Characterization of changes in therapeutic ultrasound transducer performance over time using the angular spectrum method

Abstract

Strongly focused large aperture transducers used in high-intensity focused ultrasound treatments are prone to manufacturing defects and degradation. Current methods for evaluating transducer quality measure only bulk physical changes of transducers. We have determined the pressure distribution at the transducer surface, using the angular spectrum method, to detect defects of the transducer. Three therapeutic transducers were investigated. The pressure distribution at the focal plane of each transducer was measured and input into a back-projection algorithm to calculate the pressure distribution at the transducer surface. A number of scan window sizes were used for the pressure distribution measurement at the focal plane to determine the effect on the resolution of the calculated pressure distribution at the transducer surface. Results showed that one transducer might have suffered manufacturing defects. The second transducer degraded over 1 year of use with one half of the transducer suffering a partial loss of efficiency. The third transducer remained unchanged over 1 year. The scan window of 40 mm 40 mm at the focal plane was required to identify defects 6 mm in diameter on the transducer surface. The results demonstrate that the angular spectrum method could be a useful tool for evaluating transducer quality.

Calculation of Volumetric Sound Field of Pulsed Air-Coupled Ultrasound Transducers Based on Single-Plane Measurements

“…9, for most applications the timing uncertainties are negligible. If necessary, a more accurate c estimate may be obtained by explicitly measuring the time delays of the A-scans at different distances X and consequently performing a linear fit of c. It has been previously shown [43] for water-immersed ultrasound probes that a characterization of transducer near field defects based on a far field measurement is possible with acoustic holography. This also avoids the challenges of pressure measurements directly at the transducer surface, such as microphone reverberations, or difficulties to characterize surface vibration with a laser vibrometer for heterogeneous composite layers.…”

“…Several theoretical works have shown that a pressure measurement over a closed surface is sufficient for the characterization of the full three-dimensional acoustic field (acoustic holography) [20,[36][37][38][39]. These methods have been applied to the characterization of the fields radiated by waterimmersed ultrasound transducers [40][41][42][43] and contact ultrasound probes [19]. However, since ACU is a more recent technology, ACU transducers have received considerably less attention.…”

Review of field characterization techniques for high intensity therapeutic ultrasound

“…Compared with KZK equation which is limited because of neglecting the effects of reflection and scattering, the Westervelt equation is more concise when deals with more realistic physical situations, such as multi-layered tissues and boundary conditions. Either the angular spectrum of the measured field [120] or the Rayleigh integrals over the measurement plane can be used to reconstruct the source [121,122]. Rayleigh integrals have the advantage of being inherently well suited for projection of fields onto curved surfaces and well posed in practical measurement conditions.…”

Acoustic Field Characterization of Medical Array Transducers Based on Unfocused Transmits and Single-Plane Hydrophone Measurements

“…Holography has been extensively used to characterize single-element large-area transducers, for which imperfections in the radiating surface are sought (for instance, [7,16,17,18,19]). Ghanem et al [14], Kreider et al [15] and Clement and Hynynen [20] applied holography to multi-element therapeutic arrays.…”