Orbital Fat Volumetry and Water Fraction Measurements Using T2-Weighted FSE-IDEAL Imaging in Patients with Thyroid-Associated Orbitopathy

Abstract

Background and purpose: The quantitative evaluation of orbital fat proliferation and edema and the assessment of extraocular muscles are useful for diagnosing and monitoring thyroid-associated orbitopathy. To evaluate therapy-induced quantitative changes in the orbital fat of patients with thyroid-associated orbitopathy, we performed volumetric and water fraction measurements by using T2-weighted FSE iterative decomposition of water and fat with echo asymmetry and least-squares estimation (FSE-IDEAL) imaging.

Materials and methods: Orbital FSE-IDEAL images of 30 volunteers were acquired twice within 1 week. Nine patients with thyroid-associated orbitopathy underwent FSE-IDEAL imaging before and after methylprednisolone pulse therapy, and the treatment results were assessed by using their pre- and post-methylprednisolone pulse therapy clinical activity scores. We performed volumetric and water fraction measurements of orbital fat by using FSE-IDEAL imaging and evaluated interscan differences in the volunteers. In patients with thyroid-associated orbitopathy, we compared pre- and posttherapy orbital fat measurements and assessed the correlation between the pretherapy values and clinical activity score improvement.

Results: The reproducibility of results obtained by the quantitative evaluation of orbital fat in volunteers was acceptable. After methylprednisolone pulse therapy, the water fraction in the orbital fat of patients with thyroid-associated orbitopathy was significantly decreased (P < .001). There was a significant positive correlation between the pretherapy water fraction and clinical activity score improvement (right, r = 0.82; left, r = 0.79) and a significant negative correlation between the pretherapy volume and clinical activity score improvement (bilateral, r = -0.84).

Conclusions: Volumetric and water fraction measurements of orbital fat by using FSE-IDEAL imaging are feasible and useful for monitoring the effects of therapy and for predicting the response of patients with thyroid-associated orbitopathy to methylprednisolone pulse therapy.

Fig 1.

Axial water (A) and fat (B) images of a healthy volunteer. The ROIs are encircled in yellow.

Fig 2.

Axial fat image (A) of a healthy volunteer. The selected orbital fat is light green. We separated fat tissue by using an adequate signal-intensity threshold and manually removed extraorbital fat. In this volunteer, the internal rectus muscle showed an average SI of 207 and an SD of 112, whereas the orbital fat showed an average SI of 1636 and an SD of 88. Therefore the threshold value was [(207 + 112) + (1636 − 88) / 2 = 933]. 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.

Bland-Altman analysis confirming the interscan reproducibility of the water fraction (A) and the volume of orbital fat (B) on FSE-IDEAL images of the volunteers.

Fig 4.

Comparison of the pre- and posttreatment water fraction (A) and the volume of orbital fat (B) in patients with TAO. Methylprednisolone pulse therapy significantly decreased the water fraction. The volume was unchanged.