Standing-wave suppression for transcranial ultrasound by random modulation.

Low-frequency transcranial ultrasound (<<1 MHz) is being investigated for a number of brain therapies, including stroke, tumor ablation, and localized opening of the blood-brain barrier. However, lower frequencies have been associated with the production of undesired standing waves and cavitation in the brain. Presently, we examine an approach to suppress standing waves during continuous-wave (CW) transcranial application. The investigation uses a small randomization in the frequency content of the signal for suppressing standing waves. The approach is studied in an ex-vivo human skull and a plastic-walled chamber, representing idealized conditions. The approach is compared to single-frequency CW operation as well as to a swept-frequency input. Acoustic field scans demonstrate that the swept-frequency method can suppress standing waves in the plastic chamber and skull by 3.4 and 1.6 times, respectively, compared to single-frequency CW excitation. With random modulation, standing waves were reduced by 5.6 and 2 times, respectively, in the plastic chamber and skull. It is expected that the process may play a critical role in providing a safer application of the ultrasound field in the brain and may have application in other areas where standing waves may be created.