Magnetic shielding factor for artefact-free in-beam MR imaging during proton pencil beam irradiation

Introduction
First measurements with a research prototype system for in-beam MR imaging during proton pencil beam scanning (PBS) have shown that the dynamic magnetic fringe fields of the nearby PBS magnets interfere with the static MRI (B0 =0.22 T) field, causing image ghosting artefacts [1]. Passive magnetic shielding is a possible means of eliminating the artefacts by decoupling the MR and PBS magnetic fields. The aim of this study was to determine the shielding factor required for artefact-free MR imaging during PBS dose delivery.
Materials and Methods
The change in B0 magnitude (ΔB0) due to the PBS fringe field was measured with a magnetic field camera positioned in the MR isocenter both as function of (1) the radiation field size [range 4−40 cm] and (2) the distance between the MR isocenter and the PBS isocenter [range 0.3−2.3 m]. Furthermore, images of the ACR Small Phantom were acquired during dose delivery for (1) and (2), and the percent signal ghosting ratio (PSGR) was assessed to determine the maximum ΔB0 for which the ACR action criterion of ≤0.025 was met.
Results
The magnetic field camera measurements showed that the maximum ΔB0 was 5.66 μT in the worst-case scenario of the minimum distance between MRI and PBS isocenter (0.3 m) and maximum scanning field size (40 cm). For this scenario, the PSGR test passed at a field size of 1.2 cm. Here, the maximum ΔB0 was 0.27 μT. The PSGR test was only passed for field sizes of 4 and 12 cm at distances of 1.3 m and 2.3 m between PBS and MR isocenter, respectively. In both cases, the maximum ΔB0 was 0.28 μT. Hence, a minimum shielding factor of 5.66 μT/0.28 μT = 20.22 would be required for artefact-free MR imaging during PBS dose delivery.
Conclusion
The magnetic shielding factor required for artefact-free MR imaging during PBS dose delivery was experimentally determined for the in-beam MR imaging research prototype system.
References
[1] S. Gantz et al. 2020 Phys. Med. Biol, 65(21), 215014