Department of Biomedical Engineering, University of Basel, Basel, Switzerland.
Division of Radiological Physics, Department of Radiology, University Hospital Basel, Basel, Switzerland.
Magn Reson Med. 2022 Apr;87(4):1886-1893. doi: 10.1002/mrm.29086. Epub 2021 Nov 14.
To show that for tissues the conspicuous asymmetries in the frequency response function of bSSFP can be mitigated by using a short enough TR.
Configuration theory indicates that bSSFP becomes apparently "pure" (i.e., exhibiting a symmetric profile) in the limit of TR . To this end, the frequency profile of bSSFP was measured as a function of the TR using a manganese-doped aqueous probe, as well as brain tissue that was shown to exhibit a pronounced asymmetry due to its microstructure. The frequency response function was sampled using (phantom) and (in vivo) equally distributed linear RF phase increments in the interval . Imaging was performed with 2.0 mm isotropic resolution over a TR range of 1.5-8 ms at 3 and 1.5 T.
As expected, pure substances showed a symmetric TR-independent frequency profile, whereas brain tissue revealed a pronounced asymmetry. The observed asymmetry for the tissue, however, decreases with decreasing TR and gives strong evidence that the frequency response function of bSSFP becomes symmetric in the limit of TR , in agreement with theory. The limit of apparently pure bSSFP imaging can thus be achieved for a TR 1.5 ms at 1.5 T, whereas at 3 T, tissues still show some residual asymmetry.
In the limit of short enough TR, tissues become apparently pure for bSSFP. This limit can be reached for brain tissue at 1.5 T with TR 1-2 ms at clinically relevant resolutions.
证明对于组织，通过使用足够短的 TR 可以减轻 bSSFP 频率响应函数的明显不对称性。
配置理论表明，当 TR 时，bSSFP 变得明显“纯净”（即表现出对称的轮廓）。为此，使用锰掺杂水探头以及显示出由于其微观结构而表现出明显不对称性的脑组织，测量了 bSSFP 的频率轮廓作为 TR 的函数。使用 （幻影）和 （体内）在线性 RF 相位增量 中均匀分布的频率响应函数进行采样。在 3 和 1.5 T 下，以 2.0 mm 各向同性分辨率进行 TR 范围为 1.5-8 ms 的成像。
正如预期的那样，纯物质显示出对称的与 TR 无关的频率轮廓，而脑组织则显示出明显的不对称性。然而，对于组织观察到的不对称性随着 TR 的减小而减小，这有力地证明了 bSSFP 的频率响应函数在 TR 时变得对称，这与理论相符。因此，在 1.5 T 时，TR 1.5 ms 可以实现明显纯净的 bSSFP 成像的极限，而在 3 T 时，组织仍然显示出一些残余的不对称性。
在足够短的 TR 限制内，组织对于 bSSFP 变得明显纯净。在 1.5 T 时，对于脑组织，TR 为 1-2 ms 时可以达到临床相关分辨率的这个极限。
