X-RAY TUBE FOR USE IN MAGNETIC FIELDS

Zhifei Wen, Rebecca Fahrig, Nianxiang Sun, ShanXiang Wang and Norbert Pelc Stanford University, Stanford, CA

Abstract Electron trajectory projections under different conditions: In the hybrid interventional Wx-ray system’ an x-ray tube is placed in a high magnetic field. X-rays are produced in the x-ray I A Br 0.3T, Bx = 0.03T , 111 Dr * 0.3T, Rx = 0 031 tube by bombarding a target with high-energy electrons. Ideally, the MRI system’s magnetic field and the x-ray tube’s electric field should be aligned, but in practice the electron beam may be deflected by a misaligned magnetic field*.Here we propose putting well-aligned permanent magnets inside the x-ray tube to minimize undesired deflection due to the unknown external field. Introduction Both x-ray fluoroscopy__ and MRI are powerful tools for guiding interventional procedures. Dual-modality imaging would combine, into the same gantry, the high spatial and temporal resolution of x-ray fluoroscopy for placement of catheters with the soft-tissue contrast, 3D visualization and physiological information of MRI. Figure 1 shows the electron trajectories in a computer model of our x-ray tube with no magnetic field present. The trajectories were generated by a finite-element program (Opera-3d). One half of a cylinder with radius of O.3mm and length of 3mm was used to ftigirre 3 ftlgtire 5 represent the filament. Thermionic emission was modeled using Child’s law current limit model (temperature: 2500K, work Simulations function: 4.5ev; total current: 7.5mA). Two simulations were done without permanent magnets In our Wx-ray hybrid system, the x-ray tube is placed present: one with a magnetic field B=0.3T but misaligned with the inside the magnet, in a high but uniform magnetic field (-0.3T). electric field by 5.7”, which would normally cause the focal spot to With a small misalignment angle 0 between cathode-anode axis be deflected in the x direction by -0.9mm (Fig 2); the other (therefore electric field E) and magnetic field B, the electrons simulation had only a transverse magnetic field B, of 0.02T causing the beam to be deflected in y by 1.8mm (Fig 3). principally spiral along the B field so the deflection is A=L*tanB in In the next two simulations two cylindrical (diameter 1.5cm, the direction of the magnetic field component BL perpendicular to length 2cm) SmCo permanent magnets were put 0.5cm behind the E, where L(=lcm) is the cathode-anode distance (Fig 2). filament and the anode respectively, with axes aligned with the In other applications, the x-ray tube may be placed outside cathode-anode axis. In the volume close to the cathode-anode axis, but close to the magnet, where the field may be lower (less than the magnetic field from the magnets is mostly in the +z direction, 0.02T) but where we may have little knowledge of its direction. In varying fiom -0.5T at the magnet surface to -0.2T at the center a weak B field, the deflection is approximately proportional to BI between the magnets. Because the external field is less than the in the direction of the Lorenz force q,Vx B because the velocity of intrinsic coercivity of the magnets, the external field should not the electrons V is mainly in the direction of cathode-anode axis affect the magnets and the total magnetic field is simply the vector (Fig 3) sum of both fields. In the misaligned external field, the focal spot deflection in x was reduced to 0.5mm (Fig 4), which agreed with A’=L*tan0’=Bl/(BI1+Bm,J and an average B,, -0.3T. In the transverse magnetic field Bx=0.02T, the former deflection in y was 100 mostly removed, but there was a deflection of 0.7mm in x (Fig 5). Z(mm) This is consistent with A’=L*tanB’=BI/B,,=0.02/0.3=0.67mm, further supporting the assumption that it was converted to the high- field scenario. Figure 1 Electron trajectory projection (in x-zplane) with no B field Conclusions Methods Placing properly aligned permanent magnets inside the x-ray To reduce deflection of the focal spot by the magnetic field, tube is a promising way to make the tube more immune to an one possible scheme is to create a high magnetic field Bmagin the unknown or misaligned external magnetic field. cathode-anode axis direction in the volume that the electrons pass Acknowledgments through, for example using permanent magnets. For the first case This work was supported by GE Medical Systems, NIH grant where the x-ray tube is in the high field, the angle between E and P41 RR09784, and the Lucas Foundation. B+B,,, is lower (tan0’= BI /(BI1+Bmag)),so the deflection should be reduced. In the second case, with the presence of Bmag,the References scenario has changed to that of the first case: the deflection 1. R. Fahrig, K. Butts, J.A. Rowlands, et al: JMRI 13,294-300, becomes A=L*tan0’ in the direction of the transverse component 2000. of the weak field B. 2. Z. Wen, R. Fahrig, andN.J. Pelc: Proc. ISMRM, p. 2188,2001.