Purpose: To evaluate the impact of an array of nongeographic patient and within-year temporal factors on variation in Medicare imaging utilization. Methods: Using the CMS Chronic Conditions Data Warehouse, we identified imaging events nationally per 1,000 Medicare beneficiaries from 2008 through 2014 on a quarterly basis. We also stratified imaging utilization by a variety of clinical and sociodemographic patient factors. Data were summarized descriptively. Results: On a quarterly basis from 2008 through 2014, mean and median imaging utilization were highest in the second quarter (878 and 885 imaging events per 1,000 beneficiaries, respectively) and lowest in the fourth quarter (844 and 846, respectively) of each year. Imaging events per 1,000 beneficiaries increased progressively with increasing patient comorbidities (0 conditions, 511 events; 1-3 conditions, 2,033 events; 4-5 conditions, 3,188 events; 6+ conditions, 5,774 events). Imaging utilization was also higher in dual Medicaid eligibility beneficiaries than in others (3,855 versus 3,200 events) and in those eligible for Medicare owing to end-stage renal disease versus age or disability (7,876 versus 3,225 and 3,501, respectively). Imaging utilization showed additional variation with beneficiary age, gender, and ethnic group. Conclusions: In the Medicare population, the utilization of medical imaging varies greatly in association with a variety of patient and within-year temporal factors that have previously received little attention. As radiologists embark on risk-bearing contracts, the timing and length of such arrangements should be carefully considered, as well as specific features of the patient population attributed to their practices.