117 in vivo evaluation of the cervical stiffness evolution during induced labor in ewes using elastography.

Despite numerous advances and intensive research in perinatal medicine, spontaneous preterm birth (PTB) is the leading global cause of neonatal mortality and morbidity. On the other hand, labour has to be induced in ~23% of pregnancies worldwide. Both issues may be related to the distensibility of the cervical tissue. Quantitative and objective monitoring of the cervix ripening may provide a complementary method to identify cases at risk of PTB and assess the likelihood of successful induction of labour. Currently, however, no reliable clinical tools for such a quantitative and objective evaluation exist. Elastography aims at imaging tissue stiffness. All elastography techniques rely on the same basics: an external force is applied to the tissue and the resulting movements are then followed. Supersonic shear imaging (SSI) is a dynamic method that uses the propagation of mechanical waves to excite the tissue. Its speed is tracked then by ultrafast imaging, allowing characterisation of stiffness [Bercoff et al. 2004 IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 51, 396-409]. Understanding the mechanisms that take place in normal pregnancy will allow a better comprehension of the cervical remodelling and lead to better methods of diagnosis of PTB and successful induction of labour. In this work, we propose a preliminary assessment of the evolution of stiffness during the cervical maturation process in the sheep. The main goal was to study the feasibility of elastography using SSI to quantify cervical stiffness during the maturation process and to assess the potential of this technique for diagnosis of preterm labour and for labour induction success. Cervical stiffness was quantified, by 2 different operators, in 9 pregnant ewes in vivo by using SSI. The cervical ripening was induced by a dexamethasone injection in 5 animals, and 4 animals constituted the control group. The stiffness of the second ring of the cervix was quantified over a circular region of interest of 5mm of diameter during vaginal ultrasound examination. Images were acquired every 4h during 24h to monitor the cervical maturation induced by the dexamethasone injection. Cervical stiffness was found to decrease significantly throughout the cervical ripening (from 9.5±0.9kPa to 5.0±0.8kPa; P=2.7e-5). The intraobserver and interobserver repeatability of measurements were assessed using Bland-Altman analysis with 95% CI. The principal findings of the study were that elastography measurements using SSI technique were highly reproducible in all cases. Second, stiffness of the uterine cervix decreases throughout the maturation process induced by the dexamethasone injection. Finally, it was possible to quantify the decrease of stiffness through the cervical maturation process. Elastography may be a valuable method to quantify objectively and noninvasively the cervical stiffness in vivo, and ultimately could be a useful tool for the diagnosis of PTB and the assessment of labour induction success.