Cervical Elastography for Prediction of Successful Induction of Labor at Term

Discussion

The exact process from cervical ripening to effacement and dilatation is not clearly established. However, several elements, including ecorin, hyaluronic acid, hormones,cytokines, and proteases, are involved in this process, reducing collagen levels and cell components within the cervix while increasing the water content, all leading to the softening of the cervix.  Cervix shortening may follow ripening but not always, as seen in cases of term deliveries with a reduced cervical length measured from the mid second trimester. Thus, evaluation of not only the cervical length but also the mechanical properties of the cervix should be included to predict successful induction of labor.

Only a few attempts have been made to objectively evaluate the cervical consistency with  sonography. One study  reported that cervical consistency can be evaluated by measuring the difference of echogenicity in the anterior and posterior cervical lips on a vaginal grayscale sonographic histogram. A disadvantage is that the echogenicity of the cervix can be affected by gain and artifacts such as reverberation.  Thus, another way to measure cervical consistency is needed, based on the physical properties of the cervix. The main issue with elastography of the cervix is the lack of reference tissue for comparison. Elastography is most useful when there is adjacent tissue of differing stiffness (ie, tumor imaging). Thus, elastography of malignant tumors can be useful because it increases the contrast between adjacent tissues of differing stiffness. However, the cervix is nearly uniform and changed in toto. Considering the limitations of cervical elastography, this study was performed and showed that it was possible to quantify the whole elastographic data of the cervix and that imaging analysis could be applied to cervical elastography to predict successful induction of labor in nulliparous women at term. Moreover, the intraobserver and interobserver variability for cervical elastographic data shows that imaging analysis was reliable and reproducible.

The application of elastography in the cervix of pregnant woman is at a rudimentary stage. In particular, the elastographic method used to evaluate solid tumors in the prostate, breast, and thyroid gland cannot be directly transferred to measuring the cervix in a pregnant woman.

A tumor in a solid organ is relatively round and can be compared with surrounding normal tissue. However, a normal cervix in a pregnant woman has no abnormal tissue or a typical shape that is different from round. In addition, to adequately assess the status of the cervix, data obtained from the entire cervix are needed. If the analytic method of elastography used for solid tumors is applied to the cervix of a pregnant woman, the predicted problems are as follows: the color in a cervical elastographic image is not homogeneous, and the area colored the same is not circular but very irregularly shaped. Thus, the scoring method using color in small circular areas of the cervix in previous studies seldom reflects the whole cervix and is subjective. Especially, if the uterine cervix is shortened or funneled, it is difficult to select and score the several small circular areas in the cervix. However, these problems can be resolved by the imaging analysis technique introduced in this study. By using a different imaging analysis technique, the whole cervix can be included for evaluation; the area can be selected regardless of shape; and the data are objective and automatically calculated by a computer.

During the prenatal period, the main changes in the cervix include softening, ripening, and dilatation.  If the cervical length or cervical area is correlated with cervical dilatation, the softening and ripening of the cervix can be reflected by cervical elastography. In this study, the combinations of cervical length or cervical area + mean elastographic index or cervical hard area were modeled to improve prediction. This study indicates that elastography is a technique that can be applied to examine the cervix of pregnant women.

Although the imaging analysis used in this study was able to resolve some problems originated by the application of elastography in the cervix of pregnant women, other limitations remain. There were no reference data to show the elastographic status of the cervix according to the gestational age in normal pregnant women. The physiologic modifications of breathing and arterial pulsation could play a role in the variability of tissue displacement. The elastographic image can be changed by pressing the probe with different pressure levels. To overcome this problem, we tried to apply no pressure and just touch the cervix with the probe after insertion. Of course, although we tried to maintain steady pressure, we could not stop all minimal shaking. Therefore, to evaluate whether the changes made by this minimal shaking could affect the elastographic results, we performed the intraobserver and interobserver reproducibility test for imaging analysis of the elastographic results. There were 2 limitations to the intraobserver and interobserver test in this study. For intraobserver reproducibility, a minimum 2-week interval is required between reviews of the same image to avoid recall bias. The periods between each review in our study were just 20 minutes. The other limitation was that there were some large 95% CI values. Nevertheless, imaging analysis of cervical elastography can be a good method for evaluating the cervical status when used together with the cervical length. This finding can be applied to other clinical studies, such as the prediction of preterm birth, breast cancer detection, and thyroid mass evaluation.

In conclusion, imaging analysis of cervical elastography to predict successful induction of labor in nulliparous women at term is objectively quantifiable, reliable, and reproducible. Future studies should be performed to determine the effect of the combination of cervical length and cervical elastographic parameters and to resolve the remaining limitations of cervical elastography.