Miranda Vincenzo, Ruello Giuseppe, Lattanzi Riccardo
University of Napoli Federico II, Department of Electrical Engineering and Information Technology, Naples, Italy.
Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University Grossman School of Medicine, New York, New York, USA.
Magn Reson Med. 2024 Jul;92(1):416-429. doi: 10.1002/mrm.30063. Epub 2024 Mar 5.
Recent numerical and empirical results proved that high permittivity materials (HPM) used in pads placed near the subject or directly integrated with coils can increase the SNR and reduce the specific absorption rate (SAR) in MRI. In this paper, we propose an analytical investigation of the effect on the magnetic field distribution of a layer of HPM surrounding an anatomy-mimicking cylindrical sample.
The study is based on a reformulation of the Mie scattering for cylindrical geometry, following an approach recently introduced for spherical samples. The total field in each medium is decomposed in terms of inward and outward electromagnetic waves, and the fields are expressed as series of cylindrical harmonics, whose coefficients can be interpreted as classical reflection and transmission coefficients.
Our new formulation allows a quantitative evaluation of the effect of the HPM layer for varying permittivity and thickness, and it provides an intuitive understanding of such effect in terms of propagation and scattering of the RF field.
We show how HPM can filter out the modes that only contribute to the noise or RF power deposition, resulting in higher SNR or lower SAR, respectively. Our proposed framework provides physical insight on how to properly design HPM for MRI applications.
最近的数值和实验结果证明，置于受试者附近或直接与线圈集成的高介电常数材料（HPM）可提高磁共振成像（MRI）中的信噪比（SNR）并降低比吸收率（SAR）。在本文中，我们对围绕仿解剖圆柱形样品的一层HPM对磁场分布的影响进行了分析研究。
本研究基于对圆柱形几何结构的米氏散射的重新表述，采用了最近针对球形样品引入的方法。每种介质中的总场根据向内和向外的电磁波进行分解，场表示为圆柱谐波的级数，其系数可解释为经典的反射和透射系数。
我们的新公式允许对不同介电常数和厚度的HPM层的影响进行定量评估，并从射频场的传播和散射方面直观地理解这种影响。
我们展示了HPM如何滤除仅对噪声或射频功率沉积有贡献的模式，从而分别导致更高的SNR或更低的SAR。我们提出的框架为如何针对MRI应用正确设计HPM提供了物理见解。
