Objective: To measure the impact of improvements in access to care on Medicare spending growth for diagnostic imaging (DI) services. Methods: We modeled Medicare DI spending growth as a function of growth in: enrollment; per-service payment; access to care (% using = 1 service); volume (services/user); and intensity (relative value units per service used). We then used Medicare Standard Analytic File 5% sample data from 2002-2005 to decompose DI spending growth into these factors by modality: standard (x-ray and ultrasound); and advanced (computed tomography (CT), magnetic resonance (MR) and nuclear). Results: Results: Aggregate DI service spending grew at an annual rate of 15.2% during 2002-2005, and varied substantially by modality (x-ray 10.2%, ultrasound 11.7%, CT 19.6%, MR 18.5%, nuclear 15.0%). Enrollment growth accounted for less than 15% of this increase (range: 7.2% (CT) - 13.3% (x-ray)), while the impact of payment increases was far greater and varied widely (range: 7.6% (nuclear) - 54.0% (x-ray)). The share of DI spending growth attributable to improvements in access to care was: x-ray (6.5%); ultrasound (19.1%); CT (30.4%); MR (49.0%); and nuclear (30.5%). The contribution of volume growth to overall spending growth ranged from 10.5% for MR to 24.1% for CT. Service intensity growth accounted for less than 10% of spending growth for x-ray, CT and MR; 17.9% and 33.0% of spending growth for ultrasound and nuclear were due to service intensity growth, respectively. Conclusion: Improved access to care explains approximately 30%-50% of the growth in Medicare spending for advanced diagnostic imaging services.