Ugurbil Kamil
IEEE Trans Biomed Eng. 2014 May;61(5):1364-79. doi: 10.1109/TBME.2014.2313619. Epub 2014 Mar 25.
Since the introduction of 4 T human systems in three academic laboratories circa 1990, rapid progress in imaging and spectroscopy studies in humans at 4 T and animal model systems at 9.4 T have led to the introduction of 7 T and higher magnetic fields for human investigation at about the turn of the century. Work conducted on these platforms has demonstrated the existence of significant advantages in SNR and biological information content at these ultrahigh fields, as well as the presence of numerous challenges. Primary difference from lower fields is the deviation from the near field regime; at the frequencies corresponding to hydrogen resonance conditions at ultrahigh fields, the RF is characterized by attenuated traveling waves in the human body, which leads to image nonuniformities for a given sample-coil configuration because of interferences. These nonuniformities were considered detrimental to the progress of imaging at high field strengths. However, they are advantageous for parallel imaging for signal reception and parallel transmission, two critical technologies that account, to a large extend, for the success of ultrahigh fields. With these technologies, and improvements in instrumentation and imaging methods, ultrahigh fields have provided unprecedented gains in imaging of brain function and anatomy, and started to make inroads into investigation of the human torso and extremities. As extensive as they are, these gains still constitute a prelude to what is to come given the increasingly larger effort committed to ultrahigh field research and development of ever better instrumentation and techniques.
自1990年左右三个学术实验室引入4T人体系统以来,4T人体成像与光谱研究以及9.4T动物模型系统研究的迅速进展,促使在世纪之交引入7T及更高磁场用于人体研究。在这些平台上开展的工作已证明,在这些超高场强下,信噪比和生物信息含量存在显著优势,同时也存在诸多挑战。与低场强的主要区别在于偏离了近场状态;在超高场强下与氢共振条件对应的频率下,射频在人体中表现为衰减行波,这会导致在给定样本 - 线圈配置下由于干扰而产生图像不均匀性。这些不均匀性曾被认为对高场强成像的进展不利。然而,它们有利于信号接收和并行传输的并行成像,这两项关键技术在很大程度上促成了超高场强的成功。借助这些技术以及仪器和成像方法的改进,超高场强在脑功能和解剖结构成像方面取得了前所未有的进展,并开始涉足人体躯干和四肢的研究。尽管进展广泛,但鉴于对超高场研究以及开发越来越好的仪器和技术投入了越来越大的努力,这些进展仍只是未来发展的前奏。