Fahrig R, Wen Z, Ganguly A, DeCrescenzo G, Rowlands J A, Stevens G M, Saunders R F, Pelc N J
Department of Radiology, Stanford University, Stanford, California 94305, USA.
Med Phys. 2005 Jun;32(6):1775-84. doi: 10.1118/1.1915016.
Minimally invasive procedures are increasing in variety and frequency, facilitated by advances in imaging technology. Our hybrid imaging system (GE Apollo flat panel, custom Brand x-ray static anode x-ray tube, GE Lunar high-frequency power supply and 0.5 T Signa SP) provides both x-ray and MR imaging capability to guide complex procedures without requiring motion of the patient between two distant gantries. The performance of the x-ray tube in this closely integrated system was evaluated by modeling and measuring both the response of the filament to an externally applied field and the behavior of the electron beam for field strengths and geometries of interest. The performance of the detector was assessed by measuring the slanted-edge modulation transfer function (MTF) and when placed at zero field and at 0.5 T. Measured resonant frequencies of filaments can be approximated using a modified vibrating beam model, and were at frequencies well below the 25 kHz frequency of our generator for our filament geometry. The amplitude of vibration was not sufficient to cause shorting of the filament during operation within the magnetic field. A simple model of electrons in uniform electric and magnetic fields can be used to estimate the deflection of the electron beam on the anode for the fields of interest between 0.2 and 0.5 T. The MTF measured at the detector and the DQE showed no significant difference inside and outside of the magnetic field. With the proper modifications, an x-ray system can be fully integrated with a MR system, with minimal loss of image quality. Any x-ray tube can be assessed for compatibility when placed at a particular location within the field using the models. We have also concluded that a-Si electronics are robust against magnetic fields. Detailed knowledge of the x-ray system installation is required to provide estimates of system operation.
在成像技术进步的推动下，微创手术的种类和频率不断增加。我们的混合成像系统（GE Apollo平板探测器、定制的Brand x射线静态阳极x射线管、GE Lunar高频电源以及0.5 T Signa SP磁共振成像仪）具备x射线和磁共振成像功能，可在无需患者在两个相距较远的机架之间移动的情况下引导复杂手术。通过对灯丝对外加磁场的响应以及感兴趣的场强和几何形状下电子束行为进行建模和测量，评估了该紧密集成系统中x射线管的性能。通过测量倾斜边缘调制传递函数（MTF）以及将探测器置于零场和0.5 T场强下，对探测器的性能进行了评估。对于我们的灯丝几何形状，测量得到的灯丝共振频率可以使用改进的振动梁模型进行近似，且这些频率远低于我们发生器25 kHz的频率。在磁场中运行期间，振动幅度不足以导致灯丝短路。对于0.2至0.5 T之间感兴趣的场强，可以使用均匀电场和磁场中电子的简单模型来估计电子束在阳极上的偏转。在探测器处测量的MTF以及探测器量子效率（DQE）在磁场内外均未显示出显著差异。经过适当修改，x射线系统可以与磁共振系统完全集成，同时图像质量损失最小。当将任何x射线管放置在磁场内的特定位置时，可使用这些模型评估其兼容性。我们还得出结论，非晶硅电子元件对磁场具有较强的抗性。需要详细了解x射线系统的安装情况，才能对系统运行进行估计。
