Ultrasound safety with midfrequency transcranial sonothrombolysis: preliminary study on normal macaca monkey brain

We investigated the safety of transcranial-targeting midfrequency (0.1 to 1 MHz) ultrasonic thrombolysis for acute ischemic stroke. We applied a new therapeutic and imaging transducer to healthy Macaca monkey brains via sonication of the ipsilateral middle cerebral artery through an acoustic temporal window. Young adult cynomolgus monkeys (Macaca fascicularis) were assigned to a group without sonication (control), a group maintained for 1 d after sonication (C1) and a group maintained for 7 d after sonication (C7; n = 3 for each). Two elder rhesus monkeys (Macaca mulatta) were ultrasonicated under transvenous injection of the recombinant tissue plasminogen activator alteplase (0.9 mg/kg), and maintained for 7 d (R). An automatic switching circuit alternately operated a therapeutic ultrasound beam (T-beam) generator for thrombolysis (frequency = 490 kHz; intensity = 0.72 W/cm(2)) and a diagnostic color-flow imaging ultrasound beam (D-beam; frequency = 2.5 MHz; intensity = 0.20 W/cm(2)). A 15-min protocol, comprising four repeats of a sequence of 120-s T-beam activation followed by 30-s D-beam activation and then 5-min T-beam deactivation together with D-beam activation, was repeated four times over 60 min. After confirmation of neurologic deficits, the brains were removed and investigated histologically and immunohistochemically. Three skull samples were subjected to 494-kHz continuous waveform ultrasound, the transcranial intensity was measured and the mechanical index was calculated. None of the monkeys showed neurologic deficits after ultrasonication. The transskull ultrasound intensity rate was 48 ± 12%. The intracranial mechanical index value was 0.15. The novel system did not cause tissue damage in the primate brain and no cavitation effect was detected intracranially.