Becker muscular dystrophy (BMD) is characterised by progressive muscle damage and weakness. Diffusion tensor imaging (DTI) represents a promising candidate for studying BMD; the random permeable barrier model (RPBM), in particular, gives estimates of muscle fibre diameters and membrane permeabilities. Here we study DTI and RPBM metrics in BMD patients and controls. Spin-echo and stimulated-echo DTI data were acquired in the lower leg and RPBM metrics calculated. DTI metrics showed time-dependence and intramuscular differences, but no inter-group differences. RPBM analysis, however, successfully showed differences between BMD patients and controls, with fibre diameters being larger and more variable in patients.