A computer-aided diagnosis (CAD) system for classification and visualisation of a 3D medical image comprises a classification component comprising a 2D convolutional neural network (CNN) that is configured to generate a prediction of one or more classes for 2D slices of the 3D medical image. The system also comprises a visualisation component that is configured to: determine, for a target class of said one or more classes, which slices belong to the target class; for each identified slice, determine, by back-propagation to an intermediate layer of the CNN, a contribution of each pixel of the identified slice to classification of the identified slice as belonging to the target class; and generate a heatmap that provides a visual indication of the contributions of respective pixels.