19 research outputs found
Diffraction effects and compensation in passive acoustic mapping
Over the last decade, a variety of noninvasive techniques has been developed to monitor therapeutic ultrasound procedures in support of safety or efficacy assessments. One class of methods employs diagnostic ultrasound arrays to sense acoustic emissions, thereby providing a means to passively detect, localize and quantify the strength of nonlinear sources, including cavitation. Real array element diffraction patterns may differ substantially from those presumed in existing beamforming algorithms. However, diffraction compensation has received limited treatment in passive and active imaging, and measured diffraction data has yet to be used for array response correction. The objectives of this work were to identify differences between ideal and real element diffraction patterns, and to quantify the impact of diffraction correction on cavitation mapping beamformer performance. These objectives were addressed by performing calibration measurements on a diagnostic linear array, using the results to calculate diffraction correction terms, and applying the corrections to cavitation emission data collected from soft tissue phantom experiments. Measured diffraction patterns were found to differ significantly from those of ideal element forms, particularly at higher frequencies and shorter distances from the array. Diffraction compensation of array data resulted in cavitation energy estimates elevated by as much as a factor of five, accompanied by the elimination of a substantial bias between two established beamforming algorithms. These results illustrate the importance of using measured array responses to validate analytical field models and to minimize observation biases in imaging applications where quantitative analyses are critical for assessment of therapeutic safety and efficacy
Diffraction effects and compensation in passive acoustic mapping
Over the last decade, a variety of noninvasive techniques has been developed to monitor therapeutic ultrasound procedures in support of safety or efficacy assessments. One class of methods employs diagnostic ultrasound arrays to sense acoustic emissions, thereby providing a means to passively detect, localize and quantify the strength of nonlinear sources, including cavitation. Real array element diffraction patterns may differ substantially from those presumed in existing beamforming algorithms. However, diffraction compensation has received limited treatment in passive and active imaging, and measured diffraction data has yet to be used for array response correction. The objectives of this work were to identify differences between ideal and real element diffraction patterns, and to quantify the impact of diffraction correction on cavitation mapping beamformer performance. These objectives were addressed by performing calibration measurements on a diagnostic linear array, using the results to calculate diffraction correction terms, and applying the corrections to cavitation emission data collected from soft tissue phantom experiments. Measured diffraction patterns were found to differ significantly from those of ideal element forms, particularly at higher frequencies and shorter distances from the array. Diffraction compensation of array data resulted in cavitation energy estimates elevated by as much as a factor of five, accompanied by the elimination of a substantial bias between two established beamforming algorithms. These results illustrate the importance of using measured array responses to validate analytical field models and to minimize observation biases in imaging applications where quantitative analyses are critical for assessment of therapeutic safety and efficacy
Passive acoustic mapping of extravasation following ultrasound-enhanced drug delivery
The amount and distribution of chemotherapeutic agents delivered to tumours can vary significantly due to tumour heterogeneity, even under focussed ultrasound (FUS) assisted drug delivery regimes. The ability to non-invasively localise cavitation nuclei of a similar size to therapeutic drugs, both within the vasculature and tumour tissue, may provide a useful marker of ultrasound-enhanced drug delivery and extravasation. Solid polymer based nanoscale cavitation nuclei, under FUS excitation, have previously been shown to extravasate into tissue-mimicking phantoms, and to increase drug delivery in murine tumour models in vivo. Here we show in a tissue-mimicking material that these nuclei, once extravasated under FUS excitation, are still acoustically active and can be non-invasively localised using passive acoustic mapping (PAM). By using a high resolution dual linear array setup in conjunction with adaptive beamformers, we demonstrate that the average 'maximum distance' of a PAM pixel to an extravasated particle across experiments is mm. Although the primary objective of the paper is to show that extravascular cavitation can be used as evidence of successful drug extravasation in a tissue-mimicking phantom, we also recognise the physical and computational limitations of using a high resolution dual array setup with adaptive beamformers. Thus as a secondary objective, we evaluate tradeoffs between adaptive and non-adaptive beamformers, as well as between dual and single array geometries. When compared to a conventional beamformer, adaptive beamformers reduce the maximum distance of PAM pixels to extravasated particles from an average of mm to mm in the single array case. The distance is further reduced to mm using the dual array configuration, thereby demonstrating that increasing the solid angle spanned by the PAM array aperture significantly improves drug delivery localisation. Future work will test the applicability of PAM-based monitoring of ultrasound-enhanced drug delivery in vivo
Passive acoustic mapping of extravasation following ultrasoundenhanced drug delivery
The amount and distribution of chemotherapeutic agents delivered to tumours can vary significantly due to tumour heterogeneity, even under focussed ultrasound (FUS) assisted drug delivery regimes. The ability to non-invasively localise cavitation nuclei of a similar size to therapeutic drugs, both within the vasculature and tumour tissue, may provide a useful marker of ultrasound-enhanced drug delivery and extravasation. Solid polymer based nanoscale cavitation nuclei, under FUS excitation, have previously been shown to extravasate into tissue-mimicking phantoms, and to increase drug delivery in murine tumour models in vivo. Here we show in a tissue-mimicking material that these nuclei, once extravasated under FUS excitation, are still acoustically active and can be non-invasively localised using passive acoustic mapping (PAM). By using a high resolution dual linear array setup in conjunction with adaptive beamformers, we demonstrate that the average 'maximum distance' of a PAM pixel to an extravasated particle across experiments is mm. Although the primary objective of the paper is to show that extravascular cavitation can be used as evidence of successful drug extravasation in a tissue-mimicking phantom, we also recognise the physical and computational limitations of using a high resolution dual array setup with adaptive beamformers. Thus as a secondary objective, we evaluate tradeoffs between adaptive and non-adaptive beamformers, as well as between dual and single array geometries. When compared to a conventional beamformer, adaptive beamformers reduce the maximum distance of PAM pixels to extravasated particles from an average of mm to mm in the single array case. The distance is further reduced to mm using the dual array configuration, thereby demonstrating that increasing the solid angle spanned by the PAM array aperture significantly improves drug delivery localisation. Future work will test the applicability of PAM-based monitoring of ultrasound-enhanced drug delivery in vivo
Sum-of-harmonics method for improved narrowband and broadband signal quantification during passive monitoring of ultrasound therapies
Passive Acoustic Mapping (PAM) enables real-time monitoring of ultrasound therapies by beamforming acoustic emissions emanating from the ultrasound focus. Reconstruction of the narrowband or broadband acoustic emissions component enables mapping of different physical phenomena, with narrowband emissions arising from non-linear propagation and scattering, non-inertial cavitation or tissue boiling, and broadband (generally, of significantly lower amplitude) indicating inertial cavitation. Currently, accurate classification of the received signals based on pre- defined frequency-domain comb filters cannot be guaranteed because varying levels of leakage occur as a function of signal amplitude and the choice of windowing function. This work presents a time-domain parametric model aimed at enabling accurate estimation of the amplitude of time-varying narrowband components in the presence of broadband signals. Conversely, the method makes it possible to recover a weak broadband signal in the presence of a dominant harmonic or other narrowband component. Compared to conventional comb filtering, the proposed sum-of- harmonics method enables PAM of cavitation sources that better reflect their physical location and extent
Passive acoustic mapping using data-adaptive beamforming based on higher-order statistics
Sources of nonlinear acoustic emissions, particularly those associated with cavitation activity, play a key role in the safety and efficacy of current and emerging therapeutic ultrasound applications, such as oncological drug delivery, blood-brain barrier opening and histotripsy. Passive Acoustic Mapping (PAM) is the first technique to enable real-time and non-invasive imaging of cavitation activity during therapeutic ultrasound exposure, through the recording and passive beamforming of broadband acoustic emissions using an array of ultrasound detectors. Initial limitations in PAM spatial resolution led to the adoption of optimal data-adaptive beamforming algorithms, such as the Robust Capon Beamformer (RCB), that provide improved interference suppression and calibration error mitigation compared to non-adaptive beamformers. However, such approaches are restricted by the assumption that the recorded signals have a Gaussian distribution. To overcome this limitation and further improve the source resolvability of PAM, we propose a new beamforming approach termed Robust Beamforming by Linear Programming (RLPB). Along with the variance, this optimization-based method uses higher-order-statistics of the recorded signals, making no prior assumption on the statistical distribution of the acoustic signals. RLPB is found via numerical simulations to improve resolvability over TEA and RCB. In vitro experimentation yielded improved resolvability with respect to the source-to-array distance on the order of 22% axially and 13% transversely relative to RCB, whilst successfully accounting for array calibration errors. The improved resolution and decreased dependence on accurate calibration of RLPB is expected to facilitate the clinical translation of PAM for diagnostic, including super-resolution, and therapeutic ultrasound applications
Real-time three-dimensional passive cavitation detection for clinical high intensity focussed ultrasound systems
Bubble activity during High Intensity Focussed Ultrasound (HIFU) surgery has been linked with desirable effects, such as an enhanced heat deposition caused by inertial cavitation, and undesirable effects, such as lesion migration caused by boiling bubbles. There is presently no reliable way of achieving spatiotemporal monitoring of cavitation activity during clinical HIFU treatments. In the present work, a near-acoustically-transparent two-dimensional 32-element PVDF array was designed and mounted on the therapy transducer of a clinical HIFU device (Model JC200, Chongqing Haifu) to enable detection of acoustic emissions arising from cavitation during therapy. The signal detected by each of the elements was digitized and processed in real time on a Graphical Processing Unit (GPU), and beamformed using our previously described Passive Acoustic Mapping (PAM) algorithm to produce real-time three-dimensional (3D) maps of cavitation activity with a frame rate in excess of 5 Hz. The system was initially validated in agar-based tissue-mimicking materials, demonstrating that the displayed volume of cavitation activity agreed with predictions based on in situ pressure calibrations. The system was further validated during clinical HIFU treatments of kidney tumour, liver tumour and uterine fibroid ablation, and was found to enable accurate localization of the HIFU focus at sub-lesioning intensities. © 2013 Acoustical Society of America
Passive acoustic mapping using data-adaptive beamforming based on higher-order statistics
Sources of nonlinear acoustic emissions, particularly those associated with cavitation activity, play a key role in the safety and efficacy of current and emerging therapeutic ultrasound applications, such as oncological drug delivery, blood-brain barrier opening and histotripsy. Passive Acoustic Mapping (PAM) is the first technique to enable real-time and non-invasive imaging of cavitation activity during therapeutic ultrasound exposure, through the recording and passive beamforming of broadband acoustic emissions using an array of ultrasound detectors. Initial limitations in PAM spatial resolution led to the adoption of optimal data-adaptive beamforming algorithms, such as the Robust Capon Beamformer (RCB), that provide improved interference suppression and calibration error mitigation compared to non-adaptive beamformers. However, such approaches are restricted by the assumption that the recorded signals have a Gaussian distribution. To overcome this limitation and further improve the source resolvability of PAM, we propose a new beamforming approach termed Robust Beamforming by Linear Programming (RLPB). Along with the variance, this optimization-based method uses higher-order-statistics of the recorded signals, making no prior assumption on the statistical distribution of the acoustic signals. RLPB is found via numerical simulations to improve resolvability over TEA and RCB. In vitro experimentation yielded improved resolvability with respect to the source-to-array distance on the order of 22% axially and 13% transversely relative to RCB, whilst successfully accounting for array calibration errors. The improved resolution and decreased dependence on accurate calibration of RLPB is expected to facilitate the clinical translation of PAM for diagnostic, including super-resolution, and therapeutic ultrasound applications
Dual array passive acoustic mapping for cavitation imaging with enhanced 2-D resolution
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 paper 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 7.6 cm. 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
Derepression of glomerular filtration, renal blood flow and antioxidant defence in patients with type 2 diabetes at high-risk of cardiorenal disease
Background
The role of antioxidant status on microvascular blood flow and glomerular filtration (eGFR) in patients with type 2 diabetes and hypertension whose risk of progressive renal disease varies by ethnicity is unknown.
Methods
Adult, non-Caucasian (n=101) and Caucasian (n=69) patients with type 2 diabetes, hypertension and/or microalbuminuria and an eGFR > 45 mL/min/1.73m2 were randomised to 400 IU vitamin E and/or 20 ug selenium daily or matching placebo. eGFR (CKD-EPI) was measured at baseline, 3,6 and 12 months and renal blood flow by contrast-enhanced ultrasonography in a sub-group (n=9) at baseline and 3 months by assessing the area under the time intensity curve (TIC). Circulating glutathione peroxidase 3 (GPx-3) activity was measured as a biomarker of oxidative defence status.
Results
The time to change in eGFR was shortest with combined vitamin E and selenium than usual care (5.6 [4.0-7.0] vs 8.9 [6.8-10.9 months]; p=0.006). Area under the TIC was reduced compared to baseline (38.52 [22.41- 90.49] vs 123 [86.98- 367.03]dB.s; P≤0.05 and 347 [175.88- 654.92] vs 928.03 [448.45-1683]dB.s; P≤0.05, respectively] at 3 months suggesting an increase in rate of perfusion. The proportional change in eGFR at 12 months was greater in the group whose GPx-3 activity was above, compared with those below the cohort median (360 U/L) in the non-Caucasian and the Caucasian groups (19.1(12.5 to 25.7] % vs 6.5[-3.5 to 16.5] % and 12.8 [0.7 to 24] % vs 0.2 [-6.1 to 6.5] %). .
Conclusion
In these patients with type 2 diabetes and early CKD, antioxidant treatment derepresses renal blood flow and a rise in eGFR correlated directly with GPx-3 activity.
Significance
Diabetes mellitus is the world’s leading cause of end-stage renal disease which has a predilection for black and minor ethnic groups compared with Caucasians. The differences in risk despite the benefits of conventional care may be related to oxidative stress. We found that glomerular filtration and renal blood flow is suppressed when renal function is preserved in high-risk patients with type 2 diabetes. Conventional care supplemented with selenium - the co-factor for glutathione peroxidase-3 (GPx-3) - improves renal blood flow and glomerular filtration in sub-groups with lower host antioxidant defence determined by GPx-3 activity. Circulating GPx-3 activity warrants further investigation as a novel biomarker of reversible haemodynamic changes in early diabetic kidney disease to better enable targeting of renoprotective strategies