The first days of diagnostic ultrasound.

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

A Qualitative Analysis of Clinician Perspectives of Ultrasound Biofeedback for Speech Sound Disorders.

PURPOSE: Ultrasound biofeedback therapy (UBT) is a relatively new type of technology-assisted speech-language therapy and has shown promise in remediating speech sound disorders. However, there is a current lack of understanding of the barriers and benefits that may influence the usage behavior and clinical decision making for the implementation of UBT from a clinician perspective. In this qualitative study, we explore the perspectives of speech-language pathologists (SLPs) who have used ultrasound biofeedback in programs of speech sound therapy using the unified theory of acceptance and use of technology (UTAUT) model. METHOD: Seven SLPs who had clinical experience treating speech sound disorders with UBT participated. Semistructured in-depth interviews were conducted and video-recorded. Two coders coded and categorized the transcribed data, with consensus established with a third coder. Using thematic analysis, the data were exploratorily grouped into themes along components of the UTAUT model. RESULTS: The highest number of codes was sorted into the "effort expectancy" theme, followed by "performance expectancy," "social influence," and "facilitating conditions" themes of the UTAUT model. Clinicians identified multiple perceived barriers and benefits to the use of ultrasound technology. The top identified barrier was limited accessibility, and the top benefit was the ability to visualize a client's articulatory response to cues on a display. CONCLUSIONS: Clinicians prioritized "effort expectancy" and "performance expectancy" when reflecting on the use of ultrasound biofeedback for speech sound disorders. Clinicians spoke favorably about using UBT for speech sound disorder treatment but acknowledged institutional barriers and limitations at organizational and social levels.

Reconstruction of thermoacoustic emission sources induced by proton irradiation using numerical time reversal.

Objective.Mapping of dose delivery in proton beam therapy can potentially be performed by analyzing thermoacoustic emissions measured by ultrasound arrays. Here, a method is derived and demonstrated for spatial mapping of thermoacoustic sources using numerical time reversal, simulating re-transmission of measured emissions into the medium.Approach.Spatial distributions of thermoacoustic emission sources are shown to be approximated by the analytic-signal form of the time-reversed acoustic field, evaluated at the time of the initial proton pulse. Given calibration of the array sensitivity and knowledge of tissue properties, this approach approximately reconstructs the acoustic source amplitude, equal to the product of the time derivative of the radiation dose rate, mass density, and Grüneisen parameter. This approach was implemented using two models for acoustic fields of the array elements, one modeling elements as line sources and the other as rectangular radiators. Thermoacoustic source reconstructions employed previously reported measurements of emissions from proton energy deposition in tissue-mimicking phantoms. For a phantom incorporating a bone layer, reconstructions accounted for the higher sound speed in bone. Dependence of reconstruction quality on array aperture size and signal-to-noise ratio was consistent with previous acoustic simulation studies.Main results.Thermoacoustic source distributions were successfully reconstructed from acoustic emissions measured by a linear ultrasound array. Spatial resolution of reconstructions was significantly improved in the azimuthal (array) direction by incorporation of array element diffraction. Source localization agreed well with Monte Carlo simulations of energy deposition, and was improved by incorporating effects of inhomogeneous sound speed.Significance.The presented numerical time reversal approach reconstructs thermoacoustic sources from proton beam radiation, based on straightforward processing of acoustic emissions measured by ultrasound arrays. This approach may be useful for ranging and dosimetry of clinical proton beams, if acoustic emissions of sufficient amplitude and bandwidth can be generated by therapeutic proton sources.

Classification of accurate and misarticulated /ɑr/ for ultrasound biofeedback using tongue part displacement trajectories.

Ultrasound biofeedback therapy (UBT), which incorporates real-time imaging of tongue articulation, has demonstrated generally positive speech remediation outcomes for individuals with residual speech sound disorder (RSSD). However, UBT requires high attentional demands and may therefore benefit from a simplified display of articulation targets that are easily interpretable and can be compared to real-time articulation. Identifying such targets requires automatic quantification and analysis of movement features relevant to accurate speech production. Our image-analysis program TonguePART automatically quantifies tongue movement as tongue part displacement trajectories from midsagittal ultrasound videos of the tongue, with real-time capability. The present study uses such displacement trajectories to compare accurate and misarticulated American-English rhotic /ɑr/ productions from 40 children, with degree of accuracy determined by auditory perceptual ratings. To identify relevant features of accurate articulation, support vector machine (SVM) classifiers were trained and evaluated on several candidate data representations. Classification accuracy was up to 85%, indicating that quantification of tongue part displacement trajectories captured tongue articulation characteristics that distinguish accurate from misarticulated production of /ɑr/. Regression models for perceptual ratings were also compared. The simplest data representation that retained high predictive ability, demonstrated by high classification accuracy and strong correlation between observed and predicted ratings, was displacements at the midpoint of /r/ relative to /ɑ/ for the tongue dorsum and blade. This indicates that movements of the dorsum and blade are especially relevant to accurate production of /r/, suggesting that a predictive parameter and biofeedback target based on this data representation may be usable for simplified UBT.

Effect of Overpressure on Acoustic Emissions and Treated Tissue Histology in ex Vivo Bulk Ultrasound Ablation.

Bulk ultrasound ablation is a thermal therapy approach in which tissue is heated by unfocused or weakly focused sonication (average intensities on the order of 100 W/cm2) to achieve coagulative necrosis within a few minutes exposure time. Assessing the role of bubble activity, including acoustic cavitation and tissue vaporization, in bulk ultrasound ablation may help in making bulk ultrasound ablation safer and more effective for clinical applications. Here, two series of ex vivo ablation trials were conducted to investigate the role of bubble activity and tissue vaporization in bulk ultrasound ablation. Fresh bovine liver tissue was ablated with unfocused, continuous-wave ultrasound using ultrasound image-ablate arrays sonicating at 31 W/cm2 (0.9 MPa amplitude) for either 20 min at a frequency of 3.1 MHz or 10 min at 4.8 MHz. Tissue specimens were maintained at a static overpressure of either 0.52 or 1.2 MPa to suppress bubble activity and tissue vaporization or at atmospheric pressure for control groups. A passive cavitation detector was used to record subharmonic (1.55 or 2.4 MHz), broadband (1.2-1.5 MHz) and low-frequency (5-20 kHz) acoustic emissions. Treated tissue was stained with 2% triphenyl tetrazolium chloride to evaluate thermal lesion dimensions. Subharmonic emissions were significantly reduced in overpressure groups compared with control groups. Correlations observed between acoustic emissions and lesion dimensions were significant and positive for the 3.1-MHz series, but significant and negative for the 4.8-MHz series. The results indicate that for bulk ultrasound ablation, where both acoustic cavitation and tissue vaporization are possible, bubble activity can enhance ablation in the absence of tissue vaporization, but can reduce thermal lesion dimensions in the presence of vaporization.

Reconstruction of shear wave speed in tissue-mimicking phantoms from aliased pulse-echo imaging of high-frequency wavefields.

Quantitative elasticity estimation in medical and industrial applications may benefit from advancements in reconstruction of shear wave speed with enhanced resolution. Here, shear wave speed is reconstructed from pulse-echo ultrasound imaging of elastic waves induced by high-frequency (>400 Hz), time-harmonic mechanical excitation. Particle displacement in shear wavefields is mapped from measured interframe phase differences with compensation for timing of multiple scan lines, then processed by spatial Fourier analysis to estimate the predominant wave speed and analyzed by algebraic wavefield inversion to reconstruct wave speed maps. Reconstructions of shear wave speed from simulated wavefields illustrate the accuracy and spatial resolution available with both methods, as functions of signal-to-noise ratio and sizes of windows used for Fourier analysis or wavefield smoothing. The methods are applied to shear wavefields with frequencies up to six times the Nyquist rate, thus extending the frequency range measurable by a given imaging system. Wave speed measurements in tissue-mimicking phantoms are compared with supersonic shear imaging and mechanical tensile testing, demonstrating feasibility of the wavefield measurement and wave speed reconstruction methods employed.

Numerical analysis of three-dimensional echo decorrelation imaging.

A numerical model for three-dimensional echo decorrelation imaging, a pulse-echo ultrasound method applicable to thermal ablation monitoring, is presented. Beam patterns for steered transmit and receive array apertures are combined with a three-dimensional numerical tissue model to yield beamformed scan lines in a pyramidal configuration, volumetric B-mode images, and spatial maps of normalized decorrelation between sequential image volumes. Simulated three-dimensional echo decorrelation images of random media are analyzed as estimators of local tissue reflectivity decoherence, mimicking thermal ablation effects. The estimation error is analyzed as a function of correlation window size, scan line density, and ensemble averaging of decorrelation maps.

In vivo ultrasound thermal ablation control using echo decorrelation imaging in rabbit liver and VX2 tumor.

The utility of echo decorrelation imaging feedback for real-time control of in vivo ultrasound thermal ablation was assessed in rabbit liver with VX2 tumor. High-intensity focused ultrasound (HIFU) and unfocused (bulk) ablation were performed using 5 MHz linear image-ablate arrays. Treatments comprised up to nine lower-power sonications, followed by up to nine higher-power sonications, ceasing when the average cumulative echo decorrelation within a control region of interest exceeded a predefined threshold (- 2.3, log10-scaled echo decorrelation per millisecond, corresponding to 90% specificity for tumor ablation prediction in previous in vivo experiments). This threshold was exceeded in all cases for both HIFU (N = 12) and bulk (N = 8) ablation. Controlled HIFU trials achieved a significantly higher average ablation rate compared to comparable ablation trials without image-based control, reported previously. Both controlled HIFU and bulk ablation trials required significantly less treatment time than these previous uncontrolled trials. Prediction of local liver and VX2 tumor ablation using echo decorrelation was tested using receiver operator characteristic curve analysis, showing prediction capability statistically equivalent to uncontrolled trials. Compared to uncontrolled trials, controlled trials resulted in smaller thermal ablation regions and higher contrast between echo decorrelation in treated vs. untreated regions. These results indicate that control using echo decorrelation imaging may reduce treatment duration and increase treatment reliability for in vivo thermal ablation.

The effect of 220 kHz insonation scheme on rt-PA thrombolytic efficacy in vitro.

Ultrasound-enhanced recombinant tissue plasminogen activator (rt-PA) thrombolysis is under development as an adjuvant to ischemic stroke therapy. The goal of this study was to design a pulsed ultrasound (US) exposure scheme that reduced intracranial constructive interference and tissue heating, and maintained thrombolytic efficacy relative to continuous wave (CW) insonation. Three 220 kHz US schemes were evaluated, two pulsed insonation schemes (15 cycles, 68 µs pulse duration, 33% or 62.5% duty cycle) and an intermittent CW insonation scheme (50 s active, 30 s quiescent) over a 30-min treatment period. An in silico study using a finite-difference model of transcranial US propagation was performed to estimate the intracranial acoustic field and temperature rise in the skull for each insonation scheme. In vitro measurements with flow were performed to assess thrombolysis using time-lapse microscopy. Intracranial constructive interference was not reduced with pulsed US using a pulse length of 15 cycles compared to intermittent CW US. The 33.3% duty cycle pulsed US scheme reduced heating in the temporal bone as much as 60% relative to the intermittent CW scheme. All insonation schemes promoted sustained stable cavitation in vitro and augmented thrombolysis compared to rt-PA alone (p  < 0.05). Ultraharmonic (UH) and harmonic cumulative energy over a 30 min treatment period was significantly higher (p  < 0.05) for the intermittent CW US scheme compared to either pulsed US scheme. Despite the difference in cavitation emissions, no difference was observed in the clot lysis between the three US schemes. These findings demonstrate that a 33.3% duty cycle pulsed US scheme with a 15-cycle burst can reduce bone heating and achieve equivalent thrombolytic efficacy as an intermittent CW scheme.

Tongue Part Movement Trajectories for /r/ Using Ultrasound.

PURPOSE: Because it shows the movement of different parts of the tongue in real time, ultrasound biofeedback therapy is a promising technology for speech research and remediation. One limitation is the difficulty of interpreting real-time ultrasound images of tongue motion. Our image processing system, TonguePART, tracks the tongue surface and allows for the acquisition of quantitative tongue part trajectories. METHOD: TonguePART automatically identifies the tongue contour based on ultrasound image brightness and tracks motion of the tongue root, dorsum, and blade in real time. We present tongue part trajectory data from 2 children with residual sound errors on /r/ and 2 children with typical speech, focusing on /r/ (International Phonetic Alphabet ɹ) in the phonetic context /ɑr/. We compared the tongue trajectories to magnetic resonance images of sustained vowel /ɑ/ and /r/. RESULTS: Measured trajectories show larger overall displacement and greater differentiation of tongue part movements for children with typical speech during the production of /ɑr/, compared to children with residual speech sound disorders. CONCLUSION: TonguePART is a fast, reliable method of tracking articulatory movement of tongue parts for syllables such as /ɑr/. It is extensible to other sounds and phonetic contexts. By tracking tongue parts, clinical researchers can investigate lingual coordination. TonguePART is suitable for real-time data collection and biofeedback. Ultrasound biofeedback therapy users may make more progress using simplified biofeedback of tongue movement.

Optimized Echo Decorrelation Imaging Feedback for Bulk Ultrasound Ablation Control.

Feasibility of controlling bulk ultrasound (US) thermal ablation using echo decorrelation imaging was investigated in ex vivo bovine liver. The first of two ablation and control procedures used a sequence of constant-intensity sonication cycles, ceased when the minimum echo decorrelation within a control region of interest (ROI) exceeded a predetermined threshold. The second procedure used a variable-intensity sonication sequence, with spatially averaged decorrelation as the stopping criterion. US exposures and echo decorrelation imaging were performed by a linear image-ablate array. Based on preliminary experiments, control ROIs and thresholds for the minimum-decorrelation and average-decorrelation criteria were specified. Controlled trials for the minimum-decorrelation and average-decorrelation criteria were compared with uncontrolled trials employing 9 or 18 cycles of matching sonication sequences. Lesion dimensions, treatment times, ablation rates, and areas under receiver operating characteristic curves were statistically compared. Successfully controlled trials using both criteria required significantly shorter treatment times than corresponding 18-cycle treatments, with better ablation prediction performance than uncontrolled 9-cycle and 18-cycle treatments. Either control approach resulted in greater ablation rate than corresponding 9-cycle or 18-cycle uncontrolled approaches. A post hoc analysis studied the effect of exchanging control criteria between the two series of controlled experiments. For either group, the average time needed to exceed the alternative decorrelation threshold approximately matched the average duration of successfully controlled experimental trials. These results indicate that either approach, using minimum-decorrelation or average-decorrelation criteria, is feasible for control of bulk US ablation. In addition, use of a variable-intensity sonication sequence for bulk US thermal ablation can result in larger ablated regions compared to constant-intensity sonication sequences.

Characterization of cavitation-radiated acoustic power using diffraction correction.

A method is developed for compensating absolute pressure measurements made by a calibrated passive cavitation detector (PCD) to estimate the average acoustic power radiated from a region of interest (ROI) defined to encompass all cavitating bubbles. A diffraction correction factor for conversion of PCD-measured pressures to cavitation-radiated acoustic power per unit area or volume is derived as a simple analytic expression, accounting for position- and frequency-dependent PCD sensitivity. This approach can be applied to measurements made by any PCD without precise knowledge of the number, spatial, or temporal distribution of cavitating bubbles. The diffraction correction factor is validated in simulation for a wide range of ROI dimensions and frequencies. The correction factor is also applied to emission measurements obtained during in vitro ultrasound-enhanced sonophoresis experiments, allowing comparison of stable cavitation levels between therapeutic configurations with different source center frequencies. Results incorporating sonication at both 0.41 and 2.0 MHz indicate that increases in skin permeability correlate strongly with the acoustic power of subharmonic emissions radiated per unit skin area.

Real-Time Spatiotemporal Control of High-Intensity Focused Ultrasound Thermal Ablation Using Echo Decorrelation Imaging in ex Vivo Bovine Liver.

The ability to control high-intensity focused ultrasound (HIFU) thermal ablation using echo decorrelation imaging feedback was evaluated in ex vivo bovine liver. Sonications were automatically ceased when the minimum cumulative echo decorrelation within the region of interest exceeded an ablation control threshold, determined from preliminary experiments as -2.7 (log-scaled decorrelation per millisecond), corresponding to 90% specificity for local ablation prediction. Controlled HIFU thermal ablation experiments were compared with uncontrolled experiments employing two, five or nine sonication cycles. Means and standard errors of the lesion width, area and depth, as well as receiver operating characteristic curves testing ablation prediction performance, were computed for each group. Controlled trials exhibited significantly smaller average lesion area, width and treatment time than five-cycle or nine-cycle uncontrolled trials and also had significantly greater prediction capability than two-cycle uncontrolled trials. These results suggest echo decorrelation imaging is an effective approach to real-time HIFU ablation control.

Quantitative Frequency-Domain Passive Cavitation Imaging.

Passive cavitation detection has been an instrumental technique for measuring cavitation dynamics, elucidating concomitant bioeffects, and guiding ultrasound therapies. Recently, techniques have been developed to create images of cavitation activity to provide investigators with a more complete set of information. These techniques use arrays to record and subsequently beamform received cavitation emissions, rather than processing emissions received on a single-element transducer. In this paper, the methods for performing frequency-domain delay, sum, and integrate passive imaging are outlined. The method can be applied to any passively acquired acoustic scattering or emissions, including cavitation emissions. To compare data across different systems, techniques for normalizing Fourier transformed data and converting the data to the acoustic energy received by the array are described. A discussion of hardware requirements and alternative imaging approaches is additionally outlined. Examples are provided in MATLAB.

Echo Decorrelation Imaging of Rabbit Liver and VX2 Tumor during In Vivo Ultrasound Ablation.

In open surgical procedures, image-ablate ultrasound arrays performed thermal ablation and imaging on rabbit liver lobes with implanted VX2 tumor. Treatments included unfocused (bulk ultrasound ablation, N = 10) and focused (high-intensity focused ultrasound ablation, N = 13) exposure conditions. Echo decorrelation and integrated backscatter images were formed from pulse-echo data recorded during rest periods after each therapy pulse. Echo decorrelation images were corrected for artifacts using decorrelation measured prior to ablation. Ablation prediction performance was assessed using receiver operating characteristic curves. Results revealed significantly increased echo decorrelation and integrated backscatter in both ablated liver and ablated tumor relative to unablated tissue, with larger differences observed in liver than in tumor. For receiver operating characteristic curves computed from all ablation exposures, both echo decorrelation and integrated backscatter predicted liver and tumor ablation with statistically significant success, and echo decorrelation was significantly better as a predictor of liver ablation. These results indicate echo decorrelation imaging is a successful predictor of local thermal ablation in both normal liver and tumor tissue, with potential for real-time therapy monitoring.

Dependence of ultrasound echo decorrelation on local tissue temperature during ex vivo radiofrequency ablation.

This study investigates echo decorrelation imaging, an ultrasound method for thermal ablation monitoring. The effect of tissue temperature on the mapped echo decorrelation parameter was assessed in radiofrequency ablation experiments performed on ex vivo bovine liver tissue. Echo decorrelation maps were compared with corresponding tissue temperatures simulated using the finite element method. For both echo decorrelation imaging and integrated backscatter imaging, the mapped tissue parameters correlated significantly but weakly with local tissue temperature. Receiver operating characteristic (ROC) curves were used to assess the ability of echo decorrelation and integrated backscatter to predict tissue temperature greater than 40, 60, and 80 °C. Significantly higher area under the ROC curve (AUROC) values were obtained for prediction of tissue temperatures greater than 40, 60, and 80 °C using echo decorrelation imaging (AUROC = 0.871, 0.948 and 0.966) compared to integrated backscatter imaging (AUROC = 0.865, 0.877 and 0.832).

Accuracy of a bistatic scattering substitution technique for calibration of focused receivers.

A recent method for calibrating single-element, focused passive cavitation detectors (PCD) compares bistatic scattering measurements by the PCD and a reference hydrophone. Here, effects of scatterer properties and PCD size on frequency-dependent receive calibration accuracy are investigated. Simulated scattering from silica and polystyrene spheres was compared for small hydrophone and spherically focused PCD receivers to assess the achievable calibration accuracy as a function of frequency, scatterer size, and PCD size. Good agreement between measurements was found when the scatterer diameter was sufficiently smaller than the focal beamwidth of the PCD; this relationship was dependent on the scatterer material. For conditions that result in significant disagreement between measurements, the numerical methods described here can be used to correct experimental calibrations.

Methods to calibrate the absolute receive sensitivity of single-element, focused transducers.

Absolute pressure measurements of acoustic emissions by single-element, focused passive cavitation detectors would be facilitated by improved wideband receive calibration techniques. Here, calibration methods were developed to characterize the absolute, frequency-dependent receive sensitivity of a spherically focused, single-element transducer using pulse-echo and pitch-catch techniques. Validation of these calibration methods on a focused receiver were made by generating a pulse from a small diameter source at the focus of the transducer and comparing the absolute pressure measured by a calibrated hydrophone to that of the focused transducer using the receive sensitivities determined here.

Optimization of tissue physical parameters for accurate temperature estimation from finite-element simulation of radiofrequency ablation.

Computational finite element models are commonly used for the simulation of radiofrequency ablation (RFA) treatments. However, the accuracy of these simulations is limited by the lack of precise knowledge of tissue parameters. In this technical note, an inverse solver based on the unscented Kalman filter (UKF) is proposed to optimize values for specific heat, thermal conductivity, and electrical conductivity resulting in accurately simulated temperature elevations. A total of 15 RFA treatments were performed on ex vivo bovine liver tissue. For each RFA treatment, 15 finite-element simulations were performed using a set of deterministically chosen tissue parameters to estimate the mean and variance of the resulting tissue ablation. The UKF was implemented as an inverse solver to recover the specific heat, thermal conductivity, and electrical conductivity corresponding to the measured area of the ablated tissue region, as determined from gross tissue histology. These tissue parameters were then employed in the finite element model to simulate the position- and time-dependent tissue temperature. Results show good agreement between simulated and measured temperature.

Using passive cavitation images to classify high-intensity focused ultrasound lesions.

Passive cavitation imaging provides spatially resolved monitoring of cavitation emissions. However, the diffraction limit of a linear imaging array results in relatively poor range resolution. Poor range resolution has limited prior analyses of the spatial specificity and sensitivity of passive cavitation imaging in predicting thermal lesion formation. In this study, this limitation is overcome by orienting a linear array orthogonal to the high-intensity focused ultrasound propagation direction and performing passive imaging. Fourteen lesions were formed in ex vivo bovine liver samples as a result of 1.1-MHz continuous-wave ultrasound exposure. The lesions were classified as focal, "tadpole" or pre-focal based on their shape and location. Passive cavitation images were beamformed from emissions at the fundamental, harmonic, ultraharmonic and inharmonic frequencies with an established algorithm. Using the area under a receiver operating characteristic curve (AUROC), fundamental, harmonic and ultraharmonic emissions were found to be significant predictors of lesion formation for all lesion types. For both harmonic and ultraharmonic emissions, pre-focal lesions were classified most successfully (AUROC values of 0.87 and 0.88, respectively), followed by tadpole lesions (AUROC values of 0.77 and 0.64, respectively) and focal lesions (AUROC values of 0.65 and 0.60, respectively).