MRI of superparamagnetic iron oxide (SPIO)-labeled cells has become a valuable tool for studying the in vivo trafficking of transplanted cells. Cellular detection with MRI is generally considered to be orders of magnitude less sensitive than other techniques, such as positron emission tomography (PET), single photon emission-computed tomography (SPECT), or optical fluorescence microscopy. However, an analytic description of the detection threshold for single SPIO-labeled cells and the parameters that govern detection has not been adequately provided. In the present work, the detection threshold for single SPIO-labeled cells and the effect of resolution and SNR were studied for a balanced steady-state free precession (SSFP) sequence (3D-FIESTA). Based on the results from both theoretical and experimental analyses, an expression that predicts the minimum detectable mass of SPIO (m(c)) required to detect a single cell against a uniform signal background was derivedm(c) = 5v/(K(fsl) x SNR), where v is the voxel volume, SNR is the image signal-to-noise ratio, and K(fsl) is an empirical constant measured to be 6.2 +/- 0.5 x 10(-5) microl/pgFe. Using this expression, it was shown that the sensitivity of MRI is not very different from that of PET, requiring femtomole quantities of SPIO iron for detection under typical micro-imaging conditions (100 microm isotropic resolution, SNR = 60). The results of this work will aid in the design of cellular imaging experiments by defining the lower limit of SPIO labeling required for single cell detection at any given resolution and SNR.

译文

超顺磁性氧化铁 (SPIO) 标记细胞的MRI已成为研究移植细胞体内运输的有价值的工具。MRI的细胞检测通常被认为比其他技术 (例如正电子发射断层扫描 (PET),单光子发射计算机断层扫描 (SPECT) 或光学荧光显微镜) 的敏感性低几个数量级。但是,尚未充分提供单个SPIO标记细胞的检测阈值和控制检测的参数的分析描述。在当前工作中,研究了平衡稳态自由进动 (SSFP) 序列 (3D-FIESTA) 的单个SPIO标记细胞的检测阈值以及分辨率和SNR的影响。根据理论和实验分析的结果,可以预测在均匀信号背景下检测单个细胞所需的SPIO (m(c)) 的最小可检测质量的表达式为dm(c) = 5v/(K(fsl) x SNR),其中v是体素体积,SNR是图像信噪比,K(fsl) 是测量为6.2 +/- 0.5 × 10(-5) microl/pgFe的经验常数。使用该表达式,表明MRI的灵敏度与PET的灵敏度没有太大差异,需要在典型的微成像条件下 (100微m各向同性分辨率,SNR = 60) 检测SPIO铁的飞度量。这项工作的结果将通过定义在任何给定分辨率和SNR下进行单细胞检测所需的SPIO标记的下限来帮助细胞成像实验的设计。

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