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.