Acoustic Heterogeneities Research Articles

Weighted model-based optoacoustic reconstruction for partial-view geometries.

Acoustic heterogeneities in biological samples are known to cause artifacts in tomographic optoacoustic (photoacoustic) image reconstruction. A statistical weighted model-based reconstruction approach was previously introduced to mitigate such artifacts. However, this approach does not reliably provide high-quality reconstructions for partial-view imaging systems, which are common in preclinical and clinical optoacoustics. In this article, the capability of the weighted model-based algorithm is extended to generate optoacoustic reconstructions with less distortions for partial-view geometry data. This is achieved by manipulating the weighting scheme based on the detector geometry. Using partial-view optoacoustic tomography data from a tissue-mimicking phantom containing a strong acoustic reflector, tumors grafted onto mice, and a mouse brain with intact skull, the proposed partial-view-corrected weighted model-based algorithm is shown to mitigate reflection artifacts in reconstructed images without distorting structures or boundaries, compared with both conventional model-based and the weighted model-based algorithms. It is also demonstrated that the partial-view-corrected weighted model-based algorithm has the additional advantage of suppressing streaking artifacts due to the partial-view geometry itself in the presence of a very strong optoacoustic chromophore. Due to its enhanced performance, the partial-view-corrected weighted model-based algorithm may prove useful for improving the quality of partial-view multispectral optoacoustic tomography, leading to enhanced visualization of functional parameters such as tissue oxygenation.

Modeling combined ultrasound and photoacoustic imaging: Simulations aiding device development and artificial intelligence

Combined ultrasound and photoacoustic (USPA) imaging has attracted several pre-clinical and clinical applications due to its ability to simultaneously display structural, functional, and molecular information of deep biological tissue in real time. However, the depth and wavelength dependent optical attenuation and the unknown optical and acoustic heterogeneities limit the USPA imaging performance in deep tissue regions. Novel instrumentation, image reconstruction, and artificial intelligence (AI) methods are currently being investigated to overcome these limitations and improve the USPA image quality. Effective implementation of these approaches requires a reliable USPA simulation tool capable of generating US based anatomical and PA based molecular contrasts of deep biological tissue. Here, we developed a hybrid USPA simulation platform by integrating finite element models of light (NIRFast) and ultrasound (k-Wave) propagations for co-simulation of B-mode US and PA images. The platform allows optimization of different design parameters for USPA devices, such as the aperture size and frequency of both light and ultrasound detector arrays. For designing tissue-realistic digital phantoms, a dictionary-based function has been added to k-Wave to generate various levels of ultrasound speckle contrast. The feasibility of modeling US imaging combined with optical fluence dependent multispectral PA imaging is demonstrated using homogeneous as well as heterogeneous tissue phantoms mimicking human organs (e.g., prostate and finger). In addition, we also demonstrate the potential of the simulation platform to generate large scale application-specific training and test datasets for AI enhanced USPA imaging. The complete USPA simulation codes together with the supplementary user guides have been posted to an open-source repository (https://github.com/KothapalliLabPSU/US-PA_simulation_codes).

Aberration correction in transcranial ultrasonic imaging using CT data and simulation-based focusing algorithms

The complex structure of the skull bone, reflected in particular by spatial variations in thickness and density, leads to a heterogeneity of its acoustic properties. This results in a strong attenuation as well as specific phase shifts of the ultrasonic wave during its crossing, leading to a defocusing of the beam. The improvement of the quality of brain ultrasonography requires knowledge of these acoustic heterogeneities to shape the ultrasonic wavefront during the focusing and imaging processes. CT scan realized on a phantom of a human skull associated to a fusion tool available on an ultrafast ultrasound device (Aixplorer, Supersonic Imagine, France) allows the positioning of the phased array in real-time in the CT volume. Then, for a fixed position of the probe, the real geometry of the skull region insonified by the array can be used as input data of the simulation based imaging algorithms. In particular, the Plane Wave Imaging (PWI) and Total Focusing Method (TFM) algorithms exploit a direct model of beam propagation through 3-D complex surfaces in order to simulate and correct the aberrations due to the skull crossing. We illustrate the potential of this technique on several examples of brain echography.

Thermoacoustic Range Verification in the Presence of Acoustic Heterogeneity and Soundspeed Errors– Accuracy and Robustness Relative to Live Ultrasound Images

Lack of online range verification limits efficacy of particle therapy for many tumor sites. Real-time thermoacoustic range verification could enable more aggressive treatment planning and hypofractionation for tumor sites that can be imaged with ultrasound during treatment. Our objective was to experimentally demonstrate accuracy and robustness of thermoacoustic range estimates relative to ultrasound images despite acoustic heterogeneity and discrepancies between assumed and true soundspeed. Prior results were for weak acoustic scatterers with known soundspeeds. 250 ns pulses of 0.26 Gy of 16 MeV protons and 2.3 Gy of 60 MeV helium ions were delivered to water and oil targets, respectively. Thermoacoustic signals with DC-4 MHz bandwidth were detected by a 96-channel ultrasound array placed 6-10 cm distal to the Bragg peak. One-way beamforming with an assumed soundspeed was performed to estimate range. The same soundspeed and transducer array were used to generate ultrasound images via two-way ultrasound beamforming. An air gap phantom displaced the Bragg peak by 6.5 mm to demonstrate accuracy. The scanner’s soundspeed setting was altered by ±5% to demonstrate robustness to soundspeed errors. Tissue mimicking gelatin and a 5 mm thick bone sample were introduced to demonstrate robustness to acoustic heterogeneity. Single ion pulse measurements sufficed during the helium run, but signal averaging was required for protons. Estimates of the entry point agreed with the air-target interface in ultrasound images and range estimates agreed with Monte Carlo simulations to within 300 μm microns, even when TA emissions traveled through a strong acoustic scatterer. Estimated Bragg peak locations were translated 6.5 mm by the air gap phantom and correctly identified scenarios when the beam stopped inside bone, but did not accurately estimate range in bone. Soundspeed errors dilate and acoustic heterogeneities deform ultrasound images. Thermoacoustic range estimates are transformed similarly and are robust relative to ultrasound images of underlying anatomy. When the target can be visualized with ultrasound during treatment, thermoacoustic range verification may enable real-time motion management and range verification. Therapeutic systems deliver higher (200+ MeV) energy protons using pulse durations exceeding 5 μs and generate thermoacoustic emissions with sonar bandwidths (DC-100 kHz). Therefore, custom acoustic hardware will be required to detect low frequency thermoacoustic emissions and also generate high-resolution ultrasound images.Abstract 3713; Table 1Experimentwaterwater & bonewater & air gapsafflower oilIonprotonprotonproton4HeBeam Energy (MeV)16.0±0.1516.0±0.1516.0±0.1560.7±0.4Bragg curve FWHM (μm)390240390230distal HWTM (μm)1609016080Monte Carlo Range (mm)2.471.948.952.51Thermoacoustic Range (mm)2.65±0.091.36±0.616.59±0.042.76±0.04N8887 Open table in a new tab

Thermoacoustic range verification in the presence of acoustic heterogeneity and soundspeed errors - Robustness relative to ultrasound image of underlying anatomy.

To demonstrate robustness of thermooacoustic range verification to acoustic heterogeneity and discrepancies between assumed and true propagation speed, i.e., soundspeed errors. A beam sweeper was used to deliver 250ns pulses that deposited 0.26Gy of 16MeV protons and 2.3Gy of 60MeV helium ions into water and oil targets, respectively. Thermoacoustic signals were detected by a 96-channel ultrasound array with a 1-4MHz sensitivity band (-6dB), bandpass filtered and backprojected to create thermoacoustic images in the plane of the ultrasound array. The same soundspeed and transducer array were used to estimate range and generate the ultrasound images onto which Bragg peak locations were overlaid. An air-gap phantom that displaced the Bragg peak by 6.5mm demonstrated accuracy. Robustness to soundspeed errors was demonstrated in a waterbath as the assumed propagation speed scanner setting was altered by . Tissue-mimicking gelatin and a bone sample were introduced to demonstrate robustness to acoustic heterogeneity relative to ultrasound images of the underlying morphology. Single ion pulse measurements sufficed during the helium run, but signal averaging was required for protons. Range and entry point into the target were estimated from data collected by transducers placed at least 6cm distal to the Bragg peak. When ultrasound images depicted the air-target interface where the beam enters, estimates of the entry point agreed with ultrasound images and range estimates agreed with Monte Carlo simulations to within 300μm, even when thermoacoustic emissions traveled through a strongly scattering bone sample. Estimated Bragg peak locations were translated 6.5mm by the air-gap phantom and correctly identified scenarios when the beam stopped inside the bone. Soundspeed errors dilate and acoustic heterogeneities deform ultrasound images. When thermoacoustic receivers are co-located with the ultrasound imaging array, the same transformations shift thermoacoustic range estimates. Therefore, thermoacoustic range verification is robust relative to ultrasound images of underlying anatomy. When the treatment target is visible in ultrasound, e.g., prostate, online thermoacoustic range estimates could verify that the treatment spot is inside the target.

Analysis of reservoir heterogeneities and depositional environments: a new method

A new methodology is presented to quantify reservoir heterogeneities based on a Monte Carlo parameter estimation technique from sonic logs. The acoustic reservoir heterogeneities are then quantified as an indicator to differentiate depositional environments and reservoir facies for geological interpretation in this study. Fractal statistics provides us with a framework to model reservoir heterogeneities with different Hurst numbers, correlation lengths and fluctuation standard deviations from sonic logs. These fractal parameters are derived from the von Karman autocorrelation model and estimated by an improved methodology using a Monte Carlo parameter estimation technique. Unlike the regular estimation method, the Monte Carlo parameter estimation technique is more stable. The resulting Hurst numbers, correlation lengths and root mean square (RMS) heights from 20 sonic logs are mapped in a reservoir with sandstone–mudstone sequences over complex continental deposits in north-eastern China. The spatial distribution of estimated reservoir heterogeneity parameters are then correlated with depositional facies interpretation derived from seismic attributes and petrophysical properties based on prior geological knowledge. Numerical experiments show that the Monte Carlo parameter estimation technique is successful in recovering acoustic heterogeneity parameters from sonic logs. Results from qualitative correlational analysis of sonic logs from the complex continent deposits in north-eastern China show that these parameters can be key discriminants for depositional facies and environments and may be utilised as a constraint in reservoir characterisation. Maps of Hurst numbers and correlation lengths strongly correspond with reservoir facies distributions, whereas maps of RMS heights correlate significantly with fluvial depositional patterns. The results were generally uniform even when different depth ranges within the formation of interest were used as input for parameter estimation. Numerical values of reservoir heterogeneity parameters may not fully recover the geology of a given reservoir but their distribution in space is an important indication of the morphological features of depositional environments.

Mitigation of artifacts due to isolated acoustic heterogeneities in photoacoustic computed tomography using a variable data truncation-based reconstruction method.

Photoacoustic computed tomography (PACT) is an emerging computed imaging modality that exploits optical contrast and ultrasonic detection principles to form images of the absorbed optical energy density within tissue. If the object possesses spatially variant acoustic properties that are unaccounted for by the reconstruction method, the estimated image can contain distortions. While reconstruction methods have recently been developed to compensate for this effect, they generally require the object’s acoustic properties to be known a priori. To circumvent the need for detailed information regarding an object’s acoustic properties, we previously proposed a half-time reconstruction method for PACT. A half-time reconstruction method estimates the PACT image from a data set that has been temporally truncated to exclude the data components that have been strongly aberrated. However, this method can be improved upon when the approximate sizes and locations of isolated heterogeneous structures, such as bones or gas pockets, are known. To address this, we investigate PACT reconstruction methods that are based on a variable data truncation (VDT) approach. The VDT approach represents a generalization of the half-time approach, in which the degree of temporal truncation for each measurement is determined by the distance between the corresponding ultrasonic transducer location and the nearest known bone or gas void location. Computer-simulated and experimental data are employed to demonstrate the effectiveness of the approach in mitigating artifacts due to acoustic heterogeneities.

Real-Time Model-Based Inversion in Cross-Sectional Optoacoustic Tomography.

Analytical (closed-form) inversion schemes have been the standard approach for image reconstruction in optoacoustic tomography due to their fast reconstruction abilities and low memory requirements. Yet, the need for quantitative imaging and artifact reduction has led to the development of more accurate inversion approaches, which rely on accurate forward modeling of the optoacoustic wave generation and propagation. In this way, multiple experimental factors can be incorporated, such as the exact detection geometry, spatio-temporal response of the transducers, and acoustic heterogeneities. The model-based inversion commonly results in very large sparse matrix formulations that require computationally extensive and memory demanding regularization schemes for image reconstruction, hindering their effective implementation in real-time imaging applications. Herein, we introduce a new discretization procedure for efficient model-based reconstructions in two-dimensional optoacoustic tomography that allows for parallel implementation on a graphics processing unit (GPU) with a relatively low numerical complexity. By on-the-fly calculation of the model matrix in each iteration of the inversion procedure, the new approach results in imaging frame rates exceeding 10 Hz, thus enabling real-time image rendering using the model-based approach.

Acoustic and thermal simulations of tcMRgFUS in patient specific models: validation with experiments

In tcMRgFUS, acoustic and spatial heterogeneities of the skull cause reflection, attenuation, and phase aberrations of the acoustic beams, which may cause patient-specific thermal responses to the same transducer power. In this work, we use acoustic and thermal simulations based on patient-specific 3D heterogeneous tissue models to predict heating at the center of the brain. We validate the model through comparison of the simulated temperature rises to experimentally derived energies for five patients treated using tcMRgFUS. Further, we separate the components of energy loss in the acoustic simulations into reflection-only, attenuation-only, phase aberration-only, and reflection and attenuation both to understand the cause of potential inter-patient variability in these treatments.

Coherent fiber bundle based integrated photoacoustic, ultrasound and fluorescence imaging (PAUSFI) for endoscopy and diagnostic bio-imaging applications

Recent research in diagnostic imaging and sensing focuses on deriving complementary information from the diagnosed site. From that perspective it is imperative to devise new imaging platforms where multiple distinct modalities are used either simultaneously or sequentially. Increased efforts have been devoted towards establishing such multi-modal imaging systems, which house and operate more than two imaging modalities within a single instrumentation set-up. In this context, we propose a novel multi-modal imaging platform using non-ionizing radiation that has been successfully conceptualized, established and experimentally demonstrated. This proposed GRIN lensed fiber-optic microscope and linear array transducer based PAUSFI (photoacoustic, ultrasound and fluorescence imaging) system makes use of non-ionizing radiation sources to map optical and acoustic heterogeneities (complementary information) along the depth of the tissue at multi-scale resolution (microscopic to mesoscopic). The fiber-optic assembly enables the system to perform minimally invasive remote light delivery and high resolution fluorescence and photoacoustic imaging of inaccessible areas of intact tissues or intra body cavities. It is expected that the proposed multi-modal imaging system could open up niches in bio-imaging research in the near future.

Effects of small variations of speed of sound in optoacoustic tomographic imaging.

Speed of sound difference in the imaged object and surrounding coupling medium may reduce the resolution and overall quality of optoacoustic tomographic reconstructions obtained by assuming a uniform acoustic medium. In this work, the authors investigate the effects of acoustic heterogeneities and discuss potential benefits of accounting for those during the reconstruction procedure. The time shift of optoacoustic signals in an acoustically heterogeneous medium is studied theoretically by comparing different continuous and discrete wave propagation models. A modification of filtered back-projection reconstruction is subsequently implemented by considering a straight acoustic rays model for ultrasound propagation. The results obtained with this reconstruction procedure are compared numerically and experimentally to those obtained assuming a heuristically fitted uniform speed of sound in both full-view and limited-view optoacoustic tomography scenarios. The theoretical analysis showcases that the errors in the time-of-flight of the signals predicted by considering the straight acoustic rays model tend to be generally small. When using this model for reconstructing simulated data, the resulting images accurately represent the theoretical ones. On the other hand, significant deviations in the location of the absorbing structures are found when using a uniform speed of sound assumption. The experimental results obtained with tissue-mimicking phantoms and a mouse postmortem are found to be consistent with the numerical simulations. Accurate analysis of effects of small speed of sound variations demonstrates that accounting for differences in the speed of sound allows improving optoacoustic reconstruction results in realistic imaging scenarios involving acoustic heterogeneities in tissues and surrounding media.

Integrated photoacoustic, ultrasound and fluorescence platform for diagnostic medical imaging-proof of concept study with a tissue mimicking phantom.

The structural and molecular heterogeneities of biological tissues demand the interrogation of the samples with multiple energy sources and provide visualization capabilities at varying spatial resolution and depth scales for obtaining complementary diagnostic information. A novel multi-modal imaging approach that uses optical and acoustic energies to perform photoacoustic, ultrasound and fluorescence imaging at multiple resolution scales from the tissue surface and depth is proposed in this paper. The system comprises of two distinct forms of hardware level integration so as to have an integrated imaging system under a single instrumentation set-up. The experimental studies show that the system is capable of mapping high resolution fluorescence signatures from the surface, optical absorption and acoustic heterogeneities along the depth (>2cm) of the tissue at multi-scale resolution (<1µm to <0.5mm).

Temporal and spatial variability of animal sound within a neotropical forest

Soundscape ecology aims to use biological, geophysical and anthropogenic sound to understand natural-human landscape dynamics. The analysis of natural soundscapes with no human noise is a prerequisite to understand and quantify the effects of human activity on animal ecology linked to sound. Preserved tropical forests are the location of unique, highly diverse, and animal sound. However, although the acoustic behavior of several tropical species has been examined, very few analyses have attempted tropical sounds at a spatial scale able to incorporate landscape characters. Here we analyze the acoustic structure of a neotropical forest landscape in French Guiana. We used a four dimensional synchronous acoustic sampling (three spatial dimensions and the temporal dimension) by deploying an array of 24 microphones in the understory and canopy of the Nouragues Nature Reserve during a 43day period and we undertook a detailed signal analysis to detect spatial and temporal animal acoustic heterogeneity. We identified a clear pattern of acoustic activity with four distinct periods of activity that differed by their spectral characteristics indicating acoustic heterogeneity along the 24-hourcycle but periodicity at a longer time scale. We revealed acoustic divergences between the understory and the canopy layers in terms of amplitude level and frequency content. We highlighted vertical (understory/canopy) and horizontal acoustic heterogeneities with a more diverse (frequency) patch in the north of the study area sampled and a more active (intensity) patch in the southeast of the study area. Our results show that the soundscape of a tropical forest, in the absence of human disturbance, is subtly structured in time and is heterogeneous in space. This structure is probably linked to endogenous factors that rule out the acoustic time activity of animal species, to the vertical stratification of singing communities or guilds, to horizontal variations in the distributions of species and to vegetation spatial heterogeneity. Our study emphasizes that tropical soundscapes need to be recorded and analyzed in considerable spatial and temporal detail to understand their dynamics without the presence of human produced noise. Our analysis also suggests that tropical forests are unique places for acoustic diversity, supporting the need for preservation from all perturbations including anthropic noise.

Rapid aberration correction for transcranial magnetic resonance-guided focused ultrasound surgery using a hybrid simulation and magnetic resonance-acoustic radiation force imaging method

Transcranial magnetic resonance-guided focused ultrasound surgery is a technique for causing tissue necrosis in the brain though the intact skull. Skull spatial and acoustic heterogeneities cause changes in the location, shape, and intensity of the focus. Current techniques use computed tomography (CT) imaging or MR-acoustic radiation force images (MR-ARFI) to correct these aberrations. CT-based techniques approximate acoustic parameters from Hounsfield units but suffer from co-registration concerns. MR-ARFI-based techniques use MR images as feedback to manipulate transducer phases, but require many image acquisitions (~4000) for one correction [Herbert, IEEE-TUFFC 56(11)2388–2399]. We demonstrate here a hybrid technique that uses one MR-ARFI image to improve the focal intensity. The hybrid simulation-MR-ARFI technique used an optimization routine to iteratively modify the simulation aberrations to minimize the difference between simulated and experimental radiation force patterns. Experiments were conducted by applying skull-based aberrations to a 1024-element, 550 kHz phased-array transducer. The experimental MR-ARFI image of the aberrated focus was used with the simulation pattern from the hybrid angular spectrum [Vyas, IEEE-TUFFC 59(6)1093–1100] beam propagation technique to estimate aberrations. The experiment was repeated three times. The hybrid simulation-MR-ARFI technique resulted in an average increase in focal MR-ARFI phase of 44%, and recovered 83% of the ideal MR-ARFI phase.

Acoustic Inversion in Optoacoustic Tomography: A Review.

Optoacoustic tomography enables volumetric imaging with optical contrast in biological tissue at depths beyond the optical mean free path by the use of optical excitation and acoustic detection. The hybrid nature of optoacoustic tomography gives rise to two distinct inverse problems: The optical inverse problem, related to the propagation of the excitation light in tissue, and the acoustic inverse problem, which deals with the propagation and detection of the generated acoustic waves. Since the two inverse problems have different physical underpinnings and are governed by different types of equations, they are often treated independently as unrelated problems. From an imaging standpoint, the acoustic inverse problem relates to forming an image from the measured acoustic data, whereas the optical inverse problem relates to quantifying the formed image. This review focuses on the acoustic aspects of optoacoustic tomography, specifically acoustic reconstruction algorithms and imaging-system practicalities. As these two aspects are intimately linked, and no silver bullet exists in the path towards high-performance imaging, we adopt a holistic approach in our review and discuss the many links between the two aspects. Four classes of reconstruction algorithms are reviewed: time-domain (so called back-projection) formulae, frequency-domain formulae, time-reversal algorithms, and model-based algorithms. These algorithms are discussed in the context of the various acoustic detectors and detection surfaces which are commonly used in experimental studies. We further discuss the effects of non-ideal imaging scenarios on the quality of reconstruction and review methods that can mitigate these effects. Namely, we consider the cases of finite detector aperture, limited-view tomography, spatial under-sampling of the acoustic signals, and acoustic heterogeneities and losses.

Enhancement of photoacoustic tomography by ultrasonic computed tomography based on optical excitation of elements of a full-ring transducer array

Photoacoustic computed tomography (PACT) is a hybrid technique that combines optical excitation and ultrasonic detection to provide high-resolution images in deep tissues. In the image reconstruction, a constant speed of sound (SOS) is normally assumed. This assumption, however, is often not strictly satisfied in deep tissue imaging, due to acoustic heterogeneities within the object and between the object and the coupling medium. If these heterogeneities are not accounted for, they will cause distortions and artifacts in the reconstructed images. In this Letter, we incorporated ultrasonic computed tomography (USCT), which measures the SOS distribution within the object, into our full-ring array PACT system. Without the need for ultrasonic transmitting electronics, USCT was performed using the same laser beam as for PACT measurement. By scanning the laser beam on the array surface, we can sequentially fire different elements. As a first demonstration of the system, we studied the effect of acoustic heterogeneities on photoacoustic vascular imaging. We verified that constant SOS is a reasonable approximation when the SOS variation is small. When the variation is large, distortion will be observed in the periphery of the object, especially in the tangential direction.

Focus Control Aided by Numerical Simulation in Heterogeneous Media for High-Intensity Focused Ultrasound Treatment

A focus control method based on time reversal aided by numerical simulation was implemented to correct focal errors in heterogeneous media by using a 56-element phased array transducer at a frequency of 2 MHz. Two types of acrylic phantom were employed to mimic the acoustic heterogeneities in the human body. Focal errors were significantly reduced by applying the phase correction. Moreover, the effects of amplitude correction, array configuration of the transducer, and transverse wave propagation in an elastic body were examined to improve the focal quality. The results suggested that the ultrasound was effectively converged on the target by correcting the amplitude of the ultrasound, and the accuracy of the focus control was enhanced by changing the array configuration and taking into account the shear elasticity of the elastic body in the simulation.

An ultrasound cylindrical phased array for deep heating in the breast: theoretical design using heterogeneous models

The objective of this theoretical study is to design an ultrasound (US) cylindrical phased array that can be used for hyperthermia (40–44 °C) treatment of tumours in the intact breast. Simultaneously, we characterize the influence of acoustic and thermal heterogeneities on the specific absorption rate (SAR) and temperature patterns to determine the necessity of using heterogeneous models for a US applicator design and treatment planning. Cylindrical configurations of monopole transducers are studied on their ability to generate interference patterns that can be steered electronically to the location of the target region. Hereto, design parameters such as frequency, number of transducers per ring, ring distance and number of rings are optimized to obtain a small primary focus, while suppressing secondary foci. The models account for local heterogeneities in both acoustic (wave velocity and absorption) and thermal (blood perfusion rate, heat capacity and conductivity) tissue properties. We used breast models with a central tumour (30 × 20 × 38 mm3) and an artificial thorax tumour (sphere with a radius of 25 mm) to test the design. Simulations predict that a US cylindrical phased array, consisting of six rings with 32 transducers per ring, a radius of 75 mm and 66 mm distance between the first and sixth transducer ring, operating at a frequency of 100 kHz, can be used to obtain 44 °C in the centre of tumours located anywhere in the intact breast. The dimensions of the volumes enclosed by the 41 °C iso-temperature are 19 × 19 × 21 mm3 and 21 × 21 × 32 mm3 for the central and the thorax tumours, respectively. It is demonstrated that acoustic and thermal heterogeneities do not disturb the SAR and temperature patterns.

Acoustic and petrophysical properties of a clastic deepwater depositional system from lithofacies to architectural elements’ scales

An analysis of acoustic, petrophysical, and stratigraphic heterogeneities has been completed at three scales for an outcropping/subcropping deep-water stratigraphic sequence: lithofacies (core/plug), lithostratigraphic unit (well log), and architectural element (seismic). Measurement techniques/instruments included outcrop measured sections; behind-outcrop drilling/logging/coring (and subsequent core and log analysis); ground-penetrating radar; shallow seismic reflection; and electromagnetic induction. At the lithofacies scale, four rock types are defined: (1) heterogeneous sandstones and (2) uniform sandstones, which differ in their grain composition and sedimentary structures, but do not differ significantly in average porosity, permeability, and acoustic impedance; and (3) organic-rich shales and (4) organic-poor shales, which exhibit significantly higher acoustic impedance than either sandstone type. There is an inverse relation between porosity and permeability versus acoustic impedance ofthe lithofacies at this scale. At the lithostratigraphic unit scale, three units of interbedded lithofacies are defined: (1) uniform sandstone prone, (2) heterogeneous sandstone prone, and (3) shale prone. Successive merging of thinner beds with thicker beds results in clear differences in average rock properties between the lithostratigraphic units, but there is insufficient variation about the averages to preclude statistically significant differentiation of the sandstones. Lithostratigraphic unit properties also vary laterally. At the architectural element scale, two architectural elements are channel element and lobe element. Only wellbore acoustic impedance differs significantly between these two elements. However, the internal lateral architecture of these two elements is quite different. The results highlight the difficulty in evaluating stratigraphic patterns away from the wellbore. More research in this area is warranted. Attempts to quantify lateral variability of properties in a geologically realistic manner are encouraged because lateral variability is as important to reservoir characterization and performance as is vertical variability.

Effects of acoustic heterogeneities on transcranial brain imaging with microwave-induced thermoacoustic tomography

The effects of acoustic heterogeneities on transcranial brain imaging with microwave-induced thermoacoustic tomography were studied. A numerical model for calculating the propagation of thermoacoustic waves through the skull was developed and experimentally examined. The model takes into account wave reflection and refraction at the skull surfaces and therefore provides improved accuracy for the reconstruction. To evaluate when the skull-induced effects could be ignored in reconstruction, the reconstructed images obtained by the proposed method were further compared with those obtained with the method based on homogeneous acoustic properties. From simulation and experimental results, it was found that when the target region is close to the center of the brain, the effects caused by the skull layer are minimal and both reconstruction methods work well. As the target region becomes closer to the interface between the skull and brain tissue, however, the skull-induced distortion becomes increasingly severe, and the reconstructed image would be strongly distorted without correcting those effects. In this case, the proposed numerical method can improve image quality by taking into consideration the wave refraction and mode conversion at the skull surfaces. This work is important for obtaining good brain images when the thickness of the skull cannot be ignored.