Thyroid-associated Orbitopathy: Quantitative Evaluation of the Orbital Fat Volume and Edema Using IDEAL-FSE

Abstract

Background and purpose: To compare orbital quantitative data obtained by fast spin-echo iterative decomposition of water and fat with echo asymmetry and least-squares estimation (FSE-IDEAL) in patients with thyroid-associated orbitopathy (TAO) and healthy controls and to investigate the characteristics of these data in TAO patients.

Materials and methods: Twenty-two TAO patients (4 males and 18 females; median age 51.0 years) and 22 healthy subjects (5 males and 17 females; median age 50.5 years) underwent orbital T2-weighted FSE-IDEAL. The water fraction in orbital fat was defined as the signal intensity (SI) water / (SI water + SI fat). The orbital fat volume was measured on fat images. The degree of proptosis was evaluated using in-phase imaging. Mann-Whitney U test was used to compare these quantitative data in the two groups. In TAO patients we ascertained the correlation among these values with the Spearman's rank correlation coefficient.

Results: In TAO patients, the water fraction (right and left, p = 0.04), fat volume (right and left, p = 0.03) and degree of proptosis (right and left, p < 0.01) were higher than in the controls. In TAO patients, only the water fraction and the fat volume of left orbit showed negative correlation (p = 0.01).

Conclusion: The water fraction of orbital fat, the orbital fat volume and the degree of proptosis obtained with FSE-IDEAL were higher in TAO patients than in the controls. The water fraction was a new parameter for differentiating between TAO patients and healthy subjects.

Keywords: CAS, clinical activity score; FSE-IDEAL, fast spin-echo iterative decomposition of water and fat with echo asymmetry and least-squares estimation; IQR, interquartile range; SD, standard deviation; SI, signal intensity; TAO, thyroid-associated orbitopathy; fast spin-echo iterative decomposition of water and fat with echo asymmetry and least-squares estimation (FSE-IDEAL); orbital fat volume; proptosis; quantitative evaluation; thyroid-associated orbitopathy (TAO); water fraction.

Fig. 1

Control Axial water- (A) and fat image (B). Yellow circles identify the ROIs. Using the average signal intensity (SI) in the ROIs, the water fraction in orbital fat was calculated as SI water / (SI water + SI fat).

Fig. 2

Control Axial fat image (A). The selected orbital fat is green. We separated fat tissue using an adequate signal intensity (SI) threshold and manually removed extraorbital fat. The average SI in the internal rectus muscle was 92; the standard deviation (SD) was 39. In orbital fat, the the average SI was 1306 and the SD was 54. Therefore the threshold value was [(92 + 39) + (1306 - 54)] / 2 = 691. Using FSE-IDEAL images we then produced 3D reconstruction images of the bilateral orbital fat (B) and measured the orbital fat volume on a workstation.

Fig. 3

Control Maximum-diameter slice in-phase image. The yellow horizontal line connects the bilateral frontal processes in the zygomatic bones. The distance from the top of the cornea to the horizontal line was measured to determine the degree of proptosis.

Fig. 4

Comparison of the quantitative data of 22 TAO patients and 22 healthy controls by Mann–Whitney U test. Box-and-whisker plots show water fraction of orbital fat (A), orbital fat volume (B) and proptosis (C). The plots show the minimum and maximum values (lower and upper horizontal lines, respectively), the median values (line in box) and the interquartile ranges (box) in each dataset. Significant differences are indicated with the P value. In TAO patients, the value of water fraction, fat volume and degree of proptosis were significantly higher than in the controls.

Fig. 5

Bland-Altman analysis confirming the interobserver variability or the water fraction (WF) (A), the volume of orbital fat (FV) (B) and the proptosis (C) on FSE-IDEAL images of the participants. R1 = reader1, R2 = reader2.