Various approaches for enhancing treatment of target tissue using a source of focused ultrasound while limiting damage to non-target tissue include selecting a frequency of ultrasound waves transmitted from the source of focused ultrasound for generating a focus in the target tissue; providing microbubbles having the first size distribution such that at least 50% of the microbubbles have a radius smaller than a critical radius corresponding to a resonance frequency matching the selected frequency of ultrasound waves; and applying the ultrasound waves at the selected frequency to treat the target tissue.