Lindsey C. Carlson

Nonlinear optical microscopy and ultrasound imaging of human cervical structure.

Abstract. The cervix softens and shortens as its collagen microstructure rearranges in preparation for birth, but premature change may lead to premature birth. The global preterm birth rate has not decreased despite decades of research, likely because cervical microstructure is poorly understood. Our group has developed a multilevel approach to evaluating the human cervix. We are developing quantitative ultrasound (QUS) techniques for noninvasive interrogation of cervical microstructure and corroborating those results with high-resolution images of microstructure from second harmonic generation imaging (SHG) microscopy. We obtain ultrasound measurements from hysterectomy specimens, prepare the tissue for SHG, and stitch together several hundred images to create a comprehensive view of large areas of cervix. The images are analyzed for collagen orientation and alignment with curvelet transform, and registered with QUS data, facilitating multiscale analysis in which the micron-scale SHG images and millimeter-scale ultrasound data interpretation inform each other. This novel combination of modalities allows comprehensive characterization of cervical microstructure in high resolution. Through a detailed comparative study, we demonstrate that SHG imaging both corroborates the quantitative ultrasound measurements and provides further insight. Ultimately, a comprehensive understanding of specific microstructural cervical change in pregnancy should lead to novel approaches to the prevention of preterm birth.

Estimation of shear wave speed in the human uterine cervix.

To explore spatial variability within the cervix and the sensitivity of shear wave speed (SWS) to assess softness/stiffness differences in ripened (softened) vs unripened tissue.

Changes in shear wave speed pre- and post-induction of labor: a feasibility study.

To explore the feasibility of using shear wave speed (SWS) estimates to detect differences in cervical softening pre‐ and post‐ripening in women undergoing induction of labor.

Statistical analysis of shear wave speed in the uterine cervix.

Although cervical softening is critical in pregnancy, there currently is no objective method for assessing the softness of the cervix. Shear wave speed (SWS) estimation is a noninvasive tool used to measure tissue mechanical properties such as stiffness. The goal of this study was to determine the spatial variability and assess the ability of SWS to classify ripened versus unripened tissue samples. Ex vivo human hysterectomy samples (n = 22) were collected; a subset (n = 13) were ripened. SWS estimates were made at 4 to 5 locations along the length of the canal on both anterior and posterior halves. A linear mixed model was used for a robust multivariate analysis. Receiver operating characteristic (ROC) analysis and the area under the ROC curve (AUC) were calculated to describe the utility of SWS to classify ripened versus unripened tissue samples. Results showed that all variables used in the linear mixed model were significant ( p <; 0.05). Estimates at the mid location for the unripened group were 3.45 ± 0.95 m/s (anterior) and 3.56 ± 0.92 m/s (posterior), and 2.11 ± 0.45 m/s (anterior) and 2.68 ± 0.57 m/s (posterior) for the ripened ( p <; 0.001). The AUCs were 0.91 and 0.84 for anterior and posterior, respectively, suggesting that SWS estimates may be useful for quantifying cervical softening.

Evaluating the feasibility of acoustic radiation force impulse shear wave elasticity imaging of the uterine cervix with an intracavity array: a simulation study

The uterine cervix softens, shortens, and dilates throughout pregnancy in response to progressive disorganization of its layered collagen microstructure. This process is an essential part of normal pregnancy, but premature changes are associated with preterm birth. Clinically, there are no reliable noninvasive methods to objectively measure cervical softening or assess cervical microstructure. The goal of these preliminary studies was to evaluate the feasibility of using an intracavity ultrasound array to generate acoustic radiation force impulse (ARFI) excitations in the uterine cervix through simulation, and to optimize the acoustic radiation force (ARF) excitation for shear wave elasticity imaging (SWEI) of the tissue stiffness. The cervix is a unique soft tissue target for SWEI because it has significantly greater acoustic attenuation (α = 1.3 to 2.0 dB·cm-1·MHz-1) than other soft tissues, and the pathology being studied tends to lead to an increase in tissue compliance, with healthy cervix being relatively stiff compared with other soft tissues (E ≈ 25 kPa). Additionally, the cervix can only be accessed in vivo using a transvaginal or catheter-based array, which places additional constraints on the excitation focal characteristics that can be used during SWEI. Finite element method (FEM) models of SWEI show that larger-aperture, catheter-based arrays can utilize excitation frequencies up to 7 MHz to generate adequate focal gain up to focal depths 10 to 15 mm deep, with higher frequencies suffering from excessive amounts of near-field acoustic attenuation. Using full-aperture excitations can yield ~40% increases in ARFI-induced displacements, but also restricts the depth of field of the excitation to ~0.5 mm, compared with 2 to 6 mm, which limits the range that can be used for shear wave characterization of the tissue. The center-frequency content of the shear wave particle velocity profiles ranges from 1.5 to 2.5 kHz, depending on the focal configuration and the stiffness of the material being imaged. Overall, SWEI is possible using catheter-based imaging arrays to generate adequate displacements in cervical tissue for shear wave imaging, although specific considerations must be made when optimizing these arrays for this shear wave imaging application.

Shear Wave Speed Estimation in the Human Uterine Cervix

To explore spatial variability within the cervix and the sensitivity of shear wave speed (SWS) to assess softness/stiffness differences in ripened (softened) vs unripened tissue.

Quantitative ultrasound backscatter parameters in the human cervix

Quantitative ultrasound has been investigated as a tool for monitoring cervical changes that might result in preterm birth. Backscatter parameters, specifically attenuation and the backscattered power loss (BSPL), appear to be two important parameters. Sources of potential variability such as the angle of the beam interrogating the cervix, the region within the cervix, the number of previous births, and the state of ripening, have not been systematically examined, but could contribute to bias and variance in parameter estimates. Results presented here show that attenuation was affected by angle of interrogation, region in the cervix, parity, and ripened state. BSPL in the nonpregnant cervix was affected only by cervical region.

Detecting cervical microstructure via ultrasound and optical microscopy

A novel method for characterizing the microstructural integrity of the human cervix is under development. The cervix is composed of mostly aligned collagen and variations in acoustic backscattered power as a function of beam steering is an indicator of anisotropic scatterers. Results of experiments with phantoms and hysterectomy specimens suggest that this approach is a sensitive tool for describing the microstructure.

Detection of Changes in Cervical Softness Using Shear Wave Speed in Early versus Late Pregnancy: An in Vivo Cross-Sectional Study

The aim of this study was to assess the ability of shear wave elasticity imaging (SWEI) to detect changes in cervical softness between early and late pregnancy. Using a cross-sectional study design, shear wave speed (SWS) measurements were obtained from women in the first trimester (5-14 wk of gestation) and compared with estimates from a previous study of women at term (37-41 wk). Two sets of five SWS measurements were made using commercial SWEI applications on an ultrasound system equipped with a prototype catheter transducer (128 elements, 3-mm diameter, 14-mm aperture). Average SWS estimates were 4.42 ± 0.32 m/s (n = 12) for the first trimester and 2.13 ± 0.66 m/s (n = 18) for the third trimester (p <0.0001). The area under the curve was 0.95 (95% confidence interval: 0.82-0.99) with a sensitivity and specificity of 83%. SWS estimates indicated that the third-trimester cervix is significantly softer than the first-trimester cervix. SWEI methods may be promising for assessing changes in cervical softness.

Comparison of shear wave speed estimates in Ex vivo non-pregnant vs. In vivo pregnant cervix

The cervix is primarily composed of layers of aligned collagen that reorganize through out pregnancy resulting in softening, shortening, and dilation at term. Premature changes during the softening process may be associated with preterm birth. The purpose of this study is to compare shear wave speed (SWS) estimates in the ex vivo non-pregnant and in vivo third trimester pregnant cervix and to discuss confounding factors that may influence comparisons of SWS estimates. Ex vivo hysterectomy samples (n = 22) were collected and a subset were ripened (softened) (n = 13). Multiple SWS measurements were made longitudinally along the cervical canal at 4-5 locations on both anterior and posterior halves of the bivalved cervix. For the in vivo study, patients (n = 10) undergoing cervical ripening for induction of labor were recruited. SWS measurements were made on the same patient with the probe placed on the anterior half of the cervix at mid-length between the internal and external os. Ten replicate SWS measurements were made before and 4 hours after a cervical ripening agent (misoprostol) was administered. Statistical analysis was performed to determine significance of differences in SWS as a result of ripening. The SWS estimates for the mid-length anterior position for the ex vivo study were 3.42±0.92 m/s and 2.06±0.39 m/s for unripened and ripened, respectively. Similarly, the SWS estimates for the in vivo third trimester pregnant cervix were 2.40±0.75 m/s and 1.54±0.31 m/s pre- and post-ripening. Factors such as bivalving the ex vivo specimens, differences in tissue temperature, and tissue perfusion likely affect SWS estimates, but similar similar trends were found.