CT perfusion study requires repeatedly scanning the same part of the patient's head, resulting in an increase of local radiation exposure. The purpose of this study was to assess the feasibility of the ultra-low-dose technique with a quantum de-noising filter. The newly developed quantum de-noising filter selectively reduced noise by two-thirds, while maintaining spatial resolution. The low-dose protocol using the quantum de-noising filter and slow rotation speed accomplished a 68% reduction in the local radiation dose compared with the previous standard protocol. The quantum de-noising filter is considered to be useful to reduce radiation exposure and to improve image quality in CT perfusion study.