Spatial Resolution Limits for Needle Hydrophones From 0.5 to 20 MHz With Implications for Transcranial Ultrasound
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control
|September 16, 2025
Summary
The effective diameter of needle hydrophones, crucial for spatial resolution, significantly exceeds their geometric size, especially at lower frequencies. This finding impacts acoustic measurements and ultrasound applications.
Area of Science:
- Acoustics
- Ultrasound Technology
- Hydrophone Metrology
Background:
- Hydrophone spatial resolution is governed by the frequency-dependent effective sensitive element diameter (deff(f)), not the geometric diameter (dg).
- Understanding deff(f) is critical for accurate acoustic measurements and interpreting spatial averaging effects.
Purpose of the Study:
- To quantify the average effective sensitive element diameter (deff(f)) for needle hydrophones.
- To analyze the relationship between deff(f), geometric diameter (dg), and frequency (f).
Main Methods:
- Directivity measurements were performed on 16 needle hydrophones with dg ranging from 75 to 1000 μm.
- Measurements covered frequencies from 0.5 to 20 MHz, analyzing 139 hydrophone/frequency combinations.
- Effective radii (aeff(f)) were inferred from the directivity data.
Main Results:
- Effective sensitive element diameter (deff(f)) exceeded geometric diameter (dg) by over 100% when the wavelength (λ) was greater than 4 times dg.
- For k*ag > 0.75, deff(f) aligned with rigid piston (RP) theory.
- For k*ag < 0.75, deff(f) deviated from RP theory, falling between RP and unbaffled (UB) piston predictions.
Conclusions:
- Needle hydrophone effective diameter is significantly larger than geometric diameter, particularly at lower frequencies relative to size.
- Existing models like rigid piston theory are accurate only above a certain frequency-size parameter (k*ag).
- Accurate characterization of deff(f) is essential for applications like transcranial neuromodulation.
