Study of increased radiation when an x‐ray tube is placed in a strong magnetic field

Abstract

When a fixed anode x-ray tube is placed in a magnetic field ͑B͒ that is parallel to the anode-cathode axis, the x-ray exposure increases with increasing B. It was hypothesized that the increase was caused by backscattered electrons which were constrained by B and reaccelerated by the electric field onto the x-ray tube target. We performed computer simulations and physical experiments to study the behavior of the backscattered electrons in a magnetic field, and their effects on the radiation output, x-ray spectrum, and off-focal radiation. A Monte Carlo program ͑EGS4͒ was used to generate the combined energy and angular distribution of the backscattered electrons. The electron trajectories were traced and their landing locations back on the anode were calculated. Radiation emission from each point was modeled with published data ͑IPEM Report 78͒, and thus the exposure rate and x-ray spectrum with the contribution of backscattered electrons could be predicted. The point spread function for a pencil beam of electrons was generated and then convolved with the density map of primary electrons incident on the anode as simulated with a finite element program ͑Opera-3d, Vector Fields, UK͒. The total spatial distribution of x-ray emission could then be calculated. Simulations showed that for an x-ray tube working at 65 kV, about 54% of the electrons incident on the target were backscattered. In a magnetic field of 0.5 T, although the exposure would be increased by 33%, only a small fraction of the backscattered electrons landed within the focal spot area. The x-ray spectrum was slightly shifted to lower energies and the half value layer ͑HVL͒ was reduced by about 6%. Measurements of the exposure rate, half value layer and focal spot distribution were acquired as functions of B. Good agreement was observed between experimental data and simulation results. The wide spatial distribution of secondary x-ray emission can degrade the MTF of the x-ray system at low spatial frequencies for B ഛ 0.5 T.

A study of the effect of in‐line and perpendicular magnetic fields on beam characteristics of electron guns in medical linear accelerators

“…The SCALA program does not provide an integrated Monte Carlo simulator for these phenomena. It is expected that secondary electron production is very low, although from previous experience in x-ray tube design, 28 up to % 50% of the fraction of electrons hitting a surface may backscatter. The contribution of these backscattered electrons to the primary electron beam will depend on material properties, geometry, and the external magnetic field.…”

Effect of Low Magnetic Field on Dose Distribution in the Partial-Breast Irradiation

“…The high magnetic field strength can have significant perturbations on dose distributions, such as changes to the percentage depth dose, tissue interface effects and lateral shifts in dose distributions in the photon beam radiotherapy. [15][16][17] The aim of this work was to investigate the consequences to radiation dose distributions that occur in low magnetic field strengths of 0.35 T in comparison with the zero magnetic field in PBI intensity modulated radiation therapy (IMRT) plans.…”

Energy spectra and angular distributions of charged particles backscattered from solid targets

“…The output of the user-code is the charged particle backscatter coefficient, the energy spectra at the detector locations specified by the user, and the angular distributions in the backscatter planes specified Figure 6. Comparison between simulation results using EGSnrc (our study) and EGS4 [9] for the overall energy spectrum of 65 keV electrons backscattered from a semi-infinite tungsten target.…”