Inclusion Criteria:  Patients referred to the University of Ottawa Heart Institute for clinically indicated dipyridamole stress, Rb-82 PET myocardial perfusion imaging (MPI) for diagnosis or risk stratification for myocardial ischemia. at least 18 years of age.  Exclusion Criteria:  Patients with contraindications to dipyridamole stress PET MPI including: 1) severe reactive airway disease; 2) less than 3 days post-MI/ACS presentation; 3) unstable crescendo angina; 4) high-grade AV block; 5) allergy to dipyridamole or theophyllines; 6) caffeine within 24 hours; 7) theophyllines within 48 hours; 8) severe claustrophobia; and 9) those who may be pregnant.