Ibrahim Tamer S, Tang Lin
Department of Radiology, University of Pittsburgh, B800 Presbyterian University Hospital, 200 Lothrop Street, Pittsburgh, PA 15213, USA.
J Magn Reson Imaging. 2007 Jun;25(6):1235-47. doi: 10.1002/jmri.20919.
To study the dependence of radiofrequency (RF) power deposition on B(0) field strength for different loads and excitation mechanisms.
Studies were performed utilizing a finite difference time domain (FDTD) model that treats the transmit array and the load as a single system. Since it was possible to achieve homogenous excitations across the human head model by varying the amplitudes/phases of the voltages driving the transmit array, studies of the RF power/B(0) field strength (frequency) dependence were achievable under well-defined/fixed/homogenous RF excitation.
Analysis illustrating the regime in which the RF power is dependent on the square of the operating frequency is presented. Detailed studies focusing on the RF power requirements as a function of number of excitation ports, driving mechanism, and orientations/positioning within the load are presented.
With variable phase/amplitude excitation, as a function of frequency, the peak-then-decrease relation observed in the upper axial slices of brain with quadrature excitation becomes more evident in the lower slices as well. Additionally, homogeneity optimization targeted at minimizing the ratio of maximum/minimum B(1) (+) field intensity within the region of interest, typically results in increased RF power requirements (standard deviation was not considered in this study). Increasing the number of excitation ports, however, can result in significant RF power reduction.
研究不同负载和激励机制下射频（RF）功率沉积对B(0)场强的依赖性。
利用时域有限差分（FDTD）模型进行研究，该模型将发射阵列和负载视为一个单一系统。由于通过改变驱动发射阵列的电压幅度/相位可以在人体头部模型上实现均匀激励，因此在明确界定/固定/均匀的RF激励下，可以研究RF功率/B(0)场强（频率）的依赖性。
给出了说明RF功率依赖于工作频率平方的区域的分析。还给出了针对RF功率需求作为激励端口数量、驱动机制以及负载内方向/定位的函数的详细研究。
通过可变相位/幅度激励，作为频率的函数，在正交激励下在脑上部轴向切片中观察到的先峰值后下降关系在下部切片中也变得更加明显。此外，以最小化感兴趣区域内最大/最小B(1)(+)场强之比为目标的均匀性优化通常会导致RF功率需求增加（本研究未考虑标准偏差）。然而，增加激励端口的数量可以显著降低RF功率。
