41-0.75和>0.75时分别提示一致性良好、很好。每一解剖部位,不同b值组时单、双射频源两组图像ADC测量值比较,采用对象内因子(within-subject factor)重复测量方差分析(repeated measure analysis of variance, ANOVA)。检验水准为P<0.05。
结果
(1)图像分析
每一b值下,并行双射频源主观图像质量评分均显著高于单射频源图像质量评分(每一组比较,P值均<0.0001)。两位评分者的结果间存在较好或很好的一致性(Kappa值范围,0.75～1.00)。与同b值单射频源图像比较,双射频源b值为0图像组完全消除了驻波伪影(两位评分者的记分均为4.00),b值为100图像组与高b值图像组(b-500,b-800)驻波伪影显著减轻。
每一b值下,并行双射频源图像各解剖部位的SNR均高于单射频源图像SNR。其中,仅肝左外叶两者SNR测量达到了统计学差异(P<0.001)。所有解剖部位中,脾并行双射频源扫描时SNR最高,肝左外叶单射频源扫描SNR最低。
B1场的均匀性测量显示,并行双射频源各解剖部位实际翻转角度均显著高于单射频源图像(P<0.0001),且前者各解剖部位实际翻转角度的变异度均显著低于后者(P<0.01),即较单射频源比,并行双射频源各解剖部位实际翻转角度分布更为一致、信号分布更为均匀。较单射频源比,并行双射频源不同解剖部位间的信号分布及不同研究对象间的信号分布也更趋于一致。
(2)腹部器官ADC测量值比较
主效应分析结果显示：对象内因子射频源主效应无统计学意义(P=0.074),即双射频源下,所有解剖部位在所有b值组时测得的ADC值合并后,与单射频源图像测得的值比较无统计学差异。对象内因子b值组与解剖部位的主效应均有统计学意义(P值均<0.0001),即不同b值组间ADC值、不同解剖部位间ADC值均有统计学差异。
二次交互效应分析结果显示：射频源与b值组二次交互效应有统计学意义,b值组为0/100时双射频源测得ADC值显著低于单射频源(P<0.0001),b值组为0/100/800、100/800时,双射频源测得ADC值显著高于单射频源(P值分别为0.007,0.031),b值组为0/500、0/800时,单、双射频源测得ADC值无统计学差异(P值分别为0.437,0.236)。射频源与解剖部位二次交互效应也有统计学意义,各个解剖部位中,仅脾双射频源测得ADC值显著低于单射频源(P<0.0001),余解剖部位单、双射频源测得ADC值均无统计学差异。
三次交互效应分析结果显示：射频源与解剖部位、b值组的三次交互效应有统计学意义,于是行简单效应分析：b值组为0/800(P<0.001)、100/800(P=0.001)、0/100/800(P=0.001)时,肝左外叶并行双射频源图像测得ADC值显著高于传统单射频源测值,而b值组为0/100时,肝左外叶并行双射频源图像测得ADC值显著低于传统单射频源测值(P<0.001)；b值组为0/100(P<0.001)、0/500(P=0.047)、0/800(P=0.012)时,脾并行双射频源图像测得ADC值显著低于传统单射频源测值；任意一组b值下,肝右后叶、胰腺尾部并行双射频源图像测得ADC值与传统单射频源测值比均无统计学差异。
射频源与b值二次交互效应分析并结合简单效应分析结果显示：b值组为0/500时,除脾有略微显著的统计学差异外,余各解剖部位ADC测量值在单、双射频源上均无显著差异。
结论
1、与传统单射频源比,并行双射频源技术显著提高了腹部不同b值DWI图像质量与射频场的均匀性；
2、与传统单射频源比,并行双射频源技术提高不同b值DWI图像上腹部器官的SNR,其中,肝左外叶的SNR差异达到统计学意义；
3、高b值组或低b值组时,肝左外叶和脾单、双射频源两组图像测得ADC值均有显著差异；
4、b值组为0/500时,腹部各器官单、双射频源两组图像测得ADC值差异最小
Objective
At3.0T higher-fields magnetic resonance (MR), to prospectively and intraindividually compare abdomen diffusion-weighted imaging (DWI) obtained with different b values between dual-source parallel radiofrequency (RF) transmission and conventional single-source RF transmission in terms of image quality and measured apparent diffusion coefficient (ADC).
Materials and Methods
The written informed consent was obtained from each participating volunteer and a total of eighteen healthy male volunteers (mean age,27.3years; age range,24-33years) with no prior history or findings related to abdomen disease were enrolled in our study. From July,2011to August, eighteen healthy male volunteers sequentially underwent DWI of the abdomen at a commercially available clinical3.0-T MR imaging system (Achieva3.0T TX; Philips Healthcare) equipped with a dual-source parallel RF transmission system. The system used in our study allows one to perform both dual-source and conventional single-source excitation with the quadrature body coil and thus enables an intraindividual comparative study design without repositioning.
Two engineers (Philips Healthcare) assisted in the design of the sequence and imaging protocol. Before DWI was started, Bi mapping was performed by using dual-source RF transmission and conventional single-source RF transmission, respectively. This was done for quantitative assessment of RF fields (B1) homogeneity on B1map. Acquisition parameters were as follows:repetition time/echo time (TR/TE) =517/3.3ms, field of view (FOV)=326×278mm, matrix size=64×58, voxel size (mm)=5×5×6, dual flip angle=60°and120°, bandwidth=4293.7Hz, a single section and a section thickness of6mm. The acquisition time was10s during a single breath hold.
The DW imaging protocol included a series of different b values:0/100,0/500,0/800and0/100/800s/mm2. Each b value was performed twice:one by using dual-source parallel RF transmission (with two independent RF sources), another by using conventional single-source RF transmission (with a single RF source). Respiratory triggering technique was used and the other sequence parameters were as follows:TR/TE=2400/40ms, FOV (mm)=326×278, matrix size=108×89, voxel size (mm)=3×3×6, water-fat shift (bandwidth)=9.9pixel/43.8Hz, halfscan=0.698, Echo train length (EPI factor)=39, number of acquired signals=4, sense factor=2.5, RF excitation (degrees)=90°, section thickness/intersection spacing (mm)=6/1, number of sections=24, spectral presaturation with inversion recovery (SPIR) for fat suppression.
ADC map of each DWI series was calculated on a workstation with standard software (Diffusion Calculation, Philips Medical Systems). In addition, higher b value100/800included in0/100/800DW images was selected for ADC map calculation to separate perfusion and true diffusion effects. Therefore, a total of ten ADC map calculations were made for each volunteer (five different b values, dual-source parallel RF transmission and single-source RF transmission). Data were analyzed as the following two steps:
(1) Image Analysis
Subjective Image Quality. Two radiologists (10and4years experience in body radiology, respectively) independently evaluated both the dual-source parallel RF transmission DWI images and the conventional single-source RF transmission DWI images in a randomized order during two separate reading sessions. The two sessions were separated1week. The observers were fully blinded to RF transmission and b value. Images obtained with the b-values of0,100,500, and800s/mm2were evaluated on a4-point scale (1=poor image quality, major standing-wave artifacts;2=fair image quality, moderate standing-wave artifacts;3=good image quality, mild standing-wave artifacts;4=excellent image quality, no standing-wave artifacts). Signal to Noise Ratio:Signal to noise ratio (SNR) was calculated on both the dual-source parallel RF transmission DWI images and the conventional single-source RF transmission DWI images at b values of0,100,500, and800s/mm2for posterior right hepatic lobe, lateral left hepatic lobe, spleen and pancreatic tail. Parallel imaging prohibits the use of a conventional method of measuring noise in which the region-of-interest (ROI) is placed in the air. Thus, as recommended in the literature, we used Standard Deviation (SD) of normal organ signal intensity as an estimate of local noise. In order to avoiding the interference from tissues with different properties and various artifacts, large blood vessels and biliary ducts should be avoided and regions with good homogeneity should be chosen when place ROI. Using copy and paste function to ensure the ROI concordance between dual-source and single-source RF transmission images.
Quantitative Assessment of B1Homogeneity:The actual flip angles excitated by90°RF pulse were measured and the coefficient of variation (CV, equal to standard deviation divided by mean) of the actual flip angles was calculated on dual-source parallel RF transmission Bi map and the conventional single-source RF transmission B1map for the posterior right hepatic lobe, lateral left hepatic lobe, pancreatic tail and spleen. The consistency of the actual flip angles distribution (uniformity of local signal intensity distribution) was compared between the dual-source parallel RF transmission and the conventional single-source RF transmission to assess the homogeneity of B1field.
(2) ADC Measurements of Abdominal Organs
ROI measurements were obtained in the posterior right hepatic lobe, lateral left hepatic lobe, pancreatic tail and spleen. For each anatomic location studied,3nonoverlapping elliptic ROIs were placed on the images. ADC measurements were obtained from both the dual-source parallel RF transmission images and the conventional single-source RF transmission images at each b value (0/100,0/500,0/800,0/100/800,100/800s/mm2). Therefore, a total of120data points per volunteer were collected:3ROIs per anatomic location,4anatomic locations,5different b values, dual-source parallel RF transmission and the conventional single-source RF transmission.
All statistical analyses were performed with SPSS, version17.0(SPSS, Chicago, Ill). A series of paired Wilcoxon tests were used to compare the subjective image quality, SNR of each anatomic location, the actual flip angles and CV of each anatomic location on B1map between dual-source parallel RF transmission images and the conventional single-source RF transmission images. In order to assess interobserver agreement, we calculated the kappa statistic for two observers. Kappa values of0.4or less were considered to indicate positive but poor agreement, while those of0.41-0.75and greater than0.75indicated good and excellent agreements, respectively. At each b value, the measured ADC values of each anatomic location were compared between dual-source parallel RF transmission and conventional single-source RF transmission by using the within-subject factor repeated measure analysis of variance (ANOVA). P-values less than0.05were considered statistically significant.
Results
(1) Image analysis
At each of the selected b values, subjective image quality within the abdomen was statistically better at dual-source parallel RF transmission imaging than at conventional single-source RF transmission imaging (P<0.0001for all comparisons). There was a good to excellent agreement between observers for image quality assessment (Kappa values from0.75to1.00). Comparing with the conventional single-source RF transmission, dual-source parallel RF transmission prevent the standing-wave artifacts at b-0DW images (the scores of both the observers were4.00) and significantly reduced the artifacts at b-100,500and800DW images.
At each of the selected b-values (0,100,500and800s/mm2), there was a trend toward better SNR at dual-source parallel RF transmission imaging than at conventional single-source RF transmission imaging for all the organs, despite only the lateral left hepatic lobe achieved the significant difference (P<0.001). Among the various anatomic locations, spleen has the highest SNR at dual-source parallel RF transmission imaging, while the lateral left hepatic lobe has the lowest SNR at conventional single-source RF transmission imaging.
Quantitative assessment of B1homogeneity showed that the actual flip angles of each anatomic location on dual-source parallel RF transmission Bi map were significantly higher than on the conventional single-source RF transmission B1map. CV of the actual flip angles of each anatomic location on dual-source parallel RF transmission B1map was significantly lower than on the conventional single-source RF transmission B1map, which suggested that the actual flip angle distribution (local signal intensity distribution) of each anatomic location is more consistent on dual-source parallel RF transmission images than on conventional single-source RF transmission images. Additionally, the local signal intensity distribution between different anatomic locations and between different subjects tends to be more consistent on the dual-source parallel RF transmission imaging, comparing with the conventional single-source RF transmission imaging.
(2) ADC Measurements of Abdominal Organs
The main effects analysis showed:there was no significant (P=0.074) main effect of RF transmission on the measured ADC values. It means when considering all b values and all anatomic locations together, the measured ADC values showed no significant difference between the dual-source parallel RF transmission imaging and the conventional single-source RF transmission imaging. The main effects of the other two within-subject factors, b values and anatomic locations, were significant (P<0.0001). It means the measured ADC values between different b values and between different anatomic locations achieved significant difference.
The two-way interaction effects analysis showed:RF transmission and b values obtained the significant two-way interaction effects when considering all anatomic locations together. At b value of0/100, measured ADC values on dual-source parallel RF transmission images were significantly lower than on the conventional single-source RF transmission images (P<0.0001). At b value of0/100/800and100/800, measured ADC values on dual-source parallel RF transmission images were significantly higher than on the conventional single-source RF transmission images (P value was0.007,0.031, respectively). At b value of0/500and0/800, measured ADC values between dual-source parallel RF transmission images and conventional single-source RF transmission images showed no significant difference (P value was0.437,0.236, respectively). Additionally, RF transmission and anatomic locations also obtained the significant two-way interaction effects when considering all b values together. Among all of the anatomic locations, only measured ADC values of spleen on dual-source parallel RF transmission images were significantly lower than on conventional single-source RF transmission images (P<0.0001). Measured ADC values of other anatomic locations showed no significant difference between dual-source parallel RF transmission images and conventional single-source RF transmission images.
The three-way interaction effects analysis showed:RF transmission, b values and anatomic locations obtained the significant three-way interaction effects. Thus, the simple effect was analyzed and the results were as follow:for lateral left hepatic lobe, the measured ADC values of the dual-source parallel RF transmission images were significantly higher than the conventional single-source RF transmission image at b=0/800(P<0.001),100/800(P=0.001),0/100/800(P=0.001) and lower than the conventional single-source RF transmission image at b=0/100(P<0.001); for spleen, the measured ADC values of the dual-source parallel RF transmission images were significantly lower than the conventional single-source RF transmission image at b=0/100(P<0.001),0/500(P=0.047),0/800(P=0.012); for posterior right hepatic lobe and pancreatic tail, there was no significant difference in the measured ADC values between the dual-source parallel RF transmission images and the conventional single-source RF transmission images at any b value.
Combining with the simple effect analysis and the two-way interaction effect analysis between RF transmission and b value, it showed that there was no significant difference on ADC measurements between the parallel and the conventional RF transmission images for all organs at b=0/500, except a slight statistically significant differences for spleen (P=0.047).
Conclusion
1. Comparing with the conventional single-source RF transmission, the dual-source parallel RF transmission significantly improved the image quality and the homogeneity of the RF field.
2. Comparing with the conventional single-source RF transmission, the dual-source parallel RF transmission improved the SNR for all organs at each b-value, of which only the lateral left hepatic lobe achieved the significant difference.
3. At either lower or higher b value, the measured ADC values of lateral left hepatic lobe and spleen between the dual-source parallel RF transmission and the conventional single-source RF transmission were significantly different.
4. At b value of0/500, the minimum difference on ADC measurements between the dual-source parallel RF transmission images and the conventional single-source RF transmission images was achieved for all abdominal organs.
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20. Boulanger Y, Amara M, Lepanto L, et al. Diffusion-weighted MR imaging of the liver of hepatitis C patients. NMR Biomed 2003; 16:132-136.
21. Cui XY, Chen HW. Role of diffusion-weighted magnetic resonance imaging in the diagnosis of extrahepatic cholangiocarcinoma. World J Gastroenterol 2010; 16:3196- 3201.
22. Lee SS, Byun JH, Park BJ, et al. Quantitative analysis of diffusion-weighted magnetic resonance imaging of the pancreas:usefulness in characterizing solid pancreatic masses. J Magn Reson Imaging 2008; 28:928-936.
23. Tsushima Y, Takahashi-Taketomi A, Endo K. Diagnostic utility of diffusion-weighted MR imaging and apparent diffusion coefficient value for the diagnosis of adrenal tumors. J Magn Reson Imaging 2009; 29:112-117.
24. Thoeny HC, Binser T, Roth B, et al. Noninvasive assessment of acute ureteral obstruction with diffusion-weighted MR imaging:a prospective study 1. Radiology 2009; 252:721-728.
25. Merkle EM, Dale BM, Paulson EK. Abdominal MR imaging at 3 T. Magn Reson Imaging Clin N Am 2006; 14:17-26.
26. Ramalho M, Altun E, Heredia V, Zapparoli M, Semelka R. Liver MR imaging:1.5 T versus 3 T. Magn Reson Imaging Clin N Am 2007; 15:321-347.
27. Krautmacher C, Willinek WA, Tschampa HJ, et al. Brain tumors:full- and half-dose contrast-enhanced MR imaging at 3.0 T compared with 1.5 T—initial experience. Radiology 2005; 237:1014-1019.
28. Kuo PH, Kanal E, Abu-Alfa AK, et al. Gadolinium-based MR contrast agents and nephrogenic systemic fibrosis. Radiology 2007; 242:647-649.
29. Collins CM, Liu W, Schreiber W, Yang QX, Smith MB. Central brightening due to constructive interference with, without, and despite dielectric resonance. J Magn Reson Imaging 2005; 21:192-196.
30. Lewin JS, Duerk JL, Jain VR, Petersilge CA, Chao CP, Haaga JR. Needle localization in MR-guided biopsy and aspiration:effects of feld strength, sequence design, and magnetic feld orientation. AJR Am J Roentgenol 1996; 166:1337-1345.
31. Akisik FM, Sandrasegaran K, Aisen AM, Lin C, Lall C. Abdominal MR imaging at 3.0 T. RadioGraphics 2007; 27:1433-1444; discussion 1462-1464.
32. Katscher U, Bornert P. Parallel RF transmission in MRI. NMR Biomed 2006; 19: 393-400.
33. Willinek WA, Gieseke J, Kukuk GM, et al. Dual-source parallel radiofrequency excitation body MR imaging compared with standard MR imaging at 3.0 T:initial clinical experience. Radiology 2010; 256:966-975.
34. Dow-Mu Koh, David J. Collins. Diffusion-Weighted MRI in the Body: Applications and Challenges in Oncology. AJR Am J Roentgenol 2007; 188:1622-1635.
35. Thoeny HC, De Keyzer F. Extracranial applications of diffusion-weighted magnetic resonance imaging. Eur Radiol 2007; 17:1385-1393.
36. Peng-Ju Xu, Fu-Hua Yan, Jian-Hua Wang, Jiang Lin and Yuan Ji. Added Value of Breathhold Diffusion-Weighted MR in Detection of Small Hepatocellular Carcinoma Lesions Compared With Dynamic Contrast-Enhanced MRI Alone Using Receiver Operating Characteristic Curve Analysis. J Magn Reson Imaging 2009; 29:341-349.
37. Vandecaveye V, De Keyzer F, Verslype C, et al. Diffusion-weighted MRI provides additional value to conventional dynamic contrast-enhanced MRI for detection of hepatocellular carcinoma. Eur Radiol 2009; 19:2456-2466.
38. Andrew B. Rosenkrantz, Marcel Oei, James S. Babb, Benjamin E. Niver and Bachir Taouli. Diffusion-Weighted Imaging of the Abdomen at 3.0 Tesla:Image Quality and Apparent Diffusion Coefficient Reproducibility Compared With 1.5 Tesla. J Magn Reson Imaging 2011; 33:128-135.
39. Hua Wang, Xiao-Ying Wang, Xue-Xiang Jiang, Zhao-Xiang Ye. Comparison of Diffusion-Weighted with T2-weighted Imaging for Detection of Small Hepatocellular Carcinoma in Cirrhosis:Preliminary Quantitative Study at 3-T. Acad Radiol 2010; 17:239-243.
40. Chiu FY, Jao JC, Chen CY, et al. Effect of intravenous gadolinium-DTPA on diffusion-weighted magnetic resonance images for evaluation of focal hepatic lesions. J Comput Assist Tomogr 2005; 29:176-180.
41. Lu TL, Meuli RA, Marques-Vidal PM, Bize P, Denys A, Schmidt S. Interobserver and intraobserver variability of the apparent diffusion coefficient in treated malignant hepatic lesions on a 3.0T machine:measurements in the whole lesion versus in the area with the most restricted diffusion. J Magn Reson Imaging 2010; 32:647-653.
42. Soher BJ, Dale BM, Merkle EM. A review of MR physics:3 T versus 1.5 T. Magn Reson Imaging Clin N Am 2007; 15:277-290.
43. Franklin KM, Dale BM, Merkle EM. Improvement in Bl-inhomogeneity artifacts in the abdomen at 3 T MR imaging using a radiofrequency cushion. J Magn Reson Imaging 2008; 27:1443-1447.
44. Chang KJ, Kamel IR, Macura KJ, Bluemke DA.3.0-T MR Imaging of the Abdomen:Comparison with 1.5 T. Radiographics 2008; 28:1983-1998.
45. Heverhagen JT. Noise measurement and estimation in MR imaging experiments. Radiology 2007; 245:638-639.
46. Moon WJ. Measurement of signal-to-noise ratio in MR imaging with sensitivity encoding. Radiology 2007; 243:908-909.
47. Kukuk GM, Gieseke J, Weber S, et al. Focal Liver Lesions at 3.0 T:Lesion Detectability and Image Quality with T2-weighted Imaging by Using Conventional and Dual-Source Parallel Radiofrequency Transmission. Radiology 2011; 259:421-428.
48. Kurihara Y, Yokushiji YK, Tani I, Nakajima Y, Van Cauteren M. Coil sensitivity encoding in MR imaging:advantages and disadvantages in clinical practice. AJR 2002; 178:1087-1091.
49. Bachir Taouli, Dow-Mu Koh. Diffusion-weighted MR Imaging of the Liver. Radiology 2010; 254:47-66.
50. Harsh Kandpal, Raju Sharma, K. S. Madhusudhan, Kulwant Singh Kapoor. Respiratory-Triggered Versus Breath-Hold Diffusion-Weighted MRI of Liver Lesions: Comparison of Image Quality and Apparent Diffusion Coeffcient Values. AJR 2009; 192:915-922.
51. Taouli B, Sandberg A, Stemmer A, et al. Diffusion-weighted imaging of the liver: comparison of navigator triggered and breathhold acquisitions. J Magn Reson Imaging 2009; 30:561-568.
52. Farhood Saremi, Mehdi Jalili, Sepideh Sefidbakht, et al. Diffusion-Weighted Imaging of the Abdomen at 3 T:Image Quality Comparison With 1.5-T Magnet Using 3 Different Imaging Sequences. J Comput Assist Tomogr 2011; 35:317-325.
1. Gourtsoyianni S, Papanikolaou N, Yarmenitis S, et al. Respiratory gated diffusion-weighted imaging of the liver:value of apparent diffusion coefficient measurements in the differentiation between most commonly encountered benign and malignant focal liver lesions. Eur Radiol 2008; 18:486-492.
2. Bruegel M, Holzapfel K, Gaa J, et al. Characterization of focal liver lesions by ADC measurements using a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging technique. Eur Radiol 2008; 18:477-485.
3. Koh DM, Scurr E, Collins D, et al. Predicting response of colorectal hepatic metastasis:value of pretreatment apparent diffusion coefficients. AJR 2007; 188:1001-1008.
4. Cui Y, Zhang XP, Sun YS, et al. Apparent diffusion coefficient:potential imaging biomarker for prediction and early detection of response to chemotherapy in hepatic metastases. Radiology 2008; 248:894-900.
5. Papanikolaou N, Gourtsoyianni S, Yarmenitis S, Maris T, Gourtsoyiannis N. Comparison between two-point and four-point methods for quantification of apparent diffusion coefficient of normal liver parenchyma and focal lesions:value of normalization with spleen. Eur J Radiol 2010; 73:305-309.
6. Do RK, Chandarana H, Felker E, et al. Diagnosis of liver fibrosis and cirrhosis with diffusion-weighted imaging:value of normalized apparent diffusion coefficient using the spleen as reference organ. AJR Am J Roentgenol 2010; 195:671-676.
7. Braithwaite AC, Dale BM, Boll DT, et al. Short- and midterm reproducibility of apparent diffusion coefficient measurements at 3.0-T diffusion-weighted imaging of the abdomen. Radiology 2009; 250:459-465.
8. Sasaki M, Yamada K, Watanabe Y, et al. Variability in absolute apparent diffusion coefficient values across different platforms may be substantial:a multivendor, multi-institutional comparison study. Radiology 2008; 249:624-630.
9. Kwee TC, Takahara T, Koh DM, Nievelstein RA, Luijten PR. Comparison and Reproducibility of ADC Measurements in Breathhold, Respiratory Triggered, and Free-Breathing Diffusion-Weighted MR Imaging of the Liver. J Magn Reson Imaging 2008; 28:1141-1148.
10. Bilgili MY. Reproductibility of apparent diffusion coefficients measurements in diffusion-weighted MRI of the abdomen with different b values. Eur J Radiol 2011 Jul 1. [Epub ahead of print].
11. Dale BM, Braithwaite AC, Boll DT, Merkle EM. Field strength and diffusion encoding technique affect the apparent diffusion coefficient measurements in diffusion-weighted imaging of the abdomen. Invest Radiol 2010; 45:104-108.
12. Nasu K, Kuroki Y, Fujii H, Minami M. Hepatic pseudo-anisotropy:a specific artifact in hepatic diffusion-weighted images obtained with respiratory triggering. MAGMA 2007; 20:205-211.
13. Dietrich O, Heiland S, Sartor K. Noise correction for the exact determination of apparent diffusion coefficients at low SNR. Magn Reson Med 2001; 45:448-453.
14. Andrew B. Rosenkrantz, Marcel Oei, James S. Babb, Benjamin E. Niver and Bachir Taouli. Diffusion-Weighted Imaging of the Abdomen at 3.0 Tesla:Image Quality and Apparent Diffusion Coefficient Reproducibility Compared With 1.5 Tesla. J Magn Reson Imaging 2011; 33:128-135.
15. Farhood Saremi, Mehdi Jalili, Sepideh Sefidbakht, et al. Diffusion-Weighted Imaging of the Abdomen at 3 T:Image Quality Comparison With 1.5-T Magnet Using 3 Different Imaging Sequences. J Comput Assist Tomogr 2011; 35:317-325.
16. Katscher U, Bornert P. Parallel RF transmission in MRI. NMR Biomed 2006; 19: 393-400.
17. Willinek WA, Gieseke J, Kukuk GM, et al. Dual-source parallel radiofrequency excitation body MR imaging compared with standard MR imaging at 3.0 T:initial clinical experience. Radiology 2010; 256:966-975.
18. Kukuk GM, Gieseke J, Weber S, et al. Focal Liver Lesions at 3.0 T:Lesion Detectability and Image Quality with T2-weighted Imaging by Using Conventional and Dual-Source Parallel Radiofrequency Transmission. Radiology 2011; 259:421-428.
19. Schaefer DJ. Safety aspects of radio-frequency power deposition in magnetic resonance.Magn Reson Imaging Clin N Am 1998; 6:775-789.
20. Shellock FG. Radiofrequency-induced heating during MR procedures:a review. J Magn Reson Imaging 2000; 12:30-36.
21. Mitchell DG, Crovello M, Matteucci T, Petersen RO, Miettinen MM. Benign adrenocortical masses:diagnosis with chemical shift MR imaging. Radiology 1992; 185:345-351.
22. Kim T, Murakami T, Takahashi S, Hori M, Tsuda K, Nakamura H. Diffusion-weighted single-shot echoplanar MR imaging for liver disease. AJR 1999; 173:393-398.
23. Hollingsworth KG, Lomas DJ. Influence of perfusion on hepatic MR diffusion measurement. NMR Biomed 2006; 19:231-235.
24. Pasquinelli F, Belli G, Mazzoni LN, Grazioli L, Colagrande S. Magnetic resonance diffusion-weighted imaging:quantitative evaluation of age-related changes in healthy liver parenchyma. Magn Reson Imaging 2011; 29:805-812.
2. Bammer R. Basic principles of diffusion-weighted imaging. Eur J Radiol 2003; 45: 169-184.
3. Ernst Martin-Fiori, Thierry A. G. M. Huisman. Diffusion-Weighted, Perfusion-Weighted, and Functional MR Imaging. Pediatric Neuroradiology,1st editon, Berlin, German, Springer,2005, Part 1, p1075.
4. Stejskal EO, Tanner JE. Spin diffusion measurements:spin echoes in the presence of a time-dependent field gradient. J Chem Phys 1965; 42:288-292.
5. Le Bihan D, Breton E, Lallemand D, Grenier P, Cabanis E, Laval-Jeantet M. MR imaging of intravoxel incoherent motions:application to diffusion and perfusion in neurologic disorders. Radiology 1986; 161:401-407.
6. Warach S, Chien D, Li W, et al. Fast magnetic resonance diffusion-weighted imaging of acute human stroke. Neurology 1992; 42:1717-1723.
7. Coenegrachts K, Delanote J, Ter Beek L, et al. Improved focal liver lesion detection: comparison of single-shot diffusion-weighted echo planar and single-shot T2 weighted turbo spin echo techniques. Br J Radiol 2007; 80:524-531.
8. Zech CJ, Herrmann KA, Dietrich O, et al. Black-blood diffusion-weighted EPI acquisition of the liver with parallel imaging:comparison with a standard T2-weighted sequence for detection of focal liver lesions. Invest Radiol 2008; 43:261-266.
9. Bruegel M, Gaa J, Waldt S, et al. Diagnosis of hepatic metastasis:comparison of respiration-triggered diffusion-weighted echo-planar MRI and five t2-weighted turbo spin-echo sequences. AJR Am J Roentgenol 2008; 191:1421-1429.
10. Hussain SM, De Becker J, Hop WC, et al. Can a single-shot black-blood T2-weighted spin-echo echo-planar imaging sequence with sensitivity encoding replace the respiratory-triggered turbo spin-echo sequence for the liver? An optimization and feasibility study. J Magn Reson Imaging 2005; 21:219-229.
11. Parikh T, Drew SJ, Lee VS, et al. Focal liver lesion detection and characterization with diffusion-weighted MR imaging:comparison with standard breath-hold T2-weighted imaging. Radiology 2008; 246:812-822.
12. van den Bos IC, Hussain SM, Krestin GP, et al. Liver imaging at 3.0 T: diffusion-induced black-blood echo-planar imaging with large anatomic volumetric coverage as an alternative for specific absorption rate -intensive echo-train spin-echo sequences:feasibility study. Radiology 2008; 248:264-271.
13. Bruegel M, Holzapfel K, Gaa J, et al. Characterization of focal liver lesions by ADC measurements using a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging technique. Eur Radiol 2008; 18:477-485.
14. Gourtsoyianni S, Papanikolaou N, Yarmenitis S, et al. Respiratory gated diffusion-weighted imaging of the liver:value of apparent diffusion coefficient measurements in the differentiation between most commonly encountered benign and malignant focal liver lesions. Eur Radiol 2008; 18:486-492.
15. Taouli B, Vilgrain V, Dumont E, et al. Evaluation of liver diffusion isotropy and characterization of focal hepatic lesions with two single-shot echo-planar MR imaging sequences:prospective study in 66 patients. Radiology 2003; 226:71-78.
16. Koinuma M, Ohashi I, Hanafusa K, et al. Apparent diffusion coefficient measurements with diffusion-weighted magnetic resonance imaging for evaluation of hepatic fibrosis. J Magn Reson Imaging 2005; 22:80-85.
17. Taouli B, Tolia AJ, Losada M, et al. Diffusion-weighted MRI for quantification of liver fibrosis:preliminary experience. AJR Am J Roentgenol 2007; 189:799-806.
18. Aube C, Racineux PX, Lebigot J, et al. Diagnosisand quantification of hepatic fibrosis with diffusion-weighted MR imaging:preliminary results. J Radiol 2004; 85: 301-306.
19. Low RN, Gurney J. Diffusion-weighted MRI (DWI) in the oncology patient: value of breathhold DWI compared to unenhanced and gadolinium-enhanced MRI. J Magn Reson Imaging 2007; 25:848-858.
20. Boulanger Y, Amara M, Lepanto L, et al. Diffusion-weighted MR imaging of the liver of hepatitis C patients. NMR Biomed 2003; 16:132-136.
21. Cui XY, Chen HW. Role of diffusion-weighted magnetic resonance imaging in the diagnosis of extrahepatic cholangiocarcinoma. World J Gastroenterol 2010; 16:3196- 3201.
22. Lee SS, Byun JH, Park BJ, et al. Quantitative analysis of diffusion-weighted magnetic resonance imaging of the pancreas:usefulness in characterizing solid pancreatic masses. J Magn Reson Imaging 2008; 28:928-936.
23. Tsushima Y, Takahashi-Taketomi A, Endo K. Diagnostic utility of diffusion-weighted MR imaging and apparent diffusion coefficient value for the diagnosis of adrenal tumors. J Magn Reson Imaging 2009; 29:112-117.
24. Thoeny HC, Binser T, Roth B, et al. Noninvasive assessment of acute ureteral obstruction with diffusion-weighted MR imaging:a prospective study 1. Radiology 2009; 252:721-728.
25. Merkle EM, Dale BM, Paulson EK. Abdominal MR imaging at 3 T. Magn Reson Imaging Clin N Am 2006; 14:17-26.
26. Ramalho M, Altun E, Heredia V, Zapparoli M, Semelka R. Liver MR imaging:1.5 T versus 3 T. Magn Reson Imaging Clin N Am 2007; 15:321-347.
27. Krautmacher C, Willinek WA, Tschampa HJ, et al. Brain tumors:full- and half-dose contrast-enhanced MR imaging at 3.0 T compared with 1.5 T—initial experience. Radiology 2005; 237:1014-1019.
28. Kuo PH, Kanal E, Abu-Alfa AK, et al. Gadolinium-based MR contrast agents and nephrogenic systemic fibrosis. Radiology 2007; 242:647-649.
29. Collins CM, Liu W, Schreiber W, Yang QX, Smith MB. Central brightening due to constructive interference with, without, and despite dielectric resonance. J Magn Reson Imaging 2005; 21:192-196.
30. Lewin JS, Duerk JL, Jain VR, Petersilge CA, Chao CP, Haaga JR. Needle localization in MR-guided biopsy and aspiration:effects of feld strength, sequence design, and magnetic feld orientation. AJR Am J Roentgenol 1996; 166:1337-1345.
31. Akisik FM, Sandrasegaran K, Aisen AM, Lin C, Lall C. Abdominal MR imaging at 3.0 T. RadioGraphics 2007; 27:1433-1444; discussion 1462-1464.
32. Katscher U, Bornert P. Parallel RF transmission in MRI. NMR Biomed 2006; 19: 393-400.
33. Willinek WA, Gieseke J, Kukuk GM, et al. Dual-source parallel radiofrequency excitation body MR imaging compared with standard MR imaging at 3.0 T:initial clinical experience. Radiology 2010; 256:966-975.
34. Dow-Mu Koh, David J. Collins. Diffusion-Weighted MRI in the Body: Applications and Challenges in Oncology. AJR Am J Roentgenol 2007; 188:1622-1635.
35. Thoeny HC, De Keyzer F. Extracranial applications of diffusion-weighted magnetic resonance imaging. Eur Radiol 2007; 17:1385-1393.
36. Peng-Ju Xu, Fu-Hua Yan, Jian-Hua Wang, Jiang Lin and Yuan Ji. Added Value of Breathhold Diffusion-Weighted MR in Detection of Small Hepatocellular Carcinoma Lesions Compared With Dynamic Contrast-Enhanced MRI Alone Using Receiver Operating Characteristic Curve Analysis. J Magn Reson Imaging 2009; 29:341-349.
37. Vandecaveye V, De Keyzer F, Verslype C, et al. Diffusion-weighted MRI provides additional value to conventional dynamic contrast-enhanced MRI for detection of hepatocellular carcinoma. Eur Radiol 2009; 19:2456-2466.
38. Andrew B. Rosenkrantz, Marcel Oei, James S. Babb, Benjamin E. Niver and Bachir Taouli. Diffusion-Weighted Imaging of the Abdomen at 3.0 Tesla:Image Quality and Apparent Diffusion Coefficient Reproducibility Compared With 1.5 Tesla. J Magn Reson Imaging 2011; 33:128-135.
39. Hua Wang, Xiao-Ying Wang, Xue-Xiang Jiang, Zhao-Xiang Ye. Comparison of Diffusion-Weighted with T2-weighted Imaging for Detection of Small Hepatocellular Carcinoma in Cirrhosis:Preliminary Quantitative Study at 3-T. Acad Radiol 2010; 17:239-243.
40. Chiu FY, Jao JC, Chen CY, et al. Effect of intravenous gadolinium-DTPA on diffusion-weighted magnetic resonance images for evaluation of focal hepatic lesions. J Comput Assist Tomogr 2005; 29:176-180.
41. Lu TL, Meuli RA, Marques-Vidal PM, Bize P, Denys A, Schmidt S. Interobserver and intraobserver variability of the apparent diffusion coefficient in treated malignant hepatic lesions on a 3.0T machine:measurements in the whole lesion versus in the area with the most restricted diffusion. J Magn Reson Imaging 2010; 32:647-653.
42. Soher BJ, Dale BM, Merkle EM. A review of MR physics:3 T versus 1.5 T. Magn Reson Imaging Clin N Am 2007; 15:277-290.
43. Franklin KM, Dale BM, Merkle EM. Improvement in Bl-inhomogeneity artifacts in the abdomen at 3 T MR imaging using a radiofrequency cushion. J Magn Reson Imaging 2008; 27:1443-1447.
44. Chang KJ, Kamel IR, Macura KJ, Bluemke DA.3.0-T MR Imaging of the Abdomen:Comparison with 1.5 T. Radiographics 2008; 28:1983-1998.
45. Heverhagen JT. Noise measurement and estimation in MR imaging experiments. Radiology 2007; 245:638-639.
46. Moon WJ. Measurement of signal-to-noise ratio in MR imaging with sensitivity encoding. Radiology 2007; 243:908-909.
47. Kukuk GM, Gieseke J, Weber S, et al. Focal Liver Lesions at 3.0 T:Lesion Detectability and Image Quality with T2-weighted Imaging by Using Conventional and Dual-Source Parallel Radiofrequency Transmission. Radiology 2011; 259:421-428.
48. Kurihara Y, Yokushiji YK, Tani I, Nakajima Y, Van Cauteren M. Coil sensitivity encoding in MR imaging:advantages and disadvantages in clinical practice. AJR 2002; 178:1087-1091.
49. Bachir Taouli, Dow-Mu Koh. Diffusion-weighted MR Imaging of the Liver. Radiology 2010; 254:47-66.
50. Harsh Kandpal, Raju Sharma, K. S. Madhusudhan, Kulwant Singh Kapoor. Respiratory-Triggered Versus Breath-Hold Diffusion-Weighted MRI of Liver Lesions: Comparison of Image Quality and Apparent Diffusion Coeffcient Values. AJR 2009; 192:915-922.
51. Taouli B, Sandberg A, Stemmer A, et al. Diffusion-weighted imaging of the liver: comparison of navigator triggered and breathhold acquisitions. J Magn Reson Imaging 2009; 30:561-568.
52. Farhood Saremi, Mehdi Jalili, Sepideh Sefidbakht, et al. Diffusion-Weighted Imaging of the Abdomen at 3 T:Image Quality Comparison With 1.5-T Magnet Using 3 Different Imaging Sequences. J Comput Assist Tomogr 2011; 35:317-325.
1. Gourtsoyianni S, Papanikolaou N, Yarmenitis S, et al. Respiratory gated diffusion-weighted imaging of the liver:value of apparent diffusion coefficient measurements in the differentiation between most commonly encountered benign and malignant focal liver lesions. Eur Radiol 2008; 18:486-492.
2. Bruegel M, Holzapfel K, Gaa J, et al. Characterization of focal liver lesions by ADC measurements using a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging technique. Eur Radiol 2008; 18:477-485.
3. Koh DM, Scurr E, Collins D, et al. Predicting response of colorectal hepatic metastasis:value of pretreatment apparent diffusion coefficients. AJR 2007; 188:1001-1008.
4. Cui Y, Zhang XP, Sun YS, et al. Apparent diffusion coefficient:potential imaging biomarker for prediction and early detection of response to chemotherapy in hepatic metastases. Radiology 2008; 248:894-900.
5. Papanikolaou N, Gourtsoyianni S, Yarmenitis S, Maris T, Gourtsoyiannis N. Comparison between two-point and four-point methods for quantification of apparent diffusion coefficient of normal liver parenchyma and focal lesions:value of normalization with spleen. Eur J Radiol 2010; 73:305-309.
6. Do RK, Chandarana H, Felker E, et al. Diagnosis of liver fibrosis and cirrhosis with diffusion-weighted imaging:value of normalized apparent diffusion coefficient using the spleen as reference organ. AJR Am J Roentgenol 2010; 195:671-676.
7. Braithwaite AC, Dale BM, Boll DT, et al. Short- and midterm reproducibility of apparent diffusion coefficient measurements at 3.0-T diffusion-weighted imaging of the abdomen. Radiology 2009; 250:459-465.
8. Sasaki M, Yamada K, Watanabe Y, et al. Variability in absolute apparent diffusion coefficient values across different platforms may be substantial:a multivendor, multi-institutional comparison study. Radiology 2008; 249:624-630.
9. Kwee TC, Takahara T, Koh DM, Nievelstein RA, Luijten PR. Comparison and Reproducibility of ADC Measurements in Breathhold, Respiratory Triggered, and Free-Breathing Diffusion-Weighted MR Imaging of the Liver. J Magn Reson Imaging 2008; 28:1141-1148.
10. Bilgili MY. Reproductibility of apparent diffusion coefficients measurements in diffusion-weighted MRI of the abdomen with different b values. Eur J Radiol 2011 Jul 1. [Epub ahead of print].
11. Dale BM, Braithwaite AC, Boll DT, Merkle EM. Field strength and diffusion encoding technique affect the apparent diffusion coefficient measurements in diffusion-weighted imaging of the abdomen. Invest Radiol 2010; 45:104-108.
12. Nasu K, Kuroki Y, Fujii H, Minami M. Hepatic pseudo-anisotropy:a specific artifact in hepatic diffusion-weighted images obtained with respiratory triggering. MAGMA 2007; 20:205-211.
13. Dietrich O, Heiland S, Sartor K. Noise correction for the exact determination of apparent diffusion coefficients at low SNR. Magn Reson Med 2001; 45:448-453.
14. Andrew B. Rosenkrantz, Marcel Oei, James S. Babb, Benjamin E. Niver and Bachir Taouli. Diffusion-Weighted Imaging of the Abdomen at 3.0 Tesla:Image Quality and Apparent Diffusion Coefficient Reproducibility Compared With 1.5 Tesla. J Magn Reson Imaging 2011; 33:128-135.
15. Farhood Saremi, Mehdi Jalili, Sepideh Sefidbakht, et al. Diffusion-Weighted Imaging of the Abdomen at 3 T:Image Quality Comparison With 1.5-T Magnet Using 3 Different Imaging Sequences. J Comput Assist Tomogr 2011; 35:317-325.
16. Katscher U, Bornert P. Parallel RF transmission in MRI. NMR Biomed 2006; 19: 393-400.
17. Willinek WA, Gieseke J, Kukuk GM, et al. Dual-source parallel radiofrequency excitation body MR imaging compared with standard MR imaging at 3.0 T:initial clinical experience. Radiology 2010; 256:966-975.
18. Kukuk GM, Gieseke J, Weber S, et al. Focal Liver Lesions at 3.0 T:Lesion Detectability and Image Quality with T2-weighted Imaging by Using Conventional and Dual-Source Parallel Radiofrequency Transmission. Radiology 2011; 259:421-428.
19. Schaefer DJ. Safety aspects of radio-frequency power deposition in magnetic resonance.Magn Reson Imaging Clin N Am 1998; 6:775-789.
20. Shellock FG. Radiofrequency-induced heating during MR procedures:a review. J Magn Reson Imaging 2000; 12:30-36.
21. Mitchell DG, Crovello M, Matteucci T, Petersen RO, Miettinen MM. Benign adrenocortical masses:diagnosis with chemical shift MR imaging. Radiology 1992; 185:345-351.
22. Kim T, Murakami T, Takahashi S, Hori M, Tsuda K, Nakamura H. Diffusion-weighted single-shot echoplanar MR imaging for liver disease. AJR 1999; 173:393-398.
23. Hollingsworth KG, Lomas DJ. Influence of perfusion on hepatic MR diffusion measurement. NMR Biomed 2006; 19:231-235.
24. Pasquinelli F, Belli G, Mazzoni LN, Grazioli L, Colagrande S. Magnetic resonance diffusion-weighted imaging:quantitative evaluation of age-related changes in healthy liver parenchyma. Magn Reson Imaging 2011; 29:805-812.
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