Geometric distortion and shimming considerations in a rotating MR-linac design due to the influence of low-level external magnetic fields

Medical Physics
K WachowiczB G Fallone

Abstract

This work investigates with simulation the effect of external stray magnetic fields on a recently reported MRI-linac hybrid, which by design will rotate about the patient axis during therapy. During rotation, interactions with magnetic fields from the earth or nearby ferromagnetic structures may cause unacceptable field distortions in the imaging field of view. Optimal approaches for passive shimming implementation, the degree and significance of residual distortion, and an analysis of the active shimming requirements for further correction are examined. Finite element simulations were implemented on two representative types of biplanar magnet designs. Each of these magnet designs, consisting of a 0.2 T four-post and a 0.5 T C-type unit, was simulated with and without an external field on the order of the earth's field (0.5 G) over a range of rotated positions. Through subtraction, the field distribution resulting from the external field alone could be determined. These measured distributions were decomposed into spherical harmonic components, which were then used to investigate the effect of their selective removal to simulate the effects of passive and active shimming. Residual fields after different levels of shim treatment ...Continue Reading

References

Oct 21, 2010·Acta Neurochirurgica. Supplement·Michael J LangGarnette R Sutherland

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Citations

Jun 30, 2016·International Journal of Radiation Oncology, Biology, Physics·Joseph WeygandJihong Wang
May 22, 2019·Journal of Applied Clinical Medical Physics·Andrew GodleyYi Rong

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