Soft Tissue Characterization Research Articles

Control System for a Tensile-Testing Device Using Low-Cost Hardware and Open-Source Software

The aim of this study is to verify whether a Raspberry Pi 3 B+ can be utilized as a low-cost device for controlling a tensile-testing device used for material research purposes. A list of requirements based on already-available hardware was drawn up, which the new control system had to fulfil. To connect all the necessary equipment, a connection board was constructed, and some additional hardware was acquired for the system to be able to perform all the necessary tasks. The whole controlling system was also put in a small enclosure to make it portable. The control-system software was written in C++ using the Pigpio library. The developed system was then tested, and the results were compared to a commercially available Instron 8802 device. A comparison of the results shows that the upgraded equipment can produce comparable results to commercially available devices and is sufficiently accurate to be applied for research purposes for the characterization of soft tissues and other materials.

Real-Time Nonlinear Characterization of Soft Tissue Mechanical Properties

Online soft tissue characterization is important for robotic-assisted minimally invasive surgery to achieve precise and stable robotic control with haptic feedback. This paper presents a new nonlinear recursive adaptive filtering methodology for online nonlinear soft tissue characterization. An adaptive unscented Kalman filter is developed based on the Hunt-Crossley model by windowing approximation to online estimate system and measurement noise covariances. To improve the accuracy of noise covariance estimations, a recursive formulation is subsequently developed for estimation of system and measurement noise covariances by introducing a weighting factor. This weighting factor is further modified to accommodate noise statistics of large variation which could be caused by rupture events and geometric discontinuities in robotic-assisted surgery. Simulations, experiments, and comparison analyses demonstrate that the proposed nonlinear recursive adaptive filtering methodology can characterize soft tissue parameters in the presence of system or measurement noise statistics in both small and large variations for robotic-assisted surgery. The proposed methodology can effectively estimate soft tissue parameters under system and measurement noises in both small and large variations, leading to improved filtering accuracy and robustness in comparison with UKF.

Estimation of Anisotropic Material Properties of Soft Tissue by MRI of Ultrasound-Induced Shear Waves.

This paper describes a new method for estimating anisotropic mechanical properties of fibrous soft tissue by imaging shear waves induced by focused ultrasound (FUS) and analyzing their direction-dependent speeds. Fibrous materials with a single, dominant fiber direction may exhibit anisotropy in both shear and tensile moduli, reflecting differences in the response of the material when loads are applied in different directions. The speeds of shear waves in such materials depend on the propagation and polarization directions of the waves relative to the dominant fiber direction. In this study, shear waves were induced in muscle tissue (chicken breast) ex vivo by harmonically oscillating the amplitude of an ultrasound beam focused in a cylindrical tissue sample. The orientation of the fiber direction relative to the excitation direction was varied by rotating the sample. Magnetic resonance elastography (MRE) was used to visualize and measure the full 3D displacement field due to the ultrasound-induced shear waves. The phase gradient (PG) of radially propagating "slow" and "fast" shear waves provided local estimates of their respective wave speeds and directions. The equations for the speeds of these waves in an incompressible, transversely isotropic (TI), linear elastic material were fitted to measurements to estimate the shear and tensile moduli of the material. The combination of focused ultrasound and MR imaging allows noninvasive, but comprehensive, characterization of anisotropic soft tissue.

Pilot Study of [18F] Fluorodeoxyglucose Positron Emission Tomography (FDG-PET)/Magnetic Resonance Imaging (MRI) for Staging of Muscle-invasive Bladder Cancer (MIBC).

Computed tomography (CT) has limited diagnostic accuracy for staging of muscle-invasive bladder cancer (MIBC). [18F] Fluorodeoxyglucose positron emission tomography (FDG-PET)/magnetic resonance imaging (MRI) is a novel imaging modality incorporating functional imaging with improved soft tissue characterization. This pilot study evaluated the use of preoperative FDG-PET/MRI for staging of MIBC. Twenty-one patients with MIBC with planned radical cystectomy were enrolled. Two teams of radiologists reviewed FDG-PET/MRI scans to determine: (1) presence of primary bladder tumor; and (2) lymph node involvement and distant metastases. FDG-PET/MRI was compared with cystectomy pathology and computed tomography (CT). Eighteen patients were included in the final analysis, most (72.2%) of whom received neoadjuvant chemotherapy. Final pathology revealed 10 (56%) patients with muscle invasion and only 3 (17%) patients with lymph node involvement. Clustered analysis of FDG-PET/MRI radiology team reads revealed a sensitivity of 0.80 and a specificity of 0.56 for detection of the primary tumor with a sensitivity of 0 and a specificity of 1.00 for detection of lymph node involvement when compared with cystectomy pathology. CT imaging demonstrated similar rates in evaluation of the primary tumor (sensitivity, 0.91; specificity, 0.43) and lymph node involvement (sensitivity, 0; specificity, 0.93) when compared with pathology. This pilot single-institution experience of FDG-PET/MRI for preoperative staging of MIBC performed similar to CT for the detection of the primary tumor; however, the determination of lymph node status was limited by few patients with true pathologic lymph node involvement. Further studies are needed to evaluate the potential role for FDG-PET/MRI in the staging of MIBC.

A complementary energy approach accommodates scale differences in soft tissues

The mechanics of biological entities, from single molecules to the whole organ, has been extensively analyzed during the last decades. At the smaller scales, statistical mechanics has fostered successful physical models of proteins and molecules, which have been later incorporated within constitutive models of rubber-like materials and biological tissues. At the macroscopic scale, the additive decomposition of energy functions i.e., a parallel arrangement of the tissue constituent, has been recurrently used to account for the internal heterogeneity of soft biological materials. However, it has not yet been possible to unite the mechanics at the tissue level with the actual response of the tissue components. Here, we exemplify our approach using cardiovascular tissue where the mechanical response at the tissue scale is in the range of kPa whereas the elastic modulus of collagen, the main component of the vascular tissue, is in the range of MPa GPa. In this work we develop a novel theoretical framework based on a complementary strain energy function that builds-up on a full network model. The complementary strain energy function introduces naturally an additive decomposition of the deformation gradient for the tissue constituents, i.e an arrangement in series of the constituents. We demonstrate that the macroscopic response of the tissue can be reproduced by just introducing the underlying mechanical and structural features of the micro-constituents, improving in a fundamental manner previous attempts in the mechanical characterization of soft biological tissues. The proposed theoretical framework unveils a new direction in the mechanical modeling of soft tissues and biological networks.

Speed of sound ultrasound transmission tomography image reconstruction based on Bézier curves

Speed of Sound (SoS) maps from ultrasound tomography (UST) provide valuable quantitative information for soft tissue characterization and identification of lesions, making this technique interesting for breast cancer detection. However, due to the complexity of the processes that characterize the interaction of ultrasonic waves with matter, classic and fast tomographic algorithms such as back-projection are not suitable. Consequently, the image reconstruction process in UST is generally slow compared to other more conventional medical tomography modalities. With the aim of facilitating the translation of this technique into real clinical practice, several reconstruction algorithms are being proposed to make image reconstruction in UST to be a fast and accurate process. The geometrical acoustic approximation is often used to reconstruct SoS with less computational burden in comparison with full-wave inversion methods. In this work, we propose a simple formulation to perform on-the-flight reconstruction for UST using geometrical acoustics with refraction correction based on quadratic Bézier polynomials. Here we demonstrate that the trajectories created with these polynomials are an accurate approximation to reproduce the refracted acoustic paths connecting the emitter and receiver transducers. The method is faster than typical acquisition times in UST. Thus, it can be considered a step towards real-time reconstructions, which may contribute to its future clinical translation.

Müllerian Duct Anomalies- Diagnostic Imaging in Pediatric and Adolescent Population

Abstract Müllerian duct anomalies (MDAs) are congenital entities that result from the non-development, defective vertical or lateral fusion, or the resorption failure of the Müllerian (paramesonephric) ducts. MDAs represent a more frequent entity than previously believed. Few recent meta-analyses reported a prevalence of 5.5 - 6.7%. MDAs are commonly associated with other anomalies, specially kidneys, so identification of both kidneys is important. In pediatric and adolescent population MDAs are discovered incidentally at patients observed for some other reason or because of the primary amenorrhea and low abdominal pain related to hematometra (colpos). Imaging is essential for a diagnosis, management, and reproductive counseling in patients with MDA. Patients suspected of having the MDA are often initially referred to pelvic ultrasonography (US). Field-of-view restrictions with US, patient body habitus and artifact from bowel gas may result in a request for the further MagneticRresonance Iimaging (MRI). Also US cannot help identify the type of the MDA. MRI is the imaging standard of reference because it is non-invasive, does not involve ionizing radiation, has a multiplanar capability, allows an excellent soft-tissue characterization, detailed delineation of the uterovaginal anatomy and accurate classification of the type of anomaly. This is especially true for young female patients, in whom the use of vaginal US probes is avoided. Establishing an accurate diagnosis is essential for planning treatment and management strategies. The surgical management of MDAs is specific to the type of malformation and may vary in a specific group.

Classification of breast lesions using quantitative ultrasound biomarkers

Quantitative ultrasound (QUS) parameters are gaining attention recently as non-invasive biomarkers for soft tissue characterization. In this work, we use two QUS parameters, namely effective scatterer diameter (ESD) and mean scatterer spacing (MSS), for binary classification of breast lesions. To produce improved classification results, we propose a modified frequency-domain technique for ESD estimation of breast tissues from the diffuse component of backscattered radio-frequency (RF) data. Ensemble empirical mode decomposition (EEMD) is performed to separate the diffuse component from the coherent component of the backscattered RF data. A non-parametric Kolmogorov–Smirnov (K-S) test is employed for automatic selection of the intrinsic mode functions (IMFs). A novel multi-step system effect minimization process is also used. The ESD is estimated using a novel nearest neighborhood average regression line fitting (NNARLF) algorithm. Furthermore, we use an ameliorated EEMD domain autoregressive (AR) spectral estimation technique for MSS estimation. On using the ESD for binary classification of 139 lesions, we obtain high sensitivity, specificity, accuracy values of 95.45%, 95.79%, and 95.68%, respectively. The area under the receiver operating characteristics (ROC) curve is 0.95. On combining ESD with MSS we obtain even more improved sensitivity, specificity, and accuracy values of 97.73%, 95.79%, and 96.40%, respectively. The area under the ROC also increases to 0.97. This high classification performance demonstrates the potential of the use of these QUS parameters as non-invasive biomarkers for breast cancer detection.

Inverse finite element characterization of the human thigh soft tissue in the seated position.

Pressure ulcers are localized damage to the skin and underlying tissues caused by sitting or lying in one position for a long time. Stresses within the soft tissue of the thigh and buttocks area play a crucial role in the initiating mechanism of these wounds. Therefore, it is crucial to develop reliable finite element models to evaluate the stresses caused by physiological loadings. In this study, we compared how the choice of material model and modeling area dimension affect prediction accuracy of a model of the thigh. We showed that the first-order Ogden and Fung orthotropic material models could approximate the mechanical behavior of soft tissue significantly better than neo-Hookean and Mooney-Rivlin. We also showed that, significant error results from using a semi-3D model versus a 3D model. We then developed full 3D models for 20 participants employing Ogden and Fung material models and compared the estimated material parameters between different sexes and locations along the thigh. We showed that males tissues are less deformable overall when compared to females and the material parameters are highly dependent on location, with tissues getting softer moving distally for both men and women.

Micro-indentation and optical coherence tomography for the mechanical characterization of embryos: Experimental setup and measurements on chicken embryos

The investigation of the mechanical properties of embryos is expected to provide valuable information on the phenomenology of morphogenesis. It is thus believed that, by mapping the viscoelastic features of an embryo at different stages of growth, it may be possible to shed light on the role of mechanics in embryonic development. To contribute to this field, we present a new instrument that can determine spatiotemporal distributions of mechanical properties of embryos over a wide area and with unprecedented accuracy. The method relies on combining ferrule-top micro-indentation, which provides local measurements of viscoelasticity, with Optical Coherence Tomography, which can reveal changes in tissue morphology and help the user identify the indentation point. To prove the working principle, we have collected viscoelasticity maps of fixed and live HH11-HH12 chicken embryos. Our study shows that the instrument can reveal correlations between tissue morphology and mechanical behavior. Statement of SignificanceLocal mechanical properties of soft biological tissue play a crucial role in several biological processes, including cell differentiation, cell migration, and body formation; therefore, measuring tissue properties at high resolution is of great interest in biology and tissue engineering. To provide an efficient method for the biomechanical characterization of soft biological tissues, we introduce a new tool in which the combination of non-invasive Optical Coherence Tomography imaging and depth-controlled indentation measurements allows one to map the viscoelastic properties of biological tissue and investigate correlations between local mechanical features and tissue morphology with unprecedented resolution.

Dynamic Stress-strain Compressive Response of Soft Tissue using Polymeric split- Hopkinson Pressure Bar

The characterization of soft tissues subjected to higher compressive strain rates is of increasing importance as the material properties of soft tissues were commonly used in impact applications such as automotive safety and crashworthiness for biofidible numerical modeling of the human body. Due to less availability of stain rate dependent data in the open literature and various challenges in the characterisation of soft tissues under impacts, need were felt for characterisation of soft tissues at higher strain rates. The purpose of this study is to investigate the dynamic mechanical behavior of soft tissues at varying compressive strain-rates. The viscoelastic split Hopkinson pressure bar (SHPB) is designed and developed to predict the high rate compressive behavior of soft tissues. The primary benefit in using viscoelastic bars is the reduced bar impedance allowing for high-quality measurements of the transmitted and reflected stress wave signals. The dynamic behavior of goat muscles has been measured at higher strain rates ~500-1200 s-1 using a polymeric SHPB apparatus. The dynamic stress-strain response of muscle tissue exhibits non-linear behavior under compressive loadings and is strain-rate dependent.

Imaging of non-traumatic urinary bladder emergencies.

Non-traumatic urinary bladder emergencies are rare but critical diagnoses to make in an emergency setting. Acute urinary bladder pathologies require an accurate and timely diagnosis to ensure a favorable clinical outcome. Multidetector computed tomography (CT) is the imaging modality of choice for acute and emergent conditions affecting the urinary bladder. MRI is helpful as a problem-solving modality due to better soft tissue characterization and higher in-plane resolution. The purpose of this article is to illustrate the spectrum of urinary bladder emergencies, review the imaging findings, and briefly describe the role of imaging in the evaluation of such patients. Although there are a few cases of bladder emergencies been reported separately, the literature summarizing the spectrum is lacking. The objective of this article is to review the imaging of acute emergencies involving urinary bladder that will help us to think beyond non-specific conclusion in an emergency setting. For the sake of focused discussion, traumatic bladder emergencies will be excluded in this review. In the era of highly image reliant clinical practice, radiologists must be familiar with the diagnostic strategy to approach these entities.

Measurement of shear wave absorption with correction of the diffraction effect for viscoelasticity characterization of soft tissues

An ultrasonic method for measuring shear wave (SW) propagation properties in soft tissues was investigated for viscoelasticity characterization. A continuous SW was induced into a specimen by an external vibration source. The displacements within the specimen due to the SW propagation were measured by a speckle-echo tracking method using a conventional B-mode scan of focused ultrasound. SW velocity and absorption coefficient of the specimen were retrieved from the measured SW waveforms with the compensation of the SW diffraction. The effect of positioning error of the ultrasound probe on the diffraction calculation was studied by numerical simulations. It was shown that the estimation error of the SW absorption could be reduced by selecting an appropriate SW measurement region where the diffraction estimation precision is less sensitive to the positioning error of the probe. Finally, the shear elasticity and viscosity of a soft tissue mimicking phantom were estimated.

Pilot study of [18F] fluorodexoyglucose positron emission tomography-magnetic resonance imaging (FDG-PET-MRI) for staging of muscle-invasive bladder cancer.

e16002 Background: Standard of care imaging for staging MIBC is computed tomography (CT) abdomen and pelvis despite poor diagnostic accuracy. The addition of FDG-PET to evaluate biologic activity has been shown to improve detection of metastatic bladder cancer. Better soft tissue characterization with MRI may improve detection of local disease. We hypothesized that simultaneous acquisition of FDG-PET and MRI images (PET-MRI) would allow for more accurate preoperative staging in patients with MIBC. Methods: 21 patients with MIBC and planned radical cystectomy with lymph node dissection were enrolled. Four radiologists (2 senior, 2 junior) with expertise in MRI and PET independently reviewed MRI and FDG-PET scans, respectively, to determine: 1) extent of the primary bladder tumor; and 2) involvement of local lymph nodes and distant sites of disease. Combined radiologist reads were performed (senior MRI and junior PET and vice versa). Imaging results were compared to surgical pathology. Results: 18 patients were included in the final analysis (3 PET-MRI scans excluded due to technical pitfalls with image acquisition). Final pathology revealed 10 patients (56%) with muscle invasion and only 2 patients (11%) with lymph node involvement. Combined analyses of PET-MRI accurately detected the extent of the primary tumor (sensitivity 0.84, specificity 0.50, PPV 0.70, NPV 0.70), but was less accurate for the detection of lymph node involvement (sensitivity 0.00, specificity 1.00, no calculable PPV, NPV 0.91). Senior MRI radiologist had improved PPV and NPV for extent of primary tumor compared to junior MRI radiologist (PPV 0.71 vs 0.56, NPV 0.83 vs. 0.50), and senior PET radiologist had improved PPV with similar NPV for extent of primary tumor (PPV 1.00 vs 0.78, NPV 0.50 vs 0.55). No patient was determined to have distant metastatic disease. Conclusions: To our knowledge, this is the first study exploring the use of PET-MRI for staging MIBC. This novel imaging modality demonstrated promise in detecting the extent of the primary tumor. Further investigation of FDG-PET-MRI is needed to better determine its potential utility in staging MIBC.

A comparison of hyperelastic constitutive models applicable to Shear Wave Elastography (SWE) data in tissue-mimicking materials

Shear wave elastography (SWE) techniques have received substantial attention in recent years. Strong experimental data in SWE suggest that shear wave speed changes significantly due to the known acoustoelastic effect (AE). This presents both challenges and opportunities toward the in vivo characterization of biological soft tissues. In this work, under the framework of continuum mechanics, we model a tissue-mimicking material as a homogeneous, isotropic, incompressible, hyperelastic material. Our primary objective is to quantitatively and qualitatively compare experimentally measured acoustoelastic data with model-predicted outcomes using multiple strain energy functions. Our analysis indicated that the classic neo-Hookean and Mooney-Rivlin models are inadequate for modeling the AE in tissue-mimicking materials. However, a subclass of strain energy functions containing both high-order/exponential term(s) and second-order invariant dependence showed good agreement with experimental data. Based on data investigated, we also found that discrepancies may exist between parameters inversely estimated from uniaxial compression, and SWE data though Mooney plots were consistent between the uniaxial compression and AE results. Overall, our findings may improve our understanding of the clinical SWE results.

A comparison of hyperelastic constitutive models applicable to shear wave elastography (SWE) data in tissue-mimicking materials

Shear wave elastography (SWE) techniques have received substantial attention in recent years. Strong experimental data in SWE suggest that shear wave speed changes significantly due to the known acoustoelastic effect (AE). This presents both challenges and opportunities toward in vivo characterization of biological soft tissues. In this work, under the framework of continuum mechanics, we model a tissue-mimicking material as a homogeneous, isotropic, incompressible, hyperelastic material. Our primary objective is to quantitatively and qualitatively compare experimentally measured acoustoelastic data with model-predicted outcomes using multiple strain energy functions. Our analysis indicated that the classic Neo-Hookean and Mooney–Rivlin models are inadequate for modeling the AE in tissue-mimicking materials. However, a subclass of strain energy functions containing both high-order/exponential term(s) and second-order invariant dependence showed good agreement with experimental data. Based on data investigated, we also found that discrepancies may exist between parameters inversely estimated from uniaxial compression and SWE data. Overall, our findings may improve our understanding of clinical SWE results.

Multispectral Optoacoustic Tomography: Intra- and Interobserver Variability Using a Clinical Hybrid Approach.

Multispectral optoacoustic tomography (MSOT) represents a new imaging approach revealing functional tissue information without extrinsic contrast agents. Using a clinical combined ultrasound (US)/MSOT device, we investigated the interindividual robustness and impact of intra- and interobserver variability of MSOT values in soft tissue (muscle and subcutaneous fat) of healthy volunteers. Semiquantitative MSOT values for deoxygenated (Hb), oxygenated (HbO2) and total hemoglobin (HbT), as well as oxygen saturation (sO2), were calculated for both forearms in transversal and longitudinal probe orientation (n = 3, 8 measurements per subject). For intraobserver reproducibility, the same examiner investigated three subjects twice. Mean values of left vs. right forearm and transversal vs. longitudinal probe orientation were compared using an unpaired Student’s t test. Bland Altmann plots with 95% limits of agreement for absolute averages and differences were calculated. Intraclass correlation coefficients (ICC 2,k) were computed for three different examiners. We obtained reproducible and consistent MSOT values with small-to-moderate deviation for muscle and subcutaneous fat tissue. Probe orientation and body side had no impact on calculated MSOT values (p > 0.05 each). Intraobserver reproducibility revealed equable mean values with small-to-moderate deviation. For muscular tissue, good ICC was obtained for sO2. Measurements of subcutaneous tissue revealed good-to-excellent ICCs for all calculated values. Thus, in this preliminary study on healthy individuals, clinical MSOT provided consistent and reproducible functional soft tissue characterization, independent on the investigating personnel.

In-Vivo Soft Tissues Mechanical Characterization: Volume-Based Aspiration Method Validated on Silicones

Simulating the deformations of soft tissues has gained importance in recent years due to the development of 3D patient-specific biomechanical models in the context of Computer Assisted Medical Interventions. To design such models, the mechanical behavior of each soft tissue has to be characterized in-vivo. In this paper, a volume-based aspiration method for in-vivo mechanical characterization of soft tissues was validated on synthetic materials. For this purpose, two silicones with slightly different stiffnesses were made. Samples were characterized using (1) aspiration, and, as references, two classical tests such as (2) uniaxial and (3) equibiaxial extension tests. Performing a Finite Element (FE) inverse identification on the experimental results provided Young’s moduli similar to classical tests with about 7% maximum overestimation for the two silicones. This highlighted a significant improvement of the measurement method accuracy compared to the literature (about 30% relative overestimation). Eventually, the aspiration method ability to discriminate the two silicones was also tested and proven to be similar to classical characterization tests. Based on the presented results, relative mechanical behavior mapping of soft tissues (organ or skin) is possible without requiring an inverse characterization procedure.

Role of Cardiac Computed Tomography in the Diagnosis of Left Ventricular Myocardial Diseases.

Multimodality imaging is indicated for the evaluation of left ventricular (LV) myocardial diseases. Cardiac magnetic resonance (CMR) allows morphological and functional assessment of the LV along with soft tissue characterization. Technological advances in cardiac computed tomography (CT) have led to the development of techniques for diagnostic acquisition in LV myocardial disease. Cardiac CT facilitates the characterization of LV myocardial disease based on anatomy, function, and enhancement pattern. LV regional and global functional parameters are evaluated using multi-phasic cine CT images. CT myocardial perfusion facilitates the identification of hemodynamically significant coronary artery stenosis. Cardiac CT with delayed enhancement is used to detect myocardial scarring or fibrosis in myocardial infarction and non-ischemic cardiomyopathy, and for the measurement of extracellular volume fraction in non-ischemic cardiomyopathy. In this review, we review imaging techniques and key imaging features of cardiac CT used for the evaluation of myocardial diseases, along with CMR findings.

Image-based analysis of uniaxial ring test for mechanical characterization of soft materials and biological tissues.

Uniaxial ring test is a widely used mechanical characterization method for a variety of materials, from industrial elastomers to biological materials. Here we show that the combination of local material compression, bending, and stretching during uniaxial ring test results in a geometry-dependent deformation profile that can introduce systematic errors in the extraction of mechanical parameters. We identify the stress and strain regimes under which stretching dominates and develop a simple image-based analysis approach that eliminates these systematic errors. We rigorously test this approach computationally and experimentally, and demonstrate that we can accurately estimate the sample mechanical properties for a wide range of ring geometries. As a proof of concept for its application, we use the approach to analyze explanted rat vascular tissues and find a clear temporal change in the mechanical properties of these explants after graft implantation. The image-based approach can therefore offer a straightforward, versatile, and accurate method for mechanically characterizing new classes of soft and biological materials.