Passive Acoustic Mapping for ultrasound therapy monitoring

Abstract

Passive Acoustic Mapping (PAM) is an ultrasoundbased imaging method developed for monitoring therapeutic ultrasound. By using diagnostic transducers to passively record the acoustic signals that are emitted by cavitation bubbles, the origin of the bubbles can be reconstructed and displayed as intensity maps. In this study, two matrix arrays with different aperture sizes were used for the volumetric reconstruction of simulated and experimental data. In a second step, the number of elements being used for the reconstruction was reduced by more than the factor of eight in order to assess the influence on the imaging quality. In the numerical part of the study, the image quality was greatly improved by increasing the aperture size, while a high number of elements used for the reconstruction merely offers minor improvements. The experimentally obtained results were able to confirm the numerical findings regarding the achievable reconstruction quality.

3-D Passive Cavitation Imaging Using Adaptive Beamforming and Matrix Array Transducer With Random Apodization

“…The relevance of the use of random sparse apodization was confirmed by the simulations comparing FD-DAS mapping obtained with the full and the random sparse probe. In a similar configuration, Therre et al [22] provided indications that using a reduced number of elements does not decrease the image quality of the cavitation mapping. Before this study, sparse arrays had already been used to build large aperture custom-built arrays for 3D-PAM with elements distributed over hemispherical or spherically-curved surfaces [16]- [21].…”

“…The cavitation cloud nucleated with a peak negative pressure of 8 to 9 MPa in the focal zone resulted in a ≃ 10 mm main lobe FWHM in both directions at a distance of 140 mm from the probe. With the same commercial Vermon probe driven by a 1024-channel ultrasound research platform, Therre et al [22] performed a 3D mapping with DAS beamforming of a cavitating source at a distance of 60 mm from the probe. Under these circumstances, the main lobe axial FWHM is more than 50 mm because of the large observation distance.…”

“…Most of studies were realized using custom-built hemispherical or spherically-curved arrays [16]- [21] for transcranial focused therapy and cavitation imaging. Only a few studies used more generic commercial 1024-element matrix array [13], [22].…”

“…In all cases, the use of matrix arrays with regularly spaced elements can be limited by the inter-element distance that has to be smaller than one half-wavelength in order to avoid the possible occurrence of grating lobes in the reconstructed images [23], which implies, a priori, a large number of small elements distributed over a surface of small aperture. This is the case for the 1024-element probe (Vermon, France) with a 0.3 mm pitch used in [13] and [22], whose width is only 1 cm and for which either a 1024-channel ultrasound research platform (Digital Phased Array Systems, Fraunhofer IBMT), or four synchronized 256-channel Vantage systems (Verasonics, USA) had to be used. This increases drastically the cost of such monitoring systems and the corresponding numerical cost of the processing to localize the cavitation sources.…”

“…Nevertheless, such custom-built hemispherical configurations are very specific to transcranial therapy and imaging, and the case of more versatile and generic systems like the above-mentioned commercial array used in [13] and [22] have to be considered. In a preliminary study [25], we compared 3D-PAM of a simulated harmonic point-source computed with a full matrix array similar to this commercial probe, to the one obtained with a random sparse array using only 256 elements among the 1024 available.…”