Since Roentgen discovered x-rays and used them to image his wife's fingers (along with her diamond ring), x-ray imaging has evolved to be an indispensable and powerful tool for screening, diagnosis and management of various diseases. Physicists have always been actively involved in the development and investigation of the physics principles and technology of making medical images. The names include Roentgen, Einstein, Curie, Compton, Cormack, Hunsfield, Lauterbur, Mansfield. Since 50's, medical physics has evolved into a specialized profession offering job opportunities ranging from faculty positions in university medical centers to regulatory officials in federal government. Due to limited memory size and processing speed, early development of digital imaging technology has focused on computed tomography (CT) and digital subtraction angiography (DSA) both of which require lower resolution. CT was developed as a revolutionary new modality which offers cross-sectional view of patients' anatomy and disease symptoms. Its development and commercialization became possible in the 70's due to the parallel development and commercialization of the mini computers and array processors. DSA, on the other hand, was the first digital x-ray imaging technique which successfully replaced its conventional analog predecessors- screen film and image intensifier-video camera based detectors. Rapid advances in electronics, computer and detector technology have resulted in the development of digital x-ray imaging systems which could be used in nearly all medical x-ray imaging applications including those requiring higher resolution and faster image acquisition. An example is cone beam breast CT which employs a 2000×1500 (3 mega) pixel detector to acquire projection images at a rate of up to 30 frames per second to generate 3-D breast images with a resolution of 140 µm or smaller for the detection, diagnosis and monitoring of breast cancers. Like many other new technologies, digital x-ray imaging has reached the stage that the amount of information generated has surpassed human ability to process and comprehend. While newer imaging systems with higher resolution and greater information will certainly continue to be developed, it becomes equally important to develop techniques to use computer or electronics to digest or consolidate the vast amount of information generated for radiologists to review and make diagnostic decisions.