The reversible and controllable opening and recovery of the blood-brain barrier (BBB) is crucial for the treatment of brain diseases, and it is a big challenge to noninvasively monitor these processes. In this article, dual-modal photoacoustic imaging and single-photon-emission computed tomography imaging based on ultrasmall Cu2- xSe nanoparticles (3.0 nm) were used to noninvasively monitor the opening and recovery of the BBB induced by focused ultrasound in living mice. The ultrasmall Cu2- xSe nanoparticles were modified with poly(ethylene glycol) to exhibit a long blood circulation time. Both small size and long blood circulation time enable them to efficiently penetrate into the brain with the assistance of ultrasound, which resulted in a strong signal at the sonicated site and allowed for photoacoustic and single-photon emission computed tomography imaging monitoring the recovery of the opened BBB. The results of biodistribution, blood routine examination, and histological staining indicate that the accumulated Cu2- xSe nanoparticles could be excreted from the brain and other major organs after 15 days without causing side effects. By the combination of the advantages of noninvasive molecular imaging and focused ultrasound, the ultrasmall biocompatible Cu2- xSe nanoparticles holds great potential for the diagnosis and therapeutic treatment of brain diseases.

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