The purpose of this study is to determine whether Zydena (Udenafil) has effect on cerebral blood flow and peripheral blood viscosity in normal and subcortical vascular cognitive impairment subjects.

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.

Inclusion Criteria:

for Normal control group

• Normal cognition by detailed neuropsychological tests
• Brain MRI scan with no or minimal white matter hyperintensities

for Subcortical vascular mild cognitive impairment (svMCI)

• Clinical diagnosis of svMCI
• Brain MRI scan with severe white matter hyperintensities

Exclusion Criteria:

- Contraindications to phosphodiesterase type 5 (Zydena, Udenafil)