Analysis of index modulation in microembolic Doppler signals part I: radiation force as a new hypothesis-simulations

The purpose of this study was to reveal the cause of frequency modulation (FM) present in microembolic Doppler ultrasound signals. This novel explanation should help the development of sensitive microembolus discrimination techniques. We suggest that the frequency modulation detected is caused by the ultrasonic radiation force (URF) acting directly on microemboli. The frequency modulation and the imposed displacement were calculated using a numerical dynamic model. By setting simulation parameters with practical values, it was possible to reproduce most microembolic frequency modulation signatures. The most interesting findings in this study were that: (1) the ultrasound radiation force acting on a gaseous microembolus and its corresponding cumulative displacement were far higher than those obtained for a solid microembolus, and that is encouraging for discrimination purposes; and 2) the calculated frequency modulation indices (FMIs) (≈20 kHz) were in good agreement with literature results. By taking into account the URF, the flow pulsatility, the beam-to-flow angle and both the velocity and the ultrasound beam profiles, it was possible to explain all erratic FM signatures of a microbubble. Finally, by measuring FMI from simulated Doppler signals and by using a constant threshold of 1 KHz, it was possible to discriminate gaseous from solid microemboli with ease. (E-mail: jean-marc.girault@univ-tours.fr)     

Newtonian viscous effects in ultrasonic emboli removal from blood

We have modeled the removal of emboli from cardiopulmonary bypass circuits via acoustic radiation force. Unless removed, emboli can result in cognitive deficit for those undergoing heart surgery with the use of extracorporeal circuits. There are a variety of mathematical formulations in the literature describing acoustic radiation force, but a lingering question that remains is how important viscosity of the blood and/or embolus is to the process. We implemented both inviscid and viscous models for acoustic radiation force on a sphere immersed in a fluid. We found that for this specific application, the inviscid model seems to be sufficient for predicting acoustic force upon emboli when compared with the chosen viscous model. Thus, the much simpler inviscid model could be used to optimize experimental techniques for ultrasonic emboli removal.     

Subsample volume processing of doppler ultrasound signals

Processing of Doppler signals produced by pulsed Doppler systems is based on the assumption that the phase of the received high frequency ultrasound signals changes linearly with depth. However, the random spatial distribution of scatterers is not in accordance with this basic assumption. Consequently, averaging of the demodulated signal over an observation window, covering a few periods of the received signal, does not improve the estimate for the instantaneous quadrature components of the Doppler signal originating from a given depth. Hence, the accuracy of the Doppler velocity estimate is independent of the length of the observation window employed. However, splitting the observation window in subsample volumes, each with a length of one period at the emission frequency, and combining the Doppler signals of the subsample volumes at the last stage of signal processing, i.e., mean Doppler frequency estimation using the autocorrelation technique, results in a considerable reduction of the variance of the velocity estimate. Using a computer simulation of the signal processing involved, it is demonstrated that with subsample volume processing the variance of the velocity estimate attains the same variance as is expected for the RF cross correlation technique.     

Absence of gravity-dependent modulation of straight sinus flow velocity in healthy humans

The influence of whole-body positions on the cerebral blood flow in normal subjects is unclear. Blood flow in cerebral veins and sinuses is continuous, pulsatile and proportional to cerebral blood flow. We examined young healthy volunteers to evaluate peak mean flow velocity (vm) in the straight sinus (SS) assessed by transcranial Doppler sonography in predefined variations of the whole-body pitch position relative to gravity in the presence of a normal (normocarbia) and an impaired (hypercarbia) cerebral autoregulation. A 2 MHz ultrasound probe was fixed with a headband nearby the protuberantia occipitalis externa. Fifteen subjects were seated in a motorized three-dimensional turntable. Vm-SS, blood pressure and heart rate were monitored in five whole-body pitch positions from upright (0°) to “20° head-hanging” (110°): 0, 30, 60, 90 and 110°. The experiment was repeated during the inspiration of 5% CO₂. Of 15 subjects, 14 showed reliable ultrasound data; the results of one subject with movement artifacts were excluded. Vm-SS values under normocarbia (hypercarbia) were 23.9 ± 4.2 cm/s (40.9 ± 6.7 cm/s) at 0°, 23.1 ± 5.0 cm/s (38.0 ± 5.0 cm/s) at 30°, 24.9 ± 5.1 cm/s (39.9 ± 3.3 cm/s) at 60°, 29.2 ± 8.5 cm/s (41.0 ± 4.7 cm/s) at 90° and 27.0 ± 11.6 cm/s (43.6 ± 12.1 cm/s) at 110°. Vm-SS measured under normocarbia (p = 0.09) and hypercarbia (p = 0.25) were not affected while subjects were positioned from upright toward “20° head-hanging”, whereas blood pressure and heart rate decreased (p < 0.01). Our results suggest that changes of whole-body position from upright to “20° head-hanging” do not alter cerebral blood flow in healthy subjects. (E-mail: Ralf.Baumgartner@usz.ch)     

Acoustic radiation force impulse imaging of human prostates: initial in vivo demonstration

Reliably detecting prostate cancer (PCa) has been a challenge for current imaging modalities. Acoustic radiation force impulse (ARFI) imaging is an elasticity imaging method that uses remotely generated, focused acoustic beams to probe tissue stiffness. A previous study on excised human prostates demonstrated ARFI images portray various prostatic structures and has the potential to guide prostate needle biopsy with improved sampling accuracy. The goal of this study is to demonstrate the feasibility of ARFI imaging to portray internal structures and PCa in the human prostate in vivo. Custom ARFI imaging sequences were designed and implemented using a modified Siemens Antares™ scanner with a three-dimensional (3-D) wobbler, end-firing, trans-cavity transducer, EV9F4. Nineteen patients were consented and imaged immediately preceding surgical prostatectomy. Pathologies and anatomic structures were identified in histologic slides by a pathologist blinded to ARFI data and were then registered with structures found in ARFI images. The results demonstrated that when PCa is visible, it generally appears as bilaterally asymmetric stiff structures; benign prostatic hyperplasia (BPH) appears heterogeneous with a nodular texture; the verumontanum and ejaculatory ducts appears softer compared with surrounding tissue, which form a unique ‘V’ shape; and the boundary of the transitional zone (TZ) forms a stiff rim separating the TZ from the peripheral zone (PZ). These characteristic appearances of prostatic structures are consistent with those found in our previous study of prostate ARFI imaging on excised human prostates. Compared with the matched B-mode images, ARFI images, in general, portray prostate structures with higher contrast. With the end-firing transducer used for this study, ARFI depth penetration was limited to 22 mm. Image contrast and resolution were decreased as compared with the previous ex vivo study due to the small transducer aperture. Even with these limitations, this study suggests ARFI imaging holds promise for guidance of targeted prostate needle biopsy and focal therapy, as well as aiding assessment of changes during watchful waiting/active surveillance.     

Ultrasound-Based Measurement of Molecular Marker Concentration in Large Blood Vessels: A Feasibility Study

Ultrasound molecular imaging has demonstrated efficacy in pre-clinical studies for cancer and cardiovascular inflammation. However, these techniques often require lengthy protocols because of waiting periods or additional control microbubble injections. Moreover, they are not capable of quantifying molecular marker concentration in human tissue environments that exhibit variable attenuation and propagation path lengths. Our group recently investigated a modulated acoustic radiation force-based imaging sequence, which was found to detect targeted adhesion independent of control measurements. In the present study, this sequence was tested against various experimental parameters to determine its feasibility for quantitative measurements of molecular marker concentration. Results indicated that measurements obtained from the sequence (residual-to-saturation ratio, R_resid) were independent of acoustic pressure and attenuation (p > 0.13, n = 10) when acoustic pressures were sufficiently low. The R_resid parameter exhibited a linear relationship with measured molecular marker concentration (R² > 0.94). Consequently, feasibility was illustrated in vitro, for quantification of molecular marker concentration in large vessels using a modulated acoustic radiation force-based sequence. Moreover, these measurements were independent of absolute acoustic reflection amplitude and used short imaging protocols (3 min) without control measurements.     

Analysis of contrast in images generated with transient acoustic radiation force

Several mechanical imaging methods are under investigation that use focused ultrasound (US) as a source of mechanical excitation. Images are then generated of the tissue response to this localized excitation. One such method, acoustic radiation force impulse (ARFI) imaging, utilizes a single US transducer on a commercial US system to transmit brief, high-energy, focused acoustic pulses to generate radiation force in tissue and correlation-based US methods to detect the resulting tissue displacements. Local displacements reflect relative mechanical properties of tissue. The resolution of these images is comparable with that of conventional B-mode imaging. The response of tissue to focused radiation force excitation is complex and depends upon tissue geometry, forcing function geometry (i.e., region of excitation, or ROE) and tissue mechanical and acoustic properties. Finite element method (FEM) simulations using an experimentally validated model and phantom experiments have been performed using varying systems, system configurations and tissue-mimicking phantoms to determine their impact on image quality. Image quality is assessed by lesion contrast. Due to the dynamic nature of ARFI excitation, lesion contrast is temporally-dependent. Contrast of spherical inclusions is highest immediately after force cessation, decreases with time postforce and then reverses, due to shear wave interaction with internal boundaries, differences in shear modulus between lesions and background and inertial effects. In images generated immediately after force cessation, contrast does not vary with applied force, increases with lesion stiffness and increases as the ROE size decreases relative to the size of the structure being imaged. These studies indicate that improved contrast in radiation force-generated images will be achieved as ROE size decreases; however, frame rate and thermal considerations present trade-offs with small ROE size.     

ARFI Imaging for Noninvasive Material Characterization of Atherosclerosis Part II: Toward In Vivo Characterization

Seventy percent of cardiovascular disease (CVD) deaths are attributed to atherosclerosis. Despite their clinical significance, nonstenotic atherosclerotic plaques are not effectively detected by conventional atherosclerosis imaging methods. Moreover, conventional imaging methods are insufficient for describing plaque composition, which is relevant to cardiovascular risk assessment. Atherosclerosis imaging technologies capable of improving plaque detection and stratifying cardiovascular risk are needed. Acoustic radiation force impulse (ARFI) ultrasound, a novel imaging method for noninvasively differentiating the mechanical properties of tissue, is demonstrated for in vivo detection of nonstenotic plaques and plaque material assessment in this pilot investigation. In vivo ARFI imaging was performed on four iliac arteries: (1) of a normocholesterolemic pig with no atherosclerosis as a control, (2) of a familial hypercholesterolemic pig with diffuse atherosclerosis, (3) of a normocholesterolemic pig fed a high-fat diet with early atherosclerotic plaques and (4) of a familial hypercholesterolemic pig with diffuse atherosclerosis and a small, minimally occlusive plaque. ARFI results were compared with spatially matched immunohistochemistry, showing correlations between elastin and collagen content and ARFI-derived peak displacement and recovery time parameters. Faster recoveries from ARFI-induced peak displacements and smaller peak displacements were observed in areas of higher elastin and collagen content. Importantly, spatial correlations between tissue content and ARFI results were consistent and observable in large and highly evolved as well as small plaques. ARFI imaging successfully distinguished nonstenotic plaques, while conventional B-mode ultrasound did not. This work validates the potential relevance of ARFI imaging as a noninvasive imaging technology for in vivo detection and material assessment of atherosclerotic plaques. (E-mail: russbehler@unc.edu)     

判别分析在评估动脉粥样硬化程度中的临床应用

目的 通过对动脉粥样硬化观察指标进行判别分析来简单、快捷地评估动脉粥样硬化的程度。方法 通过脑电阻图检查将研究对象分为3组，分别进行血糖、血脂、血压、内皮素、一氧化氮及脑循环动力学检查和眼底动脉的检测，通过逐步判别分析法进行分析。结果 判别分析法得出三个不同的方程式，分别评估轻度、中度和重度动脉粥样硬化。结论 内皮素、一氧化氮、患者年龄、眼底动脉的硬化分级和脑循环动力学的各项指标对评估动脉粥样硬化的程度有非常重要的作用。此判别分析法临床使用简便、快捷，有一定的实用价值。     

“四川省医院科技能力评价指标体系”的信度与效度检验及鉴别力与区分度分析

目的探讨已建立的＂四川省医院科技能力评价指标体系＂的信度与效度检验及鉴别力与区分度。方法采用分层随机抽样的方法对回收合格的调查问卷进行整理,抽取54家二、三级医院进行实证研究,采用克朗巴哈系数、因子分析分别检验指标体系的信度和效度。结果整套指标体系、1～3级指标的克朗巴哈系数分别为0.91、0.80、0.79、0.81;二级医院的整套指标体系、1～3级指标的克朗巴哈系数分别为0.89、0.70、0.68、0.78;三级医院的整套指标体系、1～3级指标的克朗巴哈系数分别为0.86、0.60、0.60、0.64。3个一级指标的因子分析结果如下：特征根大于1的公因子1个,累积贡献率为72.885%;因子负荷估计值分别为0.804、0.907、0.847。科技投入的二级指标的因子分析结果为特征根大于1的公因子1个,累积贡献率为48.810%;因子负荷估计值分别为0.824、0.689、0.558。科技管理及活动的二级指标的因子分析结果为特征根大于1的公因子1个,累积贡献率为52.377%;因子负荷估计值分别为0.600、0.647、0.764、0.857。科技产出的二级指标的因子分析结果为特征根大于1的公因子3个,累积贡献率分别为39.371%、66.633%、83.798%。在第一个公因子上,因子负荷估计值分别为0.651、0.652、0.518、0.817、0.759、0.040;在第二个公因子上,因子负荷估计值分别为0.544、0.264、0.747、-0.525、-0.627、-0.208;在第三个公因子上,因子负荷估计值分别为0.252、-0.210、0.068、-0.120、-0.003、0.950。该套指标体系除外5个指标,其余指标的变异系数均在10.00%以上,最高为19.55%。进一步分析指标的整体区分度,三级医院得分均秩为45.25,二级医院得分均秩为22.43,差异有统计学意义（P=0.001）。结论整套指标体系、1～3级指标的克朗巴哈系数均在0.6以上,表明该套指标内部一致性较好,具有较好的信度和效度;鉴别力与区分度的分析说明指标体系的所测结果既具有较好的稳定性又有较强的真实性。     

Anatomy and time course of discrimination and categorization processes in vision: an fMRI study

Studies investigating the cerebral areas involved in visual processes generally oppose either different tasks or different stimulus types. This work addresses, by fMRI, the interaction between the type of task (discrimination vs. categorization) and the type of stimulus (Latin letters, well-known geometrical figures, and Korean letters). Behavioral data revealed that the two tasks did not differ in term of percentage of errors or correct responses, but a delay of 185 ms was observed for the categorization task in comparison with the discrimination task. All conditions activated a common neural network that includes both striate and extrastriate areas, especially the fusiform gyri, the precunei, the insulae, and the dorsolateral frontal cortex. In addition, interaction analysis revealed that the right insula was sensitive to both tasks and stimuli, and that stimulus type induced several significant signal variations for the categorization task in right frontal cortex, the right middle occipital gyrus, the right cuneus, and the left and right fusiform gyri, whereas for the discrimination task, significant signal variations were observed in the right occipito-parietal junction only. Finally, analyzing the latency of the BOLD signal also revealed a differential neural dynamics according to tasks but not to stimulus type. These temporal differences suggest a parallel hemisphere processing in the discrimination task vs. a cooperative interhemisphere processing in the categorization task that may reflect the observed differences in reaction time.     

Anthropomorphic breast phantoms for assessing ultrasonic imaging system performance and for training ultrasonographers: part II

Three prototype anthropomorphic breast phantoms are discussed. The phantoms were constructed using ultrasonically tissue-mimicking materials; these materials mimic various tissue parenchymae in terms of attenuation, speed of sound, density, and scatter level. Realistic artifacts related to refraction and reflection at interfaces between different simulated parenchymae are produced. The phantoms represent premenopausal breasts, and they complement one another. Two of them represent the dense breasts of women under 30 years of age, and one represents that of a woman between 35 and 40 years of age. Of the former two, one produces what is apparently above-average refraction effects in the region of the peripheral fat layer; the other produces more typical refraction effects. Simulated tumors, cysts, and calcifications of various sizes are suspended in the glandular regions. Such phantoms are valuable for use in developmental testing of state-of-the-art ultrasound machines, quality assurance testing of clinical machines, and training of sonographers in breast imaging.     

Characterization of in vitro healthy and pathological human liver tissue periodicity using backscattered ultrasound signals

This work studied the periodicity of in vitro healthy and pathologic liver tissue, using backscattered ultrasound (US) signals. It utilized the mean scatterer spacing (MSS) as a parameter of tissue characterization, estimated by three methods: the spectral autocorrelation (SAC), the singular spectrum analysis (SSA) and the quadratic transformation method (SIMON). The liver samples were classified in terms of tissue status using the METAVIR scoring system. Twenty tissue samples were classified in four groups: F0, F1, F3 and F4 (five samples for each). The Kolmogorov-Smirnov test (applied on group pairs) resulted as nonsignificant (p > 0.05) for two pairs only: F1/F3 (for SSA) and F3/F4 (for SAC). A discriminant analysis was applied using as parameters the MSS mean (MSS) and standard deviation (sigmaMSS), the estimates histogram mode (mMSS), and the speed of US (mc(foie)) in the medium, to evaluate the degree of discrimination among healthy and pathologic tissues. The better accuracy (Ac) with SAC (80%) was with parameter group (MSS, sigmaMSS, mc(foie)), achieving a sensitivity (Ss) of 92.3% and a specificity (Sp) of 57.1%. For SSA, the group with all four parameters showed an Ac of 75%, an Ss of 78.6% and an Sp of 66.70%. SIMON obtained the best Ac of all (85%) with group (MSS, mMSS, mc(foie)), an Ss of 100%, but with an Sp of 50%.     

In vitro strain measurement in the porcine antrum using ultrasound doppler strain rate imaging

Strain rate imaging (SRI) enables study of deformation in soft tissues. The aim of this study was to evaluate the accuracy of SRI in measuring strain in the porcine antral wall in vitro. An experimental set-up enabled controlled distension of a porcine stomach in a saline reservoir. Radial strain obtained by SRI was compared with radial strain calculated from B-mode ultrasonography. Circumferential strain obtained by SRI was compared with circumferential strain calculated from sonomicrometry. The agreement between radial strain values measured by SRI and B-mode, along and across several ultrasound (US) beams, using US frequency 6.7 MHz and strain length (SL) = 1.9 mm was = -1.0 +/- 12.1% and 0.5 +/- 13.4%, respectively (mean difference +/- 2SD%) and it was better than with SL 1.2 mm. Compared with sonomicrometry, SRI-determined circumferential strain using 6.7 MHz and SL = 1.9 mm was less accurate, whether averaging along or across several US beams (-9.2 +/- 46.7% and 13.8 +/- 51.2%, respectively). In conclusion, SRI gave accurate measurement of radial strain of the antral wall, but seemed to be less accurate for measurement of circumferential strain for this in vitro set-up.     

Comparison of the performance of the RF cross correlation and doppler autocorrelation technique to estimate the mean velocity of simulated ultrasound signals

In pulsed Doppler systems the received RF (radio frequency) signal is multiplied by a quadrature reference signal and subsequently averaged over a short depth range to obtain a sample of the complex Doppler signal. The mean frequency of the sampled Doppler signal, obtained with the autocorrelation function, reflects the mean velocity of the scatterers moving through the sample volume. An alternative is to evaluate the two-dimensional cross correlation function of a short segment of the RF signals over subsequent lines, giving the mean velocity of the scatterers. Both methods of velocity estimation were applied to computer-generated RF signals with varying RF bandwidth, signal-to-noise ratio, and mean and width of the imposed velocity distribution. The length of the RF signal segment and the number of lines for velocity estimation (package length) affects the accuracy of the velocity estimate. It can be concluded that the cross correlation technique behaves superiorly especially for a low velocity dispersion. Furthermore, the standard deviation of the velocity estimate decreases for an increasing sample volume length and package length, while the performance of the conventional Doppler technique is rather independent of the length of the sample volume. The difference between both techniques decreases for a greater package length or for signals simulating a wide velocity distribution.     

Quantitative assessment of breast lesion viscoelasticity: initial clinical results using supersonic shear imaging

Abtract This paper presents an initial clinical evaluation of in vivo elastography for breast lesion imaging using the concept of supersonic shear imaging. This technique is based on the combination of a radiation force induced in tissue by an ultrasonic beam and an ultrafast imaging sequence capable of catching in real time the propagation of the resulting shear waves. The local shear wave velocity is recovered using a time-offlight technique and enables the 2-D mapping of shear elasticity. This imaging modality is implemented on a conventional linear probe driven by a dedicated ultrafast echographic device. Consequently, it can be performed during a standard echographic examination. The clinical investigation was performed on 15 patients, which corresponded to 15 lesions (4 cases BI-RADS 3, 7 cases BI-RADS 4 and 4 cases BI-RADS 5). The ability of the supersonic shear imaging technique to provide a quantitative and local estimation of the shear modulus of abnormalities with a millimetric resolution is illustrated on several malignant (invasive ductal and lobular carcinoma) and benign cases (fibrocystic changes and viscous cysts). In the investigated cases, malignant lesions were found to be significantly different from benign solid lesions with respect to their elasticity values. Cystic lesions have shown no shear wave propagate at all in the lesion (because shear waves do not propage in liquid). These preliminary clinical results directly demonstrate the clinical feasibility of this new elastography technique in providing quantitative assessment of relative stiffness of breast tissues. This technique of evaluating tissue elasticity gives valuable information that is complementary to the B-mode morphologic information. More extensive studies are necessary to validate the assumption that this new mode potentially helps the physician in both false-positive and false-negative rejection. (E-mail: Mickael.tanter@espci.fr).     

Shear-wave generation using acoustic radiation force: in vivo and ex vivo results

Acoustic radiation force impulse (ARFI) imaging involves the mechanical excitation of tissue using localized, impulsive radiation force. This results in shear-wave propagation away from the region of excitation. Using a single diagnostic transducer on a modified commercial ultrasound (US) scanner with conventional beam-forming architecture, repeated excitations with multiple look directions facilitate imaging shear-wave propagation. Direct inversion methods are then applied to estimate the associated Young's modulus. Shear-wave images are generated in tissue-mimicking phantoms, ex vivo human breast tissue and in vivo in the human abdomen. Mean Young's modulus values of between 3.8 and 5.6 kPa, 11.7 kPa and 14.0 kPa were estimated for fat, fibroadenoma and skin, respectively. Reasonable agreement is demonstrated between structures in matched B-mode and reconstructed modulus images. Although the relatively small magnitude of the displacement data presents some challenges, the reconstructions suggest the clinical feasibility of radiation force induced shear-wave imaging. (E-mail: kathy.nightingale@duke.edu)     

Acoustic Radiation Force for Vascular Cell Therapy: In Vitro Validation

Cell-based therapeutic approaches are attractive for the restoration of the protective endothelial layer in arteries affected by atherosclerosis or following angioplasty and stenting. We have recently demonstrated a novel technique for the delivery of mesenchymal stem cells (MSCs) that are surface-coated with cationic lipid microbubbles (MBs) and displaced by acoustic radiation force (ARF) to a site of arterial injury. The objective of this study was to characterize ultrasound parameters for effective acoustic-based delivery of cell therapy. In vitro experiments were performed in a vascular flow phantom where MB-tagged MSCs were delivered toward the phantom wall using ARF generated with an intravascular ultrasound catheter. The translation motion velocity and adhesion of the MB-cell complexes were analyzed. Experimental data indicated that MSC radial velocity and adhesion to the vessel phantom increased with the time-averaged ultrasound intensity up to 1.65 W/cm², after which no further significant adhesion was observed. Temperature increase from baseline near the catheter was 5.5 ± 0.8°C with this setting. Using higher time-averaged ultrasound intensities may not significantly benefit the adhesion of MB-cell complexes to the target vessel wall (p = NS), but could cause undesirable biologic effects such as heating to the MB-cell complexes and surrounding tissue. For the highest time-averaged ultrasound intensity of 6.60 W/cm², the temperature increase was 11.6 ± 1.3°C.     

Improving the Robustness of Time-of-Flight Based Shear Wave Speed Reconstruction Methods Using RANSAC in Human Liver in vivo

The stiffness of tissue can be quantified by measuring the shear wave speed (SWS) within the medium. Ultrasound is a real-time imaging modality capable of tracking the propagation of shear waves in soft tissue. Time-of-flight (TOF) methods have previously been shown to be effective for quantifying SWS from ultrasonically tracked displacements. However, the application of these methods to in vivo data is challenging due to the presence of additional sources of error, such as physiologic motion or spatial inhomogeneities in tissue. This article introduces the use of random sample consensus (RANSAC), a model fitting paradigm robust to the presence of gross outlier data, for estimating the SWS from ultrasonically tracked tissue displacements in vivo. SWS reconstruction is posed as a parameter estimation problem and the RANSAC solution to this problem is described. Simulations using synthetic TOF data show that RANSAC is capable of good stiffness reconstruction accuracy (mean error 0.5 kPa) and precision (standard deviation 0.6 kPa) over a range of shear stiffness (0.6–10 kPa) and proportion of inlier data (50%–95%). As with all TOF SWS estimation methods, the accuracy and precision of the RANSAC reconstructed shear modulus decreases with increasing tissue stiffness. The RANSAC SWS estimator was applied to radiation force induced shear wave data from 123 human patient livers acquired with a modified SONOLINE Antares ultrasound system (Siemens Healthcare, Ultrasound Business Unit, Mountain View, CA, USA) in a clinical setting before liver biopsy was performed. Stiffness measurements were not possible in 19 patients due to the absence of shear wave propagation inside the liver. The mean liver stiffness for the remaining 104 patients ranged from 1.3 to 24.2 kPa and the proportion of inliers for the successful reconstructions ranged between 42% to 99%. Using RANSAC for SWS estimation improved the diagnostic accuracy of liver stiffness for delineating fibrosis stage compared with ordinary least squares (OLS) without outlier removal (AUROC = 0.94 for F ≥ 3 and AUROC = 0.98 for F = 4). These results show that RANSAC is a suitable method for estimating the SWS from noisy in vivo shear wave displacements tracked by ultrasound. (E-mail: michael.h.wang@duke.edu)     

Acoustic Radiation Force Impulse Imaging of Human Prostates Ex Vivo

It has been challenging for clinicians using current imaging modalities to visualize internal structures and detect lesions inside human prostates. Lack of contrast among prostatic tissues and high false positive or negative detection rates of prostate lesions have limited the use of current imaging modalities in the diagnosis of prostate cancer. In this study, acoustic radiation force impulse (ARFI) imaging is introduced to visualize the anatomical and abnormal structures in freshly excised human prostates. A modified Siemens Antares ultrasound scanner (Siemens Medical Solutions USA Inc., Malvern, PA) and a Siemens VF10-5 linear array were used to acquire ARFI images. The transducer was attached to a three-dimensional (3-D) translation stage, which was programmed to automate volumetric data acquisition. A depth dependent gain (DDG) method was developed and applied to 3-D ARFI datasets to compensate for the displacement gradients associated with spatially varying radiation force magnitudes as a function of depth. Nine human prostate specimens were collected and imaged immediately after surgical excision. Prostate anatomical structures such as seminal vesicles, ejaculatory ducts, peripheral zone, central zone, transition zone and verumontanum were visualized with high spatial resolution and in good agreement with McNeal's zonal anatomy. The characteristic appearance of prostate pathologies, such as prostate cancerous lesions, benign prostatic hyperplasia, calcified tissues and atrophy were identified in ARFI images based upon correlation with the corresponding histologic slides. This study demonstrates that ARFI imaging can be used to visualize internal structures and detecting suspicious lesions in the prostate and appears promising for image guidance of prostate biopsy. (E-mail:liang.zhai@duke.edu)