Low Flow Low Gradient Aortic Stenosis Relevance of the (re)Search for a Contractile Reserve

Inclusion Criteria: * Patient from 45 to 90 years old * Patient agreeing freely to participate in the study * Aortic surface area suspected to be \< 1 cm² or \< 0.6 cms²/m² * Low aortic gradient defined by a mean pressure gradient \< 40 mmHg * Aortic low flow defined by: a stroke volume indexed \< 35 ml/m2 * Any contra-indication for the realization of the MRI Exclusion Criteria: * Heart rhythm disorders * Presence of comorbidity altering the 2-year prognosis of a patient (cancer, terminal renal insufficiency (GFR 30 ml / min / 1,73m2), liver cirrhosis, respiratory insufficiency (VEMS 1 L) * Presence of a concomitant valvular heart disease with stenosis or regurgitation \> moderated * Persons subject to major legal protection (safeguarding justice, guardianship, trusteeship), persons deprived of liberty