本文探讨1.5 T磁共振化学交换饱和转移（Chemical Exchange Saturation Transfer，CEST）成像的影响因素.通过试管模型和临床病例，采用GE Signa HDe 1.5 T磁共振成像（Magnetic Resonance Imaging，MRI）扫描仪分别进行不同矩阵、激励次数、翻转角、磁化传递翻转角的CEST成像对比分析，以及不同激励次数、磁化传递翻转角的Z谱分析，并从成像组织、成像设备、成像技术等方面对原始图信号、酰胺质子转移（Amide Proton Transfer，APT）信号及Z谱进行分析研究.实验结果表明1.5 T MRI扫描仪的CEST图像信噪比相对较低，且磁场稳定性及均匀度影响了CEST成像的效果.在其他参数不变的情况下，降低采集矩阵和增加激励次数与翻转角可以增加原始图像信噪比.磁化传递翻转角为105°时，CEST成像效果最好.激励次数为2、磁化传递翻转角为105°时，所得数据符合组织Z谱情况.模型Z谱在磁化传递频率为-294~-194 Hz范围可显示30%谷氨酸（Glu）、碘剂（I320）、纯水（H2O）、肌酸（Cr）的信号差异，与H2O差异最大处在-244~-214 Hz.原始图像信号30% I320明显高于Glu、H2O、Cr，Cr略低于Glu，APT图Cr略低于Glu.25例脑肿瘤的APT图呈高信号、12例脑梗塞的APT图呈低信号，CEST原始图像均可区分病变区域.有12例因采集时间、患者配合情况、环境及室温等影响导致CEST成像的失败.由此得出1.5 T场强下，CEST技术受到成像组织、设备、技术等因素的影响，需要进行多方面优化.在保证磁场稳定性及均匀度的情况下，优化参数的CEST成像和Z谱成像可以区分代谢物及其浓度.
郭岗
,
蔡聪波
,
杨永贵
,
陈忠
. 1.5 T磁共振化学交换饱和转移成像的影响因素分析[J]. 波谱学杂志, 2017
, 34(3)
: 275
-282
.
DOI: 10.11938/cjmr20162553
Acquisition parameters for chemical exchange saturation transfer (CEST) imaging were optimized on a GE Signa HDe 1.5 T magnetic resonance imaging (MRI) scanner with phantoms and clinical cases. The effects of matrix size, number of averages (NEX) and flip angles on the quality of CEST images were assessed. It was shown that the signal-to-noise ratio (SNR) of the CEST images acquired on the 1.5 T scanner was relatively low, and the stability and uniformity of the B0 field affected the outcome significantly. Reducing matrix size and increasing NEX improved the SNR of the CEST images. Optimal flip angle for magnetization transfer was found to be 105°. With a NEX of 2, usable Z spectra could be obtained. The Z spectra indicated that, with the saturation pulse frequency centered at -294~-194 Hz, signal differences could be observed for 30% Glu, I320, H2O, and Cr. Maximal signal differences were observed when the saturation pulse applied at -244~-214 Hz. Amide proton transfer (APT) imaging on patients showed that 25 cases of brain tumor had high CEST signals, 12 cases of cerebral infarction had low CEST signals. It was therefore possible to differentiate brain tumor from infarction with CEST imaging. There were also 12 cases which failed due to long acquisition time, patient movements, and temperature changes in the scanner room.
[1] WARD K M, ALETRAS A H, BALABAN R S. A new class of contrast agents for MRI based on proton chemical exchange dependent saturation transfer (CEST)[J]. J Magn Reson, 2000, 143(1):79-87.
[2] HUA J, JONES C K, BLAKELEY J, et al. Quantitative description of the asymmetry in magnetization transfer effects around the water resonance in the human brain[J]. Magn Reson Med, 2007, 58(4):786-793.
[3] YANG Y G, SHEN Z W, WU R H, et al. Research of chemical exchange saturation transfer in brain[J]. Chin J Magn Reson Imaging, 2016, 7(4):81-84. 杨永贵, 沈智威, 吴仁华, 等. 脑部化学交换饱和转移成像研究[J]. 磁共振成像, 2016, 7(4):81-84.
[4] VAN ZIJL P C M, YADAV N N. Chemical exchange saturation transfer (CEST):What is in a name and what isn't?[J]. Magn Reson Med, 2011, 65(4):927-948.
[5] WANG S, JARSO S, VAN ZIJL P C, et al. Role of amide proton transfer (APT)-MRI of endogenous proteins and peptides in brain tumor imaging[M]. New York:Springer, 2014:171-181.
[6] MORI S, ELEFF S M, PILATUS U, et al. Proton NMR spectroscopy of solvent-saturable resonances:A new approach to study pH effects in Situ[J]. Magn Reson Med, 1998, 40(1):36-42.
[7] SUN P Z, WANG E, CHEUNG J S, et al. Simulation and optimization of pulsed radio frequency (RF) irradiation scheme for chemical exchange saturation transfer(CEST) MRI-demonstration of pH-weighted pulsed-amide proton CEST MRI in an animal model of acute cerebral ischemia[J]. Magn Reson Med, 2011, 66(4):1042-1048
[8] SUN P Z, ZHOU J, HUANG J, et al. Simplified quantitative description of amide proton transfer (APT) imaging during acute ischemia[J]. Magn Reson Med, 2007, 57(2):405-410.
[9] SUN P Z, ZHOU J, SUN W, et al. Detection of the ischemic penumbra using pH-weighted MRI[J]. J Cereb Blood Flow Metab, 2007, 27(6):1129-1136.
