Cardiac diffusion tensor imaging: helix angle (HA) healthy statistical average technique for HA quantification in vivo by Ferreira PF, Nielles-Vallespin S, Gatehouse PD, de Silva R, Keegan J, Speier P, Feiweier T, Reese TG, Ismail TF, Scott A, Mekkaoui C, Sosnovik DE, Firmin D