Quantification and correction of distortion in diffusion-weighted-MRI at 1.5 and 3 T in a muscle-invasive bladder cancer phantom for radiotherapy planning.

Limited visibility of post-resection muscle-invasive bladder cancer (MIBC) on CT hinders radiotherapy dose escalation of the residual tumour. Diffusion-weighted magnetic resonance imaging (DW-MRI) visualizes areas of high tumour burden and is increasingly used within diagnosis and as a biomarker for cancer. DW-MRI could, therefore, facilitate dose escalation, potentially via dose-painting and/or accommodating response. However, the distortion inherent in DW-MRI could limit geometric accuracy. Therefore, this study aims to quantify DW-MRI distortion via imaging of a bladder phantom.

A phantom was designed to mimic MIBC and imaged using CT, DW-MRI and T2-weighted (T2w) MRI. Fiducial marker locations were compared across modalities and publicly-available software was assessed for correction of magnetic susceptibility-related distortion.

Fiducial marker locations on CT and T2w-MRI agreed within 1.2 mm at 3 T and 1.8 mm at 1.5 T. The greatest discrepancy between CT and Apparent Diffusion Coefficient (ADC) maps was 6.3 mm at 3 T, reducing to 1.8 mm when corrected for distortion. At 1.5 T these values were 3.9 mm and 1.7 mm respectively.

Geometric distortion in DW-MRI of a model bladder was initially >6 mm at 3 T and >3 mm at 1.5 T, however, established correction methods reduced this to <2 mm in both cases.

A phantom designed to mimic MIBC has been produced and used to show distortion in DW-MRI can be sufficiently mitigated for incorporation into the radiotherapy pathway. Further investigation is therefore warranted to enable individually-adaptive image-guided radiotherapy of MIBC based upon DW-MRI.

The British journal of radiology. 2020 Jul 24 [Epub ahead of print]

Jane Rogers, Victoria Sherwood, Sarah C Wayte, Jonathan A Duffy, Spyros Manolopoulos

Department of Physics, University of Warwick CV4 7AL, Warwick, United Kingdom., Clinical Physics and Bioengineering, University Hospitals Coventry and Warwickshire NHS Trust CV2 2DX, Warwick, United Kingdom., Department of Medical Physics and Biomedical Engineering, University College, London WC1E 6BT, United Kingdom.