Evaluating the feasibility of acoustic radiation force impulse shear wave elasticity imaging of the uterine cervix with an intracavity array: a simulation study

Abstract

The uterine cervix softens, shortens, and dilates throughout pregnancy in response to progressive disorganization of its layered collagen microstructure. This process is an essential part of normal pregnancy, but premature changes are associated with preterm birth. Clinically, there are no reliable noninvasive methods to objectively measure cervical softening or assess cervical microstructure. The goal of these preliminary studies was to evaluate the feasibility of using an intracavity ultrasound array to generate acoustic radiation force impulse (ARFI) excitations in the uterine cervix through simulation, and to optimize the acoustic radiation force (ARF) excitation for shear wave elasticity imaging (SWEI) of the tissue stiffness. The cervix is a unique soft tissue target for SWEI because it has significantly greater acoustic attenuation (α = 1.3 to 2.0 dB·cm−1·MHz−1) than other soft tissues, and the pathology being studied tends to lead to an increase in tissue compliance, with healthy cervix being relatively stiff compared with other soft tissues (E ≈ 25 kPa). Additionally, the cervix can only be accessed in vivo using a transvaginal or catheter-based array, which places additional constraints on the excitation focal characteristics that can be used during SWEI. Finite element method (FEM) models of SWEI show that larger-aperture, catheter-based arrays can utilize excitation frequencies up to 7 MHz to generate adequate focal gain up to focal depths 10 to 15 mm deep, with higher frequencies suffering from excessive amounts of near-field acoustic attenuation. Using full-aperture excitations can yield ~40% increases in ARFI-induced displacements, but also restricts the depth of field of the excitation to ~0.5 mm, compared with 2 to 6 mm, which limits the range that can be used for shear wave characterization of the tissue. The center-frequency content of the shear wave particle velocity profiles ranges from 1.5 to 2.5 kHz, depending on the focal configuration and the stiffness of the material being imaged. Overall, SWEI is possible using catheter-based imaging arrays to generate adequate displacements in cervical tissue for shear wave imaging, although specific considerations must be made when optimizing these arrays for this shear wave imaging application.

In vivo estimation of perineal body properties using ultrasound quasistatic elastography in nulliparous women

“…Recently, researchers have begun using shearwave elastography to evaluate cervical tissue stiffness (for example, Hernandez-Andrade, 2014; Palmeri et al 2013) because of its advantage that the generation of the mechanical impulse is operator independent and the stiffness measurement is an absolute measurement. However, morphological factors can influence shear-wave propagation such as complex tissue composition and boundary condition.…”

Using speckle statistics to improve attenuation estimates for cervical assessment

“…In our study of ultrasound attenuation assessment of human cervix tissue, ROI size was varied from 10.5 pulse length to The cervix is mainly composed of collagen with relatively few smooth muscle bundles. The alignment of collagen varies from one layer to another with inner and outer layer having collagen arranged longitudinally, however middle layer has circumferentially aligned collagen [15,41,42]. As the cervix remodels for birth the cervix softens before the cervical length decreases (after 30 weeks of gestation age) which happens along with change of collagen concentration [43].…”

“…Studies have shown that as the delivery time approaches the collagen rich cervix is filled with water and enzymes which results in a lower attenuation value and a softening of the cervix as it prepares for delivery [23][24][25]. Our earlier work showed that attenuation of the cervix decreased as the cervix prepared for delivery [8][9][10][11][12] while the softening of the cervix can also be quantified using shear wave elastography [13][14][15].…”

“…Our earlier work showed that attenuation of the cervix decreased as the cervix prepared for delivery [8][9][10][11][12]. The softening of the cervix can also be quantified using shear wave elastography [13][14][15]. In this method a stress is created and then the strain in the tissue is observed from that stress.…”

A Hilbert Transform Based Shear Wave Speed Estimation Method for Shear Wave Elasticity Imaging

“…Shear wave is monitored outside the region of excitation (ROE) within focal zone [17][18][19]. Many researchers have quantified tissue stiffness from shear wave speed (SWS) estimation from dynamic displacement data.…”