Croft Laura R, Goodwill Patrick W, Konkle Justin J, Arami Hamed, Price Daniel A, Li Ada X, Saritas Emine U, Conolly Steven M
Department of Bioengineering, University of California, Berkeley, Berkeley, California 94720-1762.
Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120.
Med Phys. 2016 Jan;43(1):424. doi: 10.1118/1.4938097.
Magnetic particle imaging (MPI) is a new imaging technology that directly detects superparamagnetic iron oxide nanoparticles. The technique has potential medical applications in angiography, cell tracking, and cancer detection. In this paper, the authors explore how nanoparticle relaxation affects image resolution. Historically, researchers have analyzed nanoparticle behavior by studying the time constant of the nanoparticle physical rotation. In contrast, in this paper, the authors focus instead on how the time constant of nanoparticle rotation affects the final image resolution, and this reveals nonobvious conclusions for tailoring MPI imaging parameters for optimal spatial resolution.
The authors first extend x-space systems theory to include nanoparticle relaxation. The authors then measure the spatial resolution and relative signal levels in an MPI relaxometer and a 3D MPI imager at multiple drive field amplitudes and frequencies. Finally, these image measurements are used to estimate relaxation times and nanoparticle phase lags.
The authors demonstrate that spatial resolution, as measured by full-width at half-maximum, improves at lower drive field amplitudes. The authors further determine that relaxation in MPI can be approximated as a frequency-independent phase lag. These results enable the authors to accurately predict MPI resolution and sensitivity across a wide range of drive field amplitudes and frequencies.
To balance resolution, signal-to-noise ratio, specific absorption rate, and magnetostimulation requirements, the drive field can be a low amplitude and high frequency. Continued research into how the MPI drive field affects relaxation and its adverse effects will be crucial for developing new nanoparticles tailored to the unique physics of MPI. Moreover, this theory informs researchers how to design scanning sequences to minimize relaxation-induced blurring for better spatial resolution or to exploit relaxation-induced blurring for MPI with molecular contrast.
磁粒子成像（MPI）是一种直接检测超顺磁性氧化铁纳米颗粒的新型成像技术。该技术在血管造影、细胞追踪和癌症检测等方面具有潜在的医学应用价值。在本文中，作者探讨了纳米颗粒弛豫如何影响图像分辨率。从历史上看，研究人员通过研究纳米颗粒物理旋转的时间常数来分析纳米颗粒的行为。相比之下，在本文中，作者关注的是纳米颗粒旋转的时间常数如何影响最终的图像分辨率，这为调整MPI成像参数以实现最佳空间分辨率揭示了一些不明显的结论。
作者首先扩展了x空间系统理论，将纳米颗粒弛豫纳入其中。然后，作者在多个驱动场幅度和频率下，测量了MPI弛豫仪和3D MPI成像仪中的空间分辨率和相对信号水平。最后，利用这些图像测量结果来估计弛豫时间和纳米颗粒的相位滞后。
作者证明，以半高宽测量的空间分辨率在较低驱动场幅度下会提高。作者进一步确定，MPI中的弛豫可以近似为与频率无关的相位滞后。这些结果使作者能够准确预测在广泛的驱动场幅度和频率范围内的MPI分辨率和灵敏度。
为了平衡分辨率、信噪比、比吸收率和磁刺激要求，驱动场可以采用低幅度和高频。继续研究MPI驱动场如何影响弛豫及其不利影响，对于开发适合MPI独特物理特性的新型纳米颗粒至关重要。此外，该理论还告知研究人员如何设计扫描序列，以最小化弛豫引起的模糊，从而获得更好的空间分辨率，或者利用弛豫引起的模糊来实现具有分子对比度的MPI。
