Writing and rewriting a text such as Medical Imaging Physics over several editions presents two challenges. The first is to keep the information fresh and relevant. This is a particular challenge in medical imaging, because the field is evolving so rapidly. The third edition of this text was published in 1992, just 10 short years ago. Yet in that text no mention was made of topics such as photodiode or direct conversion digital x-ray imagers; digital mammography; digital fluoroscopy; power Doppler ultrasound; functional magnetic resonance imaging; elastography; or helical CT scanning. This is just a partial list of imaging approaches that must be covered today in any text of imaging.