The feasibility of quantitative MRI of extra‐ocular muscles in myasthenia gravis and Graves' orbitopathy

Abstract

Although quantitative MRI can be instrumental in the diagnosis and assessment of disease progression in orbital diseases involving the extra‐ocular muscles (EOM), acquisition can be challenging as EOM are small and prone to eye‐motion artefacts. We explored the feasibility of assessing fat fractions (FF), muscle volumes and water T2 (T2 water ) of EOM in healthy controls (HC), myasthenia gravis (MG) and Graves' orbitopathy (GO) patients. FF, EOM volumes and T2 water values were determined in 12 HC (aged 22‐65 years), 11 MG (aged 28‐71 years) and six GO (aged 28‐64 years) patients at 7 T using Dixon and multi‐echo spin‐echo sequences. The EOM were semi‐automatically 3D‐segmented by two independent observers. MANOVA and t‐tests were used to assess differences in FF, T2 water and volume of EOM between groups ( P < .05). Bland–Altman limits of agreement (LoA) were used to assess the reproducibility of segmentations and Dixon scans. The scans were well tolerated by all subjects. The bias in FF between the repeated Dixon scans was −0.7% (LoA: ±2.1%) for the different observers; the bias in FF was −0.3% (LoA: ±2.8%) and 0.03 cm 3 (LoA: ± 0.36 cm 3 ) for volume. Mean FF of EOM in MG (14.1% ± 1.6%) was higher than in HC (10.4% ± 2.5%). Mean muscle volume was higher in both GO (1.2 ± 0.4 cm 3 ) and MG (0.8 ± 0.2 cm 3 ) compared with HC (0.6 ± 0.2 cm 3 ). The average T2 water for all EOM was 24.6 ± 4.0 ms for HC, 24.0 ± 4.7 ms for MG patients and 27.4 ± 4.2 ms for the GO patient. Quantitative MRI at 7 T is feasible for measuring FF and muscle volumes of EOM in HC, MG and GO patients. The measured T2 water was on average comparable with skeletal muscle, although with higher variation between subjects. The increased FF in the EOM in MG patients suggests that EOM involvement in MG is accompanied by fat replacement. The unexpected EOM volume increase in MG may provide novel insights into underlying pathophysiological processes.

Evaluation of lateral pterygoid muscle in patients with temporomandibular joint anterior disk displacement using T1-weighted Dixon sequence: a retrospective study

“…Studies have found that the fat fraction of skeletal muscle (thigh or calf) in healthy people was ranged from 2 to 5% [ 26 , 27 ], and our fat fraction value of NP group just belonged to the range that described above. Recently, keene et al [ 17 ] reported on the temporal muscle, which is also a masticatory muscle, with a fat fraction of 9.3%, slightly higher than the fat fraction of LPM (ranged from 3.65% to 4.63%) we observed in this study. The closer to the orbital fat, the more noise bias is likely to occur in the imaging of nearby muscles, and resulting in an overestimation of fat fraction.…”

“…The head has rich tissue-air interfaces, which would cause magnetic field inhomogeneity and thus affecting the imaging quality in conventional MRI [ 15 ]. Owing to its short acquisition time and low sensitivity to susceptibility, Dixon-based method is very suitable for head muscle imaging and has been applied preliminary in recent years [ 15 – 17 ]. Latest literature showed that Dixon-based sequence presented better image quality for extraocular muscles [ 15 ].…”

“…Latest literature showed that Dixon-based sequence presented better image quality for extraocular muscles [ 15 ]. Moreover, it was also found the fat fraction of extraocular muscles is higher even in early stage orbitopathy patients than that in healthy people by using Dixon technique [ 17 ]. Since there are abundant tissue-air interfaces around LPM as well as the extraocular muscles, these studies hinted that the Dixon-based technique also has the potential to be used in the assessment of LPM.…”

Radiological Characteristics of Extraocular Muscles in Myasthenia Gravis Patients with Ocular Manifestations: A Case–Control Study

“…The findings indicate fat replacement in the EOMs in the early phase of the disease. 6 Moreover, muscle atrophy also extends beyond the EOMs. There was MRI evidence of early muscle atrophy in anti-AChR-Ab MG in the posterior cervical, paraspinal, sternocleidomastoid, and masseter muscles.…”

Expanding diagnostic tools for dysthyroid optic neuropathy: how quantitative MRI can be used to visualize and measure orbital inflammation

“…Another promising approach is to perform a quantitative analysis of the MRI. It provides quantitative data, such as fat fractions, muscle volumes, and water T2 value for assessing extraocular muscle inflammation, or diameter and crosssectional area of the optic nerve and its sheath, water fraction of the optic nerve, and volume of the fluid in the optic nerve sheath for assessing optic neuropathy [10]. All these quantitative data might be used as biomarkers to diagnose and follow up extraocular muscle inflammation or optic neuropathy in TED.…”