MRF based B1+ mapping for 7T Magnetic Resonance Electrical Properties Tomography and RF pulse design

虽然在20世纪80年代早期推出的第一台临床磁共振成像（MRI）扫描仪仍然在0.2-0.5特斯拉(T)的磁场强度下工作，但今天的临床MRI扫描仪通常在1.5到3 T的磁场强度下工作。此外，所谓的超高场（UHF） MRI设备在7 T及以上的磁场强度下工作，越来越多地用于临床应用和科学目的。高场强的优势在于具有较高的信噪比，从而提高了图像的时空分辨率，加快了图像采集的速度。然而，超高频磁共振成像的一个主要挑战是空间不均匀的对比度和图像的不均匀信号。这是由射频（RF）场的空间不均匀分布引起的，特别是它们的磁性成分（B1+），它负责激发核自旋。虽然这种不均匀分布可以通过使用几个发射机（“射频线圈”）和特殊的射频脉冲来补偿，但这种方法需要对单个发射机的空间三维B1+场进行非常精确的映射。除了补偿这种不均匀性的可能性之外，B1+映射还提供了获得有关人体组织电学特性信息的高电位，即电导率（作为导电电流可能性的度量）和介电常数（作为电极化率的度量）。这些是令人感兴趣的，因为它们作为组织类型和细胞组成的功能变化很大，因此可以检测和分化肿瘤组织。然而，尽管具有潜力，“电性质断层扫描”（EPT）迄今尚未得到广泛应用。造成这一事实的原因之一是B1+野图对高信噪比的要求，以及缺乏具有足够统计意义的大型患者研究。本提案的目的是开发一种准确和精确的方法来绘制人类头部和身体的B1+场，这将受益于7 t的高信噪比。为此，将使用“磁共振指纹识别”（MRF）的原理，该原理在定量MRI中变得越来越重要，因为它承诺高编码效率。将获得的地图与现有的但存在偏差的方法进行比较，并分析新技术对导出的电性能值和对UHF身体成像中实现的空间均匀性的影响。从长远来看，开发的技术可以研究超高频身体成像的新方法，并为未来肿瘤患者的EPT研究服务。

While the first clinical magnetic resonance imaging (MRI) scanners introduced in the early 1980s still operated at a magnetic field strength of 0.2-0.5 Tesla (T), today's clinical MRI scanners typically operate at a field strength of 1.5 and 3 T. In addition, so-called ultra high field (UHF) MRI devices operating at 7 T and more are increasingly being investigated for clinical use and scientific purposes. The advantage of the high field strengths is especially the higher signal-to-noise ratio (SNR), which allows a higher spatial and temporal resolution of the images and which can accelerate the image acquisition process. However, a major challenge of UHF MRI is the spatially inhomogeneous contrast and the inhomogeneous signal of the image. These are caused by the spatially inhomogeneous distribution of the radio frequency (RF) fields, especially their magnetic component (B1+), which is responsible for the excitation of the nuclear spins. Although this inhomogeneous distribution can be compensated by using several transmitters (the "RF coils") and special RF pulses, this approach requires a very precise mapping of the spatial three-dimensional B1+ fields of the individual transmitters. Besides the possibility to compensate this inhomogeneity, B1+ mapping also offers high potential to obtain information about the electrical properties of the human tissue, i.e. conductivity (as a measure of the possibility of conducting electric currents) and permittivity (as a measure of the electric polarizability). These are of interest because they vary greatly as a function of tissue type and cell composition, and thus tumor tissue can possibly be detected and differentiated. Despite its potential, however, 'electrical properties tomography' (EPT) has not been widely used so far. Among the reasons for this fact is the demand for high SNR in the B1+ field maps, and also the lack of large patient studies with sufficient statistical significance.The aim of this proposal is to develop an accurate and precise method for mapping the B1+ fields in the human head and the body that will benefit from the high SNR available at 7 T. For this purpose, the principle of "Magnetic Resonance Fingerprinting" (MRF) will be used, which is becoming increasingly important in quantitative MRI as it promises high encoding efficiency. The obtained maps will be compared to an existing but biased method and the impact of the novel technique i) on the derived electric properties values and ii) on the achieved spatial homogeneity in UHF body imaging will be analyzed. In the long term, the developed technique enables the investigation of novel methods for UHF body imaging and serves for future EPT studies with tumor patients.