Wust P, Fähling H, Wlodarczyk W, Seebass M, Gellermann J, Deuflhard P, Nadobny J
Center of Radiation Medicine, Charité Medical School, Humboldt University at Berlin, Germany.
Med Phys. 2001 Aug;28(8):1793-805. doi: 10.1118/1.1388220.
In multiantenna applicators such as the SIGMA-60 or SIGMA-Eye, which consist of 4 or 12 pairs of antennas shunt to 4 or 12 amplifiers ("antenna couplets"), phases and amplitudes in the feed points of these antennas under certain conditions can significantly differ from the values selected at the multichannel amplifier (forward parameters), mainly due to coupling. In the SIGMA-Eye, this interaction is particularly affected by the transforming networks between the generators and the feed points, thus hampering the control of the feed point parameters. In this work, we perform measurements at existing applicators, present a formalism to describe the facts numerically, and investigate modifications of the transforming networks to improve the performance.
We prepared an experimental setup for the SIGMA-Eye applicator that is fed by forward waves of a 12-channel amplifier system. In this setup, we made the water bolus, the interior of the tissue-equivalent phantom, and the entire transforming network accessible for measuring probes. Then, we constructed various alternative transforming networks such as Pawsey loops, LC matching networks, and power dividers and compared them with the original matching network of the SIGMA-Eye applicator. In particular, we utilized a high-resistive probe to determine the disturbances and influences caused by some channels with respect to some selected feed points of the SIGMA-Eye dipoles.
In the original SIGMA-Eye applicator, the influences of coupling channels on the phases and voltages in the feed point of a particular antenna are largest for adjacent longitudinal channels. Here, the +/- 10 degrees phase shift and +/- 30% voltage change were observed if the reference channel (i.e., the disturbed channel) and disturbing channel are equally powered. The changes eminently increased to -30 degrees to + 100 degrees phase shift and -80% to +50% voltage change if the reference channel is fed with much lower power (four to eight-fold) than the disturbing channel. The disturbance from distant channels is less but still significant, reaching shifts of -10 degrees to +50 degrees and -50% to +20%, respectively. Using Pawsey loops instead of the original ferrite rings in the SIGMA-Eye network, the efficacy of the baluns was improved by a more than a factor of 4. Using an LC matching network, dependencies on frequency and external arrangements can be reduced significantly. Applying a power divider circuit, the coupling between antennas combined to one channel is considerably diminished (down to <-25 dB).
Coupling between resonators (pairs of antennas including the matching network) reduces the control of the SIGMA-Eye applicator, i.e., it causes deviations between the selection of forward parameters at the amplifier and the total actual parameters in the feed points of the antennas. Modified transformation networks can improve the control, in particular by reducing sheath currents and asymmetries. There is a linear but variable relationship between selected (amplifiers) and actually given (feed points) parameters. This linear mapping (described by a matrix) and its characteristics need further investigation.
在多天线施加器中，如SIGMA - 60或SIGMA - Eye，它们由4对或12对天线并联到4个或12个放大器（“天线对”）组成，在某些条件下，这些天线馈电点处的相位和幅度可能与多通道放大器处选择的值（正向参数）有显著差异，主要是由于耦合。在SIGMA - Eye中，这种相互作用特别受发生器与馈电点之间的变换网络影响，从而妨碍了对馈电点参数的控制。在本研究中，我们对现有的施加器进行测量，提出一种形式主义以数值方式描述这些事实，并研究变换网络的修改以改善性能。
我们为SIGMA - Eye施加器准备了一个实验装置，该装置由一个12通道放大器系统的正向波馈电。在这个装置中，我们使水模、组织等效体模内部以及整个变换网络都可供测量探头使用。然后，我们构建了各种替代变换网络，如Pawsey环、LC匹配网络和功率分配器，并将它们与SIGMA - Eye施加器的原始匹配网络进行比较。特别是，我们使用高电阻探头来确定某些通道对SIGMA - Eye偶极子的一些选定馈电点所造成的干扰和影响。
在原始的SIGMA - Eye施加器中，对于相邻纵向通道，耦合通道对特定天线馈电点处的相位和电压的影响最大。在此，如果参考通道（即受干扰通道）和干扰通道具有相同功率，则观察到±10度的相位偏移和±30%的电压变化。如果参考通道的馈电功率比干扰通道低得多（四到八倍），则变化显著增加到 - 30度至 + 100度的相位偏移和 - 80%至 + 50%的电压变化。来自远处通道的干扰较小但仍然显著，分别达到 - 10度至 + 50度的偏移和 - 50%至 + 20%的变化。在SIGMA - Eye网络中使用Pawsey环代替原始的铁氧体环，平衡 - 不平衡变换器的效率提高了4倍以上。使用LC匹配网络，可以显著降低对频率和外部布置的依赖性。应用功率分配器电路，组合到一个通道的天线之间的耦合会大大减小（降至< - 25 dB）。
谐振器（包括匹配网络的天线对）之间的耦合降低了SIGMA - Eye施加器的控制能力，即它导致放大器处的正向参数选择与天线馈电点处的总实际参数之间存在偏差。修改后的变换网络可以改善控制，特别是通过减少鞘电流和不对称性。选定的（放大器）参数与实际给定的（馈电点）参数之间存在线性但可变的关系。这种线性映射（由矩阵描述）及其特性需要进一步研究。
