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Objective: ADC (Apparent Diffusion Coefficient) derived from Diffusion-Weighted Imaging (DWI) has shown promise as a non-invasive quantitative imaging biomarker in Wilms' tumours. However, many non-Gaussian models could be applied to DWI. This study aimed to compare the suitability of four diffusion models (mono exponential, IVIM [Intravoxel Incoherent Motion], stretched exponential, and kurtosis) in Wilms' tumours and the unaffected contralateral kidneys., Materials and Methods: DWI data were retrospectively reviewed (110 Wilms' tumours and 75 normal kidney datasets). The goodness of fit for each model was measured voxel-wise using Akaike Information Criteria (AIC). Mean AIC was calculated for each tumour volume (or contralateral normal kidney tissue). One-way ANOVAs with Greenhouse-Geisser correction and post hoc tests using the Bonferroni correction evaluated significant differences between AIC values; the lowest AIC indicating the optimum model., Results: IVIM and stretched exponential provided the best fits to the Wilms' tumour DWI data. IVIM provided the best fit for the normal kidney data. Mono exponential was the least appropriate fitting method for both Wilms' tumour and normal kidney data., Discussion: The diffusion weighted signal in Wilms' tumours and normal kidney tissue does not exhibit a mono-exponential decay and is better described by non-Gaussian models of diffusion.

Objectives: Volume of necrosis in Wilms tumour is informative of chemotherapy response. Contrast-enhanced T 1 -weighted MRI (T 1 w) provides a measure of necrosis using gadolinium. This study aimed to develop a non-invasive method of identifying non-enhancing (necrotic) tissue in Wilms tumour., Methods: In this single centre, retrospective study, post-chemotherapy MRI data from 34 Wilms tumour patients were reviewed (March 2012-March 2017). Cases with multiple b value diffusion-weighted imaging (DWI) and T 1 w imaging pre- and post-gadolinium were included. Fractional T 1 enhancement maps were generated from the gadolinium T 1 w data. Multiple linear regression determined whether fitted parameters from a mono-exponential model (ADC) and bi-exponential model (IVIM - intravoxel incoherent motion) (D, D*, f) could predict fractional T 1 enhancement in Wilms tumours, using normalised pre-gadolinium T 1 w (T 1 w norm ) signal as an additional predictor. Measured and predicted fractional enhancement values were compared using the Bland-Altman plot. An optimum threshold for separating necrotic and viable tissue using fractional T 1 enhancement was established using ROC., Results: ADC and D (diffusion coefficient) provided the strongest predictors of fractional T 1 enhancement in tumour tissue (p < 0.001). Using the ADC-T 1 w norm model (adjusted R 2  = 0.4), little bias (mean difference = - 0.093, 95% confidence interval = [- 0.52, 0.34]) was shown between predicted and measured values of fractional enhancement and analysed via the Bland-Altman plot. The optimal threshold for differentiating viable and necrotic tissue was 33% fractional T 1 enhancement (based on measured values, AUC = 0.93; sensitivity = 85%; specificity = 90%)., Conclusions: Combining ADC and T 1 w imaging predicts enhancement in Wilms tumours and reliably identifies and measures necrotic tissue without gadolinium., Key Points: • Alternative method to identify necrotic tissue in Wilms tumour without using contrast agents but rather using diffusion and T 1 weighted MRI. • A method is presented to visualise and quantify necrotic tissue in Wilms tumour without contrast. • The proposed method has the potential to reduce costs and burden to Wilms tumour patients who undergo longitudinal follow-up imaging as contrast agents are not used.