🔊🧠 The hardest tissue for focused ultrasound to get through is the one protecting everything it is aimed at. To reach a deep brain target precisely without surgery, a transducer sends ultrasound energy through gel, through the skull, and into the brain. The beam is supposed to converge on a spot a few millimeters wide. The physics makes it possible. Yet the skull makes that goal hard in two ways. First, bone takes a large share of the energy. It reflects some energy back toward the transducer and absorbs some energy. In the model in this video, about three quarters of the energy never makes it past the skull. Second, bone carries sound about twice as fast as brain, and the skull’s curvature and thickness change from one spot to the next. Each part of the beam gets a different head start through the bone. This video runs the same burst through the same skull twice, at the same time. Without aberration correction (top), the waves leave the bone out of step, and the energy scatters around the target. With aberration correction (bottom), the system maps the skull’s geometry, usually from a CT scan, and delays each element by just enough to cancel its head start. The waves arrive together, and the energy concentrates where it was aimed. Same transducer, same skull, same target. Without correction, the target only gets about a fifth of the energy it gets with correction. Accounting for skull geometry is among the hardest parts of focused ultrasound. The skull will keep protecting the brain, so the timing has to work with it. Correcting for the skull and its geometry brings the focus back to the target! This 2D model for educational purposes, not a treatment plan. Whether for histotripsy (destroying tissue) or neuromodulation (changing neural activity), Aberration correction keeps us beaming inside and out with the energy of focused ultrasound!