The shape of magnetic nanoparticles is of great importance in determining their contrast abilities for magnetic resonance imaging. Various magnetic nanoparticles have been developed to achieve high T₁ or T₂ relaxivities, but the mechanism on how morphology influences the water proton relaxation process is still unrevealed. Herein we synthesize manganese-doped iron oxide (MnIO) nanoparticles of the same volume with six different shapes and reveal the relationship between morphologies and T₁/T₂ relaxation rates. The morphology of magnetic nanoparticles largely determines the effective radius and the gradient of stray field, which in turn affects the transverse relaxation rate. The longitudinal relaxivity has positive correlation with the surface-area-to-volume ratio and the occupancy rate of effective metal ions on exposed surfaces of magnetic nanoparticles. These findings together with the summary of r₂/r₁ ratios could help to guide the screening for the optimal shapes of promising T₁ or T₂ contrast agents. Varying effective radii could be utilized to change negative contrast abilities. The surface-area-to-volume ratio and the amount of effective metal ions on exposed surface are instrumental for tuning positive contrast abilities. These principles could serve as guidelines for design and development of high-performance nanoparticle-based contrast agents.

中文翻译：

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形态对磁性纳米粒子弛豫速率的作用

磁性纳米粒子的形状对于确定其磁共振成像的对比能力非常重要。已经开发出各种磁性纳米颗粒以实现高的T ₁或T ₂弛豫性，但是关于形态如何影响水质子弛豫过程的机理仍未揭示。在这里我们合成具有六个不同形状的相同体积的锰掺杂氧化铁（MnIO）纳米粒子，并揭示形态与T ₁ / T ₂之间的关系放松率。磁性纳米粒子的形态在很大程度上决定了有效半径和杂散场的梯度，进而影响了横向弛豫率。纵向弛豫率与磁性纳米颗粒的暴露表面上的表面积/体积比和有效金属离子的占有率呈正相关。这些发现以及r ₂ / r ₁比的总结可帮助指导筛选有前途的T ₁或T ₂的最佳形状造影剂。可以利用变化的有效半径来改变负对比能力。表面积/体积比和暴露表面上有效金属离子的量有助于调节正对比度。这些原则可以用作设计和开发基于纳米粒子的高性能造影剂的指南。