Assessment of Structural Heterogeneity and Viscosity in the Cervix Using Shear Wave Elasticity Imaging: Initial Results from a Rhesus Macaque Model

Abstract

Shear wave elasticity imaging has shown promise in evaluation of the pregnant cervix. Changes in shear wave group velocity have been attributed exclusively to changes in stiffness. This assumes homogeneity within the region of interest and purely elastic tissue behavior. However, the cervix is structurally/microstructurally heterogeneous and viscoelastic. We therefore developed strategies to investigate these complex tissue properties. Shear wave elasticity imaging was performed ex vivo on 14 unripened and 13 misoprostol-ripened cervix specimens from rhesus macaques. After tests of significant and uniform shear wave displacement, as well as reliability of estimates, group velocity decreased significantly from the distal (vaginal) to proximal (uterine) end of unripened, but not ripened, specimens. Viscosity was quantified by the slope of the phase velocity versus frequency. Dispersion was observed in both groups (median: 5.5 m/s/kHz, interquartile range: 1.5-12.0 m/s/kHz), also decreasing toward the proximal cervix. This work suggests that comprehensive assessment of complex tissues such as cervix requires consideration of structural heterogeneity and viscosity.

Keywords: Cervix; Shear Wave Elasticity; Viscoelasticity.

Figure 1

Analysis of uniformity and statistical significance of shear wave motion in a ROI deemed as heterogeneous. (a) Average displacement in microns as function of the axial and lateral position. (b) Empirical cumulative distribution function (CDF) of the laterally-averaged displacement in microns in the reference frames. (c) Comparison of the laterally-averaged average displacement with the reference-based threshold.

Figure 2

Examples of (a) Velocity field (normalized to the maximum velocity value) and (b) Velocity power spectrum.

Figure 3

Maximum Velocity Power Spectrum (MVPS) as a function of temporal frequency. The 0.4kHz bandwidth used to determine the noise level is shown in blue. The green and red dashed lines indicate the noise level and the 10dB threshold above the noise level, respectively. Arrows indicate the cutoff frequencies for the analysis bandwidth.

Figure 4

Left: Analysis of the spatial variability of viscoelastic properties in (a) Huang et al. (2016) and (b) the current work. In (a) the cervix is divided into five regions with respect to its entire length (simple linear scaling). In (b) the bulbous distal cervix is disregarded, and the transition zone is taken as reference.

Figure 5

Group velocity c_g as a function of distance from transition zone (proximally, toward the internal os) in unripened ((a) and (c)) and ripened ((b) and (d)) groups. Top: Anterior, Bottom: Posterior, Left: Unripened, Right: Ripened. Boxes indicate the median (middle line), the 25%ile and 75%iles (upper and lower ends), the range (whiskers), and outliers based on 1.5 times the interquartile range above the 75%ile. P values correspond to Kruskal-Wallis test with respect to the values at the transition zone (0mm).

Figure 6

Non-parametric estimates of the phase velocity as a function of the frequency at the transition zone in the unripened (black) vs. ripened (red) groups. The thick lines indicate the mean between specimens, and shadowed areas indicate ± one standard deviation.

Figure 7

Spectral centroid frequency as a function of distance from transition zone in unripened (a) and (c) vs. ripened (b) and (d) groups. Top: Anterior, Bottom: Posterior, Left: Unripened, Right: Ripened. Note that the higher frequency content of the shear waves agrees with the observation that shear wave speed is higher closer to the transition zone. P values correspond to Kruskal-Wallis test with respect to the values at the transition zone (0mm).

Figure 8

Parametric c_p,LM at 0.53kHz as a function of distance from the transition zone. Top: Anterior, Bottom: Posterior, Left: Unripened, Right: Ripened. P values correspond to Kruskal-Wallis test with respect to the values at the transition zone (0mm).

Figure 9

Slope of the phase velocity vs. frequency as a function of the distance from the transition zone. Top: Anterior, Bottom: Posterior, Left: Unripened, Right: Ripened. P values correspond to Kruskal-Wallis test with respect to the values at the transition zone (0mm).

Figure 10

Ratio of the velocity-based and the displacement-based group velocities as a function of the phase velocity slope vs. frequency in unripened (black) and ripened (red) samples.