Quantitative assessment of cervical softening during pregnancy with shear wave elasticity imaging: an in vivo longitudinal study

Fatigue loading and volumetric microscopy demonstrate changes to the mouse cervix throughout and after pregnancy

INTRODUCTION

The cervix plays important mechanical roles in pregnancy and regulating the timing of parturition. Dysfunction of the cervix is implicated in disorders of parturition including spontaneous preterm birth, failed induction of labor and post term pregnancies. To address these disorders, it is imperative to first understand the function of the cervix throughout a normal pregnancy. However, current knowledge on the response of the cervix to mechanical fatigue and the underlying microstructural changes throughout a pregnancy is lacking.

METHODS

In this study, mechanical fatigue properties were measured at different stages of pregnancy using uniaxial fatigue testing that simulated circumferential hoop stresses in the cervix. Collagen microstructure was quantified using second harmonic generation imaging and three-dimensional orientation analysis.

RESULTS

The stiffness and modulus of the cervix during fatigue testing were dramatically reduced in all stages of pregnancy, and pregnant samples experienced greater peak strain before failure. All mechanical properties recovered postpartum despite persistent changes in cervix size. Microstructural analysis demonstrated increased local collagen alignment in postpartum samples, which may indicate a mechanism that serves to improve material properties after childbirth.

DISCUSSION

Altogether, conclusions from this study enhance our understanding of how properties of the cervix change with pregnancy and lay the foundation for future work investigating how alterations from this healthy function can lead to spontaneous preterm birth and other reproductive complications.

Cervical function in pregnancy and disease: new insights from single-cell analysis

The uterine cervix plays an essential role in regulating fertility, maintaining pregnancy, remodeling in preparation for parturition, and protecting the reproductive tract from infection. A compromise in cervical function contributes to adverse clinical outcomes. Understanding molecular events that drive the multifunctional and temporally defined roles of the cervix is necessary to effectively treat infertility, reproductive tract infections, preterm birth, labor dystocia, and cervical cancer. The application of single-cell technologies to study cervical pathophysiology, while in its infancy, underscores the potential of these approaches in developing clinically relevant biomarkers of disease and preventative therapies. This review focuses on insights gained from single-cell transcriptomic studies in human and mouse cervical tissue and highlights outstanding questions in the field. One collective advance from single-cell analysis is the dynamic plasticity of cervical epithelial cells during the reproductive cycle in health and disease. Single-cell comparisons between upper and lower regions of the reproductive tract also highlight the distinct and divergent immunological responses elicited in the cervix during the reproductive lifespan. These findings may reconcile prior controversies in the role of proinflammatory mediators during parturition. In addition to providing obstetric insights, single-cell technologies elucidate the molecular pathways that drive cervical cancer progression. Thus far, these technologies have uncovered cellular heterogeneity in the tumor microenvironment and have identified potential cancer stem cells. While single-cell technology alone will not uncover all the molecular underpinnings contributing to preterm birth or cervical cancer, the insights derived from this valuable technology will accelerate our understanding of cervical biology in health and disease, which ultimately will help develop biomarkers for disease prediction and prevention therapies.

Shear wave elastography primer for the abdominal radiologist

PURPOSE

Shear wave elastography (SWE) provides a means for adding information about the mechanical properties of tissues to a diagnostic ultrasound examination. It is important to understand the physics and methods by which the measurements are made to aid interpretation of the results as they relate to disease processes.

METHODS

The components of how ultrasound is used to generate shear waves and make measurements of the induced motion are reviewed. The physics of shear wave propagation are briefly described for elastic and viscoelastic tissues. Additionally, shear wave propagation in homogeneous and inhomogeneous cases is addressed.

RESULTS

SWE technology has been implemented by many clinical vendors with different capabilities. Various quality metrics are used to define valid measurements based on aspects of the shear wave signals or wave velocity estimates.

CONCLUSION

There are many uses for SWE in abdominal imaging, but it is important to understand how the measurements are performed to gauge their utility for diagnosis of different conditions. Continued efforts to make the technology robust in complex clinical situations are ongoing, but many applications actively benefit from added information about tissue mechanical properties for a more holistic view of the patient for diagnosis or assessment of prognosis and treatment management.

A Study on the Effects of Depth-Dependent Power Loss on Speckle Statistics Estimation

Characterization of the interference patterns observed in B-mode images (i.e., speckle statistics) is a valuable tool in tissue characterization. However, changes in echo amplitudes unrelated to speckle, including power loss due to attenuation and diffraction, can bias these metrics, undermining their utility. Tissue with high attenuation such as the uterine cervix are especially affected. The purpose of this study was to demonstrate and quantify the effects of attenuation and diffraction on speckle statistics and to propose methods of compensation. Analysis was performed on simulated diffuse scattering phantoms of varying attenuation with simulated transducers at 9 and 5 MHz center frequency. Application in the in vivo macaque cervix using a clinical scanner is also presented. Nakagami and homodyned K distribution parameters were calculated in parameter estimation regions (PERs) of varying size within simulations and experiments. Changes in speckle statistics parameters with respect to PER size and depth were compared with and without two different compensation schemes. It has been shown that compensation for attenuation and diffraction is necessary to produce speckle statistics estimates that do not depend on medium attenuation or PER size. Reducing the dependence on these factors connects speckle statistics estimates more closely with the microstructure of the probed medium.

Quantification of Cervical Elasticity During Pregnancy Based on Transvaginal Ultrasound Imaging and Stress Measurement

Strain elastography and shear wave elastography are commonly used to quantify cervical elasticity. However, the absence of stress information in strain elastography causes difficulty in inter-session elasticity comparison, and the robustness of shear wave elastography is compromised by cervical tissue's high inhomogeneity.

OBJECTIVE

To overcome these limitations, we develop a quantitative cervical elastography system by adding a stress sensor to a clinically used transvaginal ultrasound imaging system.

METHODS

We record the cervical deformation in B-mode images and measure the probe-surface stress through the sensor. Then we quantify the strain using a customized algorithm and estimate the cervical Young's modulus through stress-strain linear regression.

RESULTS

In phantom experiments, we demonstrate the system's high accuracy (alignment with the quasi-static compression method, p-value = 0.369 > 0.05), robustness (alignment between 60°- and 90°-contact measurements, p-value = 0.638 > 0.05), repeatability (consistency of single sonographers' measurements, coefficient of variation < 0.06), and reproducibility (alignment between two sonographers' measurements, Pearson correlation coefficient = 0.981). Applying it to pregnant participants, we observe significant cervical softening (p-value < 0.001): Young's modulus decreases 3.95% weekly and its geometric mean value during the first (11 to 13 weeks), second, and third trimesters are 13.07 kPa, 7.59 kPa, and 4.40 kPa, respectively.

CONCLUSION

The proposed system is accurate, robust, and safe, and enables longitudinal and inter-examiner comparisons.

SIGNIFICANCE

The system applies to different ultrasound machines with minor software updates, which allows for studies of cervical softening patterns in pregnancy for larger populations, facilitating insights into conditions such as preterm birth.

Whole cervix imaging of collagen, muscle, and cellularity in term and preterm pregnancy

Cervical softening and dilation are critical for the successful term delivery of a fetus, with premature changes associated with preterm birth. Traditional clinical measures like transvaginal ultrasound and Bishop scores fall short in predicting preterm births and elucidating the cervix's complex microstructural changes. Here, we introduce a magnetic resonance diffusion basis spectrum imaging (DBSI) technique for non-invasive, comprehensive imaging of cervical cellularity, collagen, and muscle fibers. This method is validated through ex vivo DBSI and histological analyses of specimens from total hysterectomies. Subsequently, retrospective in vivo DBSI analysis at 32 weeks of gestation in ten term deliveries and seven preterm deliveries with inflammation-related conditions shows distinct microstructural differences between the groups, alongside significant correlations with delivery timing. These results highlight DBSI's potential to improve understanding of premature cervical remodeling and aid in the evaluation of therapeutic interventions for at-risk pregnancies. Future studies will further assess DBSI's clinical applicability.

Uterus and cervix anatomical changes and cervix stiffness evolution throughout pregnancy

The coordinated biomechanical performance, such as uterine stretch and cervical barrier function, within maternal reproductive tissues facilitates healthy human pregnancy and birth. Quantifying normal biomechanical function and detecting potentially detrimental biomechanical dysfunction (e.g., cervical insufficiency, uterine overdistention, premature rupture of membranes) is difficult, largely due to minimal data on the shape and size of maternal anatomy and material properties of tissue across gestation. This study quantitates key structural features of human pregnancy to fill this knowledge gap and facilitate three-dimensional modeling for biomechanical pregnancy simulations to deeply explore pregnancy and childbirth. These measurements include the longitudinal assessment of uterine and cervical dimensions, fetal weight, and cervical stiffness in 47 low-risk pregnancies at four time points during gestation (late first, middle second, late second, and middle third trimesters). The uterine and cervical size were measured via 2-dimensional ultrasound, and cervical stiffness was measured via cervical aspiration. Trends in uterine and cervical measurements were assessed as time-course slopes across pregnancy and between gestational time points, accounting for specific participants. Patient-specific computational solid models of the uterus and cervix, generated from the ultrasonic measurements, were used to estimate deformed uterocervical volume. Results show that for this low-risk cohort, the uterus grows fastest in the inferior-superior direction from the late first to middle second trimester and fastest in the anterior-posterior and left-right direction between the middle and late second trimester. Contemporaneously, the cervix softens and shortens. It softens fastest from the late first to the middle second trimester and shortens fastest between the late second and middle third trimester. Alongside the fetal weight estimated from ultrasonic measurements, this work presents holistic maternal and fetal patient-specific biomechanical measurements across gestation.

Quantitative strain elastography of the uterine cervix assessed by the GE Voluson E10 system in combination with a force-measuring device

OBJECTIVE

During pregnancy, the stiffness of the cervical tissue decreases long before the cervical length decreases. Therefore, several approaches have been proposed in order to ensure a more objective assessment of cervical stiffness than that achieved by digital evaluation. Strain elastography has shown promising results. This technique is based on an ultrasound assessment of the tissue deformation that occurs when the examiner applies pressure on the tissue with the ultrasound probe. However, the results are only semi-quantitative as they depend on the unmeasured force used by the examiner. We, therefore, hypothesized that a force-measuring device applied to the handle of the ultrasound probe may render the technique quantitative. With this approach, the stiffness is the force (measured by the device) divided by the compression (measured by the elastography platform). One perspective is the early identification of women at risk of preterm birth in whom cervical stiffness may decrease long before cervical shortening. Another perspective is cervical evaluation when planning labor induction. In this feasibility study, we aimed to evaluate how quantitative strain elastography performs when a commercially available strain elastography platform (by which the algorithm is unavailable) is combined with a custom-made, force-measuring device. We studied how the assessments were associated with the gestational age in women with uncomplicated pregnancies and how they were associated with cervical dilatation time from 4 to 10 cm in women undergoing labor induction.

METHODS

In the analysis, we included quantitative strain elastography assessments from 47 women with uncomplicated singleton pregnancies, with gestational age between 12⁺⁰ and 40⁺⁰, and from 27 singleton term-pregnant women undergoing labor induction. The force-measuring device was mounted on the handle of a transvaginal probe. The strain values (i.e. the compression of the cervical tissue) were obtained by the elastography software of the ultrasound scanner (GE Voluson E10). The region of interest was placed within the central part of the anterior cervical lip. Based on the force data and strain values, we calculated the outcomes cervical elastography index_GE (CEI_GE) and the cervical strength index_GE (CEI_GE x cervical length: CSI_GE).

RESULTS

The average CEI_GE was 0.24 N at week 12 and 0.15 N at week 30-34. For CSI_GE these figures were 8.2 and 4.7 N mm, respectively (p = 0.002). Among women undergoing labor induction, the CEI_GE was associated with a cervical dilatation time (4-10 cm) beyond 7 h. For nulliparous women, this area under the ROC curve was 0.94.

CONCLUSION

Quantitative strain elastography may constitute a tool for the evaluation of a uterine cervix with normal length in women at risk of preterm birth and in women undergoing labor induction. The performance of this tool deserves evaluation in larger clinical trials.

Bioengineering and the cervix: The past, current, and future for addressing preterm birth

The uterine cervix plays two important but opposing roles during pregnancy - as a mechanical barrier that maintains the fetus for nine months and as a compliant structure that dilates to allow for the delivery of a baby. In some pregnancies, however, the cervix softens and dilates prematurely, leading to preterm birth. Bioengineers have addressed and continue to address the lack of reduction in preterm birth rates by developing novel technologies to diagnose, prevent, and understand premature cervical remodeling. This article highlights these existing and emerging technologies and concludes with open areas of research related to the cervix and preterm birth that bioengineers are currently well-positioned to address.

Reproducibility and usability assessment of the novel Fine Birth device for threatened preterm labor diagnosis

BACKGROUND

Preterm delivery is considered the leading cause of mortality worldwide in children under 5 years old. Approximately 45 million pregnant women are hospitalized yearly for threatened preterm labor. However, only 50% of pregnancies complicated by threatened preterm labor end in delivery before the estimated date, classifying the rest as false threatened preterm labor. The ability of current diagnostic methods to predict threatened preterm labor is low (low positive predictive value), ranging between 8% and 30%. This highlights the need for a solution that accurately detects and differentiates between false and real threatened preterm labors in women who attend obstetrical clinics and hospital emergency departments with delivery symptoms.

OBJECTIVE

Primarily, this aimed to assess the reproducibility and usability of a novel medical device, the Fine Birth, aimed at accurately diagnosing threatened preterm labor through the objective quantification of pregnant women's cervical consistency. Secondarily, this study aimed to evaluate the effect of training and the incorporation of a lateral microcamera on the device's reliability and usability outcomes.

STUDY DESIGN

A total of 77 singleton pregnant women were recruited during their follow-up visits to the obstetrical and gynecologic departments at 5 Spanish hospitals. The eligibility criteria included pregnant women aged ≥18 years; women with a normal fetus and uncomplicated pregnancy; women without prolapse of membranes, uterine anomalies, previous cervical surgery, or latex allergy; and women signing the informed written consent. Cervical tissue stiffness was assessed using the Fine Birth device, whose technology is based on the propagation of torsional waves through the studied tissue. Cervical consistency measurements were taken for each woman until obtaining 2 valid measurements by 2 different operators. The intraobserver and interobserver reproducibilities of the Fine Birth measurements were assessed using the intraclass correlation coefficients with a 95% confidence interval and the Fisher test P value. The usability was evaluated on the basis of the clinicians' and participants' feedback.

RESULTS

There was good intraobserver reproducibility (intraclass correlation coefficient, 0.88; 95% confidence interval, 0.84-0.95; Fisher test P value<.05). As the results obtained for the interobserver reproducibility did not reach the desired acceptable values (intraclass correlation coefficient of <0.75), a lateral microcamera was added to the Fine Birth intravaginal probe, and the operators involved in the clinical investigation received the corresponding training with the modified device. The analysis of 16 additional subjects demonstrated excellent interobserver reproducibility (intraclass correlation coefficient, 0.93; 95% confidence interval, 0.78-0.97) and an improvement after the intervention (P<.0001).

CONCLUSION

The robust reproducibility and usability results obtained after the insertion of a lateral microcamera and the corresponding training make the Fine Birth a promising novel device to objectively quantify the patient's cervical consistency, diagnose threatened preterm labor, and, thus, predict the risk of spontaneous preterm birth. Further research is needed to demonstrate the clinical utility of the device.

Progesterone and its receptor signaling in cervical remodeling: Mechanisms of physiological actions and therapeutic implications

The remodeling of the cervix from a closed rigid structure to one that can open sufficiently for passage of a term infant is achieved by a complex series of molecular events that in large part are regulated by the steroid hormones progesterone and estrogen. Among hormonal influences, progesterone exerts a dominant role for most of pregnancy to initiate a loss of tissue strength yet maintain competence in a phase termed softening. Equally important are the molecular events that abrogate progesterone function in late pregnancy to allow a loss of tissue competence and strength during cervical ripening and dilation. In this review, we focus on current understanding by which progesterone receptor signaling for the majority of pregnancy followed by a loss/shift in progesterone receptor action at the end of pregnancy, collectively ensure cervical remodeling as necessary for successful parturition.

Mechanical Response of Mouse Cervices Lacking Decorin and Biglycan During Pregnancy

Cervical remodeling is critical for a healthy pregnancy. The proper regulation of extracellular matrix (ECM) turnover leads to remodeling throughout gestation, transforming the tissue from a stiff material to a compliant, extensible, viscoelastic tissue prepared for delivery. Small leucine-rich proteoglycans (SLRPs) regulate structural fiber assembly in the cervical ECM and overall tissue material properties. To quantify the SLRPs' mechanical role in the cervix, whole cervix specimens from nonpregnant and late pregnant knockout mice of SLRPs, decorin and biglycan, were subjected to cyclic load-unload, ramp-hold, and load-to-failure mechanical tests. Further, a fiber composite material model, accounting for collagen fiber bundle waviness, was developed to describe the cervix's three-dimensional large deformation equilibrium behavior. In nonpregnant tissue, SLRP knockout cervices have the same equilibrium material properties as wild-type tissue. In contrast, the load-to-failure and ramp-hold tests reveal SLRPs impact rupture and time-dependent relaxation behavior. Loss of decorin in nonpregnant (NP) cervices results in inferior rupture properties. After extensive remodeling, cervical strength is similar between all genotypes, but the SLRP-deficient tissue has a diminished ability to dissipate stress during a ramp-hold. In mice with a combined loss of decorin and biglycan, the pregnant cervix loses its extensibility, compliance, and viscoelasticity. These results suggest that decorin and biglycan are necessary for crucial extensibility and viscoelastic material properties of a healthy, remodeled pregnant cervix.

Advancements in the Application of Ultrasound Elastography in the Cervix

Ultrasound elastography is a modern imaging technique that has developed rapidly in recent years. It enables objective measurement of tissue stiffness, a physical property intuitive to the human sense of touch. This novel technology has become a hotspot and plays a major role in scientific research and academic practice. Presently, ultrasound elastography has been used in the identification of benign and malignant tumors in superficial organs, such as breast and thyroid, providing clinically accurate diagnosis and treatment. The method has also been widely used for the liver, kidney, prostate, lymph nodes, blood vessels, skin and muscle system. In the application of cervical lesions, ultrasound elastography can distinguish normal cervix from abnormal cervix and differentiate benign from malignant lesions. It can significantly improve the diagnostic specificity for cervical cancer and is also useful for assessing infiltration depth and stage of cervical cancer, as well as predicting chemoradiotherapy treatment response. For cervical evaluation during pregnancy, ultrasound elastography is useful for assessing cervical softening and predicting premature delivery and outcome of induced labor. This article reviews the principles of ultrasound elastography as well as the current status and limitations in its application for cervical lesions and the cervix during pregnancy.

Tapering of the interventricular septum can affect ultrasound shear wave elastography: An in vitro and in silico study

Shear wave elastography (SWE) has the potential to determine cardiac tissue stiffness from non-invasive shear wave speed measurements, important, e.g., for predicting heart failure. Previous studies showed that waves traveling in the interventricular septum (IVS) may display Lamb-like dispersive behaviour, introducing a thickness-frequency dependency in the wave speed. However, the IVS tapers across its length, which complicates wave speed estimation by introducing an additional variable to account for. The goal of this work is to assess the impact of tapering thickness on SWE. The investigation is performed by combining in vitro experiments with acoustic radiation force (ARF) and 2D finite element simulations, to isolate the effect of the tapering curve on ARF-induced and natural waves in the heart. The experiments show a 11% deceleration during propagation from the thick to the thin end of an IVS-mimicking tapered phantom plate. The numerical analysis shows that neglecting the thickness variation in the wavenumber-frequency domain can introduce errors of more than 30% in the estimation of the shear modulus, and that the exact tapering curve, rather than the overall thickness reduction, determines the dispersive behaviour of the wave. These results suggest that septal geometry should be accounted for when deriving cardiac stiffness with SWE.

Biomechanical Cervical Assessment Using 2-Dimentional Transvaginal Shear Wave Elastography in Nonpregnant and Pregnant Women: A Prospective Pilot Study

ABSTRACT

This study evaluated the technical feasibility of 2-dimensional transvaginal shear wave elastography to quantify cervical stiffness in nonpregnant and pregnant women and established normal values in each group. With institutional review board approval, we performed a prospective study with an age-matched historical control design. Sixteen premenopausal nonpregnant women without cervical pathology and 17 low-risk pregnant women (gestational age 17-33 weeks) were enrolled. Cervical shear wave speeds were measured on a SuperSonic Aixplorer machine. The mean shear wave speeds of anterior cervix were 4.96 ± 1.96 m/s in nonpregnant women and 1.92 ± 0.31 m/s in pregnant women. No significant stiffness difference was found between the anterior and posterior cervix (P = 0.15). The upper cervix was stiffer than the lower cervix in the pregnant women (P = 0.00012). Transvaginal shear wave elastography reveals that cervix at a midterm gestation is significantly softer than nonpregnant cervix (P < 0.0001) and suggests a spatial stiffness gradient along the length of the cervix, consistent with histopathology and limited elastography literature. Our results indicate the potential of transvaginal shear wave elastography to provide objective and quantitative estimates of cervical stiffness, especially during pregnancy.

Predictive value of cervical length by ultrasound and cervical strain elastography in labor induction at term

OBJECTIVE

This study aimed to examine whether addition of cervical elastographic parameters measured by ElastoScan for the cervix (E-cervix) improves the predictive value of cervical length (CL) in induction of labor at term by dinoprostone.

METHODS

We conducted a prospective, observational study between January 2020 and June 2020 in term primiparous women (n = 73) who were scheduled for labor induction by a 10-mg dinoprostone vaginal insert. The time intervals from the start of labor induction to regular uterine contractions and to vaginal delivery were calculated as the primary outcomes. We divided subjects into two groups using a threshold of 24 hours. Ultrasound measurements were compared between the two groups and the area under the curve (AUC) of the prediction model was calculated.

RESULTS

Women who delivered vaginally within 24 hours had a shorter CL and softer cervix than those who delivered after 24 hours. The combination of CL and elastographic parameters increased the AUC to 0.672 compared with CL alone (AUC = 0.637).

CONCLUSIONS

Measurement by E-cervix is relatively reproducible. Addition of cervical strain elastography slightly improves the predictive performance of CL in vaginal delivery within 24 hours. This technique is a promising ancillary tool for use with ultrasound.

Shear wave dispersion as a potential biomarker for cervical remodeling during pregnancy: evidence from a non-human primate model

Shear wave dispersion (variation of phase velocity with frequency) occurs in tissues with layered and anisotropic microstructure and viscous components, such as the uterine cervix. This phenomenon, mostly overlooked in previous applications of cervical Shear Wave Elasticity Imaging (SWEI) for preterm birth risk assessment, is expected to change drastically during pregnancy due to cervical remodeling. Here we demonstrate the potential of SWEI-based descriptors of dispersion as potential biomarkers for cervical remodeling during pregnancy. First, we performed a simulation-based pre-selection of two SWEI-based dispersion descriptors: the ratio R of group velocities computed with particle-velocity and particle-displacement, and the slope S of the phase velocity vs. frequency. The pre-selection consisted of comparing the contrast-to-noise ratio (CNR) of dispersion descriptors in materials with different degrees of dispersion with respect to a low-dispersive medium. Shear waves induced in these media by SWEI were simulated with a finite-element model of Zener viscoelastic solids. The pre-selection also considered two denoising strategies to improve CNR: a low-pass filter with automatic frequency cutoff determination, and singular value decomposition of shear wave displacements. After pre-selection, the descriptor-denoising combination that produced the largest CNR was applied to SWEI cervix data from 18 pregnant Rhesus macaques acquired at weeks 10 (mid-pregnancy stage) and 23 (late pregnancy stage) of the 24.5-week full pregnancy. A maximum likelihood linear mixed-effects model (LME) was used to evaluate the dependence of the dispersion descriptor on pregnancy stage, maternal age, parity and other experimental factors. The pre-selection study showed that descriptor S combined with singular value decomposition produced a CNR 11.6 times larger than the other descriptor and denoising strategy combinations. In the Non-Human Primates (NHP) study, the LME model showed that descriptor S significantly decreased from mid to late pregnancy (-0.37 ± 0.07 m/s-kHz per week, p <0.00001) with respect to the base value of 15.5 ± 1.9 m/s-kHz. This change was more significant than changes in other SWEI features such as the group velocity previously reported. Also, S varied significantly between the anterior and posterior portions of the cervix (p =0.02) and with maternal age (p =0.008). Given the potential of shear wave dispersion to track cervical remodeling, we will extend its application to ongoing longitudinal human studies.

Longitudinal ultrasonic dimensions and parametric solid models of the gravid uterus and cervix

Tissue mechanics is central to pregnancy, during which maternal anatomic structures undergo continuous remodeling to serve a dual function to first protect the fetus in utero while it develops and then facilitate its passage out. In this study of normal pregnancy using biomechanical solid modeling, we used standard clinical ultrasound images to obtain measurements of structural dimensions of the gravid uterus and cervix throughout gestation. 2-dimensional ultrasound images were acquired from the uterus and cervix in 30 pregnant subjects in supine and standing positions at four time points during pregnancy (8-14, 14-16, 22-24, and 32-34 weeks). Offline, three observers independently measured from the images of multiple anatomic regions. Statistical analysis was performed to evaluate inter-observer variance, as well as effect of gestational age, gravity, and parity on maternal geometry. A parametric solid model developed in the Solidworks computer aided design (CAD) software was used to convert ultrasonic measurements to a 3-dimensional solid computer model, from which estimates of uterine and cervical volumes were made. This parametric model was compared against previous 3-dimensional solid models derived from magnetic resonance frequency images in pregnancy. In brief, we found several anatomic measurements easily derived from standard clinical imaging are reproducible and reliable, and provide sufficient information to allow biomechanical solid modeling. This structural dataset is the first, to our knowledge, to provide key variables to enable future computational calculations of tissue stress and stretch in pregnancy, making it possible to characterize the biomechanical milieu of normal pregnancy. This vital dataset will be the foundation to understand how the uterus and cervix malfunction in pregnancy leading to adverse perinatal outcomes.

Why Are Viscosity and Nonlinearity Bound to Make an Impact in Clinical Elastographic Diagnosis?

The adoption of multiscale approaches by the biomechanical community has caused a major improvement in quality in the mechanical characterization of soft tissues. The recent developments in elastography techniques are enabling in vivo and non-invasive quantification of tissues' mechanical properties. Elastic changes in a tissue are associated with a broad spectrum of pathologies, which stems from the tissue microstructure, histology and biochemistry. This knowledge is combined with research evidence to provide a powerful diagnostic range of highly prevalent pathologies, from birth and labor disorders (prematurity, induction failures, etc.), to solid tumors (e.g., prostate, cervix, breast, melanoma) and liver fibrosis, just to name a few. This review aims to elucidate the potential of viscous and nonlinear elastic parameters as conceivable diagnostic mechanical biomarkers. First, by providing an insight into the classic role of soft tissue microstructure in linear elasticity; secondly, by understanding how viscosity and nonlinearity could enhance the current diagnosis in elastography; and finally, by compounding preliminary investigations of those elastography parameters within different technologies. In conclusion, evidence of the diagnostic capability of elastic parameters beyond linear stiffness is gaining momentum as a result of the technological and imaging developments in the field of biomechanics.

Bioengineering in women's health, volume 2: pregnancy—from implantation to parturition

This special issue of Interface Focus is the second of two sets of articles on the topic of bioengineering in women's health. This second issue in the series focuses on pregnancy, a dynamic time in a women's life that involves dramatic physiologic changes within a relatively small timeframe. Pregnancy demands endurance and resilience of one's body and represents a critical component of women's health research. The health of an individual leading up to, during and after pregnancy is paramount for reproductive health and the lifelong health of offspring. The articles in this issue explore physiological events that support reproduction spanning from embryo implantation, through gestation, to delivery and parturition. Specifically, the articles highlight essential developments in placenta, fetal membranes, cervix, pelvic floor and anthropometry research. The featured bioengineering disciplines deployed to study such complex biological processes are diverse, with articles detailing the latest advancements in computational modelling at various biological length-scales, biomaterial design, material modelling, non-invasive diagnostic techniques, microfluidic devices and experimental mechanics. This second issue continues the first in this series, on the physiology of the non-pregnant woman.