 This study is being done to compare the blood flow measurements obtained with the SPECT and PET imaging scans. Measurements such as myocardial blood flow MBF and myocardial flow reserve MFR are analyzed on PET scans. These measurements give the doctors more precise information in diagnosing heart problems. SPECT scans, with new camera technology and computer software, now have the potential to also give this additional information. The study is investigating how well the blood flow measurements from SPECT compared to PET. 