Abstract
Passive acoustic mapping (PAM) techniques have been developed for the purposes of detecting, localizing, and quantifying cavitation activity during therapeutic ultrasound procedures. Implementation with conventional diagnostic ultrasound arrays has allowed planar mapping of bubble acoustic emissions to be overlaid with B-mode anatomical images, with a variety of beamforming approaches providing enhanced resolution at the cost of extended computation times. However, no passive signal processing techniques implemented to date have overcome the fundamental physical limitation of the conventional diagnostic array aperture that results in point spread functions with axial/lateral beamwidth ratios of nearly an order of magnitude. To mitigate this problem, the use of a pair of orthogonally oriented diagnostic arrays was recently proposed, with potential benefits arising from the substantially expanded range of observation angles. This article presents experiments and simulations intended to demonstrate the performance and limitations of the dual-array system concept. The key finding of this study is that source pair resolution of better than 1 mm is now possible in both dimensions of the imaging plane using a pair of 7.5-MHz center frequency conventional arrays at a distance of 7.6cm. With an eye toward accelerating computations for real-time applications, channel count reductions of up to a factor of eight induce negligible performance losses. Modest sensitivities to sound speed and relative array position uncertainties were identified, but if these can be kept on the order of 1% and 1 mm, respectively, then the proposed methods offer the potential for a step improvement in cavitation monitoring capability.
Highlights
A S THE scope of therapeutic ultrasound applications in clinical medicine continues to expand in areas ranging from pain reduction [1], [2] to drug delivery to the brain [3] and solid tumors [4], [5], so too does the need for treatment monitoring methods that can help reduce risk and enhance efficacy
In order to determine the impact of the added array on passive acoustic mapping (PAM) resolution, a series of experiments was conducted with a pair of identical diagnostic arrays to monitor cavitation emanating from one or more tubes containing a flowing ultrasound cavitation agent
Distances, frequency ranges, and source pairs studied, PAM resolution of better than 1 mm is possible in both dimensions of the imaging plane
Summary
A S THE scope of therapeutic ultrasound applications in clinical medicine continues to expand in areas ranging from pain reduction [1], [2] to drug delivery to the brain [3] and solid tumors [4], [5], so too does the need for treatment monitoring methods that can help reduce risk and enhance efficacy. Current noninvasive options include techniques based on magnetic resonance (thermometry, elastography) and ultrasound (conventional B-mode, elastography, passive cavitation detection) [6]. Absent the transmissions required for active methods, passive techniques may provide a cavitation monitoring capability at any time during the application of a therapeutic ultrasound field, without concern for waveform interference [7]. Studies of passive cavitation detection employed a single element receiver that enabled identification of characteristic bubble emissions within the detector beam pattern [8], [9]. More flexible approaches employed conventional diagnostic arrays [11], [12], allowing the temporal, spatial, and spectral characteristics of individual cavitation events to be separated and tracked over the course of a therapeutic procedure. By implementing arraybased processing schemes, passive acoustic mapping (PAM) allows emitted acoustic energy in a specified frequency band to be quantified and overlaid with anatomic information from conventional B-mode or other imaging modalities
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