Magnetic Particle Imaging (MPI) is a promising new tracer modality with zero attenuation in tissue, high contrast and sensitivity, and an excellent safety profile. However, the spatial resolution of MPI is currently around 1 mm in small animal scanners. Especially considering tradeoffs when scaling up MPI scanning systems to human size, this resolution needs to be improved for clinical applications such as angiography and brain perfusion. One method to improve spatial resolution is to increase the magnetic core size of the superparamagnetic nanoparticle tracers. The Langevin model of superparamagnetism predicts a cubic improvement of spatial resolution with magnetic core diameter. However, prior work has shown that the finite temporal response, or magnetic relaxation, of the tracer increases with magnetic core diameter and eventually leads to blurring in the MPI image. Here we perform the first wide ranging study of 5 core sizes between 18-32 nm with experimental quantification of the spatial resolution of each. Our results show that increasing magnetic relaxation with core size eventually opposes the expected Langevin behavior, causing spatial resolution to stop improving after 25 nm. Different MPI excitation strategies were experimentally investigated to mitigate the effect of magnetic relaxation. The results show that magnetic relaxation could not be fully mitigated for the larger core sizes and the cubic resolution improvement predicted by the Langevin was not achieved. This suggests that magnetic relaxation is a significant and unsolved barrier to achieving the high spatial resolutions predicted by the Langevin model for large core size SPIOs.

中文翻译：

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弛豫壁：通过增加纳米粒子核心尺寸来提高 MPI 空间分辨率的实验限制

磁粒子成像 (MPI) 是一种很有前途的新型示踪剂模式，具有组织中的零衰减、高对比度和灵敏度以及出色的安全性。然而，目前在小动物扫描仪中 MPI 的空间分辨率约为 1 毫米。尤其是在将 MPI 扫描系统放大到人体尺寸时考虑权衡，需要改进该分辨率以用于临床应用，例如血管造影和脑灌注。提高空间分辨率的一种方法是增加超顺磁性纳米颗粒示踪剂的磁芯尺寸。超顺磁性的朗之万模型预测空间分辨率随磁芯直径的三次提高。然而，先前的工作表明，有限的时间响应，或磁弛豫，示踪剂随着磁芯直径的增加而增加，最终导致 MPI 图像模糊。在这里，我们对 18-32 nm 之间的 5 个核心尺寸进行了第一次广泛的研究，并对每个的空间分辨率进行了实验量化。我们的结果表明，随着核心尺寸的增加磁弛豫最终与预期的朗之万行为相反，导致空间分辨率在 25 nm 后停止提高。实验研究了不同的 MPI 激发策略以减轻磁弛豫的影响。结果表明，对于较大的磁芯尺寸，不能完全减轻磁弛豫，并且没有实现朗之万预测的立方分辨率提高。