In silico study of low-frequency transcranial ultrasound fields in acute ischemic stroke patients

Ultrasound in the sub-megahertz range enhances thrombolysis and may be applied transcranially to ischemic stroke patients. The consistency of transcranial insonification needs to be evaluated. Acoustic and thermal simulations based on computed-tomography (CT) scans of 20 patients were performed. An unfocused 120-kHz transducer allowed homogeneous insonification of the thrombus, and positioning based on external landmarks performed similarly to an optimized placement based on CT data. With a weakly focused 500-kHz transducer, the landmark-based positioning underperformed. The predicted inter-patient variation of in situ acoustic pressure was similar with both the 120 and 500-kHz transducers for the optimized placement (18.0-26.4% relative standard deviation). The simulated maximum acoustic pressure in intervening tissues was 2.6 ± 0.6 and 2.0 ± 0.7 times the pressure in the thrombus for the 120-kHz and 500-kHz transducers, respectively. A 1 W/cm(2) insonification of the thrombus caused a 3.8 ± 2.2 °C increase in the bone for the 120-kHz transducer, and a 13.4 ± 3.3 °C increase for the 500-kHz transducer. Contralateral local maxima up to 1.1 times the pressure amplitude in the targeted zone were predicted for the 120-kHz transducer. We established two transducer placement approaches, one based on analysis of a head CT and the other using simple external, visible landmarks. Both approaches allowed consistent insonification of the thrombus.