I. Introduction
Digital image enhancement is necessary to expose critical details that are essential but not clearly seen in the image at hand. Image enhancement focuses on the improvement of digital image quality for visual inspection or for machine analysis, without knowledge about the source of degradation. In medical imaging (e.g., computer tomography and magnetic resonance), three-dimensional images (or a stack of two-dimensional images) of different organs and tissues are produced. However, various sources of interference during image acquisition (e.g., movement of a patient, insufficient performance and noise of imaging devices) make the quality of such images too poor to be directly used for diagnostic purposes. In such cases, to discern the concealed but important information in the images, it is deemed necessary to use various image enhancement methods such as enhancing edges, emphasizing the differences, or reducing the noise [1]–[8].