Left atrial 4D flow cardiovascular magnetic resonance: a reproducibility study in sinus rhythm and atrial fibrillation by Spartera, M, Pessoa-Amorim, G, Stracquadanio, A, Von Ende, A, Fletcher, A, Manley, P, Neubauer, S, Ferreira, VM, Casadei, B, Hess, AT, Wijesurendra, RS