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.
