低场三维超短回波时间磁共振成像的优化  

Optimization of Three-dimensional Ultrashort Echo Time Magnetic Resonance Imaging at a Low Field

作　　者：黄预立 杜一平[1] 

机构地区：[1]浙江大学生物医学工程与仪器科学学院,杭州310027

出　　处：《生物医学工程学杂志》2015年第6期1239-1243,共5页Journal of Biomedical Engineering

摘　　要：传统磁共振序列回波时间至少为1ms,能够形成良好的软组织对比度。然而一些短T2组织由于衰减太快,在传统磁共振图像中呈现暗信号。近年来,超短回波时间序列因其对短T2组织的成像能力成为了磁共振领域的研究热点,绝大多数研究是在高场下进行的,但在发展中国家仍有大量的低场磁共振系统在使用。本文在0.35T永磁磁共振系统中实现了三维超短回波时间序列,研究采样与读出梯度间的延时、参与计算R2*图像的回波时间和欠采样率对结果的影响。通过改变成像和重建参数发现,采样与读出梯度间的微小延时会对原始图像产生较大的影响;用不同时刻的回波图像计算出的R2*图像也有较大的差别;而过度的欠采样会直接影响对短T2组织的显示。上述结果显示了在低场下进行三维超短回波时间成像时选择合适成像参数的重要性,通过选择适当的参数,短T2组织可以在低场下得到成像。Conventional magnetic resonance （MR） pulse sequences typically have an echo time （TE） of 1 ms or lon- ger, providing an excellent contrast between different soft tissues. However, some short T2 tissues appear dark in conventional MR images because the signal from these tissues has decayed to nearly zero before the center of k space is acquired. Because of the ability of directly imaging short T2 tissues, ultrashort echo time technique has been wide- ly studied in recent years. An overwhelming majority of the studies were carried out at high fields, while many low- field scanner systems are still used in developing countries. To investigate the effects of the delay between analog-to- digital converter sampling and the readout gradient, the TE of the second echo used to calculate the R2 ＊ map, and the undersampling ratio on the results of three-dimensional （3D） ultrashort echo time imaging at a low field, we im- plemented a 3D ultrashort echo time sequence on a 0.35T scanner. Different parameters were used and the recon structed images and R2 ~ maps were compared. Images reconstructed with slightly varying delays appeared quite dif- ferent. Different contrast between short and long T2 tissues were found in R2 ~ maps calculated with different echoes. The result of undersampling study indicated that excessive undersampling could cause unwanted blurring, mak- ing it difficult to better visualize the short T2 tissues in the R2 ＊ map. The results suggested that cautions should be taken in the choice of these parameters in 3D ultrashort echo time imaging. Short T2 tissues can be visualized with appropriate imaging parameters at this low field.

关 键 词：磁共振成像 超短回波时间 短T2组织 

分 类 号：R445.2[医药卫生—影像医学与核医学]