Perfusion by Arterial Spin labelling following Single dose Tadalafil In Small vessel disease (PASTIS): study protocol for a randomised controlled trial oleh Mathilde M. H. Pauls, Natasha Clarke, Sarah Trippier, Shai Betteridge, Franklyn A. Howe, Usman Khan, Christina Kruuse, Jeremy B. Madigan, Barry Moynihan, Anthony C. Pereira, Debbie Rolfe, Egill Rostrup, Caroline E. Haig, Thomas R. Barrick, Jeremy D. Isaacs, Atticus H. Hainsworth