The size of molecules penetrating the blood-brain barrier can be controlled using acoustic pressure -— the pressure of an ultrasound beam -— to let specific molecules through, a new technique has demonstrated for the first time. This innovative method may help improve drug delivery to the brain. Most small -- and all large -- molecule drugs do not currently penetrate the blood-brain barrier that sits between the vascular bed and the brain tissue. "As a result," one researcher explains, "all central nervous system diseases remain undertreated at best."