Udenafil is a newly developed selective phosphodiesterase type 5 inhibitor (PDE5i). Since PDE5i is known to relax smooth muscle cells, it is suggested that PDE5i may dilate cerebral vessels and lead to an increase in cerebral blood flow. The main mechanism of subcortical vascular cognitive impairment are hypoperfusion of the brain. Therefore, in this study, the investigators will investigate whether Udenafil can increase cerebral blood flow and also lower peripheral blood viscosity in normal and subcortical vascular cognitive impairment subjects.