Computer Assisted Clinical Medicine, Heidelberg University, Mannheim, Germany.
Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
NMR Biomed. 2016 Feb;29(2):129-36. doi: 10.1002/nbm.3292. Epub 2015 Apr 21.
Tissue sodium content (TSC) is a sensitive measure of pathological changes and can be detected non-invasively by MRI. For the absolute quantification of TSC, B1 inhomogeneities must be corrected, which is not well established beyond research applications. An in-depth analysis of B1 mapping methods which are suitable for application in TSC quantification is presented. On the basis of these results, a method for simultaneous B1 mapping and imaging is proposed in order to enhance accuracy and to reduce measurement time at clinical field strengths. The B1 mapping techniques used were phase-sensitive (PS), Bloch-Siegert shift (BSS), double-angle (DAM) and actual flip-angle imaging (AFI) methods. Experimental and theoretical comparisons demonstrated that the PS technique yields the most accurate field profiles and exhibits the highest signal-to-noise ratio (SNR). Simultaneous B1 mapping and imaging was performed for the PS method, employing both degrees of freedom of the MR signal: the B1 field is encoded into signal phase and the amplitude provides the concentration information. In comparison with the more established DAM, a 13% higher SNR was obtained and field effects could be corrected more accurately without the need for additional measurement time. The protocol developed was applied to measure TSC in the healthy human head at an isotropic resolution of 4 mm. TSC was determined to be 35 ± 1 mM in white matter and 134 ± 3 mM in vitreous humor. By employing the proposed simultaneous characterization of the B1 field and acquisition of the spin density-weighted sodium signal, the accuracy of the non-invasive measurement of TSC is enhanced and the measurement time is reduced. This should allow (23)Na MRI to be better incorporated into clinical studies and routine.
组织钠含量 (TSC) 是病理变化的敏感指标，可以通过 MRI 进行非侵入性检测。为了对 TSC 进行绝对定量，必须校正 B1 不均匀性，但在研究应用之外，这尚未得到很好的确立。本文深入分析了适用于 TSC 定量的 B1 映射方法。在此基础上，提出了一种同时进行 B1 映射和成像的方法，以提高准确性并减少临床场强下的测量时间。所使用的 B1 映射技术包括相位敏感 (PS)、Bloch-Siegert 偏移 (BSS)、双角 (DAM) 和实际翻转角成像 (AFI) 方法。实验和理论比较表明，PS 技术产生的场分布最准确，具有最高的信噪比 (SNR)。对 PS 方法同时进行 B1 映射和成像，利用 MR 信号的两个自由度：B1 场被编码到信号相位中，而幅度则提供浓度信息。与更为成熟的 DAM 相比，PS 方法获得了 13%更高的 SNR，并且可以更准确地校正场效应，而无需额外的测量时间。所开发的方案应用于在各向同性分辨率为 4mm 的健康人头部测量 TSC。结果表明，白质中的 TSC 为 35±1mM，玻璃体液中的 TSC 为 134±3mM。通过采用同时对 B1 场进行特征描述和对自旋密度加权的钠信号进行采集的方法，提高了 TSC 无创测量的准确性，同时缩短了测量时间。这应该使 (23)Na MRI 更好地应用于临床研究和常规。
