(color online) (a1)–(a3) The results from conventional imaging when V = 5.424 mm/s, V = 10.771 mm/s, V = 19.556 mm/s, respectively. (a4) Relationship between the SNRs of conventional imaging and V. (b1)–(b3) The corresponding results in RGI. (b4) The dependence of SNRs of RGI on V.