This measure calculates the percentage of acute ischemic stroke or hemorrhagic stroke patients who arrive at the ED within two hours of the onset of symptoms and have a head computed tomography (CT) or magnetic resonance imaging (MRI) scan interpreted within 45 minutes of ED arrival. This measure is included in the CMS Hospital Outpatient Quality Reporting (HOQR) system measure portfolio. HOQR is a quality data-reporting program, implemented by CMS for outpatient hospital services. Under this program, hospitals report data using standardized measures of care to receive the full annual update to their Outpatient Prospective Payment System (OPPS) payment rate, effective for payments beginning in calendar year 2009. To meet HOQR Program requirements and receive the full Annual Payment Update (APU) under the OPPS, hospitals must meet administrative, data collection and submission, and data validation requirements. Participating hospitals agree that they will allow CMS to publicly report data for the quality measures (as stated in the current OPPS Final Rule). In the context of this quality measure reporting program, NQF #0661 is referred to as ´OP-23.´