Wust P, Fähling H, Helzel T, Kniephoff M, Wlodarczyk W, Mönich G, Felix R
Department of Radiology, Charité Medical School-Campus Virchow-Klinikum, Humboldt University at Berlin, Germany.
Int J Hyperthermia. 1998 Sep-Oct;14(5):459-77. doi: 10.3109/02656739809018248.
The clinical relevance of the radiofrequency regional hyperthermia (RF-RHT) as an adjuvant cancer therapy grows continuously. Simulation studies for optimization of RF-RHT based on the annular phased array systems have shown a significant improvement of power deposition patterns with increasing number of channels. However, this probably requires higher phase accuracy and amplitude stability than are provided by presently used clinical systems, e.g. BSD-2000. Measurements performed on the BSD-200 electronic revealed phase inaccuracies up to +/- 20 degrees and errors in the power registration of +/- 20 W (up to +/- 50 W in the low power range). These errors are further enhanced by the mismatching of the external load (antenna applicator) and thermal instabilities. To achieve the required phase accuracy and long-term stability in the prototype of a new amplifier system, single-sideband (SSB) mixing in combination with direct digital synthesizers (DDS), in-phase and quadrature-phase (IQ) processing and phase-lock loop (PLL) were used. In the DDS's the actual phase of the output signal of each channel is calculated in real-time. No analogue control loop is involved that may cause thermal offset or drift problems. Each DDS operates at a low intermediate frequency (IF) of 1 MHz. To transform the phase information of this IF signal into the desired RF band, SSB mixing-up is performed. A second frequency source, operating as a local oscillator (LO) in the RF band, is required for this technique. Also, the frequency adjustment of the desired RF signal is performed in the LO. These phase and frequency adjustment units are followed by the high efficiency AB-class solid state amplifier unit. The phase and power level stability of the amplifier are controlled by means of digital PLL structures in conjunction with look-up tables. For this control test signals are coupled out by means of directional couplers. The phase control is based on very sensitive phase comparison. These digital control loops are programmable and allow the implementation of different control algorithms. The achieved long-term accuracy (95% confidence interval) is +/- 1-3 W for output power levels ranging from 10-100 W, and +/- 1 degree for phase differences between each channel and a reference signal at a constant power level, and +/- 1.5 degrees for phase difference values at variable power levels between 10-100 W. In conclusion, the new amplifier system is smaller and more efficient than presently available commercial systems.
射频区域热疗（RF - RHT）作为一种辅助癌症治疗方法，其临床相关性在不断增加。基于环形相控阵系统对RF - RHT进行优化的模拟研究表明，随着通道数量的增加，功率沉积模式有显著改善。然而，这可能需要比目前临床使用的系统（如BSD - 2000）更高的相位精度和幅度稳定性。在BSD - 200电子设备上进行的测量显示，相位误差高达±20度，功率记录误差为±20 W（在低功率范围内高达±50 W）。外部负载（天线 applicator）的失配和热不稳定性会进一步加剧这些误差。为了在新放大器系统的原型中实现所需的相位精度和长期稳定性，采用了单边带（SSB）混频与直接数字合成器（DDS）、同相和正交（IQ）处理以及锁相环（PLL）相结合的方法。在DDS中，每个通道输出信号的实际相位是实时计算的。不涉及可能导致热偏移或漂移问题的模拟控制回路。每个DDS在1 MHz的低中频（IF）下工作。为了将该IF信号的相位信息转换到所需的射频频段，进行SSB上变频。此技术需要一个在射频频段作为本地振荡器（LO）工作的第二频率源。此外，所需射频信号的频率调整在LO中进行。这些相位和频率调整单元之后是高效AB类固态放大器单元。放大器的相位和功率电平稳定性通过数字PLL结构结合查找表来控制。对于此控制，测试信号通过定向耦合器耦合输出。相位控制基于非常灵敏的相位比较。这些数字控制回路是可编程的，允许实现不同的控制算法。对于10 - 100 W的输出功率电平，实现的长期精度（95%置信区间）为±1 - 3 W；在恒定功率电平下，每个通道与参考信号之间的相位差为±1度；在10 - 100 W的可变功率电平下，相位差值为±1.5度。总之，新放大器系统比目前可用的商业系统更小、更高效。
