Synopsis

Summary: Exophthalmos caused by muscle thickening and fat increasing in orbit is the typical symptom in hyperthyroidism, which impairs multiple systems as the auto-immune disease. From this study, we found the increase of percentage of fat and T2* value using the fat quantification analysis package, which indicate the accumulation and differentiation of fat-oriented cells, inflammation cells infiltration and collagen mucous change. Therefore, the characteristics of fat change in orbit can be evaluated by the transverse six-echo proton density fat fraction (PDFF) sequence (mDIXON-quant) technique in early-stage hyperthyroidism without significant manifestation of eyes.

Characteristics of fat in orbits of patients with early-stage hyperthyroidism

Introduction: Exophthalmos caused by muscle thickening and fat increasing in orbit is the typical symptom in hyperthyroidism, which impairs multiple systems as the auto-immune disease. Although there were lots of literature studied the morphology of orbit in hyperthyroidism by magnetic resonance imaging (MRI), research on quantification of fat in early-stage hyperthyroidism without significant exophthalmos is rare.

Purpose: To explore the characteristics of fat in orbits of early-phase hyperthyroidism patients without obvious abnormality in eyes.

Methods: 15 patients (30 eyes) diagnosed as early-phase hyperthyroidism without significant exophthalmos were recruited and 15 normal subjects (30 eyes) were involved as the control. All subjects were scanned in a whole body 3T (Ingenia, Philips Healthcare, Best, the Netherlands) MR with a 16ch ds head coil. A transverse six-echo proton density fat fraction (PDFF) sequence (mDIXON-quant) was employed with following parameters: TE/TR = shortest/9 ms, FA = 3.0°, matrix = 228 * 278, slice thickness = 3mm, slices = 20, NSA = 2. the degree of exophthalmos (Figure 1)¹, fat thickness was measured by calculating number of slices with fat. Fat fraction and T₂* values of fat were measured by drawing ROIs on the maximum slice with fat .The t test was used to compare the differences in contiguous data, and χ² test for the categorical data.

Results: 1.There was no significant differences in age between study and control groups (38.27 ± 9.96 vs. 37.80 ± 10.21 years, t = 0.127, P = 0.900 > 0.05), so was the sex (male 8 vs. 7 and female 7 vs. 8, χ2 = 0.133，P = 0.715 > 0.05). 2.The quantification of exophthalmos in study and control group were 16.10 ± 1.53 mm, 15.28 ± 1.99 mm, respectively. There was no significant difference (t = 1.746, p = 0.086﹥0.05).3. The thickness of fat in orbit in study and control group were 4.29 ± 0.54 mm, 4.06 ± 0.54 mm, respectively. There was no significant difference (t = 1.681, p = 0.099﹥0.05).4. The percentage of fat in orbit in study and control group were 87.96 ± 4.14 %, 83.75 ± 5.58 %, respectively. There was significant difference (t = 3.23, p = 0.002 < 0.05). The T₂* value in study and control group were 28.31 ± 23.03 ms, 17.77 ± 10.11 ms, respectively. There was significant difference (t = 2.159, p = 0.035﹤0.05). (Table 1)(Figure 2. Figure 3)

Discussion: Fat in orbit increased in the early-stage of hyperthyroidism without remarkable manifestation, which signify the accumulation and differentiation of fat-oriented cells. Meanwhile, inflammation cells infiltration and collagen mucous change may cause the rise of T₂* value.²

Conclusion: Orbits with early-stage hyperthyroidism have increased fat fraction and T2* value. The PDFF technique can be employed to diagnose the early-stage hyperthyroidism.

Acknowledgements

References

1.Comerci M, Elefante A, Strianese D, et al. Semiautomatic regional segmentation to measure orbital fat volumes in thyroid-associated ophthalmopathy. A validation study. Neuroradiol J. 2013; 26(4):373-379.

2.Garrity J, Bahn R. Pathogenesis of graves ophthalmopathy: implications for prediction, prevention, and treatment. Am J Ophthalmol. 2006; 142(1):147-153.