By Kalpana Kanal, PhD, ABR Trustee, and J. Anthony Seibert, PhD, ABR Governor X-rays and their use for human imaging were discovered by Wilhelm Roentgen in 1895, using equipment available in most physics departments. Initially, most radiographs were taken by physicists because the equipment was difficult to use. Due to the nature of the systems used, many human radiation injuries occurred. At first, the radiation output of the machines was low, preventing the imaging of thicker body parts. This changed rapidly in 1913 when William Coolidge was able to incorporate tungsten into X-ray tubes and light bulbs. The latter transformed the use of electric lighting; the former revolutionized medical imaging. The output became much higher and more stable, so acute injuries became rare. Since most diagnostic radiology and radiation oncology departments were usually housed together, medical physicists were available and devoted much of their activity to radiation therapy. Physicists continued to do valuable research that improved diagnostic imaging but there was little call for them in the day-to-day operation of diagnostic imaging equipment. The Radiological Society of North American (RSNA) began certifying medical physicists in 1934. Certification required a knowledge of both radiation therapy and diagnostic radiology. This important function was transferred to the American Board of Radiology (ABR), where it resides to this day. The role of the diagnostic medical physicist changed rapidly in the years following World War II for two reasons. Nuclear development made the community aware that, in addition to deterministic radiation injuries (mostly radiation skin injuries), genetic mutations and cancer induction were possible. Secondly, the introduction of the image intensifier to fluoroscopic imaging transformed the field. It was immediately clear that medical physicists were needed, both for the testing and maintenance of fluoroscopy equipment and to ensure that radiation doses were kept at suitable levels. These needs fostered the growth of medical physics research in image quality and joint research with radiation biologists on radiation effects. This also led to the development of medical physicists whose principal role was to insure the image quality and safe operation of radiology departments. Image-intensified fluoroscopy was first in the armamentarium of ways to do medical imaging. It was soon followed by technologies that revolutionized medicine. Both CT and MRI are complex technologies where close cooperation between physicians and physicists leads to better image quality and safer imaging. In the early 1990s, the role of mammography in the early detection of breast cancer became well established. Unfortunately, some of the mammography imaging was not of sufficient quality to meet the needs of patients and radiologists. The American College of Radiology (ACR) stepped in and took a leadership role in improving the situation. As part of this effort, medical physicists assumed a defined role in mammography. This intense effort led to improved image quality. Since then the field has seen the replacement of most analog imaging with digital imaging, replacement of film interpretation with all digital technology, and many other improvements. All of these changes benefit from a close partnership between radiologists and physicists. In modern medical imaging, the physicist has several key roles:

• close cooperation with radiologists to select protocols that maximize image quality and keeping radiation dose at safe levels;
• measuring radiation doses for the various imaging techniques and comparing them to national standards;
• ensuring safe operation in IR/DR imaging suites;
• oversight of quality control programs that assure image quality and patient safety, and
• instruction of radiologists, radiology residents, and radiologic technologists in the imaging principles and safety considerations for all imaging modalities

While physicists continue to perform traditional quality control testing as required by accrediting bodies, state agencies, federal agencies, and best practice guidance, their major function is to partner with radiologists to continually improve the practice for the benefit of patients.