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Upgrading the MRI-Linac Prototype to a Gantry-Based System: Impact On Magnetic Field Homogeneity

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S Crijns

S Crijns*, B Raaymakers, J Lagendijk, University Medical Center Utrecht, Deparment of Radiotherapy, Utrecht, The Netherlands

WE-G-WAB-5 Wednesday 4:30PM - 6:00PM Room: Wabash Ballroom

In cooperation with Elekta AB (Stockholm, Sweden) and Philips (Best, The Netherlands) the UMC Utrecht has constructed a prototype MRI-linear accelerator. This prototype was recently upgraded to a continuously rotating gantry-based system with a 160-leaf MLC. Ferro-magnetic parts on the gantry around the MR scanner may induce gantry position dependent magnetic field inhomogeneities. Here the aim is to investigate the influence of the field inhomogeneity variations on geometrical accuracy of images as well as the feasibility of possible solutions.

Magnetic field maps were acquired with and without additional shimming at a gantry position resolution of 5° for static gantry positions using a dual-echo GRE-based sequence. From these maps the minimum read-out gradient strength GR,min to achieve an accuracy δ at position (x,y) is Gread,min(x,y)=ΔB(x,y)/δ. Moreover, the measured maps were used as input for a for simulation of a dynamic shimming solution, that updates the scanner shims in real-time while the gantry is rotating.

With shimming, GR,min was below 10mTm⁻¹ for δ=1.0mm for all measurements. Shim settings for the x and y directions displayed a sinusoidal pattern with varying gantry position and are highly reproducible. The z-shim remained gantry position independent. The simulations showed a great reduction of image artifacts due to dynamic shimming in case of gantry rotation during image acquisition.

Gantry position dependent field inhomogeneity in our upgraded MRI-linac prototype can be mitigated by shimming in combination with a clinically feasible minimum read-out gradient strength. The shim settings are reproducible, such that look-up-table-based shimming is feasible for static gantry positions, reducing preparation time needed before imaging after changing the gantry angle. Simulations show that for simultaneous gantry rotation and MR imaging, a look-up-table-based dynamic shimming solution could be used to preserve image quality.

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