Mcgary J, Chen J, Xiong Z
University of Houston, Houston, Tx.
University of Houston, Houston, TX.
Med Phys. 2012 Jun;39(6Part8):3689. doi: 10.1118/1.4734997.
Real-time, electromagnetic tumor tacking during 4DCT is an unsolved problem. The underlying problem is due to eddy current magnetic fields generated within the conducting surfaces in response to the source's alternating magnetic fields.To solve this problem, we developed a method to separate the source from the background fields, which can be measured with simple search coil sensors, such as those used by the Calypso (Seattle, WA) tracking system.
We modeled the environment using ANSYS Maxwell electromagnetic simulation software. The gantry was modeled as a 5 mm thick × 1.2 m diameter stainless-steel cylinder, with variable length. The transponder solenoid was modeled as a 10 mm × 1 mm conducting cylinder, with azimuth directed current, which was assumed to have a frequency range of 300-500 kHz. The search coil configuration was assumed to be a 5 × 5 array of 5-7 cm square current loops with 6.7 cmseparation. An algorithm based on free space calculations and measurements was developed to calculate the solenoid position within the cylinder, in the presences of relatively large eddy magnetic fields that were generated at the same frequency as the source.
Of the various methods and sensor configurations investigated, we found a method that localized the transponder solenoid within 1 mm over all solenoid locations and gantry lengths. We also found that gradient techniques did not significantly increase localization accuracy as expected. Complex solutions were found but not suitable for rapid clinical implementation.
This method can be used to localize a Calypso® Beacon transponder during 4DCT to accurately track tumor positions. Furthermore, the method was based on inexpensive search coils with comparable dimensions to preserve the source-sensor distance. The next task is to create a system and investigate clinical implementation.
在4DCT期间进行实时电磁肿瘤追踪是一个尚未解决的问题。根本问题在于导电表面内响应源交变磁场而产生的涡流磁场。为了解决这个问题，我们开发了一种将源与背景场分离的方法，背景场可用简单的搜索线圈传感器测量，如Calypso（华盛顿州西雅图）追踪系统所使用的传感器。
我们使用ANSYS Maxwell电磁仿真软件对环境进行建模。机架被建模为一个5毫米厚×1.2米直径的不锈钢圆柱体，长度可变。应答器螺线管被建模为一个10毫米×1毫米的导电圆柱体，带有方位定向电流，假定其频率范围为300 - 500千赫。搜索线圈配置假定为一个5×5阵列，由边长5 - 7厘米的方形电流环组成，间距为6.7厘米。开发了一种基于自由空间计算和测量的算法，用于在与源频率相同的相对较大涡流磁场存在的情况下，计算圆柱体内部螺线管的位置。
在研究的各种方法和传感器配置中，我们找到了一种方法，能在所有螺线管位置和机架长度范围内将应答器螺线管定位在1毫米以内。我们还发现梯度技术并未如预期那样显著提高定位精度。找到了复杂的解决方案，但不适合快速临床应用。
该方法可用于在4DCT期间定位Calypso®信标应答器，以准确追踪肿瘤位置。此外，该方法基于尺寸相当的廉价搜索线圈，以保持源 - 传感器距离。下一步任务是创建一个系统并研究临床应用。
