New severity indices for quantifying single-suture metopic craniosynostosis

Abstract

Objective: To describe novel severity indices with which to quantify severity of trigonocephaly malformation in children diagnosed with isolated metopic synostosis.

Methods: Computed tomographic scans of the cranium were obtained from 38 infants diagnosed with isolated metopic synostosis and 53 age-matched control patients. Volumetric reformations of the cranium were used to trace two-dimensional planes defined by the cranium-base plane and well-defined brain landmarks. For each patient, novel trigonocephaly severity indices (TSI) were computed from outline cranium shapes on each of these planes. The metopic severity index based on measurements of interlandmark distances was also computed and a receiver operating characteristic analysis used to compare the accuracy of classification based on TSIs versus that based on the metopic severity index.

Results: The proposed TSIs are a sensitive measure of trigonocephaly malformation that can provide a classification accuracy of 96% with a specificity of 95%, in contrast with 82% of the metopic severity index at the same specificity level.

Conclusions: We completed exploratory analysis of outline-based severity measurements computed from computed tomographic image planes of the cranium. These TSIs enable quantitative analysis of cranium features in isolated metopic synostosis that may not be accurately detected by analytic tools derived from a sparse set of traditional interlandmark and semilandmark distances.

Figures

FIGURE 1

Volumetric reformations of the cranium of a patient affected with isolated metopic synostosis.

FIGURE 2

Image analysis planes (A, F, and M). Trigonocephaly severity measures are computed from computed tomographic (CT) image slices located at three different image planes. The A plane is at the top of the lateral ventricle, the F plane is at the foramina of Munro, and the M plane is at the level of the maximal dimension of the fourth ventricle. Computer enhancement of ventricles is provided for illustration purposes. NS, nasa suture; O, opisthion.

FIGURE 3

CT image of a patient affected with metopic synostosis that shows the linear distances used to compute the metopic severity index. The distances are as follows: the bifrontal diameter at the level of the coronal sutures (B), the bifrontal diameter at half the distance from the coronal sutures to the anteriormost portion of the frontal bones (A), and the biparietal diameter (C).

FIGURE 4

Standard image processing techniques can be used to segment a bone CT image (A) to compute an outline (B) whose origin is defined by its corresponding centroid (O). The outline is represented in polar coordinates (r, θ). The coordinate r is the axial distance from the origin to the p (the length of the line segment Op). The coordinate θ is the angle that makes the line segment Op with the polar axis OR. The orientation of p is defined by the direction of its corresponding tangent (T) and normal (N) vectors. The normalized (ρ, θ) coordinates (with ρ = r/α) can be represented in a scatterplot of θ versus ρ (C) to produce normalized polar representation (NPR). This representation of outline shape enables the computation of severity indices that are scale invariant. The normalization constant α is defined as the head length measured in A.

FIGURE 5

Trigonocephaly severity indices (TSI) are calculated from NPRs (B and D), which are derived from outline cranium shapes (A and C). A and B show the outline shape and NPR of a patient affected with isolated metopic synostosis. C and D show the same plots of a nonsynostotic control. Note that the NPR corresponding to the affected patient (B) resembles an isosceles triangle, unlike the shape shown in the NPR of the control (D). The angle γ and the ordinates hM and hC are parameters that are used in the computation of the TSIs (see Fitting Isosceles Triangles to the Normalized Coordinates under Patients and Methods). The arrows labeled CS in A indicate the approximate location of the coronal sutures.

FIGURE 6

A, NPR of an outline shape. B, TSI is computed as the base of an isosceles triangle that approximates the profile of the NPR’s main section (curve above shaded area), which encodes the shape information for the frontal bones. The base is computed as 2A/(180)hM, where 180 is a normalization factor that ensures that the TSI takes values between 0 and 1. C, triangle fitted to the outline representation.

FIGURE 7

Receiver operating characteristic (ROC) curves for classification of metopic synostosis versus nonsynostotic cranium outlines by using the TSIs (TSI-A, -F, and -M) and the metopic severity index.