Acoustic radiation force-based elasticity imaging methods

Computational predictions of magnetic resonance acoustic radiation force imaging for breast cancer focused ultrasound therapy

PURPOSE

In magnetic resonance-guided focused ultrasound (MRgFUS) breast therapies, the focal location must be characterized to guide successful treatment. Focal characterization is difficult because heterogeneous breast tissues introduce phase aberrations that blur and shift the focus and traditional guidance methods do not work in adipose tissues. The purpose of this work is to evaluate numerical simulations of MRgFUS that predict the focal location. Those simulations are compared to clinical magnetic resonance acoustic radiation force imaging (MR-ARFI) data collected during in vivo treatment of breast tumors.

METHODS

The focal location was evaluated before MRgFUS treatment with MR-ARFI in five patients. The hybrid angular spectrum method (HAS) was applied to simulate pressure fields which were converted to forces, then convolved with a 3D Green's function (with time-of-arrival weighting) to produce a simulation of the MR-ARFI tissue displacement.

RESULTS

The focal locations found by the simulations and the MR-ARFI measurements were on average separated by 3.7 mm (SD: 0.9 mm). Characterization of the focal zone spatial distributions had a normalized root mean squared difference of 8.1% (SD: 2.5%). The displacement magnitudes of the simulations underestimated the MR-ARFI measurements by 82% (SD: 5.6%).

CONCLUSIONS

The agreement between MR-ARFI measurements and simulations demonstrates that HAS can predict the in vivo focal location in heterogeneous tissues, though accurate patient-specific properties are needed to improve predictions of tissue displacement magnitude. Tools developed in this study could be used to streamline MRgFUS treatment planning and optimization, for biomechanical property estimation, and in developing phase aberration correction techniques.

Mitigating high frame rate demands in shear wave elastography using radial basis function-based reconstruction: An experimental phantom study

BACKGROUND

Shear wave elastography (SWE) is a technique that quantifies tissue stiffness by assessing the speed of shear waves propagating after being excited by acoustic radiation force. SWE allows the quantification of elastic tissue properties and serves as an adjunct to conventional ultrasound techniques, aiding in tissue characterization. To capture this transient propagation of the shear wave, the ultrasound device must be able to reach very high frame rates.

METHODOLOGY

In this paper, our aim is to relax the high frame rate requirement for SWE imaging. To this end, we propose lower frame rate SWE imaging followed by employing a 2-dimensional (2D) radial basis functions (RBF)-based interpolation. More specifically, the process involves obtaining low frame rate data and then temporal upsampling to reach a synthetic high frame rate data by inserting the 'UpS-1' image frames with missing values between two successive image frames (UpS: Upsampling rate). Finally, we apply the proposed interpolation technique to reconstruct the missing values within the incomplete high frame rate data.

RESULTS AND CONCLUSION

The results obtained from employing the proposed model on two experimental datasets indicate that we can relax the frame rate requirement of SWE imaging by a factor of 4 while maintaining shear wave speed (SWS), group velocity, and phase velocity estimates closely align with the high frame rate SWE model so that the error is less than 3%. Furthermore, analysis of the structural similarity index (SSIM) and root mean squared error (RMSE) on the 2D-SWS maps highlights the efficacy of the suggested technique in enhancing local SWS estimates, even at a downsampling (DS) factor of 4. For DS≤4, the SSIM values between the 2D-SWS maps produced by the proposed technique and those generated by the original high frame rate data consistently remain above 0.94. Additionally, the RMSE values is below 0.37 m/s, indicating promising performance of the proposed technique in reconstruction of SWS values.

Predicting the high intensity focused ultrasound focus in vivo using acoustic radiation force imaging

BACKGROUND

One big challenge in the noninvasive high-intensity focused ultrasound (HIFU) surgery is that the location and shape of its focus is unpredictable at the preoperative stage due to the complexity of sound wave propagation. The Acoustic Radiation Force Impulse (ARFI) imaging is a potential solution to this problem, but artifacts resulting from shear wave propagation remain to be solved.

PURPOSE

In this study, we proposed avoiding those artefacts by applying the ARFI technique at a high imaging frame rate within a very short time before the shear waves start to propagate.

METHODS

Using single transmission with a convex imaging probe, two ultrafast imaging modalities (the diverging wave and the wide beam), were developed in the ARFI framework, and their reliabilities were validated on a nylon string phantom by the centroid tracking method borrowed from ultrasound localization microscopy (ULM). The proposed ARFI method was tested on a clinically equivalent HIFU system under different acoustic radiation intensities by in-vitro, ex-vivo and in-vivo experiments. In three experimental scenarios, we delivered short HIFU stimulation pulses at varying acoustic powers to induce tissue motion within the focal region. At each experimental site, both diverging wave and wide-beam imaging techniques were employed for motion estimation. Based on the focus prediction derived from the motion estimation, HIFU ablation treatment was performed. The treated samples were then incised to examine the damaged areas. Additionally, ultrasound B-mode images were acquired before and after the procedure and saved for analysis.

RESULTS

Quantitative analysis showed that the ARFI with wide beam imaging was able to predict the HIFU focus preoperatively, only with 1 to 3 mm of errors in focal central location, and less than 23% of percentage errors in focal area in most cases. However, the diverging wave imaging failed to predict the HIFU focus due to its low signal-to-noise ratio.

CONCLUSIONS

In conclusion, the inherent shear wave artefacts in ARFI for predicting the HIFU focus can be successfully avoided by carefully designing the imaging strategy and its working sequence. This ARFI technique was validated through a series of experiments on a clinically equivalent HIFU system, which demonstrated its capability in assisting surgical planning.

The role of strain wave elastography in the evaluation of renal fibrosis in patients with kidney diseases

BACKGROUND

A renal biopsy represents the gold standard in the diagnosis, prognosis and management of patients with chronic kidney disease and glomerulonephritis. Strain wave elastography (SE) is a developing technique to assess tissue elasticity. The aim of this study was to correlate between the strain index value of renal parenchyma and degree of renal fibrosis detected with renal biopsy.

METHOD

For 68 patients who were referred for a kidney biopsy, SE test was performed. The Banff scoring system was utilized to classify the IFTA grading of kidney fibrosis that assigns a severity level of mild, moderate, or severe. Receiver operating characteristic curve (ROC) was utilized to correlate between the severity of renal fibrosis and the grade of renal elasticity determined by SE.

RESULTS

In total, 38 males and 30 females, the echogenicity, qualitative and semiquantitative elastography showed significant positive correlation with serum creatinine, percentage of fibrosis, G score and tubular atrophy and significant negative correlation with eGFR. ROC curve of SE for diagnosis of interstitial fibrosis shown that echogenicity has sensitivity 100.0%, specificity 62.5%, positive predictive value (PPV) 75.0%, negative predictive value (NPV) 100.0% with area under curve (AUC) 0.906, while qualitative elastography has sensitivity 77.8%, specificity 75.0%, PPV 77.8%, NPV 75.0%, AUC 0.833, semi quantitative elastography has sensitivity 83.3%, specificity 93.8%, PPV 93.8%, NPV 83.3% with AUC 0.915.

CONCLUSION

SE approach is simple to use, and can differentiate between varying stages of renal fibrosis. However, further research is required before it can be frequently used in clinical practice.

Feasibility using a compact fiber optic Sagnac interferometer for non-contact soft tissue surface mechanical wave speed detection

The Sagnac interferometer offers distinct advantages in vibrational wave detection. In this study, an air-coupled transducer and a compact fiber-optic Sagnac interferometer were developed for non-contact elasticity characterization in biological tissues. Given the challenge of limited light collection by a compact fiber optic Sagnac interferometer in biological tissues, this study aims to explore the potential of using a compact Sagnac interferometer to measure vibrational waves in biological tissues. The speeds of the generated vibrational surface waves in tissue-mimic phantoms were measured. Measurement errors caused by cross-correlation wave tracking were analyzed, and the performance of the integrated system was characterized. The results demonstrated the effectiveness of the integrated system and the cross-correlation algorithm in tracing the speed of vibrational surface waves in tissue-mimicking phantoms. They suggested potential applications for the non-invasive, contactless characterization of the mechanical properties in soft biological tissues.

Advances and Prospects in Liquid Biopsy Techniques for Malignant Tumor Diagnosis and Surveillance

Liquid biopsy technology provides invaluable support for the early diagnosis of tumors and surveillance of disease course by detecting tumor-related biomarkers in bodily fluids. Currently, liquid biopsy techniques are mainly divided into two categories: biomarker and label-free. Biomarker liquid biopsy techniques utilize specific antibodies or probes to identify and isolate target cells, exosomes, or molecules, and these techniques are widely used in clinical practice. However, they have certain limitations including dependence on tumor markers, alterations in cell biological properties, and high cost. In contrast, label-free liquid biopsy techniques directly utilize physical or chemical properties of cells, exosomes, or molecules for detection and isolation. These techniques have the advantage of not needing labeling, not impacting downstream analysis, and low detection cost. However, most are still in the research stage and not yet mature. This review first discusses recent advances in liquid biopsy techniques for early tumor diagnosis and disease surveillance. Several current techniques are described in detail. These techniques exploit differences in biomarkers, size, density, deformability, electrical properties, and chemical composition in tumor components to achieve highly sensitive tumor component identification and separation. Finally, the current research progress is summarized and the future research directions of the field are discussed.

Effect of maneuvers, diuresis, and fluid administration on ultrasound-measured liver stiffness after Fontan

BACKGROUND

To determine the effect of stress maneuvers/interventions on ultrasound liver stiffness measurements (LSMs) in patients with Fontan circulation and healthy controls.

METHODS

In this prospective, IRB-approved study of 10 patients after Fontan palliation and 10 healthy controls, ultrasound 2D shear-wave elastography LSMs were acquired at baseline and after maximum inspiration, expiration, standing, handgrip, aerobic exercise, i.v. fluid (500 mL normal saline) administration, and i.v. furosemide (20 mg) administration. Absolute and percent change in LSM were compared between baseline and each maneuver, and then from fluid infusion to after diuresis.

RESULTS

Median ages were 25.5 and 26 years in the post-Fontan and control groups (p = 0.796). LSMs after Fontan were higher at baseline (2.6 vs. 1.3 m/s) and with all maneuvers compared to controls (all p < 0.001). Changes in LSM with maneuvers, exercise, fluid, or diuresis were not significant when compared to baseline in post-Fontan patients. LSM in controls increased with inspiration (+0.02 m/s, 1.6%, p = 0.03), standing (+0.07 m/s, 5.5%, p = 0.03), and fluid administration (+0.10 m/s, 7.8%, p = 0.002), and decreased 60 minutes after diuretic administration (-0.05 m/s, -3.9%, p = 0.01) compared to baseline. LSM after diuretic administration significantly decreased when compared to after i.v. fluid administration at 30 minutes (-0.79 m/s, -26.5%, p = 0.004) and 60 minutes (-0.78 m/s, -26.2%, p = 0.017) for patients after Fontan and controls at 15 minutes (-0.12 m/s, -8.70%, p = 0.002), 30 minutes (-0.15 m/s, -10.9%, p = 0.003), and 60 minutes (-0.1 m/s, -10.9%, p = 0.005).

CONCLUSIONS

LSM after Fontan is higher with more variability compared to controls. Diuresis is associated with significantly decreased liver stiffness in both patients after Fontan and controls, with the suggestion of a greater effect in Fontan patients.

Focal Volume, Acoustic Radiation Force, and Strain in Two-Transducer Regimes

Transcranial ultrasound stimulation (TUS) holds promise for noninvasive neural modulation in treating neurological disorders. Most clinically relevant targets are deep within the brain (near or at its geometric center), surrounded by other sensitive regions that need to be spared clinical intervention. However, in TUS, increasing frequency with the goal of improving spatial resolution reduces the effective penetration depth. We show that by using a pair of 1-MHz orthogonally arranged transducers, we improve the spatial resolution afforded by each of the transducers individually, by nearly 40 folds, achieving a subcubic millimeter target volume of [Formula: see text]. We show that orthogonally placed transducers generate highly localized standing waves with acoustic radiation force (ARF) arranged into periodic regions of compression and tension near the target. We further present an extended capability of the orthogonal setup, which is to impart selective pressures-either positive or negative, but not both-on the target. Finally, we share our preliminary findings that strain can arise from both particle motion (PM) and ARF with the former reaching its maximum value at the focus and the latter remaining null at the focus and reaching its maximum around the focus. As the field is investigating the mechanism of interaction in TUS by way of elucidating the mapping between ultrasound parameters and neural response, orthogonal transducers expand our toolbox by making it possible to conduct these investigations at much finer spatial resolutions, with localized and directed (compression versus tension) ARF and the capability of applying selective pressures at the target.

Quantitative Viscoelastic Response (QVisR): Direct Estimation of Viscoelasticity With Neural Networks

We present a machine learning method to directly estimate viscoelastic moduli from displacement time-series profiles generated by viscoelastic response (VisR) ultrasound excitations. VisR uses two colocalized acoustic radiation force (ARF) pushes to approximate tissue viscoelastic creep response and tracks displacements on-axis to measure the material relaxation. A fully connected neural network is trained to learn a nonlinear mapping from VisR displacements, the push focal depth, and the measurement axial depth to the material elastic and viscous moduli. In this work, we assess the validity of quantitative VisR (QVisR) in simulated materials, propose a method of domain adaption to phantom VisR displacements, and show in vivo estimates from a clinically acquired dataset.

In vivo endoscopic optical coherence elastography based on a miniature probe

Optical coherence elastography (OCE) is a functional extension of optical coherence tomography (OCT). It offers high-resolution elasticity assessment with nanoscale tissue displacement sensitivity and high quantification accuracy, promising to enhance diagnostic precision. However, in vivo endoscopic OCE imaging has not been demonstrated yet, which needs to overcome key challenges related to probe miniaturization, high excitation efficiency and speed. This study presents a novel endoscopic OCE system, achieving the first endoscopic OCE imaging in vivo. The system features the smallest integrated OCE probe with an outer diameter of only 0.9 mm (with a 1.2-mm protective tube during imaging). Utilizing a single 38-MHz high-frequency ultrasound transducer, the system induced rapid deformation in tissues with enhanced excitation efficiency. In phantom studies, the OCE quantification results match well with compression testing results, showing the system's high accuracy. The in vivo imaging of the rat vagina demonstrated the system's capability to detect changes in tissue elasticity continually and distinguish between normal tissue, hematomas, and tissue with increased collagen fibers precisely. This research narrows the gap for the clinical implementation of the endoscopic OCE system, offering the potential for the early diagnosis of intraluminal diseases.

Three-Dimensional Shear Wave Elastography Using Acoustic Radiation Force and a 2-D Row-Column Addressing (RCA) Array

Acoustic radiation force (ARF)-based shear wave elastography (SWE) is a clinically available ultrasound imaging mode that noninvasively and quantitatively measures tissue stiffness. Current implementations of ARF-SWE are largely limited to 2-D imaging, which does not provide a robust estimation of heterogeneous tissue mechanical properties. Existing 3-D ARF-SWE solutions that are clinically available are based on wobbler probes, which cannot provide true 3-D shear wave motion detection. Although 3-D ARF-SWE based on 2-D matrix arrays have been previously demonstrated, they do not provide a practical solution because of the need for a high channel-count ultrasound system (e.g., 1024-channel) to provide adequate volume rates and the delicate circuitries (e.g., multiplexers) that are vulnerable to the long-duration "push" pulses. To address these issues, here we propose a new 3-D ARF-SWE method based on the 2-D row-column addressing (RCA) array which has a much lower element count (e.g., 256), provides an ultrafast imaging volume rate (e.g., 2000 Hz), and can withstand the push pulses. In this study, we combined the comb-push shear elastography (CUSE) technique with 2-D RCA for enhanced SWE imaging field-of-view (FOV). In vitro phantom studies demonstrated that the proposed method had robust 3-D SWE performance in both homogenous and inclusion phantoms. An in vivo study on a breast cancer patient showed that the proposed method could reconstruct 3-D elasticity maps of the breast lesion, which was validated using a commercial ultrasound scanner. These results demonstrate strong potential for the proposed method to provide a viable and practical solution for clinical 3-D ARF-SWE.

Shear-wave elastography to predict hepatocellular carcinoma after hepatitis C virus eradication: A systematic review and meta-analysis

BACKGROUND

Direct-acting antiviral agents (DAAs) are highly effective treatment for chronic hepatitis C (CHC) with a significant rate of sustained virologic response (SVR). The achievement of SVR is crucial to prevent additional liver damage and slow down fibrosis progression. The assessment of fibrosis degree can be performed with transient elastography, magnetic resonance elastography or shear-wave elastography (SWE). Liver elastography could function as a predictor for hepatocellular carcinoma (HCC) in CHC patients treated with DAAs.

AIM

To explore the predictive value of SWE for HCC development after complete clearance of hepatitis C virus (HCV).

METHODS

A comprehensive literature search of clinical studies was performed to identify the ability of SWE to predict HCC occurrence after HCV clearance. In accordance with the study protocol, a qualitative and quantitative analysis of the evidence was planned.

RESULTS

At baseline and after 12 wk of follow-up, a trend was shown towards greater liver stiffness (LS) in those who go on to develop HCC compared to those who do not [baseline LS standardized mean difference (SMD): 1.15, 95% confidence interval (95%CI): 020-2.50; LS SMD after 12 wk: 0.83, 95%CI: 0.33-1.98]. The absence of a statistically significant difference between the mean LS in those who developed HCC or not may be related to the inability to correct for confounding factors and the absence of raw source data. There was a statistically significant LS SMD at 24 wk of follow-up between patients who developed HCC vs not (0.64; 95%CI: 0.04-1.24).

CONCLUSION

SWE could be a promising tool for prediction of HCC occurrence in patients treated with DAAs. Further studies with larger cohorts and standardized timing of elastographic evaluation are needed to confirm these data.

Focused Shear Wave Beam Propagation in Tissue-Mimicking Phantoms

OBJECTIVE

Ultrasound transient elastography (TE) technologies for liver stiffness measurement (LSM) utilize vibration of small, flat pistons, which generate shear waves that lack directivity. The most common cause for LSM failure in practice is insufficient shear wave signal at the needed depths. We propose to increase shear wave amplitude by focusing the waves into a directional beam. Here, we demonstrate the generation and propagation of focused shear wave beams (fSWBs) in gelatin.

METHODS

Directional fSWBs are generated by vibration at 200-400 Hz of a concave piston embedded near the surface of gelatin phantoms and measured with high-frame-rate ultrasound imaging. Five phantoms with a range of stiffnesses are employed. Shear wave speeds assessed by fSWBs are compared with those by radiation-force-based methods (2D SWE). fSWB amplitudes are compared to predictions using an analytical model.

RESULTS

fSWB-derived shear wave speeds are in good agreement with 2D SWE. The amplitudes of fSWBs are localized to the LSM region and are significantly greater than unfocused shear waves. Overall agreement with theory is observed, with some discrepancies in the theoretical source condition.

CONCLUSION

Focusing shear waves can increase the signal in the LSM region for TE. Challenges for translation include coupling piston vibration with the patient skin and increased attenuation in vivo compared to the phantoms employed here.

SIGNIFICANCE

Fibrosis is the most predictive measure of patient outcome in non-alcoholic fatty liver disease. Increased shear wave amplitude in the LSM region can reduce fibrosis assessment failure rates by TE, thus reducing the need for invasive methods like biopsy.

Adipose tissue fibrosis: the unwanted houseguest invited by obesity

The prevalence of obesity is increasing exponentially across the globe. The lack of effective treatment options for long-term weight loss has magnified the enormity of this problem. Studies continue to demonstrate that adipose tissue holds a biological memory, one of the most important determinant of long-term weight maintenance. This phenomenon is consistent with the metabolically dynamic role of adipose tissue: it adapts and expands to store for excess energy and serves as an endocrine organ capable of synthesizing a number of biologically active molecules that regulate metabolic homeostasis. An important component of the plasticity of adipose tissue is the extracellular matrix, essential for structural support, mechanical stability, cell signaling and function. Chronic obesity upends a delicate balance of extracellular matrix synthesis and degradation, and the ECM accumulates in such a way that prevents the plasticity and function of the diverse cell types in adipose tissue. A series of maladaptive responses among the cells in adipose tissue leads to inflammation and fibrosis, major mechanisms that explain the link between obesity and insulin resistance, risk of type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease. Adipose tissue fibrosis persists after weight loss and further enhances adipose tissue dysfunction if weight is regained. Here, we highlight the current knowledge of the cellular events governing adipose tissue ECM remodeling during the development of obesity. Our goal is to delineate the relationship more clearly between adipose tissue ECM and metabolic disease, an important step toward better defining the pathophysiology of dysfunctional adipose tissue.

Technical note: Evaluation of the acoustic radiation force imaging for predicting HIFU focus with in vitro and ex vivo experiments

BACKGROUND

High-intensity focused ultrasound (HIFU) is currently used for the treatment of various diseases, but it still lacks a reliable technique in the preoperative stage to accurately place its "energy blade" onto diseased targets. Acoustic radiation force imaging (ARFI) was recently introduced to tackle this issue, but its applicability and limitations were not clear.

PURPOSE

The aim of this study was to evaluate the performance of ARFI method in prediction of HIFU focal location at the preoperative stage.

METHODS

A point spread function (PSF) localization method, which was borrowed from the ultrasound super resolution field, was used to validate the core autocorrelation-based motion estimation algorithm in the ARFI procedure. Accuracy of the ARFI method for estimating the HIFU focus were tested with in vitro and ex vivo experiments with a clinically equivalent HIFU system. Comparisons were made between the estimated focal locations and those of the damaged area after the testing objects were cut open.

RESULTS

Results showed that the PSF localization was able to serve as a validating method for motion detection only when the tissue displacement was large. With the ARFI method, location of the HIFU focus could be accurately predicted by a 2D motion map preoperatively, and the axial spatial errors were less than 0.5 mm. However, the derived 2D motion maps can only be valuable when the acoustic stimulation in ARFI were strong enough, which was probably due to the fact that the HIFU focal locations were at large depths and the ultrasound imaging signal had low signal to noise ratio.

CONCLUSION

The ARFI method was indeed an accurate technique for preoperatively predicting HIFU focus in vitro and ex vivo. If clinical applications were to be considered, particularly in deep tissues, efforts might need to be made to improve ability of the ultrasound motion estimation technique.

Real-Time Nondestructive Viscosity Measurement of Soft Tissue Based on Viscoelastic Response Optical Coherence Elastography

Viscoelasticity of the soft tissue is an important mechanical factor for disease diagnosis, biomaterials testing and fabrication. Here, we present a real-time and high-resolution viscoelastic response-optical coherence elastography (VisR-OCE) method based on acoustic radiation force (ARF) excitation and optical coherence tomography (OCT) imaging. The relationship between displacements induced by two sequential ARF loading-unloading and the relaxation time constant of the soft tissue-is established for the Kelvin-Voigt material. Through numerical simulation, the optimal experimental parameters are determined, and the influences of material parameters are evaluated. Virtual experimental results show that there is less than 4% fluctuation in the relaxation time constant values obtained when various Young's modulus and Poisson's ratios were given for simulation. The accuracy of the VisR-OCE method was validated by comparing with the tensile test. The relaxation time constant of phantoms measured by VisR-OCE differs from the tensile test result by about 3%. The proposed VisR-OCE method may provide an effective tool for quick and nondestructive viscosity testing of biological tissues.

Sonothrombolysis with an acoustic net-assisted boiling histotripsy: A proof-of-concept study

Whilst sonothrombolysis is a promising and noninvasive ultrasound technique for treating blood clots, bleeding caused by thrombolytic agents used for dissolving clots and potential obstruction of blood flow by detached clots (i.e., embolus) are the major limitations of the current approach. In the present study, a new sonothrombolysis method is proposed for treating embolus without the use of thrombolytic drugs. Our proposed method involves (a) generating a spatially localised acoustic radiation force in a blood vessel against the blood flow to trap moving blood clots (i.e., generation of an acoustic net), (b) producing acoustic cavitation to mechanically destroy the trapped embolus, and (c) acoustically monitoring the trapping and mechanical fractionation processes. Three different ultrasound transducers with different purposes were employed in the proposed method: (1) 1-MHz dual focused ultrasound (dFUS) transducers for capturing moving blood clots, (2) a 2-MHz High Intensity Focused Ultrasound (HIFU) source for fractionating blood clots and (3) a passive acoustic emission detector with broad bandwidth (10 kHz to 20 MHz) for receiving and analysing acoustic waves scattered from a trapped embolus and acoustic cavitation. To demonstrate the feasibility of the proposed method, in vitro experiments with an optically transparent blood vessel-mimicking phantom filled with a blood mimicking fluid and a blood clot (1.2 to 5 mm in diameter) were performed with varying the dFUS and HIFU exposure conditions under various flow conditions (from 1.77 to 6.19 cm/s). A high-speed camera was used to observe the production of acoustic fields, acoustic cavitation formation and blood clot fragmentation within a blood vessel by the proposed method. Numerical simulations of acoustic and temperature fields generated under a given exposure condition were also conducted to further interpret experimental results on the proposed sonothrombolysis. Our results clearly showed that fringe pattern-like acoustic pressure fields (fringe width of 1 mm) produced in a blood vessel by the dFUS captured an embolus (1.2 to 5 mm in diameter) at the flow velocity up to 6.19 cm/s. This was likely to be due to the greater magnitude of the dFUS-induced acoustic radiation force exerted on an embolus in the opposite direction to the flow in a blood vessel than that of the drag force produced by the flow. The acoustically trapped embolus was then mechanically destructed into small pieces of debris (18 to 60 μm sized residual fragments) by the HIFU-induced strong cavitation without damaging the blood vessel walls. We also observed that acoustic emissions emitted from a blood clot captured by the dFUS and cavitation produced by the HIFU were clearly distinguished in the frequency domain. Taken together, these results can suggest that our proposed sonothrombolysis method could be used as a promising tool for treating thrombosis and embolism through capturing and destroying blood clots effectively.

Longitudinal motion of focused shear wave beams in soft elastic media

Shear waves are employed in medical ultrasound imaging because they reveal variations in viscoelastic properties of soft tissue. Frequencies below 1 kHz are required due to the substantially higher attenuation and lower propagation speeds than for compressional waves. Shear waves exhibiting particle motion in the direction of propagation, referred to as longitudinal shear waves, can be generated with longitudinal motion of a circular disk on the surface of a soft elastic medium. This approach permits imaging of the longitudinal shear wave with a conventional ultrasound transducer that is coaxial with the source of the shear wave. Presented here is a mathematical model describing the complete wave field generated by displacement at the surface of an isotropic elastic half-space. Numerical simulations are shown for longitudinal, transverse, torsional, and radial source polarizations, with emphasis on focused longitudinal shear waves. Predictions are consistent with measurements of light beams revealing that the longitudinal electric field component produces a smaller focal spot than the transverse field component [Dorn, Quabis, and Leuchs, Phys. Rev. Lett. 91, 233901 (2003)]. Simulations are compared with preliminary measurements of a focused longitudinal shear wave beam generated in a soft tissue phantom by longitudinal motion of a spherically concave piston.

Differences in Medial and Lateral Gastrocnemius Stiffness after Exercise-Induced Muscle Fatigue

Muscles are affected at the cellular level by exercised-induced fatigue, inducing changes in their stiffness. Examining muscle stiffness can improve the knowledge of various pathologic conditions, such as pain and injury. The objective of this study was to examine the stiffness of the medial gastrocnemius (MG) muscle and the lateral gastrocnemius (LG) muscle to determine the changes in stiffness, and to assess the differences in the stiffness between the MG and the LG, as affected by muscle fatigue measured using shear wave elastography (SWE) and a MyotonPRO after inducing muscle fatigue. A total of 35 healthy young adults participated in the study. The stiffness of the MG and the LG were assessed before and after a muscle fatigue protocol (MFP), which included three sets of 50 eccentric contractions of the calf muscles of the dominant leg, at rest, and at maximum voluntary contraction (MVC). The measurements were taken with SWE and the MyotonPRO simultaneously. Compared to baseline, the resting stiffness of the MG and the LG significantly increased immediately, 24 h, and 48 h after muscle fatigue (p < 0.05); however, during MVC, the stiffness of the MG decreased (p < 0.05) and that of the LG showed no change (p > 0.05). When the stiffness of the MG and the LG were compared before and after the MFP, changes in the stiffness of the MG were significantly greater than those in the LG (p < 0.05). This signifies that the MG was more affected by the exercise-induced muscle fatigue than was the LG. The assessment of musculoskeletal tissue and its characteristics, before and after eccentric exercise, is crucial in the prevention of overuse injuries associated with repeated exposure to both low and high levels of force.

Analysis of influencing factors of shear wave elastography of the superficial tissue: A phantom study

Shear wave elastography (SWE) is widely used in clinical work. But there is no standard protocol and operation specification for SWE acquisition methods, which impacts the diagnosis and clinical staging. This study aimed to investigate the influence factors of diameter, depth, and stiffness on SWE using different probes at superficial depths and discuss SWE differences with two machines at superficial depths. We performed SWE on two elastic phantoms that each phantom contained six subjects with two stiffness (41.06 ± 4.62 kpa and 57.30 ± 4.31 kpa), three diameters (10, 15, and 18 mm), and two depths (15 and 25 mm). A total of 240 measurements were obtained by using two ultrasound machines (SuperSonic Imagine Aixplorer and Mindray Resona 7) and 4 probes (SL15-4 and SL10-2, L11-3, and L14-5). The measurements were compared among 4 probes, 3 diameters, and 2 depths. There was no significant difference in SWE measurements among the probes from the same machine. The SWE measurements were affected by diameter, and the degree of influence was related to the stiffness. The SWE measurements were unaffected at a 15–25 mm depth range.

Three-Dimensional Shear Wave Elastography Using a 2D Row Column Addressing (RCA) Array

Objective. To develop a 3D shear wave elastography (SWE) technique using a 2D row column addressing (RCA) array, with either external vibration or acoustic radiation force (ARF) as the shear wave source. Impact Statement. The proposed method paves the way for clinical translation of 3D SWE based on the 2D RCA, providing a low-cost and high volume rate solution that is compatible with existing clinical systems. Introduction. SWE is an established ultrasound imaging modality that provides a direct and quantitative assessment of tissue stiffness, which is significant for a wide range of clinical applications including cancer and liver fibrosis. SWE requires high frame rate imaging for robust shear wave tracking. Due to the technical challenges associated with high volume rate imaging in 3D, current SWE techniques are typically confined to 2D. Advancing SWE from 2D to 3D is significant because of the heterogeneous nature of tissue, which demands 3D imaging for accurate and comprehensive evaluation. Methods. A 3D SWE method using a RCA array was developed with a volume rate up to 2000 Hz. The performance of the proposed method was systematically evaluated on tissue-mimicking elasticity phantoms and in an in vivo case study. Results. 3D shear wave motion induced by either external vibration or ARF was successfully detected with the proposed method. Robust 3D shear wave speed maps were reconstructed for phantoms and in vivo. Conclusion. The high volume rate 3D imaging provided by the 2D RCA array provides a robust and practical solution for 3D SWE with a clear pathway for future clinical translation.

Full Characterization of in vivo Muscle as an Elastic, Incompressible, Transversely Isotropic Material Using Ultrasonic Rotational 3D Shear Wave Elasticity Imaging

Using a 3D rotational shear wave elasticity imaging (SWEI) setup, 3D shear wave data were acquired in the vastus lateralis of a healthy volunteer. The innate tilt between the transducer face and the muscle fibers results in the excitation of multiple shear wave modes, allowing for more complete characterization of muscle as an elastic, incompressible, transversely isotropic (ITI) material. The ability to measure both the shear vertical (SV) and shear horizontal (SH) wave speed allows for measurement of three independent parameters needed for full ITI material characterization: the longitudinal shear modulus μL , the transverse shear modulus μT , and the tensile anisotropy χE . Herein we develop and validate methodology to estimate these parameters and measure them in vivo, with μL = 5.77±1.00 kPa, μT = 1.93±0.41 kPa (giving shear anisotropy χμ = 2.11±0.92 ), and χE = 4.67±1.40 in a relaxed vastus lateralis muscle. We also demonstrate that 3D SWEI can be used to more accurately characterize muscle mechanical properties as compared to 2D SWEI.

Controllable Angle Shear Wavefront Reconstruction Based on Image Fusion Method for Shear Wave Elasticity Imaging

The generation and measurement of shear waves are critical in ultrasonic elasticity imaging. Generally, the resulting wavefront direction is very important for accurately measuring the shear speed and estimating the medium elasticity. In this article, the proposed method can generate a compound shear wavefront with the same direction as speed reconstruction and zero angles between the wavefront and the focus direction, which can improve the estimation accuracy of shear wave velocity. Also, this method, called time-division multipoint excitation image fusion (TDMPEIF), can reconstruct the shear wave propagation images acquired at different depths of a medium according to the frame sequence to produce the shear waves front with a regulable angle. Moreover, the shear wave speed and the elasticity of a medium can be mapped quantitatively with this method. The results demonstrate that the TDMPEIF can improve the quality of the shear wave velocity images, which has wide application value and good promotion prospects for quantitative evaluation of tissue elasticity.

Shear Wave Elasticity Imaging Using Nondiffractive Bessel Apodized Acoustic Radiation Force

The acoustic radiation force impulse (ARFI) has been widely used in transient shear wave elasticity imaging (SWEI). For SWEI based on focused ARFI, the highest image quality exists inside the focal zone due to the limitation of the depth of focus and diffraction. Consequently, the areas outside the focal zone and in the near field present poor image quality. To address the limitations of the focused beam, we introduce Bessel apodized ARFI that enhances image quality and improves the depth of focus. The objective of this study is to evaluate the feasibility of SWEI based on Bessel ARF in simulation and experiment. We report measurements of elastogram image quality and depth of field in tissue-mimicking phantoms and ex vivo liver tissue. Our results demonstrate improved depth of field, image quality, and shear wave speed (SWS) estimation accuracy using Bessel push beams. As a result, Bessel ARF enlarges the field of view of elastograms. The signal-to-noise ratio (SNR) of Bessel SWEI is improved 26% compared with focused SWEI in homogeneous phantom. The estimated SWS by Bessel SWEI is closer to the measured SWS from a clinical scanner with an error of 0.3% compared to 2.4% with a focused beam. In heterogeneous phantoms, the contrast-to-noise ratios (CNRs) of shallow and deep inclusions are improved by 8.79 and 3.33 dB, respectively, under Bessel ARF. We also compare the results between Bessel SWEI and supersonic shear imaging (SSI), and the SNR of Bessel SWEI is improved by 8.1%. Compared with SSI, Bessel SWEI shows more accurate SWS estimates in high stiffness inclusions. Finally, Bessel SWEI can generate higher quality elastograms with less energy than conventional SSI.

Effects of Loading and Boundary Conditions on the Performance of Ultrasound Compressional Viscoelastography: A Computational Simulation Study to Guide Experimental Design

Most biomaterials and tissues are viscoelastic; thus, evaluating viscoelastic properties is important for numerous biomedical applications. Compressional viscoelastography is an ultrasound imaging technique used for measuring the viscoelastic properties of biomaterials and tissues. It analyzes the creep behavior of a material under an external mechanical compression. The aim of this study is to use finite element analysis to investigate how loading conditions (the distribution of the applied compressional pressure on the surface of the sample) and boundary conditions (the fixation method used to stabilize the sample) can affect the measurement accuracy of compressional viscoelastography. The results show that loading and boundary conditions in computational simulations of compressional viscoelastography can severely affect the measurement accuracy of the viscoelastic properties of materials. The measurement can only be accurate if the compressional pressure is exerted on the entire top surface of the sample, as well as if the bottom of the sample is fixed only along the vertical direction. These findings imply that, in an experimental validation study, the phantom design should take into account that the surface area of the pressure plate must be equal to or larger than that of the top surface of the sample, and the sample should be placed directly on the testing platform without any fixation (such as a sample container). The findings indicate that when applying compressional viscoelastography to real tissues in vivo, consideration should be given to the representative loading and boundary conditions. The findings of the present simulation study will provide a reference for experimental phantom designs regarding loading and boundary conditions, as well as guidance towards validating the experimental results of compressional viscoelastography.

Shear wave-based sound touch elastography in liver fibrosis assessment for patients with autoimmune liver diseases

BACKGROUND

Shear wave-based ultrasonic elastography (USE) has been widely used for the assessment of liver fibrosis in patients with chronic liver diseases (CLD). However, diagnostic criteria and accuracy vary between different etiologies and specific elastography techniques. We aimed to evaluate the tissue stiffness measured by shear wave-based sound touch elastography (STE) in staging liver fibrosis in patients with autoimmune liver diseases (AILD).

METHODS

One hundred and two AILD patients who had undergone STE liver stiffness measurements (LSMs) by using a Resona 7 ultrasound system were retrospectively studied. With the Scheuer liver fibrosis staging system as the reference, we investigated the diagnostic performance and cutoff values of STE measured liver stiffness in staging liver fibrosis through receiver operating characteristic (ROC) curve analysis. Moreover, comparisons of areas under the curve (AUCs) were made between LSMs and calculated biomarker scores, including the aspartate aminotransferase (AST)-to-platelet ratio index (APRI), and fibrosis-4 (FIB-4) index.

RESULTS

Median LSMs increased with the advancing fibrosis stages with values of 6.89 kPa (1.51 m/s), 8.00 kPa (1.63 m/s), 9.60 kPa (1.79 m/s), 11.37 kPa (1.95 m/s) and 14.50 kPa (2.20 m/s), from stage 0 to stage 4 respectively. The cutoff values of STE for identifying significant fibrosis (≥ stage 2), severe fibrosis (≥ stage 3) and cirrhosis (stage 4) were 9.07 kPa (1.74 m/s), 9.97 kPa (1.82 m/s) and 10.48 kPa (1.87 m/s), respectively, with corresponding sensitivity of 79.1%, 93.3%, and 100%; specificity of 80.0%, 70.8% and 71.8%. The AUCs of LSMs in identifying fibrosis ≥ stage 2, ≥ stage 3 and stage 4 (0.82, 0.87, and 0.91, respectively) were significantly higher than that of APRI (0.67, 0.64, and 0.72, respectively) and FIB-4 (0.70, 0.68, and 0.75, respectively) (all P<0.05).

CONCLUSIONS

LSM obtained by STE exhibited its good capability to evaluate liver fibrosis stages in patients with AILD. As a noninvasive modality for liver fibrosis staging, STE is superior to APRI and FIB-4 biomarker scores.

Multiparametric Ultrasound for Targeting Prostate Cancer: Combining ARFI, SWEI, QUS and B-Mode

Diagnosing prostate cancer through standard transrectal ultrasound (TRUS)-guided biopsy is challenging because of the sensitivity and specificity limitations of B-mode imaging. We used a linear support vector machine (SVM) to combine standard TRUS imaging data with acoustic radiation force impulse (ARFI) imaging data, shear wave elasticity imaging (SWEI) data and quantitative ultrasound (QUS) midband fit data to enhance lesion contrast into a synthesized multiparametric ultrasound volume. This SVM was trained and validated using a subset of 20 patients and tested on a second subset of 10 patients. Multiparametric US led to a statistically significant improvements in contrast, contrast-to-noise ratio (CNR) and generalized CNR (gCNR) when compared with standard TRUS B-mode and SWEI; in contrast and CNR when compared with MF; and in CNR when compared with ARFI. ARFI, MF and SWEI also outperformed TRUS B-mode in contrast, with MF outperforming B-mode in CNR and gCNR as well. ARFI, although only yielding statistically significant differences in contrast compared with TRUS B-mode, captured critical qualitative features for lesion identification. Multiparametric US enhanced lesion visibility metrics and is a promising technique for targeted TRUS-guided prostate biopsy in the future.

Quantifying Dysfunction of the Lumbar Multifidus Muscle After Radiofrequency Neurotomy and Fusion Surgery: A Preliminary Study

The multifidus is an important muscle for the active stabilization of the spine. Unfortunately, clinical procedures such as posterior lumbar fusion (PLF) and radio frequency neurotomy (RFN) cause injury to these muscles affecting their function. However, evaluating multifidus function using traditional biomechanical methods is challenging due to its unique anatomical features. The change in muscle shear modulus during contraction has been corrected to force generation for several skeletal muscles. Therefore, the change in shear modulus can be used to quantify muscle contraction. The objective of this study was to evaluate multifidus dysfunction by comparing changes in shear modulus during muscle contraction in healthy individuals and patients who received RFN and PLF in the lumbar spine. We used our recently developed protocol which consists of measuring changes of multifidus shear modulus at lying prone, sitting up, and sitting up with the arms lifted. In healthy individuals, the median multifidus shear modulus increased progressively from prone, sitting, and sitting with arms raised: 18.55 kPa, 27.14 kPa, and 38.45 kPa, respectively. A moderate increase in shear modulus for these body positions was observed in PLF patients: 9.81 kPa, 17.26 kPa, and 21.85 kPa. In RFN patients, the shear modulus remained relatively constant: 14.44 kPa, 16.57 kPa, and 17.26 kPa. Overall, RFN and PLF caused a reduction in the contraction of multifidus muscles. However, the contraction of multifidus muscle slightly increased during multifidus activation in PLF patients, while it did not change in RFN patients. These preliminary measurements suggest that the proposed protocol using SWE can provide important information about the function of individual spine muscles to guide the design and evaluation of postsurgical rehabilitation protocols.

Role of intrarenal resistive index and ElastPQ® renal shear modulus in early diagnosis and follow-up of diabetic nephropathy: A prospective study

Introduction

We aimed to establish baseline normal values of ElastPQ^® (Philips Healthcare, Bothell, Washington, USA) renal shear modulus, evaluate changes in intrarenal resistive index and renal shear modulus in various stages of diabetic nephropathy, their diagnostic potential and role in follow-up.

Methods

Our prospective observational study was performed over two years. In total, 130 adult cases with diabetic nephropathy and 130 normal adult controls were selected. Diabetic nephropathy was confirmed by persistent albuminuria on 24-hour urinary albumin testing at three month intervals and staged by albuminuria quantification. Measurement of intrarenal resistive index and renal shear modulus in all subjects was performed and their variation with stage of nephropathy was statistically analyzed using Pearson's correlation. Receiver operating characteristic curves were plotted and their individual and combined diagnostic potentials were assessed. Statistical significance was tested using t tests and analysis of variance. Interrater agreement was tested using Cohen's kappa coefficient.

Results

Mean intrarenal resistive index was significantly higher for cases (mean 0.72 ± 0.05) than controls (mean 0.62 ± 0.04) and showed significant age variation (p < 0.05). Normal values of ElastPQ^® renal shear modulus ranged from 3.87 to 4.72 kPa and was significantly higher for cases (mean 8.59 ± 1.77 kPa) than controls (mean 4.32 ± 0.45 kPa) and showed significant differences between each stage of nephropathy, being highest in stage 2. Maximum diagnostic accuracy was at 0.65 (sensitivity 90%, specificity 76.2%, area under curve 0.916) for intrarenal resistive index and at 5.31 kPa (sensitivity 90.8%, specificity 84.6%, area under curve 0.923) for renal shear modulus. Combination of the two further improved diagnostic performance (highest accuracy of 89%, sensitivity 81.7%, specificity 96.3%).

Conclusions

Normal range of ElastPQ^® renal shear modulus values could be established. Intrarenal resistive index and renal shear modulus can be used as imaging parameters for early diagnosis and follow-up of diabetic nephropathy.

A direct comparison of natural and acoustic-radiation-force-induced cardiac mechanical waves

Natural and active shear wave elastography (SWE) are potential ultrasound-based techniques to non-invasively assess myocardial stiffness, which could improve current diagnosis of heart failure. This study aims to bridge the knowledge gap between both techniques and discuss their respective impacts on cardiac stiffness evaluation. We recorded the mechanical waves occurring after aortic and mitral valve closure (AVC, MVC) and those induced by acoustic radiation force throughout the cardiac cycle in four pigs after sternotomy. Natural SWE showed a higher feasibility than active SWE, which is an advantage for clinical application. Median propagation speeds of 2.5-4.0 m/s and 1.6-4.0 m/s were obtained after AVC and MVC, whereas ARF-based median speeds of 0.9-1.2 m/s and 2.1-3.8 m/s were reported for diastole and systole, respectively. The different wave characteristics in both methods, such as the frequency content, complicate the direct comparison of waves. Nevertheless, a good match was found in propagation speeds between natural and active SWE at the moment of valve closure, and the natural waves showed higher propagation speeds than in diastole. Furthermore, the results demonstrated that the natural waves occur in between diastole and systole identified with active SWE, and thus represent a myocardial stiffness in between relaxation and contraction.

Feasibility and safety of antepartum tactile imaging

Introduction and hypothesis

Quantitative characterization of the birth canal and critical structures before delivery may provide risk assessment for maternal birth injury. The objective of this study was to explore imaging capability of an antepartum tactile imaging (ATI) probe.

Methods

Twenty randomly selected women older than 21 years with completed 35th week of pregnancy and a premise of vaginal delivery were enrolled in the feasibility study. The biomechanical data were acquired using the ATI probe with a double-curved surface, shaped according to the fetal skull and equipped with 168 tactile sensors and an electromagnetic motion tracking sensor. Software package COMSOL Multiphysics was used for finite element modeling. Subjects were asked for assessment of pain and comfort levels experienced during the ATI examination.

Results

All 20 nulliparous women were successfully examined with the ATI. Mean age was 27.8 ± 4.1 years, BMI 30.7 ± 5.8, and week of pregnancy 38.8 ± 1.4. Biomechanical mapping with the ATI allowed real-time observation of the probe location, applied load to the vaginal walls, and a 3D tactile image composition. The nonlinear finite element model describing the stress–strain relationship of the pelvic tissue was developed and used for calculation of Young’s modulus (E). Average perineal elastic modulus was 11.1 ± 4.3 kPa, levator ani 4.8 ± 2.4 kPa, and symphysis–perineum distance was 30.1 ± 6.9 mm. The pain assessment level for the ATI examination was 2.1 ± 0.8 (scale 1–4); the comfort level was 2.05 ± 0.69 (scale 1–3).

Conclusions

The antepartum examination with the ATI probe allowed measurement of the tissue elasticity and anatomical distances. The pain level was low and the comfort level was comparable with manual palpation.

Ramp-Creep Ultrasound Viscoelastography for Measuring Viscoelastic Parameters of Materials

Several ultrasound-based methods have been developed to evaluate the viscoelastic properties of materials. The purpose of this study is to introduce a novel viscoelastography method based on ultrasound acoustic radiation force for measuring the parameters relevant to the viscoelastic properties of materials, named ramp-creep ultrasound viscoelastography (RC viscoelastography). RC viscoelastography uses two different ultrasound excitation modes to cause ramp and creep strain responses in the material. By combining and analyzing the information obtained from these two modes of excitation, the viscoelastic parameters of the material can be quantitatively evaluated. Finite element computer simulation demonstrated that RC viscoelastography can accurately evaluate the viscoelastic parameters of the material, including the relaxation and creep time constants as well as the ratio of viscous fluids to solids in the material, except for the region near the top surface of the material. The novelty of RC viscoelastography is that there is no need to know the magnitude of acoustic radiation force and induced stress in the material in order to evaluate the viscoelastic parameters. In the future, experiments are necessary to test the performance of RC viscoelastography in real biomaterials and biological tissues.

Effects of hyperlipidemia on patellar tendon stiffness: A shear wave elastography study

BACKGROUND

Recent studies presented that increased adiposity and hyperlipidemia may cause tendon pathology. The aim of this study was to evaluate the effect of hyperlipidemia on the patellar tendon stiffness by shear wave elastography.

METHODS

A total of 51 participants (19 female, 32 male) were included. Participants were divided into two groups, according to their low-density lipoprotein levels, as the study group (hyperlipidemia, n = 24) and the control group (non-hyperlipidemia, n = 27). The patellar tendon and rectus femoris muscle shear wave velocities were measured by shear wave elastography.

FINDINGS

Patellar tendon shear wave velocities was 5.02 (SD: 0.78) m/s in the control group and 5.98 (SD: 1.19) m/s in the hyperlipidemia group (ES = 0.95, P = .001). There was a positive moderate statistically significant correlation between patellar tendon shear wave velocity and low-density lipoprotein (r = 0.432, p < .002). In the multiple linear regression analysis, only low-density lipoprotein was found as a significant predictor of patellar tendon shear wave velocity (CI: 0.005-0.028, P = .007).

INTERPRETATION

We evaluated the effects of hyperlipidemia and body mass index on patellar tendon mechanical properties with shear wave elastography. We found that the blood low-density lipoprotein level had an impact on patellar tendon stiffness independently of body mass index. Accordingly, it is important to evaluate individuals' low-density lipoprotein levels when examining risk factors for tendon pathology.

On the Challenges Associated with Obtaining Reproducible Measurements Using SWEI in the Median Nerve

This work discusses challenges we have encountered in acquiring reproducible measurements of shear wave speed (SWS) in the median nerve and suggests methods for improving reproducibility. First, procedural acquisition challenges are described, including nerve echogenicity, transducer pressure and transmit focal depth. Second, we present an iterative, radon sum-based algorithm that was developed specifically for measuring the SWS in median nerves. SWSs were measured using single track location shear wave elasticity imaging (SWEI) in the median nerves of six healthy volunteers and six patients diagnosed with carpal tunnel syndrome. Unsuccessful measurements were associated with several challenges including reverberation artifacts, low signal-to-noise ratio and temporal window limitations for tracking the velocity wave. To address these challenges, an iterative convergence algorithm was implemented to identify an appropriate temporal processing window that removed the reverberation artifacts while preserving shear wave signals. Algorithmically, it was important to consider the lateral regression kernel size and position and the temporal window. Procedurally, both nerve echogenicity and transducer compression were determined to impact the measured SWS. Shear waves were successfully measured in the median nerve proximal to the carpal tunnel, but SWEI measurements were significantly compromised within the carpal tunnel itself. The velocity-based SWSs were statistically significantly higher than the displacement SWSs (p < 0.0001), demonstrating for the first time dispersion in the median nerve in vivo using SWEI.

Role of Ultrasound Acoustic Radiation Force Impulse in Differentiating Benign from Malignant Superficial Lymph Nodes

Objective:

The purpose of this study was to evaluate the diagnostic performance of acoustic radiation force impulse (ARFI) imaging in differentiating benign from malignant peripheral lymphadenopathy.

Materials and Methods:

This was a prospective study approved by the Institutional Review Board with financial grant for the same. Ultrasound and ARFI imaging of peripheral lymph nodes were performed and correlated with pathological results, which were used as the reference standard. The virtual touch tissue imaging and virtual touch tissue quantification parameters of ARFI were analyzed in 86 lymph nodes, of which 78 were included in the study. Using receiver operating characteristic curve analysis, the diagnostic usefulness of ARFI values were evaluated with respect to their sensitivity, specificity, and area under the curve.

Results:

The mean area ratio of benign lymph nodes was 0.88 (±0.2) and that of malignant lymph nodes was 1.17 (±0.14). The mean shear wave velocities (SWV) of benign and malignant lymph nodes were 2.02 m/s (±0.94) and 3.7 m/s (±2.27), respectively. The sensitivity and specificity of virtual touch imaging area ratio in differentiating benign from malignant lymph nodes was 97% and 77%, of SWV was 71% and 70%, and of SWV ratio was 68% and 79%, respectively.

Conclusion:

As ARFI was found to have a superior diagnostic performance over conventional ultrasound and color Doppler in the characterization of lymph nodes, we recommend its routine use in differentiating benign from malignant nodes.

Ultrasound Real-Time Tissue Elastography Improves the Diagnostic Performance of the ACR Thyroid Imaging Reporting and Data System in Differentiating Malignant from Benign Thyroid Nodules: A Summary of 1525 Thyroid Nodules

BACKGROUND

To explore the correlation between the ultrasound elasticity score (ES) of real-time tissue elastography (RTE) and the malignant risk stratification of the Thyroid Imaging Reporting and Data System (TI-RADS) and to evaluate the added value of RTE to TI-RADS in differentiating malignant nodules from benign ones.

METHODS

A total of 1,498 patients (885 women and 613 men; mean age of 43.5 ± 12.4 years) with 1,525 confirmed thyroid nodules (D = maximum diameter, D ≤ 2.5 cm) confirmed by fine-needle aspiration (FNA) and/or surgery were included. The nodules were divided into four groups based on their sizes (D ≤ 0.5 cm, 0.5 < D ≤ 1.0 cm, 1.0 < D ≤ 2.0 cm, and 2.0 < D ≤ 2.5 cm). We assigned an ES of RTE and malignant risk stratification of the TI-RADS category to each nodule. The correlation between the ES of RTE and the malignant risk stratification of TI-RADS category was analyzed by the Spearman's rank correlation. The diagnostic performances of RTE, TI-RADS, and their combination were compared by the receiver operator characteristic (ROC) analysis.

RESULTS

The ES of RTE and the malignant risk stratification of TI-RADS showed a strong correlation in the size intervals of 0.5 < D ≤ 1.0 cm, 1.0 < D ≤ 2.0 cm, and 2.0 < D ≤ 2.5 cm (r = 0.768, 0.711, and 0.743, respectively). The diagnostic performance of their combination for each size interval was always better than RTE or TI-RADS alone (for all, P < 0.001).

CONCLUSIONS

Overall, The ES of RTE was strongly correlated with the malignant risk stratification of TI-RADS. The diagnostic performance of the combination of RTE and TI-RADS outperformed RTE or TI-RADS alone. Therefore, RTE may be an adjunctive tool to the current TI-RADS system for differentiating malignant from benign thyroid nodules.

Inter- and intra-reader reproducibility of shear wave elastography measurements for musculoskeletal soft tissue masses

OBJECTIVE

To determine inter- and intra-reader reproducibility of shear wave elastography measurements for musculoskeletal soft tissue masses.

MATERIALS AND METHODS

In all, 64 patients with musculoskeletal soft tissue masses were scanned by two readers prior to biopsy; each taking five measurements of shear wave velocity (m/s) and stiffness (kPa). A single lesion per patient was scanned in transverse and cranio-caudal planes. Depth measurements (cm) and volume (cm³) were recorded for each lesion, for each reader. Linear mixed modelling was performed to assess limits of agreement (LOA), inter- and intra-reader repeatability, including analyses for measured depth and volume.

RESULTS

Of the 64 lesions scanned, 24 (38%) were malignant. Bland-Altman plots demonstrated negligible bias with wide LOA for all measurements. Transverse velocity was the most reliable measure-intraclass correlation (95% CI) = 0.917 (0.886, 1)-though reader 1 measures could be between 38% lower and 57% higher than reader 2 [ratio-scale bias (95% LOA) = 0.99 (0.64, 1.55)]. Repeatability coefficients indicated most disagreement resulted from poor within-reader reproducibility. LOA between readers calculated from means of five repeated measurements were narrower-transverse velocity ratio-scale bias (95% LOA) = 1.00 (0.74, 1.35). Depth affected both estimated velocity and repeatability; volume also affected repeatability.

CONCLUSION

This study found poor repeatability of measurements with wide LOA due mostly to intra-reader variability. Transverse velocity was the most reliable measure; variability may be affected by lesion depth. At least five measurements should be reported with LOA to assist future comparability between shear wave elastography systems in evaluating soft tissue masses.

Relationship between Lower Urinary Tract Symptoms and Prostatic Urethral Stiffness Using Strain Elastography: Initial Experiences

We attempted to visualize the periurethral stiffness of prostatic urethras using strain elastography in the midsagittal plane of transrectal ultrasonography (TRUS) and to evaluate periurethral stiffness patterns in relation to lower urinary tract symptoms (LUTS). A total of 250 men were enrolled. The stiffness patterns of the entire prostate and individual zones were evaluated using strain elastography during a TRUS examination. After excluding 69 men with inappropriate elastography images, subjects were divided according to periurethral stiffness into either group A (low periurethral stiffness, N = 80) or group B (high periurethral stiffness, N = 101). There were significant differences in patient age (p = 0.022), transitional zone volume (p = 0.001), transitional zone index (p = 0.33), total international prostate symptom score (IPSS) (p < 0.001), IPSS-voiding subscore (p < 0.001), IPSS-storage subscore (p < 0.001), and quality of life (QoL) score (p = 0.002) between groups A and B. After adjusting for relevant variables, significant differences in total IPSS, IPSS-voiding subscore, and QoL score were maintained. Men with high periurethral stiffness were associated with worse urinary symptoms than those with low periurethral stiffness, suggesting that periurethral stiffness might play an important role in the development of LUTS.

Pulmonary Capillary Hemorrhage Induced by Super Sonic Shear Wave Elastography in Rats

The occurrence of the pulmonary capillary hemorrhage (PCH) bioeffect of diagnostic ultrasound in rats was investigated for a SuperSonic Imagine shear wave elastography system (Aixplorer, Supersonic Imagine, Aix-en-Provence, France). The elastography imaging repeated at 1 Hz and consisted of widely spaced B-mode image pulses, supersonic push (SSP) pulses and shear wave imaging (SWI) pulses. Groups of rats anesthetized with ketamine and xylazine, or with ketamine only, were imaged on their right side in a warm water bath for one frame, 30 s and 300 s. The image focus and region of interest were adjusted to give exposure only with the background B-mode imaging, or primarily with the SSP and SWI pulses. A sham group had only low power aiming scans. The lungs were removed 5 min after exposure and evaluated for PCH area and volume. The B mode was notably ineffective and produced significant PCH only at the maximum 0 dB output. The SSP pulses together with the SWI pulses produced significant PCH for 300 s, 30 s and even single image exposures. Peak rarefactional pressure amplitude PCH thresholds for 300 s exposure were the same with or without the B-mode pulses at 1.5 MPa (in situ mechanical index, MI_IS = 0.67). A 30 s duration resulted in a slightly increased threshold of 1.7 MPa (MI_IS = 0.76). The omission of xylazine from the anesthetic, which reduces the sensitivity of rat lung to PCH occurrence, resulted in an increased threshold of 2.1 MPa (MI_IS = 0.94). The unique SSP pulses were much more effective than the B mode, but thresholds were comparable to previous results with other diagnostic ultrasound modes on other systems.

Pulmonary Capillary Hemorrhage Induced by Acoustic Radiation Force Impulse Shear Wave Elastography in Ventilated Rats

OBJECTIVES

Diagnostic ultrasound (DUS) imaging can induce pulmonary capillary hemorrhage (PCH), possibly related to the ultrasonic radiation surface pressure arising from reflection at the lung blood-air interfaces. Acoustic radiation force impulse (ARFI) elastography is a relatively new DUS mode with high-energy "push pulses" used to move tissue and generate shear waves. The objective of this study was to characterize PCH induced by the ARFI elastographic mode for comparison with other previously tested DUS modes.

METHODS

Pulmonary capillary hemorrhage induction was examined for ARFI elastographic frames with 5.7-MHz push pulses (Acuson S3000; Siemens Medical Solutions, Mountain View, CA), which had a derated PRPA of 2.6 MPa. Groups of rats with tracheal tube placement had no ventilation (spontaneous breathing), intermittent positive pressure ventilation (IPPV), or IPPV plus 8 cm H₂ O of positive end-expiratory pressure (PEEP). Exposure was to 1 or 20 manually triggered image frame acquisitions. The PCH area was measured on the lung surface.

RESULTS

All 20-frame exposure groups, and even the single-frame group, had significant PCH relative to shams. Single-frame exposures produced significantly less PCH (P = .002) than 20-frame exposures in rats with a tracheal tube only (spontaneous breathing). The PEEP inhibited the PCH and produced about half of the PCH area induced for IPPV without PEEP (P = .014).

CONCLUSIONS

The PCH results were comparable with those from a previous study using B-mode or color Doppler exposure for 5 minutes; however, these modes delivered many more pulses for continuous imaging frames, suggesting that the ARFI elastographic frames were individually much more effective.

Combined Acoustic Radiation Force Impulse and Conventional Ultrasound in the Quantitative Assessment of Immunoglobulin a Nephropathy

We investigated the value of combined acoustic radiation force impulse (ARFI) imaging and conventional ultrasound (US) in identifying renal histopathological fibrosis with immunoglobulin A nephropathy. A total of 146 patients with immunoglobulin A nephropathy, pathologically confirmed by renal biopsy were grouped according to Oxford classification and Katafuchi grading, were included in the test group, and 39 healthy volunteers were included in the control group. Receiver operating characteristic (ROC) curves were constructed to compare the diagnostic accuracy of ARFI, renal lengths, parenchymal thicknesses and interlobular arterial resistance index (RI) and their combinations in identifying Katafuchi grading at renal biopsy. Shear wave velocity (SWV), renal length, renal parenchyma thickness and the interlobular arterial RI were correlated with Katafuchi grading, mesangial hypercellularity (M) and tubular atrophy/interstitial fibrosis (T) (r = -0.504 to -0.407, p < 0.01) but were not correlated with endocapillary hypercellularity (E) or segmental glomerulosclerosis (S). The area under the curves of SWV value + conventional US index (renal length, renal parenchyma thickness and interlobular arterial RI) was higher than those of the SWV value or of the conventional US index alone. The combination of ARFI imaging and conventional US can improve the diagnostic performance in quantitative evaluation pathologic damage in patients with immunoglobulin A nephropathy.

Ultrasound Neuromodulation: A Review of Results, Mechanisms and Safety

Ultrasonic neuromodulation is a rapidly growing field, in which low-intensity ultrasound (US) is delivered to nervous system tissue, resulting in transient modulation of neural activity. This review summarizes the findings in the central and peripheral nervous systems from mechanistic studies in cell culture to cognitive behavioral studies in humans. The mechanisms by which US mechanically interacts with neurons and could affect firing are presented. An in-depth safety assessment of current studies shows that parameters for the human studies fall within the safety envelope for US imaging. Challenges associated with accurately targeting US and monitoring the response are described. In conclusion, the literature supports the use of US as a safe, non-invasive brain stimulation modality with improved spatial localization and depth targeting compared with alternative methods. US neurostimulation has the potential to be used both as a scientific instrument to investigate brain function and as a therapeutic modality to modulate brain activity.

Use of Shear Wave Elastography in the Sonographic Triage of Thyroid Nodules: Feasibility Study in a Series of Lesions Already Selected for Fine Needle Aspiration

OBJECTIVES

The aim of this study was to evaluate the application of shear wave elastography (SWE) in the routine management of thyroid nodules, as a possible additional tool to the standard sonographic triage.

METHODS

A total of 248 consecutive patients scheduled for ultrasound-guided thyroid fine-needle aspiration were included in the study. The presence of a pure colloid lesion was an exclusion criterion. Absolute and relative SWE stiffness measurements on color-coded elastograms, expressed in kilopascals and meters per second, were correlated with radiologic and pathologic features.

RESULTS

SWE values in thyroid nodules were significantly higher than normal thyroid tissue (P = .0001), proving the different elastic properties of the pathologic tissues. Regarding the radiologic characteristics of the nodules, SWE highest values were associated with the largest lesions (P = .0105) but independent from sonographic and Doppler findings. The SWE elasticity was not influenced by the characteristics of the biopsy smears. The final correlation between the SWE results and the pathologic diagnoses showed a trend in stiffness from tender tumors (follicular adenoma) to papillary thyroid carcinoma (P = .016).

CONCLUSIONS

SWE allows the identification of nodules within normal parenchyma; however, the present study does not confirm the potential role in differentiating between benign and malignant thyroid nodules.

A preliminary study on ultrasound techniques applied to cicatricial alopecia

BACKGROUND

Cicatricial alopecia (CA) is a group of disorder characterized by permanent destruction of the hair follicle. Shear wave elastography (SWE) and superb microvascular imaging (SMI) are noninvasive ultrasonic techniques which evaluate thickness, stiffness, and vascular index of tissues.

OBJECTIVES

This study explored the ranges of cicatricial alopecia area and normal scalp with a combined modality of SMI and SWE and investigated the feasibility of their use in assessing these diseases.

METHODS

Seventeen patients diagnosed with CA and twenty healthy controls were included in the study. SWE and SMI were performed with an Aplio 500 ultrasound system.

RESULTS

The mean age of patient group was 37.00 ± 13.16, and the mean age of the healthy controls were 36.00 ± 11.79. SWE results as m/s in cicatricial plaques ( x ¯  = 5191) were higher than non-alopecic scalp areas ( x ¯  = 4460) in patient group (t (16) = 2260; P = 0.038 < 0.05). SMI values in patient group (cicatricial plaques) ( x ¯  = 1200) were significantly higher than control group SMI values ( x ¯  = 0.005) (t (35) = 3.075; P = 0.012 < 0.05).

CONCLUSIONS

To the best of our knowledge, this is the first study to evaluate SWE and SMI scores in cicatricial alopecia. We found higher stiffness and vascularity in patient group. We conclude that SWE and SMI can show fibrosis and inflammation like previous studies. Especially, SWE as m/s is more sensitive than as kPa for cicatricial alopecia.

Machine Learning Prediction of Liver Stiffness Using Clinical and T2-Weighted MRI Radiomic Data

OBJECTIVE. The purpose of this study is to develop a machine learning model to categorically classify MR elastography (MRE)-derived liver stiffness using clinical and nonelastographic MRI radiomic features in pediatric and young adult patients with known or suspected liver disease. MATERIALS AND METHODS. Clinical data (27 demographic, anthropomorphic, medical history, and laboratory features), MRI presence of liver fat and chemical shift-encoded fat fraction, and MRE mean liver stiffness measurements were retrieved from electronic medical records. MRI radiomic data (105 features) were extracted from T2-weighted fast spin-echo images. Patients were categorized by mean liver stiffness (< 3 vs ≥ 3 kPa). Support vector machine (SVM) models were used to perform two-class classification using clinical features, radiomic features, and both clinical and radiomic features. Our proposed model was internally evaluated in 225 patients (mean age, 14.1 years) and externally evaluated in an independent cohort of 84 patients (mean age, 13.7 years). Diagnostic performance was assessed using ROC AUC values. RESULTS. In our internal cross-validation model, the combination of clinical and radiomic features produced the best performance (AUC = 0.84), compared with clinical (AUC = 0.77) or radiomic (AUC = 0.70) features alone. Using both clinical and radiomic features, the SVM model was able to correctly classify patients with accuracy of 81.8%, sensitivity of 72.2%, and specificity of 87.0%. In our external validation experiment, this SVM model achieved an accuracy of 75.0%, sensitivity of 63.6%, specificity of 82.4%, and AUC of 0.80. CONCLUSION. An SVM learning model incorporating clinical and T2-weighted radiomic features has fair-to-good diagnostic performance for categorically classifying liver stiffness.

For Whom the Bubble Grows: Physical Principles of Bubble Nucleation and Dynamics in Histotripsy Ultrasound Therapy

Histotripsy is a focused ultrasound therapy for non-invasive tissue ablation. Unlike thermally ablative forms of therapeutic ultrasound, histotripsy relies on the mechanical action of bubble clouds for tissue destruction. Although acoustic bubble activity is often characterized as chaotic, the short-duration histotripsy pulses produce a unique and consistent type of cavitation for tissue destruction. In this review, the action of histotripsy-induced bubbles is discussed. Sources of bubble nuclei are reviewed, and bubble activity over the course of single and multiple pulses is outlined. Recent innovations in terms of novel acoustic excitations, exogenous nuclei for targeted ablation and histotripsy-enhanced drug delivery and image guidance metrics are discussed. Finally, gaps in knowledge of the histotripsy process are highlighted, along with suggested means to expedite widespread clinical utilization of histotripsy.