Quantitative MRI of extra-ocular muscles in the clinical evaluation of systemic diseases

Abstract

Purpose : Most ophthalmic techniques cannot image extra-ocular tissues. MRI could be extremely useful for this purpose. In Graves orbitopathy, for example, the muscles and orbital fat are inflamed, while in myasthenia gravis, the patients suffer from dysfunction of these muscles. Quantitative MRI data could facilitate more targeted treatment selection and a direct method to determine treatment response, which varies greatly between patients.
In this study we develop and evaluate the clinical value of the DIXON-technique, which enables a reliable determination of the water/fat ratio.

Methods : All scans were performed on a 7Tesla MRI, using a cued-blinking paradigm to minimize eye-motion. The DIXON was acquired with a 0.7x0.7x1.0mm³ resolution, TR/TE/FA:2.4ms/10ms/3.
5 healthy subjects (24-49 years, 2M, 3F), 1 myasthenia (68 years, F) and 1 Graves orbitopathy (32 years, M) patient were scanned. The muscles, optic nerve and orbital fat were manually segmented on the first coronal slice posterior to the globe. The mean water and fat signal intensity and surface area were determined for all ROIs.

Results : The scans proved to be robust against eye-motion for all participants, Fig 1. The cross-sectional area of the different muscles was similar for all participants, Fig 2D. The cross-sectional area of the orbital fat was, however, almost twice as high for both patients. The muscular fat fraction of the patients (26%, 23%) was higher than the healthy subjects (11%), Fig 2H.
Although only a small number of patients have been included so far, the DIXON data already are highly promising in terms of clinical relevance. The high fat content of the ocular muscles of the myasthenia patient was unexpected and warrants further study. The high water-content of the orbital fat in the Graves patient quantifies the presence of inflammation. These pilot data were based on the evaluation of one standard slice, but volumetric MRI-data enables an evaluation of complete muscles, Fig 1JKL.

Conclusions : Quantitative MRI can provide key clinical information on the status of extra-ocular muscles and orbital fat for patients with systemic diseases, which are not available with the conventional techniques.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

(A-I) 3D DIXON MRI quantifies water and fat content.
(JKL) Pilot 3D segmentation.

 

(A-D)Crosssectional areas of the muscles. Values indicate the healthy subject average.
(ED)Water/fat ratio maps show increased fat levels in the patients' muscles.