  Transcranial focused ultrasound stimulation (TUS) holds promise for
non-invasive neural modulation in treating neurological disorders. Most
clinically relevant targets are deep within the brain, surrounded by other
sensitive regions that need to be spared clinical intervention. However, in
TUS, increasing frequency with the goal of improving spatial resolution reduces
the effective penetration depth. We show that by using a pair of 1 MHz,
orthogonally arranged transducers we improve the spatial resolution afforded by
each of the transducers individually, by nearly $40$ fold, achieving a
sub-cubic millimeter target volume of $0.24\ mm^3$ deep within the brain. We
show that orthogonally placed transducers generate highly localized standing
waves with Acoustic Radiation Force (ARF) arranged into periodic regions of
compression and tension near the target. We further present an extended
capability of the orthogonal setup, which is to impart selective
pressures--either positive or negative, but not both--on the target. Lastly, we
share our preliminary findings that strain can arise from both particle motion
and ARF with the former reaching its maximum value at the focus, and the latter
remaining null at the focus and reaching its maximum around the focus.
  As the field is investigating the mechanism of interaction in TUS by way of
elucidating the mapping between ultrasound parameters and neural response,
orthogonal transducers expand our toolbox by making it possible to conduct
these investigations at much finer spatial resolutions, with localized and
directed (compression vs. tension) ARF and the capability of applying selective
pressures at the target.
