Parametric Solid Models of the At-Term Uterus From Magnetic Resonance Images

Abstract

Birthing mechanics are poorly understood, though many injuries during childbirth are mechanical, like fetal and maternal tissue damage. Several biomechanical simulation models of parturition have been proposed to investigate birth, but many do not include the uterus. Additionally, most solid models rely on segmenting anatomical structures from clinical images to generate patient geometry, which can be time-consuming. This work presents two new parametric solid modeling methods for generating patient-specific, at-term uterine three-dimensional geometry. Building from an established method of modeling the sagittal uterine shape, this work improves the uterine coronal shape, especially where the fetal head joins the lower uterine wall. Solid models of the uterus and cervix were built from five at-term patients' magnetic resonance imaging (MRI) sets. Using anatomy measurements from MRI-segmented models, two parametric models were created-one that employs an averaged coronal uterine shape and one with multiple axial measurements of the coronal uterus. Through finite element analysis, the two new parametric methods were compared to the MRI-segmented high-fidelity method and a previously published elliptical low-fidelity method. A clear improvement in the at-term uterine shape was found using the two new parametric methods, and agreement in principal Lagrange strain directions was observed across all modeling methods. These methods provide an effective and efficient way to generate three-dimensional solid models of patient-specific maternal uterine anatomy, advancing possibilities for future research in computational birthing biomechanics.

ParametricModelDifferences

Fig. 1

Patient-specific models generated for one patient. (a) The MRI segmented model was created by hand-segmenting MRI image stacks, whereas the multimeasured, averaged, and elliptical models are based on parametric dimension measurements. All parametric models are based on previous methods for the uterine sagittal shape, with different approaches to creating the coronal shape [21]. (b) The multimeasured model uses 45 measurements to define the coronal shape (15 intra-uterine diameters, 15 right uterine wall thicknesses, and 15 left uterine wall thicknesses). (c) The averaged model uses the same measurements outlined in previous work (one intra-uterine diameter and one uterine wall thickness) but utilizes an averaged at-term uterine shape, [21]. (d) The elliptical model uses an elliptical coronal shape, as is assumed in previous work [21]. Dimension measurements taken from clinical images are denoted by solid black lines, and locations in the averaged parametric based on measurement trends are denoted by a dashed black line.

Fig. 2

Measurements of sagittal uterine shape were measured as described previously [21]. To capture the shape of the posterior uterine wall, the intra-uterine diameters that measure between the inferior-superior axis (UD1, horizontal dashed line) and the posterior wall on the superior half (UD3a) and inferior half (UD3b) of the uterus were defined as the extrema of the posterior uterine wall. Locator dimensions along the inferior-superior axis were also taken, UD1a and UD1b, respectively. The vertical dashed red lines show where equidistant measurements would be placed.

Fig. 3

Locations where additional left-right intra-uterine diameter, left uterine wall thickness, and right uterine wall thickness measurements were taken in the MRI segmented models (left). The left-right intra-uterine diameter was taken as the longest horizontal diameter, and the left and right uterine wall thicknesses were taken at the ends of the intra-uterine diameter (right).

Fig. 4

The inferior half model for the MRI Segmented (left) and parametric multimeasure, averaged, and elliptical models (right) is fixed in all directions at the cut plane and a contraction level. An intra-uterine pressure is applied to the intra-uterine surface and along the cervical canal.

Fig. 5

The uterus was divided into 12 sections for quantitative similarity analysis, with sections in the (S)uperior, (M)iddle, (I)nferior, (A)nterior, (P)osterior, (L)eft, and (R)ight uterus. The superior most 10% of UD1 of the uterus and entire cervix were excluded from analysis. (a) Sagittal and (b) coronal view of the sectioned uterus.

Fig. 6

Representative schematic for differences between (a) sagittal and (b) axial measurements taken from MRI and MRI segmented models. The average percentage error is shown in the color of the dimension line, with darker colors indicating larger errors. The dimension line is solid if the measurement in MRI is more often larger than in the MRI segmented model and dashed if vice versa.

Fig. 7

Scaled left-right intra-uterine diameter measurements from all patients' (a) MRI-segmented models and (b) MRI images along the scaled inferior-superior intra-uterine diameter, with 0% indicating the inferior end of the uterus and 100% the superior end of the uterus

Fig. 8

Left uterine wall thickness measurements from all patients' (a) MRI-segmented models and (b) MRI images along the scaled inferior-superior intra-uterine diameter, with 0% indicating the inferior end of the uterus and 100% the superior end of the uterus.

Fig. 9

Right uterine wall thickness measurements from all patients' (a) MRI-segmented models and (b) MRI images along the scaled inferior-superior intra-uterine diameter, with 0% indicating the inferior end of the uterus and 100% the superior end of the uterus.

Fig. 10

The resulting patient-specific solid models produced using the MRI segmentation method (column 1), multimeasured parametric model (column 2), averaged parametric model (column 3), and elliptical parametric model (column 4).

Fig. 11

Finite element analysis 1st principal Lagrange strain magnitude and direction heat maps for all patients and modeling methods in the posterior uterus, as viewed from the anterior. Black lines within the uterine body note the strain direction.

Fig. 12

Finite element analysis 1st principal Lagrange strain magnitude and direction heat maps for all patients and modeling methods in the anterior uterus, as viewed from the anterior. Black lines within the uterine body note the strain direction.