  Transcranial ultrasonic stimulation (TUS) has the potential to usher in a new
era for human neuroscience by allowing spatially precise and high-resolution
non-invasive targeting of both deep and superficial brain regions. Currently,
fundamental research on the mechanisms of interaction between ultrasound and
neural tissues is progressing in parallel with application-focused research.
However, a major hurdle in the wider use of TUS is the selection of optimal
parameters to enable safe and effective neuromodulation in humans. In this
paper, we will discuss the major factors that determine both the safety and
efficacy of TUS. We will discuss the thermal and mechanical biophysical effects
of ultrasound, which underlie its biological effects, in the context of their
relationships with tunable parameters. Based on this knowledge of biophysical
effects, and drawing on concepts from radiotherapy, we propose a framework for
conceptualising TUS dose.
