Department of Bioengineering, University of California, Berkeley, CA 94720, USA.
IEEE Trans Med Imaging. 2012 Dec;31(12):2335-42. doi: 10.1109/TMI.2012.2217979. Epub 2012 Sep 7.
Magnetic particle imaging (MPI) is a new imaging modality that noninvasively images the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIOs). MPI has demonstrated high contrast and zero attenuation with depth, and MPI promises superior safety compared to current angiography methods, X-ray, computed tomography, and magnetic resonance imaging angiography. Nanoparticle relaxation can delay the SPIO magnetization, and in this work we investigate the open problem of the role relaxation plays in MPI scanning and its effect on the image. We begin by amending the x-space theory of MPI to include nanoparticle relaxation effects. We then validate the amended theory with experiments from a Berkeley x-space relaxometer and a Berkeley x-space projection MPI scanner. Our theory and experimental data indicate that relaxation reduces SNR and asymmetrically blurs the image in the scanning direction. While relaxation effects can have deleterious effects on the MPI scan, we show theoretically and experimentally that x-space reconstruction remains robust in the presence of relaxation. Furthermore, the role of relaxation in x-space theory provides guidance as we develop methods to minimize relaxation-induced blurring. This will be an important future area of research for the MPI community.
磁共振粒子成像（MPI）是一种新的成像方式，可无创地对超顺磁性氧化铁纳米颗粒（SPIOs）的空间分布进行成像。MPI 具有高对比度和零衰减的特性，与目前的血管造影方法、X 射线、计算机断层扫描和磁共振成像血管造影相比，MPI 具有更高的安全性。纳米颗粒弛豫会延迟 SPIO 的磁化，在这项工作中，我们研究了弛豫在 MPI 扫描中所扮演的角色及其对图像的影响这一开放性问题。我们首先对 MPI 的 x 空间理论进行修正，以纳入纳米颗粒弛豫效应。然后，我们使用伯克利 x 空间弛豫计和伯克利 x 空间投影 MPI 扫描仪的实验对修正后的理论进行了验证。我们的理论和实验数据表明，弛豫会降低信噪比，并使扫描方向上的图像不对称地模糊。虽然弛豫效应对 MPI 扫描有不利影响，但我们从理论和实验上表明，在弛豫存在的情况下，x 空间重建仍然具有鲁棒性。此外，弛豫在 x 空间理论中的作用为我们开发最小化弛豫诱导模糊的方法提供了指导。这将是 MPI 领域未来的一个重要研究方向。
