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George W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology
Atlanta, GA, 30318
United States

Costas D. Arvanitis
Georgia Institute of Technology and Emory University

Popular version of 3pBAa7 – Breaking the Skull Barrier: Parametric Array Enable Non-Invasive Monitoring of Transcranial Focused Ultrasound
Presented at the 189th ASA Meeting
Read the abstract at https://eppro02.ativ.me/web/index.php?page=Session&project=ASAASJ25&id=3982986&nohistory&nohistory=true

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

The Challenge of Treating the Brain
Focused Ultrasound (FUS) is a revolutionary, incision-free technology that promises to treat brain disorders, such as tumors and Parkinson’s disease. It works by concentrating high-frequency sound waves to a precise point deep within the brain, much like a magnifying glass focuses sunlight. However, this promising therapy faces a major obstacle: the human skull. The skull is a thick, bony barrier that scrambles, reflects, and weakens these high-frequency waves. This makes it incredibly difficult for doctors to monitor the treatment in real-time and confirm that the energy is actually reaching the intended target. This uncertainty limits the safety and effectiveness of FUS brain therapies.

Figure 1: a – Conceptual illustration of the technique. A transmitter (bottom) sends high-frequency (1 MHz) therapeutic ultrasound waves through the skull. Where these waves interact at the focus, they generate a 50kHz low frequency “parametric Array” signal that easily passes through the skull to a receiver (top). The HASPA framework uses this detected signal to map the therapy. b- The reconstructed (first order) 1 MHz high-frequency and 100 kHz low frequency parametric field using HASPA framework with 3,6, and 9 dB contours.

The skull is a thick, bony barrier that scrambles, reflects, and weakens these high-frequency waves. This makes it incredibly difficult for doctors to monitor the treatment in real-time and confirm that the energy is actually reaching the intended target. This uncertainty limits the safety and effectiveness of FUS brain therapies.d

An Acoustic “Trick” to Overcome the Barrier
Researchers at Georgia Tech and Emory University have developed a new computational framework called HASPA (Heterogeneous Angular Spectrum Parametric Array) that exploits a nonlinear acoustic “trick” known as the “parametric array effect.” When two high-frequency ultrasound beams around 1 MHz beams used for therapy meet at the target inside the brain, they interact nonlinearly and mix. This interaction generates a brand-new sound wave at a much lower difference frequency (around 50-100 kHz).

Think of it this way: High-frequency sounds, like a faint whistle, are easily blocked by a thick wall (the skull). However, low-frequency sounds, like the thumping bass from a neighbor’s stereo, travel through walls easily. In this new approach, the therapeutic “whistles” create a localized “bass” beat exactly where the treatment is happening. This low-frequency signal acts as a messenger, traveling cleanly back out through the skull to be detected by external sensors.

Decoding the Message: The HASPA Framework
The challenge is translating this low-frequency message back into a high-resolution picture of the high-frequency treatment zone inside the brain.

To achieve this, the team developed a novel computational framework called HASPA (Heterogeneous Angular Spectrum Parametric Array) and an associated inverse algorithm (iHASPA).

iHASPA analyzes the low-frequency signal measured outside the skull and mathematically reconstructs a map of the original therapy beams deep inside the brain. Crucially, the framework accounts for the complex ways sound travels through the specific properties of the patient’s skull and brain tissue, correcting for distortions.

Impact and Future
By leveraging this nonlinear acoustic effect, the HASPA framework allows us to “see” through the skull using sound. This new technique enables real-time, non-invasive monitoring of ultrasound beams inside the brain, paving the way for safer, more precise, and more effective focused ultrasound therapies for debilitating neurological disorders.