N oninvasively applied pulses of ultrasound and microbubbles can locally deliver molecules to the brain (1,2). Ultrasound has been shown to shrink brain tumors and improve cognitive function in animal models of Alzheimer disease (3-5). However, despite promising results, there remain concerns about potentially harmful effects from permeability changes to the blood-brain barrier (6,7). Focused ultrasound delivers molecules from the bloodstream to the brain by mechanically stimulating the vessels with acoustically active microbubbles. The microbubbles, composed of a lipid shell and stabilized gas core (1-10 µm), are administered intravenously (8). Pressure oscillations from the ultrasound drive the microbubbles to expand and contract, transporting molecules across the blood-brain barrier. In animals and in clinical trials, the most common method to drive microbubbles to disrupt the blood-brain barrier is with long-pulse sequencing (9-11), characterized by long pulses emitted in a slow sequence (6,12-14). Although parameters within this regimen attain the best performance-safety balance through thorough optimization, limitations exist. First, treatment is unequal within the beam (12,15), resulting in drugs delivered in some, but not all, targeted areas. Second, unwanted biologic responses can be triggered, such as neuronal damage, red blood cell extravasation, and hemorrhage (6,16,17). A recent clinical study has shown the presence of potential microhemorrhages (shown as hypointense areas on T2 MRI scans) using pulses that last in the range of milliseconds in humans (18). Third, it takes 4-48 hours for the permeability of the blood-brain barrier to return to normal (14,19),

Nanoparticles have been widely studied as versatile platforms for in vivo imaging and therapy. However, their use to image and/or treat the brain is limited, as they are often unable to cross the blood-brain barrier (BBB). To overcome this problem, herein we report the use of focused ultrasound in vivo to successfully deliver DNA-coated gold nanoparticles to specific locations in the brains of mice.

Passive acoustic mapping (PAM) is an algorithm that reconstructs the location of acoustic sources using an array of receivers. This technique can monitor therapeutic ultrasound procedures to confirm the spatial distribution and amount of microbubble activity induced. Current PAM algorithms have an excellent lateral resolution, but have a poor axial resolution, making it difficult to distinguish acoustic sources within the ultrasound beams. With recent studies demonstrating that shortlength and low-pressure pulsesacoustic waveletshave therapeutic function, we hypothesized that the axial resolution could be improved with a quasi-pulse-echo approach and that the resolution improvement would depend on the wavelet's pulse length. This paper describes an algorithm that resolves acoustic sources axially using time of flight and laterally using delay-andsum beamforming, which we named axial temporal position passive acoustic mapping (ATP-PAM). The algorithm accommodates a rapid short pulse (RaSP) sequence that can safely deliver drugs across the blood-brain barrier. We developed our algorithm with simulations (k-wave) and in vitro experiments for 1-, 2-, and 5-cycle pulses, comparing our resolution against that of two current PAM algorithms. We then tested ATP-PAM in vivo and evaluated whether the reconstructed acoustic sources mapped to drug delivery within the brain. In simulations and in vitro, ATP-PAM had an improved resolution for all pulse lengths tested. In vivo, experiments in mice indicated that ATP-PAM could be used to target and monitor drug delivery into the brain. With acoustic wavelets and time of flight, ATP-PAM can locate acoustic sources with a vastly improved spatial resolution.

• In a mouse model, rapid short-pulse sequences show the potential to deliver drugs at a comparable dose to long pulses across the blood-brain barrier with a uniform distribution and into neurons. • These sequences also have the potential to deliver drugs with minimal damage to the bloodbrain barrier function and the neuronal microenvironment as indicated by a very shortduration (< 10 minutes) permeability change and a 3.4-fold lower amount of albumin released into the brain parenchyma when compared to long pulses.