An alternative approach to contrast-enhanced imaging: diffusion-weighted imaging and T 1 -weighted imaging identifies and quantifies necrosis in Wilms tumour.

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.