Orbital volume measurements from magnetic resonance images using the techniques of manual planimetry and stereology

Abstract

Introduction: Current volume measurement techniques, for the orbit, are time-consuming and involve complex assessments, which prevents their routine clinical use. In this study, we evaluate the applicability and efficacy of stereology and planimetry in orbital volume measurements using magnetic resonance imaging (MRI).

Materials and methods: Prospective imaging study using MRI. Sheep craniums and human subjects were evaluated. Water-filling measurements were performed in animal skulls, as the standard validation technique. Planimetry and stereology techniques were used in each dataset. Intraobserver and interobserver reliability testing were applied.

Results: In stereology customization, 1/6 systematic sampling scheme was determined as optimal with acceptable coefficient of error (3.09%) and low measurement time (1.2 min). In sheep craniums, the mean volume measured by water displacement, planimetry, and stereology was 17.81 ± 0.59 cm³, 18.53 ± 0.24 cm³, and 19.19 ± 0.17 cm³, respectively. Planimetric and stereological methods were highly correlated (r = 0.94; P ≈ 0.001). The mean difference of the orbital volume using planimetry and stereology was 0.316 ± 0.168 cm³. In human subjects, using stereology, the mean orbital volume was found to be 19.62 ± 0.2 cm³ with a CE of 3.91 ± 0.15%.

Conclusions: The optimized stereological method was found superior to manual planimetry in terms of user effort and time spent. Stereology sampling of 1/6 was successfully applied in human subjects and showed strong correlation with manual planimetry. However, optimized stereological method tended to overestimate the orbital volume by about 1 cc, a considerable limitation to be taken in clinical practice.

Keywords: Eye; orbit; planimetry; skull; stereology.

Figure 1

(a) Standard enucleation procedure of the soft tissue from the orbital cavity. The entire orbital contents were removed en bloc down to the optic canal. (b) The optic nerve was dissected from the canal part at the optic foramen and was isolated with the eye globe. (c) The orbital socket was immobilized in a perfectly horizontal direction. Then it was filled with water to measure the exact orbital volume

Figure 2

(a) Manual delineation of the orbital cavity on axial slicer by applying three-dimensional volume rendering in three-dimensional Slicer. (b) Three-dimensional model of the segmented orbit for measuring its total volume. (c) Stereological measurement of the orbital volume using the analyze software. A grid is placed over the slice and the green points which lie within the orbit are selected by the user. The total volume is then estimated based on the total number of point counts

Figure 3

(a) Model of the segmented orbit within the human skull on three-dimensional Slicer. (b) Manual delineation of the orbital cavity on an axial magnetic resonance imaging slice, to be used for manual planimetry measurement. (c) Stereological technique, using 1/6 sampling for calculating the orbital volume based on the selected point count

Figure 4

Bland–Altman plots for comparisons between methods. Upper plot: Differences in orbital volume estimates as defined by manual planimetry and the water-filling method. The mean difference is presented with the solid line whereas the 95% limits of agreement are shown with the dotted lines. Middle plot: Differences in orbital volume estimates as defined by the optimized stereological approach and the water-filling method. Lower plot: Differences in orbital volume measurement as defined by manual planimetry and stereology