In this study researchers will use a new method, contrast-enhanced ultrasound (CEU), to visualize tiny blood vessels in cardiac and skeletal muscle. Changes in CEU measurements before, during and after administration of regadenoson will be examined. Contrast-enhanced ultrasounds will also be performed in individuals who are not having a sickle cell crisis. Some of those individuals will also undergo CEU while they are having a sickle cell crisis to compare those measurements . Lastly CEU results will be compared between individuals with sickle cell anemia and healthy African-Americans. These CEU's will be used to determine if there are changes in the blood flow of tiny blood vessels in certain conditions.