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Ultrasound-accelerated thrombolysis is a promising approach toward acute stroke treatment. In previous in vitro studies, we demonstrated enhanced thrombus destruction induced by 20-kHz ultrasound. However, little is known about biological interactions of low-frequency ultrasound with brain tissue. The aim of this in vivo MRI study was to assess safety aspects of transcranial low-frequency ultrasound in rats.The cranium of 33 male Wistar rats was sonificated for 20 minutes (20-kHz continuous wave). Power output was varied between 0 and 2.6 W/cm2. Tympanal and rectal temperature was monitored. Diffusion-weighted imaging and T2-weighted imaging was performed before and 4 hours, 24 hours, and 5 days after sonification. Apparent diffusion coefficients (ADCs) and T2 relaxation time (T2-RT) were measured in regions of interest in the cortex and the basal ganglia. The animals were euthanized for histological evaluation thereafter.Tympanal temperature increased significantly during insonation with 1.1 and 2.6 W/cm2. ADCs decreased significantly at 0.5 and 1.1 W/cm2, indicating cytotoxic edema. T2-RT increased significantly in the 0.5 and 1.1 W/cm2 group, consistent with vasogenic edema. No changes were detectable in the low-power output group (0.2 W/cm2). After sonification with 2.6 W/cm2, a significant loss of neurons could be detected on histopathology. Furthermore, 3 animals developed circumscript cortical lesions that could be identified as parenchymal necrosis.Low-frequency ultrasound caused vasogenic and cytotoxic brain edema and intracerebral necrosis in a dose-dependent fashion. This study indicates therapeutic low-frequency ultrasound as being potentially harmful and underlines the necessity of careful evaluation in further animal models.