MRIFlow: Magnetic resonance image super-resolution based on normalizing flow and frequency prior

Super-resolution (SR) is a computer vision task that involves recovering high-resolution (HR) images from low-resolution (LR) ones. While SR is applied to various disciplines, it is particularly important in the medical field which requires accurate diagnosis. L1 and L2 loss-based SR methods produce high values for the peak signal-to-noise ratio and structural similarity index measure but do not have high perceptual quality because SR methods are trained with the average of plausible HR predictions. In addition, SR is an ill-posed problem because only one LR image can be mapped to various HR images. This is crucial because poorly generated HR images can lead to misdiagnosis. In this paper, we propose MRIFlow, a novel method based on normalizing flow that transforms LR magnetic resonance (MR) images into HR MR images. MRIFlow contains frequency affine injectors to reflect frequency information. The frequency affine injector receives the output of a pre-trained LR encoder as the input and obtains frequency information from a wavelet transform based on ScatterNet. Using this method, its inverse operation is possible. MRIFlow has two versions based on the type of ScatterNet employed. In this paper, MRIFlow is compared with normalizing flow-based SR methods by using various MR image datasets such as IXI dataset, NYU fastMRI dataset, and LGG dataset and is demonstrated to produce better quantitative and qualitative results.