Objective: Strain elastography and shear wave elastography are two commonly
used methods to quantify cervical elasticity; however, they have limitations.
Strain elastography is effective in showing tissue elasticity distribution in a
single image, but the absence of stress information causes difficulty in
comparing the results acquired from different imaging sessions. Shear wave
elastography is effective in measuring shear wave speed (an intrinsic tissue
property correlated with elasticity) in relatively homogeneous tissue, such as
in the liver. However, for inhomogeneous tissue in the cervix, the shear wave
speed measurement is less robust. To overcome these limitations, we develop a
quantitative cervical elastography system by adding a stress sensor to an
ultrasound imaging system.
Methods: In an imaging session for quantitative cervical elastography, we use
the transvaginal ultrasound imaging system to record B-mode images of the
cervix showing its deformation and use the stress sensor to record the
probe-surface stress simultaneously. We develop a correlation-based automatic
feature tracking algorithm to quantify the deformation, from which the strain
is quantified. After each imaging session, we calibrate the stress sensor and
transform its measurement to true stress. Applying a linear regression to the
stress and strain, we obtain an approximation of the cervical Young's modulus.
Results: We validate the accuracy and robustness of this elastography system
using phantom experiments. Applying this system to pregnant participants, we
observe significant softening of the cervix during pregnancy (p-value < 0.001)
with the cervical Young's modulus decreasing 3.95% per week. We estimate that
geometric mean values of cervical Young's moduli during the first (11 to 13
weeks), second, and third trimesters are 13.07 kPa, 7.59 kPa, and 4.40 kPa,
respectively.Comment: 26 pages, 8 figures, 1 tabl