Inclusion Criteria:  10 healthy volunteers and 10 patients with known significant (defined as > 70% stenosis) single or 2 vessel CAD Male or female > 18 years of age Females will be non-pregnant and non-lactating  Exclusion Criteria:  Contraindication to MRI (including claustrophobia) History of any significant lung disease including asthma and COPD History of type II respiratory failure Second degree and higher atrio-ventricular conduction delay Patients taking Dipyridamole, or theophylline-based medication Significant left main stem coronary artery disease Recent myocardial infarction (within 2 months) Unstable angina Abnormal heart rhythm e.g. atrial fibrillation, atrial flutter, atrial or ventricular bigeminy Pregnancy/breast-feeding. Women of childbearing potential (not >2 years post- menopausal and/or not surgically sterilised) must have a negative blood serum pregnancy test.