Although broad-based data are sparse at best, the total numbers of procedures and rates per 1,000 population have certainly increased in the intervening years. In addition, the relative distribution among types of procedures has undoubtedly changed in response to new imaging modalities and shifts in the settings in which radiological examinations are done.

In 1980, approximately 80 to 85 percent of all radiological procedures were estimated to have been performed in hospitals (for both inpatients and outpatients), and the remaining 15 to 20 percent were done in physician offices or outpatient imaging centers. By 1990, approximately 25 to 35 percent of all imaging procedures were estimated to occur outside of hospitals (Sunshine et al., 1991). The trend towards nonhospital-based imaging has accelerated since 1990, as health care providers have reacted to market and reimbursement forces by moving imaging procedures outside the hospital setting.

No studies have updated the distribution of procedures at the level of detail reflected in Table 2.2 Some more recent information on specific diagnostic modalities is available, however. The frequency of CT use has greatly increased, from about 2 percent of radiological procedures (excluding ultrasound and MRI) in 1980 to about 7 percent in 1990. The rates of some procedures, such as plain films of the skull or lumbar spine and contrast studies of the GI tract, have probably decreased.

Over the years, several agencies, associations, and individuals have developed data on the total volume of imaging procedures. These include the FDA, the NCRP, and the American College of Radiology (ACR) (see Johnson and Abernathy, 1983; NCRP, 1989; Sunshine et al., 1991; Mettler et al., 1993). Since the compilation of the information presented in Table 2.2, less detailed surveys (which have not broken down rates for specific imaging procedures) have been published. Table 2.3 gives information for hospital-based procedures for 1970, 1980, and 1990. In this 20-year period, the total number of imaging procedures in hospitals rose by two and a half times, from just under 82 million