The Nationwide Evaluation of X-Ray Trends, Part 2: US Trends for CT.

The use of CT for routine diagnostic imaging continues to grow, with modern CT equipment capable of providing an ever increasing array of features and tools to aid clinicians. One might even argue that it was the first truly digital diagnostic x-ray imaging modality because of the computational nature of producing a final set of tomographic images. Since its introduction into clinical practice in the early 1970s, CT has become the largest contributor to total collective effective dose in the United States frommedical-related sources of radiation dose, with a rate of annual growth greater than 10% between 1983 and 2006 [1]. The FDA Center for Devices and Radiological Health conducted a survey of US hospitals in 1980, which showed that approximately 2.2 million CT examinations were performed in the United States during that year, with a rate of 1 CT examination per 100 members of the US population [2]. Of those CT examinations, 73% were of the head. A similar study of diagnostic radiologic procedures conducted in 1979 to 1980 found that 3.3 million CT procedures were performed annually, with the difference between that and the FDA study accounting for CT examinations conducted at sites other than hospitals [3]. After the introduction of CT into clinical practice, efforts were begun to understand and quantify the dosimetry of this new imaging modality. The