*4.2.1 B-mode ultrasound elastography: strain/static elastography: shear wave speed estimation elastography/dynamic elastography*

Elastography association with conventional/standard ultrasound aimed to bring new data on cervical stiffness, and sPTB, being two types of elastography used in clinical practice: strain elastography-SE (static elastography) and shear wave speed estimation elastography-shear wave speed (SWS) (dynamic elastography).

Strain elastography, a direct biomechanical testing, measures cervix deformation percentage, expressed as a strain value, computer analyzed on a color map (produced by a specialized software), when gentle transducer movements compress a region of interest (ROI: IO, EO, entire cervix [98, 99]) during ordinary B-mode ultrasound vaginal scanning (mid-sagital plan) for CL. Increased strain reflects increased deformation (softer tissue), decreasing strain reflects reduced deformation (stiffer tissue); dichotomous variables are usually constructed to describe whether each patient's measurement is in the bottom quartile (≤25th centile) for each cervical region [98]. The USA results sustained that strain measurements obtained in a cross-sectional view of IO were significantly associated with sPTD (<37 weeks) [98]. The strain values corresponding to 25th centile in IO were 0.19 for the endocervical canal and 0.14 for the entire cervix. Women with strain values ≤25th centile were approximately 80% less likely to subsequently deliver preterm than those whose values were > 25th centile (entire cervix: OR: 0.17; 95% CI: 0.03–0.9; endocervical canal OR: 0.2; 95% CI: 0.03–0.96). Patient's characteristics influence the results (parity, prior preterm deliveries, gestational age at examination, CL, mainly when it is <25 mm [100]).

Despite new real-time quantitative data on cervix morphology and architecture, there are some limitations regarding distance more than 1.5 cm from the transducer, if tissue is inhomogeneous, and if ROI is placed at an angle above 20° from the compression direction; it does not allow quantification of tissue stiffness, when the intensity of force (stress) is unknown, but new devices have press indicator; interobserver variability.

Shear wave speed (SWS)/dynamic shear wave elasticity image (SWEI), can differentiate between soft and firm cervices by measuring ultrasound speed through a tissue: SWS is faster through firm, and slower through soft tissues [101, 102] (**Figure 4**). SWEI assesses women's quantitative stiffness from early (5–14 weeks) to late (40 weeks) gestation, with assessment at EO [104], mid *cervix* (at bladder reflection level), at IO proximity [105], being recognized the depth importance on SWEI estimations in IO, and EO along pregnancy [106]. It is a constant fractional reduction in SWS (average 4%/week), considered the most critical value for birth timing prediction, increasing with gestational age (GA), and a spatial gradient along CL: the most important softness is at the proximal end—9%/week, and 2%/week at distal end [107]. There are differences in SWS reduction reports from first to third trimester: 52% [107], 42% [108], or only a 12% decrease [104], being demonstrated that SWS declines progressively with GA between 18 and 24 weeks, being higher at IO than at EO, at each GA, IO is the best area to assess [103], because collagen fibers longitudinal alignment along endocervical canal, and at it is proximal part, not circumferential as they are near cervix edge, as microscopy revealed on specimens collected at hysterectomy, and reduced by 1.4 at every 10 years of maternal age, not by BMI [109].

Women with a soft cervix were 3.3 times more likely to have a short cervix, and 1.5 times more likely to have had a prior sPTB than cases who did not have a soft cervix compared to a cervix that was neither short nor soft. Softness increases by 1.5 times the risk of repeating sPTB. Prevalence of sPTB is higher by 18 fold <37 weeks, and by 120 fold <34 weeks, when a soft cervix (C) (<25th percentile for GA defined by SWEI at IO) is associated with short C (cut-off <25 mm) compared to normal CL, at 18–24 weeks [103]. Study conclusion was that softness is an independent risk factor for sPTB <37 and < 34 weeks, regardless of shortness presence, or sPTB history. **Table 1** shows the superior specificity and positive likelihood ratio (LR) values for sPTB (<34 and <37 weeks), when softness and shortness are combined*.*

*Abnormal Cervical Remodeling Early Depiction by Ultrasound Elastography: Potential… DOI: http://dx.doi.org/10.5772/intechopen.113314*

#### **Figure 4.**

*Shear wave elastography in the cross-sectional planes of the external cervical OS of two patients at 21 weeks of gestation; (a) a non-soft cervix (shear wave speed (SWS) = 2.3 m/s [SWS 25th percentile = 1.72 m/s]); (b) a soft cervix with a SWS of 1.6 m/s (≤25th percentile). High SWS is shown in blue and low SWS in red. Adapted from Hernandez-Andrade et al. [103]. Free PMC article, HHS Public Access.*


#### **Table 1.**

*Sensitivity, specificity, LR for PTB when softness, shortness combination vs. short cervix. Results from Hernandez-Andrade et al. [103].*
