Investigation of electron trajectories of an x-ray tube in magnetic fields of MR scanners

Department of Radiology, Stanford University, Stanford, California 94305, USA.
Medical Physics (Impact Factor: 2.64). 06/2007; 34(6):2048-58. DOI: 10.1118/1.2733798
Source: PubMed


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.

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    • "V, PowerTen, Elgar Electronics Corp., San Diego, CA) to generate a known field at the location of the focal spot of our static anode X-ray tube. The resulting deflection has a measured slope of $0.25 mm/A, which agrees well with the perturbation theory calculation of 0.28 mm/A and can be converted to deflection per tesla using the Helmholtzcoil field-current coefficient of 1.870 Â 10 3 T/A to get a slope of 150 mm/T or $2 mm per 0.0150 T [25]. Using this value, we calculated the expected deflection of the e-beam in the rotating-anode X-ray tube due to a Br in the range of 0.015 to 0.1000 T. For the largest Br of 0.1022 T, the total deflection could be as high as 15 mm, which could easily drive the focal spot off the anode track in a rotating anode tube with similar electron optic geometry. "
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    ABSTRACT: Image-guided minimally invasive procedures have made a substantial impact in improving patient management, reducing the cost, morbidity, and mortality of treatments and making therapies available to patients who would otherwise have no option. X-ray fluoroscopy and magnetic resonance imaging (MRI) are two powerful tools for guiding interventional procedures but with very different strengths and weaknesses. X-ray fluoroscopy offers very high spatial and temporal resolution and is excellent for guiding and deploying devices. MRI offers tomographic imaging with complete freedom of plane orientation, outstanding soft tissue discrimination, and the ability to portray physiological responses during treatment. We have shown that it is feasible to fully integrate an X-ray fluoroscopy system into the bore of an interventional MR scanner to provide a single congruent field of view, with integration requiring minor modifications to the flat-panel digital detector, and using a static-anode X-ray tube. Given the limited availability of the MR scanner platform (0.5T GE Signa SP magnet), and the X-ray fluence limitations of the static-anode X-ray tube, we are now investigating the technology developments required to place a rotating-anode digital flat-panel X-ray system immediately adjacent to a closed-bore MRI system. These types of hybrid systems could have enormous impact in the diagnosis and treatment of oncologic, cardiovascular, and other disorders.
    Full-text · Article · Apr 2008 · Proceedings of the IEEE
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    • "In the case where E and B are aligned along the same axis the electrons travel in a helical motion along the axis of E and B. This motion can change the size and shape of the focal spot on the target of x-ray tube (increase or decrease depending on the strength of B relative to E and the distance from anode to cathode), but does not alter the center of mass of the focal spot. However, when B is perpendicular to E and the electron velocity is still in the direction of E the component of the Lorentz force due to the magnetic field (i.e. the v x B term in equation 1) will cause a deflection in the direction perpendicular to the plane containing E and B leading to a deflection of the center of mass of the focal spot [2]. "
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    ABSTRACT: In order to achieve a truly hybrid, high quality X-ray/MR system one must have a rotating anode x-ray source as close as possible to the bore of the high-field MR magnet. Full integration between a closed bore MR system and an x-ray fluoroscopy system presents two main challenges that must be addressed: x-ray tube motor operation and efficiency in an external field, and focal spot deflection. Regarding the first challenge our results have shown that an AC induction motor operating in external fields will experience a drop off in efficiency. Specifically, fields on the order of 100 Gauss perpendicular to the rotor decrease the rotation speed from 2450 RPM to below 1800 RPM. We are currently developing an alternate brushless DC motor design that would exploit the presence of the external MR fringe field and our initial finite element results indicate that the necessary amount of torque is produced. Regarding the second challenge our results show that an external field of 195 Gauss perpendicular to the anode-cathode axis (BR direction) produces a focal spot deflection of 5 mm. For the fields at which we want to operate the x-ray tube (~to 1000 Gauss along BR) this deflection will be significantly larger than 5 mm and must be corrected for. We propose a design that includes active deflection coils which serve to counteract the presence of the external field and reduce the focal spot deflection to less than 1 mm in our simulations.
    Full-text · Article · Apr 2008 · Proceedings of SPIE - The International Society for Optical Engineering
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    • "Accurate characterization of charged particles (electrons and positrons) backscattered from solid surfaces is important to many kilovoltage medical physics applications. Such applications include predicting dose perturbations due to high-Z inhomogeneities in tissue (Verhaegen 2002, Buffa and Verhaegen 2004), studying the performance of an x-ray tube when placed in the magnetic field of an MR scanner in hybrid CT/MRI systems (Wen et al 2007a, 2007b), and quantifying the effect of off-focal radiation on the output of x-ray systems (Ali and Rogers 2008b). However, accurate charged particle backscatter simulations are one of the most challenging tasks for any Monte Carlo radiation transport code that uses the condensed history technique (Kawrakow 2000). "
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    ABSTRACT: This study benchmarks the EGSnrc Monte Carlo code in the energy range of interest to kilovoltage medical physics applications (5-140 keV) against experimental measurements of charged particle backscatter coefficients. The benchmark consists of experimental data from 20 different published experiments (1954-2007) covering 35 different elements (4<or=Z<or=92), electron and positron backscatter, normal and oblique incidence, and backscatter from thin films. EGSnrc simulation results show excellent agreement with the vast majority of the experimental data. Possible experimental and computational uncertainties explaining the few noted discrepancies are discussed. This study concludes that for the energy range of interest to kilovoltage medical physics application, EGSnrc produces backscatter results within approximately 4% of the average of the majority of published experimental data. A documented EGSnrc user-code customized for backscatter calculations is available from the authors at
    Preview · Article · Mar 2008 · Physics in Medicine and Biology
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