I. Introduction
Over the past several years, deep neural networks have been widely and successfully applied to various imaging tasks such as segmentation [1], object detection [2] and image synethesis [3], by demonstrating better performance than state-of-the-art methods when large amounts of data sets are available. For medical imaging tasks such as lesion detection and region-of-interest (ROI) quantification, obtaining high quality diagnostic images is essential. Recently the neural network method has been applied to transform low-quality images into the images with improved signal-to-noise ratio (SNR).