The aim of this study is to demonstrate that cerebral velocities assessed by transcranial doppler (TCD)are more significantly decreased by SCT than by long-term transfusion program A multicenter, national, non-randomized, prospective study of paired cohort will be conducted, with 2 groups of exposed (SCT) and non-exposed (TP) patients. Sixty-three patients will be enrolled, namely 21 sickle-cell patients in the "SCT" group and 42 in the "Transfusion Program (TP)" group