Technical Basis

Functional ultrasound (fUS) applies high-frequency ultrasound pulses to measure blood flow changes reflecting neural activity, analogous to functional MRI but using ultrasound instead of magnetic resonance. The approach detects microvasculature blood flow changes that correlate with local neural population firing, enabling spatial mapping of brain activity. fUS is a “reading” modality: it captures signals out of the brain without electrical or mechanical stimulation.

Spatial and Temporal Resolution

fUS achieves 2 to 10 Hz temporal resolution, substantially slower than intracortical recordings (kilohertz) but faster than fMRI (~0.5 Hz). Spatial resolution is approximately 100 micrometers, intermediate between EEG and intracortical approaches. This resolution-speed tradeoff suits many BCI applications without penetrating brain tissue.

Advantages Over Implants

The potential advantage of fUS is that no brain-penetrating implant is required. The skull scatters and absorbs ultrasound, so the 2024 human fUS BCI demonstration (Rabut et al., Science Translational Medicine) imaged through an implanted acoustically transparent cranial window; wearable through-skull systems, the goal of Bridge Neurotech and Aleph Neuro, would remove surgical risk. This noninvasive property could enable widespread BCI adoption if functional decoding capability proves sufficient.

Current Development Status

Merge Labs is the most-capitalised company pursuing fUS as a primary BCI modality, with a $252M seed round in January 2026 in which OpenAI wrote the largest cheque and a co-founder roster (Tyson Aflalo, Sumner Norman, Mikhail Shapiro) anchored on foundational fUS BCI research at Caltech. Merge Labs is combining fUS imaging with gene therapy and molecular engineering, in an attempt to read high-bandwidth neural activity without tissue implantation. On 1 September 2026 Merge named Butterfly Network its exclusive chip-based ultrasound partner. Other fUS developers include Arbor Neuroscience (the September 2026 rebrand of Forest Neurotech) and Bridge Neurotech, which raised $13.5 million in October 2026.

Important distinction: fUS vs tFUS

Functional ultrasound (fUS, imaging) is distinct from transcranial focused ultrasound (tFUS, stimulation). tFUS uses focused ultrasound waves to mechanically modulate neural activity at depth, a “writing” modality that delivers stimulation. Gestala, the Chinese ultrasound BCI startup founded by Tianqiao Chen and Phoenix Peng in 2025/2026, combines focused-ultrasound neuromodulation with functional ultrasound imaging and blood-brain-barrier opening, so it spans both categories; Shanghai’s BCI-Sonics likewise pairs tFUS stimulation with fUS reading. Both are noninvasive ultrasound technologies but address opposite ends of the read-write spectrum.

Fundamental Limitations

fUS shares fundamental limitations with other hemodynamic approaches: recording lags neural firing by several seconds due to neurovascular coupling kinetics, and spatial resolution remains coarser than direct electrical recording. Despite limitations, the noninvasive property positions fUS as a promising long-term BCI pathway.