Collins C M, Smith M B
Department of Radiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
Magn Reson Med. 2001 Apr;45(4):684-91. doi: 10.1002/mrm.1091.
Calculations of the RF magnetic (B(1)) field as a function of frequency between 64 and 345 MHz were performed for a head model in an idealized birdcage coil. Absorbed power (P(abs)) and SNR were calculated at each frequency with three different methods of defining excitation pulse amplitude: maintaining 90 degrees flip angle at the coil center (center alpha = pi/2), maximizing FID amplitude (Max. A(FID)), and maximizing total signal amplitude in a reconstructed image (Max. A(image)). For center alpha = pi/2 and Max. A(image), SNR increases linearly with increasing field strength until 260 MHz, where it begins to increase at a greater rate. For these two methods, P(abs) increases continually, but at a lower rate at higher field strengths. Above 215 MHz in MRI of the human head, the use of FID amplitude to set B(1) excitation pulses may result in apparent decreases in SNR and power requirements with increasing static field strength. Magn Reson Med 45:684-691, 2001.
针对理想化鸟笼线圈中的头部模型，计算了64至345 MHz频率范围内的射频磁场（B(1)）。采用三种不同的定义激发脉冲幅度的方法，在每个频率下计算吸收功率（P(abs)）和信噪比（SNR）：在线圈中心保持90度翻转角（中心α = π/2）、最大化自由感应衰减（FID）幅度（最大A(FID)）以及最大化重建图像中的总信号幅度（最大A(image)）。对于中心α = π/2和最大A(image)，信噪比随场强增加呈线性增加，直至260 MHz，此后开始以更快的速率增加。对于这两种方法，吸收功率持续增加，但在较高场强下增加速率较低。在人体头部磁共振成像中，高于215 MHz时，使用FID幅度来设置B(1)激发脉冲可能会导致随着静磁场强度增加，信噪比和功率需求明显降低。《磁共振医学》45:684 - 691, 2001。
