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Mid-trimester sonographic cervical length is a powerful predictor of spontaneous preterm birth (NEJM 1996) and also identifies primiparous women at risk for cesarean delivery, due to poor progress in labor (NEJM 2008). Cervical remodeling is an important component of the common pathway of parturition. Therefore, assessing changes in cervical biochemistry could provide information about cervical status beyond that provided by cervical length and elastography. Photoacoustic imaging is a non-invasive ultrasound-based technology that captures acoustic signals emitted by the components of cervical tissue in response to laser pulses, which allows for the determination of the collagen-to-water ratio (CWR). The purpose of this study was to determine whether photoacoustic profiling of CWR can distinguish structural changes between pregnant and non-pregnant cervices.
This was a prospective cohort study in which cervical biopsies were obtained at the time of hysterectomy in pre- menopausal, non-pregnant women (n=10) and at scheduled cesarean delivery in pregnant women at term who were not in labor (n=10). Specimens were imaged using a probe complemented by a fiber-optic light delivery system. The photoacoustic signals acquired within the range of wavelengths that cover the peak absorption of collagen and water were used to determine the CWR.
1) A decrease in collagen content and an increase in water content were observed in cervical specimens from pregnant women, compared to those obtained from non-pregnant women (Figure, a, b); 2) the CWR was significantly lower in pregnant than non-pregnant uterine cervices (23.3% vs 63.1%, respectively; p<0.05); 3) microscopic images from non-pregnant cervices show compact collagen bundles with minimal edema (Figure, c), confirmed with Sirius Red Stain (Figure, e, f), while specimens of pregnant women demonstrated disorganized loose collagen fibers lacking discrete bundles with intervening edema (Figure, d).
Photoacoustic imaging represents a novel technique to characterize tissue composition. We propose that this noninvasive imaging technique could complement sonographic evaluation of cervical length in women at risk for preterm birth, as well as prior to the induction of labor. Moreover, this imaging method could be used to assess response to therapy (progesterone, cerclage, pessary).