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MRI distortion: considerations for MRI based radiotherapy treatment planning


Distortion in magnetic resonance images needs to be taken into account for the purposes of radiotherapy treatment planning (RTP). A commercial MRI grid phantom was scanned on four different MRI scanners with multiple sequences to assess variations in the geometric distortion. The distortions present across the field of view were then determined. The effect of varying bandwidth on image distortion and signal to noise was also investigated. Distortion maps were created and these were compared to the location of patient anatomy within the scanner bore to estimate the magnitude and distribution of distortions located within specific clinical regions. Distortion magnitude and patterns varied between MRI sequence protocols and scanners. The magnitude of the distortions increased with increasing distance from the isocentre of the scanner within a 2D imaging plane. Average distortion across the phantom generally remained below 2.0 mm, although towards the edge of the phantom for a turbo spin echo sequence, the distortion increased to a maximum value of 4.1 mm. Application of correction algorithms supplied by each vendor reduced but did not completely remove distortions. Increasing the bandwidth of the acquisition sequence decreased the amount of distortion at the expense of a reduction in signal-to-noise ratio of 13.5 across measured bandwidths. Imaging protocol parameters including bandwidth, slice thickness and phase encoding direction, should be noted for distortion investigations in RTP since each can influence the distortion. The magnitude of distortion varies across different clinical sites.

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The authors would like to thank Michael Jameson from Liverpool Cancer Therapy Centre for useful discussions and access to some in-house code utilised in part of this project, as well as Peter Greer from the Calvary Mater Hospital Newcastle for access to the phantom. Special thanks to Matthew Hundy (Wollongong hospital), Joseph Turner (Campbelltown hospital) and Michael Serratore (Liverpool hospital) for their time in allowing access to the clinical MRI scanners to conduct this study.

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Correspondence to Amy Walker.

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Walker, A., Liney, G., Metcalfe, P. et al. MRI distortion: considerations for MRI based radiotherapy treatment planning. Australas Phys Eng Sci Med 37, 103–113 (2014).

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  • MRI
  • Geometric distortion
  • Bandwidth
  • Phantom
  • Radiotherapy treatment planning