Purpose: To examine geographic variation in providers' use of diagnostic imaging to identify potential targets for quality improvement initiatives after adjusting for imaging referral across hospital referral regions (HRRs). Materials and Methods: Using two Centers for Medicare and Medicaid Services datasets, the authors included all claims for beneficiaries enrolled in the Medicare fee-for-service program. Diagnostic imaging procedures were selected on the basis of common procedure coding system codes, excluding interventional procedures. The authors assessed providers' use of imaging for each HRR after creating an imaging referral index (IRI) to adjust for imaging referral rates across HRRs. Relative risk statistics were used to assess geographic variation. The authors calculated two imaging measures for computed tomography (CT) and magnetic resonance (MR) imaging: IRI-adjusted utilization intensity (number of examinations per 1000 beneficiaries) and total payments (in dollars, after deducting deductibles and coinsurances) in each HRR. High-impact regions were defined as those in the highest deciles for both imaging intensity and payment. Results: For 34 million Medicare beneficiaries, 124 million unique diagnostic imaging services (totaling $5.6 billion) were performed in 2012. The average adjusted CT utilization intensity ranged from 330.4 studies per 1000 beneficiaries in the lowest decile to 684.0 in the highest decile (relative risk, 2.1); adjusted MR imaging utilization intensity varied from 105.7 studies per 1000 beneficiaries to 256.3 (relative risk, 2.4). The most common CT and MR imaging procedures were head CT and lumbar spine MR imaging. Conclusion: With use of public data, the authors identified a wide variation in imaging use across the United States. Potential targets for future imaging quality improvement initiatives include head CT and lumbar spine MR imaging.