Werner F, Gdaniec N, Knopp T
Section for Biomedical Imaging, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. Institute for Biomedical Imaging, Hamburg University of Technology, Hamburg, Germany.
Phys Med Biol. 2017 May 7;62(9):3407-3421. doi: 10.1088/1361-6560/aa6177. Epub 2017 Feb 20.
Magnetic particle imaging (MPI) is a quantitative imaging modality that allows us to determine the distribution of superparamagnetic nanoparticles. Sampling is achieved by moving a field-free point (FFP) along a specific trajectory through the volume of interest. The magnetic material that lies along the path or in the close vicinity of the FFP changes its magnetization and induces a voltage in the surrounding receiver coils. Various trajectories for the FFP are conceivable, but most experimental MPI scanners either use a Cartesian or a Lissajous sampling trajectory. For the first time, this study compares both sampling methods experimentally using an MPI scanner that allows us to implement both sampling patterns. By default, the scanner is capable of scanning 2D and 3D field of views using a Lissajous trajectory. But since it also has a 1D mode, it is possible to perform Cartesian measurements by shifting the 1D scan line in a perpendicular direction to the FFP movement using the focus field. These line scans are jointly reconstructed to obtain a 2D image. In a further step, the unidirectional Cartesian trajectory is improved by interchanging the excitation and the focus-field direction leading to a bidirectional Cartesian trajectory. Our findings reveal similar results for the bidirectional Cartesian and Lissajous trajectory concerning the overall image quality and sensitivity. In a more detailed view, the bidirectional Cartesian trajectory achieves a slightly higher spatial center resolution, whereas the Lissajous trajectory is more efficient regarding the temporal resolution since less acquisition time is needed to reach an adequate image quality.
磁粒子成像（MPI）是一种定量成像方式，它使我们能够确定超顺磁性纳米颗粒的分布。通过沿着特定轨迹在感兴趣的体积内移动无场点（FFP）来实现采样。位于FFP路径上或其紧邻区域的磁性材料会改变其磁化强度，并在周围的接收线圈中感应出电压。FFP的各种轨迹都是可以想象的，但大多数实验性MPI扫描仪要么使用笛卡尔采样轨迹，要么使用李萨如图形采样轨迹。本研究首次使用一台允许我们实现两种采样模式的MPI扫描仪，对这两种采样方法进行了实验比较。默认情况下，该扫描仪能够使用李萨如图形轨迹扫描二维和三维视野。但由于它也具有一维模式，因此可以通过使用聚焦场将一维扫描线沿垂直于FFP移动的方向移动来进行笛卡尔测量。将这些线扫描联合重建以获得二维图像。在进一步的步骤中，通过互换激励方向和聚焦场方向来改进单向笛卡尔轨迹，从而得到双向笛卡尔轨迹。我们的研究结果表明，双向笛卡尔轨迹和李萨如图形轨迹在整体图像质量和灵敏度方面的结果相似。更详细地看，双向笛卡尔轨迹实现了略高的空间中心分辨率，而李萨如图形轨迹在时间分辨率方面更高效，因为达到足够图像质量所需的采集时间更少。
