565 secure in the long term, however. First, it is important to realize that on an individual CPT code basis, Medicare pay ments for physicians are at the same level as they were in 2001, even with the conversion factor update [3]. Second, overall market trends may compel a downward trend in imaging reimbursement in the future, regard less of small Medicare conversion factor fluctuations. The most important overall macro eco nomic factor influencing imaging re imburse ment is the inevitable truth that the volume of imaging is increasing at a rate that may not allow current percase reimbursement levels to be held constant. In general, the sustainable growth rate for Medicare limits the growth of spending for health care services on a yearly basis, and when expensive tests such as CT and MRI increase at large rates, the Medicare system is taxed, raising questions about its longterm financial solvency [4]. According to testimony delivered in 2005 by the Medicare Payment Advisory Com mis sion, an independent agency advising Congress, government payments for imaging increased 45% from 1999 to 2003, more than twice the 22% increase in physician pay ments overall during the same period [5]. Furthermore, the bulk of the payment in creases for imaging went toward more com plex, technically sophisticated highercost procedures such as CT and MRI [5]. A more recent report by the U.S. General Accountability Office reached a similar conclusion, finding that Medicare Part B physician payments for imaging services more than doubled from 2000 to 2006, with spending on CT, MRI, and nuclear medicine experiencing an average growth rate of 17% per year during that period. In com parison, physician payments for sonog raphy and general radiology pro cedures increased an average of 9% during the same time period [4]. The Four S’s: A Response to Market Forces Influencing Imaging