Wear, K A; Fury, C R; Alvarenga, A V (2025) 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, 72 (11). pp. 1489-1496. ISSN 0885-3010

Preview

Text eid10365.pdf - Published Version Available under License Creative Commons Attribution Non-commercial No Derivatives. Download (5MB) | Preview

Abstract

Hydrophone spatial resolution and spatial averaging effects are determined by frequency dependent effective sensitive element diameter deff(f), which can be much larger (e.g., by over 100%) than geometrical (≈ nominal) sensitive element diameter dg at low frequencies. The objective of this work was to quantify average deff(f) for needle hydrophones as a function of dg and frequency. Estimates of effective radii aeff(f)=deff(f)/2 were inferred from directivity measurements from 0.5 to 20 MHz on 16 needle hydrophones with dg=2ag ranging from 75—1000 μm (139 hydrophone / frequency combinations). For kag > 0.7 (where k=2π/λ and λ=wavelength), deff(f) was consistent with “rigid piston” (RP) theory, reinforcing a previous report from our laboratories. However, for kag < 0.7, deff(f) showed noticeable deviations from RP theory. This behavior can be explained by a susceptibility (at low frequencies) and a resistance (at high frequencies) to normal vibrations of sensitive element surfaces (with the latter case being a condition of the RP model). Examples: 1) For a needle hydrophone with dg = 75 μm at 1 MHz (kag = 0.16), the data imply that average deff = 502 μm. 2) For a needle hydrophone with dg = 400 μm at 500 kHz (common parameters for human transcranial neuromodulation; kag = 0.42), the data imply that average deff = 1215 μm.

Item Type:	Article
Subjects:	Acoustics > Ultrasound
Divisions:	Medical, Marine & Nuclear
Identification number/DOI:	10.1109/TUFFC.2025.3610361
Last Modified:	27 Mar 2026 11:33
URI:	https://eprintspublications.npl.co.uk/id/eprint/10365