The purpose of this study was to examine the differential myocardial signal responses due to the blood oxygen level dependent (BOLD) effect in magnetic resonance imaging (MRI) under differing conditions of myocardial oxygen supply and demand. The signal response was measured when myocardial blood flow was increased in excess of oxygen demand or when flow was increased in response to increased myocardial oxygen demand. Normal volunteers were studied using a segmented, interleaved, double-echo, gradient-echo sequence at baseline conditions and during pharmacological stress with either dipyridamole (n = 5) or dobutamine (n = 6). Changes in T2* in the myocardium during stress were calculated. Peak coronary flow velocity was measured at rest and during stress using a breath-hold phase contrast technique. Administration of dipyridamole induced a 124 +/- 27% increase in coronary blood flow which resulted in a 46 +/- 22% increase in T2*, consistent with a decrease in myocardial venous deoxyhemoglobin concentration as myocardial oxygen supply exceeds demand. In contrast, the administration of dobutamine resulted in a 41 +/- 25% increase in coronary blood flow but no significant change in T2* (-5 +/- 19%), consistent with a lack of change in myocardial venous deoxyhemoglobin concentration and balanced oxygen supply and demand. Thus, alterations in the relationship between myocardial oxygen supply and demand appear to be detectable using BOLD MRI.

Contractile reserve depends, in part, on the level of myocardial blood flow at rest and during inotropic stimulation.

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Inherited defects in myocardial long-chain fatty acid metabolism are increasingly recognized as a cause of cardiomyopathy and sudden death in children. To evaluate whether the phenotypic expression of these genetic diseases could be delineated using positron emission tomography (PET), 11 patients with inherited defects in fatty acid metabolism were evaluated and results were compared with those of 6 nonaffected siblings. Myocardial perfusion, myocardial oxygen consumption (MVO2), and long-chain fatty acid metabolism were determined noninvasively with PET using quantitative mathematical models. There were no differences in haemodynamics, perfusion, MVO2 or plasma substrate levels between groups. Patients with defects in enzymes of fatty acid beta-oxidation (acyl-CoA dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase deficiencies) (n = 5) had diminished myocardial palmitate oxidation compared with healthy siblings (3.2 +/- 3.0 vs. 13.0 +/- 5.6 nmol/g per min, p < 0.03) and a decrease in the percentage of MVO2 accounted for by palmitate (2% +/- 3% vs. 9% +/- 5%, p < 0.04). In these patients, extracted palmitate was shunted into a slow-turnover compartment (predominantly reflecting esterification to triglycerides) with expansion of palmitate in that pool (185 +/- 246 compared with 27 +/- 67 nmol/g in healthy siblings,p < 0.02). In contrast, myocardium of patients with carnitine deficiency (n = 6) (all on oral carnitine therapy) had normal palmitate extraction but expansion of the interstitial/cytosolic fatty acid pool (617 +/- 399 vs. 261 +/- 73 nmol/g in healthy siblings, p < 0.04), suggesting different mechanisms for handling upstream fatty acyl intermediates. Thus, PET can be used to noninvasively assess abnormal myocardial handling of fatty acids in patients with inherited defects of metabolism. This approach should be useful in the assessment of altered myocardial fatty acid metabolism associated with cardiomyopathy as well as for evaluating the efficacy of therapeutic interventions in affected patients.

No abstract