Session 11

W. G. Lotz
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引用次数: 5

Abstract

The session on high power, pulsed radio-frequency fields consisted of six papers that addressed issues of dosimetry, cellular effects, and thermophysiological effects that are associated with the radio-frequency (RF) signals used in nuclear magnetic resonance imaging (MRI) or spectroscopy (MRS). These papers provided a stimulating, insightful, up-to-date overview of this particular area of bioelectromagnetics. Four of the papers were concerned with theoretical dosimetry and employed numerical methods to predict the specific absorption rate (SAR) in the body during MRI. The other two papers were concerned with the biological responses to these fields in cells or in animals and humans where their effect on thermal physiology is of primary importance. The dosimetry papers included two presentations [(i) Gandhi and Chen and (ii) Grandolfo et a/ .] of research using numerical analysis of a complex, anatomically realistic model of the human body composed of several thousand individual cells or compartments (typically 1-1.5 cm in length). This modeling technique is known as the “impedance method” because each cell is characterized by its electrical impedance. Two other theoretical dosimetry papers [(i) Bottomley and Roemer and (ii) Boesiger ef al.] were based on numerical analyses of simpler geometric models of spheres and cylinders. Initial work in the field of dosimetry of MRI used the simpler geometric models, whereas the application of the impedance method to MRI is a new contribution. The individual papers discussed the assumptions, strengths, and weaknesses of each method. These dosimetry methods are directed toward the analysis of the primary known effect of radio-frequency exposure, namely, heating of tissues. The techniques must take into account or make assumptions for the many complex factors affecting the absorption of RF energy by the body, including frequency, variations in time and space of the intensity of the magnetic field, coupling efficiency between the R F coil and the body, duty cycle and waveform of the specific pulse sequence used in imaging, electrical properties of different tissues, geometry, and orientation of the body with respect to the polarity of the field. All of the authors deal with both average and local (or peak) S A R s for various MRI conditions. Considerations of local SARs in different regions of the body (e.g., skin) are the most difficult to determine and thus are the ones that received the most discussion. The impedance method has the capability to provide much more detailed information about internal current distribution and local S A R than the simpler geometric models. However, one of the points that stimulated the most discussion, in this session, was the difference of opinion over the significance of eddy currents in determining the SAR. Bottomley and Roemer presented challenging arguments
会议11
关于高功率脉冲射频场的会议包括六篇论文,讨论了与核磁共振成像(MRI)或光谱学(MRS)中使用的射频(RF)信号相关的剂量学,细胞效应和热生理效应。这些论文提供了一个刺激的,有见地的,最新的概述,这一特定领域的生物电磁学。其中四篇论文涉及理论剂量学,并采用数值方法预测MRI期间体内的比吸收率(SAR)。另外两篇论文涉及细胞或动物和人类对这些电场的生物反应,其中它们对热生理的影响是最重要的。剂量学论文包括两份报告[(i) Gandhi和Chen以及(ii) Grandolfo等人],内容是对由数千个单独细胞或隔室(通常长度为1-1.5厘米)组成的复杂的、解剖学上真实的人体模型进行数值分析。这种建模技术被称为“阻抗法”,因为每个电池都有其电阻抗特征。另外两篇理论剂量学论文[(i) Bottomley和Roemer和(ii) Boesiger等]是基于较简单的球体和圆柱体几何模型的数值分析。磁共振成像剂量学领域的初步工作采用了较简单的几何模型,而阻抗法在磁共振成像中的应用是一个新的贡献。个别论文讨论了每种方法的假设、优点和缺点。这些剂量测定方法的目的是分析射频暴露的主要已知效应,即组织加热。这些技术必须考虑或假设影响人体吸收射频能量的许多复杂因素,包括频率、磁场强度在时间和空间上的变化、射频线圈与人体之间的耦合效率、成像中使用的特定脉冲序列的占空比和波形、不同组织的电学特性、几何形状和人体相对于磁场极性的方向。所有的作者都处理了不同MRI条件下的平均和局部(或峰值)S - A - R。对身体不同部位(如皮肤)局部非典型肺炎的考虑是最难确定的,因此也是讨论最多的。与简单的几何模型相比,阻抗方法能够提供更详细的内部电流分布和局部sa - R信息。然而,在本次会议上,引起讨论最多的一点是对涡流在确定SAR中的重要性的不同意见。Bottomley和Roemer提出了具有挑战性的论点
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