A 3D printed phantom to assess MRI geometric distortion.

Magnetic Resonance has become a standard imaging modality for target volume delineation and treatment planning in radiation oncology. Geometric distortions, however, have the potential to detrimentally affect both tumour definition and the dose delivered to the target volume. We report the design, fabrication and imaging of a 3D printed unibody MR distortion phantom along with quantitative image analysis. METHODS The internal cavity of the phantom is an orthogonal three-dimensional planar lattice, composed of 3mm diameter rods spaced equidistantly at a 20mm centre-centre offset repeating along the X, Y and Z axes. The phantom featured an overall length of 308.5 mm, a width of 246 mm and a height of 264 mm with lines on the external surface for phantom positioning matched to external lasers. The MR phantom was 3D printed in Nylon-12 using an advancement on traditional selective laser sintering (SLS) (HP Jet Fusion 3D - 4200 machine). The phantom was scanned on a Toshiba Aquilion CT scanner to check the integrity of the 3D print and to correct for any resultant issues. The phantom was then filled with NiSO4 solution and scanned on a 3T PET-MR Siemens scanner for selected T1 and T2 sequences, from which distortion vectors were generated and analysed using in-house software written in Python. RESULTS All deviations were less than 1 mm, with an average displacement of 0.228 mm. The majority of the deviations are smaller than the 0.692 mm pixel size for this dataset. CONCLUSION A cost-effective, 3D printed MRI-phantom was successfully printed and tested for assessing geometric distortion on MRI scanners. The custom phantom with markings for phantom alignment may be considered for radiotherapy departments looking to add MR scanners for simulation and image guidance.