Wen Zhifei, Fahrig Rebecca, Conolly Steven, Pelc Norbert J
Department of Radiology, Stanford University, Stanford, California 94305, USA.
Med Phys. 2007 Jun;34(6):2048-58. doi: 10.1118/1.2733798.
A hybrid x-ray/MR system combining an x-ray fluoroscopic system and an open-bore magnetic resonance (MR) system offers advantages from both powerful imaging modalities and thus can benefit numerous image-guided interventional procedures. In our hybrid system configurations, the x-ray tube and detector are placed in the MR magnet and therefore experience a strong magnetic field. The electron beam inside the x-ray tube can be deflected by a misaligned magnetic field, which may damage the tube. Understanding the deflection process is crucial to predicting the electron beam deflection and avoiding potential damage to the x-ray tube. For this purpose, the motion of an electron in combined electric (E) and magnetic (B) fields was analyzed theoretically to provide general solutions that can be applied to different geometries. For two specific cases, a slightly misaligned strong field and a perpendicular weak field, computer simulations were performed with a finite-element method program. In addition, experiments were conducted using an open MRI magnet and an inserted electromagnet to quantitatively verify the relationship between the deflections and the field misalignment. In a strong (B >> E/c; c: speed of light) and slightly misaligned magnetic field, the deflection in the plane of E and B caused by electrons following the magnetic field lines is the dominant component compared to the deflection in the E X B direction due to the drift of electrons. In a weak magnetic field (B < or = E/c), the main deflection is in the E x B direction and is caused by the perpendicular component of the magnetic field.
一种将X射线荧光透视系统与开放式磁共振（MR）系统相结合的混合X射线/MR系统兼具两种强大成像模式的优势，因此可使众多图像引导介入手术受益。在我们的混合系统配置中，X射线管和探测器置于MR磁体中，因而会受到强磁场作用。X射线管内的电子束可能会因磁场失准而发生偏转，这可能会损坏X射线管。了解偏转过程对于预测电子束偏转以及避免对X射线管造成潜在损坏至关重要。为此，从理论上分析了电子在组合电场（E）和磁场（B）中的运动，以提供可应用于不同几何结构的通用解决方案。对于两种特定情况，即轻微失准的强磁场和垂直的弱磁场，使用有限元方法程序进行了计算机模拟。此外还进行了实验，使用开放式MRI磁体和插入式电磁铁来定量验证偏转与磁场失准之间的关系。在强磁场（B >> E/c；c：光速）且轻微失准的情况下，与电子漂移导致的E×B方向的偏转相比，电子沿磁力线运动在E和B平面内引起的偏转是主要成分。在弱磁场（B ≤ E/c）中，主要偏转方向为E×B方向，是由磁场的垂直分量引起的。
