QTZ500塔式起重机——臂架优化设计【cad高清图纸和说明书全套】.zip

河北建筑工程学院 毕业设计（论文）外文资料翻译 系别： 机械工程系 专业： 机械设计制造及其自动化 班级： 机 053 班 姓名： 张勇杰 学号： 22 号 外 文 出 处 ： European Journal of Radiology 附 件 ： 1、 外 文 原 文 ； 2、 外 文 资 料 翻 译 译 文 。 指导教师评语： 签字： 年 月 日 MR imaging at high magnetic fields Masaya Takahashi a, *, Hidemasa Uematsu b , Hiroto Hatabu a a Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA b Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, PA, USA Received 12 November 2002; received in revised form 13 November 2002; accepted 14 November 2002 Abstract Recently, more investigators have been applying higher magnetic field strengths (3C1/4 Tesla) in research and clinical settings. Higher magnetic field strength is expected to afford higher spatial resolution and/or a decrease in the length of total scan time due to its higher signal intensity. Besides MR signal intensity, however, there are several factors which are magnetic field dependent, thus the same set of imaging parameters at lower magnetic field strengths would provide differences in signal or contrast to noise ratios at 3 T or higher. Therefore, an outcome of the combined effect of all these factors should be considered to estimate the change in usefulness at different magnetic fields. The objective of this article is to illustrate the practical scientific applications, focusing on MR imaging, of higher magnetic field strength. First, we will discuss previous literature and our experiments to demonstrate several changes that lead to a number of practical applications in MR imaging, e.g. in relaxation times, effects of contrast agent, design of RF coils, maintaining a safety profile and in switching magnetic field strength. Second, we discuss what will be required to gain the maximum benefit of high magnetic field when the current magnetic field (5/1.5 T) is switched to 3 or 4 T. In addition, we discuss MR microscopy, which is one of the anticipated applications of high magnetic field strength to understand the quantitative estimation of the gain benefit and other considerations to help establish a practically available imaging protocol. # 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Magnetic resonance imaging; Higher magnetic field strength; Contrast agent 1. Introduction Thanks to recent technological development, whole- body magnetic resonance (MR) scanners at higher magnetic field strengths (]/3T)have been introduced into research and clinical settings. In the beginning, one of the main reasons to install higher fields was its higher sensitivity to the blood oxygenation level-dependent effect for functional MR imaging of the brain [1]. Recently, more investigators applied these higher mag- netic field strengths to both research and conventional clinical settings. The expectation for higher magnetic fields in MRI is the improvement in signal-to-noise ratio (SNR) due to higher signal intensity (SI), where