Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Purpose: The physical integration of MRI with proton therapy (PT) into an MR-integrated PT (MRiPT) system is expected to improve the targeting accuracy of PT for the treatment of moving tumors. The objective of this project was to develop the world’s first MRiPT system providing high soft-tissue contrast real-time imaging and beam gating, combining an open 0.5 T MRI scanner with a horizontal proton pencil beam scanning (PBS) research beamline to enable gated proton beam treatments of moving tumors delivered inside the in-beam MRI scanner. This contribution provides an overview of this unique MRiPT system and presents first results showcasing its performance.

Methods: The novel MRiPT setup consists of an open 0.5 T MRI scanner (P3, ASG Superconductors SpA, Genoa, Italy) positioned in close proximity to the treatment nozzle of a horizontal proton PBS research beamline (Figure 1). The liquid-Helium free, high-temperature superconducting MgB2-based magnet was installed on a 90° rotating gantry (MagnetTx Oncology Solutions, Ltd., Edmonton, Canada), enabling its static magnetic field (B0) to be either perpendicular or parallel to the central axis of the proton beam. The 35 ton system was designed to be portable inside the radiation bunker through its installation on two separable air cushion platforms also carrying a custom-designed compact aluminum Faraday cabin and a 4-degrees of freedom patient couch.
At the location of the beam exit window of the nozzle, a recess having a beam entrance opening was incorporated in the wall of the Faraday cabin to achieve close positioning relative to the MRI isocenter and thus enable high proton beam quality. The beam entrance window was sealed by a thin (30 µm) aluminum foil to combine high RF attenuation and small lateral spreading of the traversing proton beam. The maneuvering of the assembly into treatment position was visually guided by room lasers that intersect at the beam isocenter and project onto the outer wall of the cabin. The magnet was shimmed in treatment position close to ferromagnetic components of the nozzle. The B0 field homogeneity was measured using a magnetic field camera (MFC3048, Metrolab Technology SA, Geneva, Switzerland). For preliminary acceptance tests, the MR image quality was assessed using the MRI Accreditation Large Phantom Tests by ACR (American College of Radiology, Virginia, USA) with T1w and T2w spin echo (SE) imaging using a 2-channel head coil. For real-time imaging, a FISP and True FISP sequence was acquired at 4 Hz sampling rate.

Fig. 1 [left] View from outside the RF cage with the MRI scanner rotated at 90°, positioned in proximity to the proton PBS nozzle. [right] View from inside the MRiPT system with the 4 DoF patient couch visible and the MRI scanner rotated at 90°. Image curtesy Uniklinikum Carl-Gustav Carus Dresden.

Results: A rotatable, whole-body, 0.5 T in-beam MRI scanner capable of single-plane real-time imaging at 4 frames per second was successfully installed in front of a horizontal proton PBS beamline, setting up a research platform for MRiPT of moving targets. The MRI isocenter was thereby accurately aligned with the height of the central proton beam at 125 cm above floor level. The positioning accuracy and precision of the in-beam MRI system were below 1 mm. A peak-to-peak B0 field homogeneity of <48 ppm over a 30 cm diameter spherical volume (DSV) around the MR isocenter was achieved during shimming, which can be further optimized through an active 2nd order shimming. The ACR QA protocol revealed a signal-to-noise ratio (SNR) of >90 in T1 SE and > 70 in T2 SE. The geometric distortion as measured according to the ACR protocol was found to be of <1.5 mm over a 19 cm DSV around the MR isocenter, without an additionally available retrospective geometrical image distortion correction algorithm. The high spatial resolution test revealed a sufficient in-plane resolution of <1 mm in T1 SE and < 0.9 mm in T2 SE.

Fig. 2. T1 / T2 SE images of the ACR Large phantom showing the satisfactory geometric accuracy, SNR and the high spatial resolution. Small image artefacts due to an air bubble and the physical degradation of the phantom can be seen in the images.

Conclusion: The installation of a rotatable whole-body and real-time imaging capable MRI scanner at a horizontal proton PBS nozzle was successful. Preliminary acceptance tests showed a satisfactory image performance with MR imaging possible in both 0° and 90° orientation of the scanner. Further acceptance and commissioning tests are still outstanding focusing on the image quality of the real-time sequences and the electromagnetic interaction of the MRI scanner with the proton beam line.