MRI for the Evaluation of Inflammatory Myopathy

By Michael Rubin, MD

Professor of Clinical Neurology, Weill Cornell Medical College

SYNOPSIS: Magnetic resonance imaging (MRI) of large muscles is an important adjunctive diagnostic test for inflammatory myopathies, in conjunction with serum muscle enzymes and electrophysiology. MRI also can identify the highest-yielding section of muscle to target a muscle biopsy.

SOURCE: Marth AA, Hosse C, Yamamura J, et al. The value of non-enhanced MRI in the evaluation of patients with suspected idiopathic inflammatory myopathy. Muscle Nerve 2024;69:334-339.

Laboratory diagnosis of idiopathic inflammatory myopathy (IIM) proceeds along several avenues. Elevated muscle enzymes will be found in most patients with IIM, and, within the IIM spectrum, myositis-specific autoantibodies are associated with particular clinical syndromes. Myopathic changes on electromyography (EMG) will help distinguish myopathic from neuropathic causes of weakness, and magnetic resonance imaging (MRI) can demonstrate areas of muscle inflammation over large areas of muscle, avoiding problems of sampling error associated with muscle biopsy. Is a contrast-enhanced MRI necessary in the evaluation of possible IIM or is a non-enhanced MRI equally efficacious?

In this retrospective study, performed at the Departments of Radiology, Charité-Universitätsmedizin Berlin, and Universitätsklinikum Eppendorf, Hamburg, Germany, records of all patients who underwent MRI examination of the upper thigh for suspected IIM between 2008 and 2020 were collected. Those who did not undergo muscle biopsy or had an ambiguous biopsy or incomplete examinations were subsequently excluded.

All biopsies were from the upper thigh, with MRI imaging studies guiding which muscle to biopsy. MRI scans included a T1-weighted sequence in axial orientation, a fat-suppressed (fluid-sensitive) turbo inversion recovery magnitude (TIRM) sequence in axial orientation, and a fat-suppressed, contrast-enhanced, T1-weighted sequence in axial orientation. Three independent, blinded radiologists evaluated the MRI scans, in random order, for evidence of IIM based on imaging features, including muscle or fascial edema in fluid-sensitive images, and fatty muscle infiltration in T1-weighted images. Criteria used for IIM diagnosis were those established by the 2017 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR). Based on these criteria, a diagnosis of definite, probable, or possible IIM was established. Statistical analysis comprised Levene’s test, the Kolmogorov-Smirnov test, and Chi-square testing, with a P value < 0.05 considered statistically significant.

Among 80 patients included in the study, 54 (67.5%) satisfied ACR/EULAR criteria for IIM. The remaining 24 patients were diagnosed as having muscular dystrophy, metabolic myopathy, or endocrine myopathy. MRI images were acquired at 1.5 T in 33 patients and at 3 T in 47 patients, with gadoteric acid used in 48 (60%) patients and gadobutrol in 32 (40%) patients. No statistical difference in MRI field strength was present between those who did or did not receive contrast.

Sensitivity and specificity for MRI detection of IIM was 87.1% and 83.3%, respectively, in the non-enhanced MRI group, and 87% and 63%, respectively, in the contrast-enhanced MRI group. Overall, sensitivity and specificity for IIM diagnosis were not significantly different between the two groups. Non-enhanced MRI scans have an acceptable diagnostic accuracy in IIM.

COMMENTARY

Widely used in diagnosing, staging, and monitoring response in cancer care, 18F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) is a scanning method capable of localizing increased FDG to specific normal or abnormal anatomic locations. PET/CT might be able to evaluate muscle involvement in IIM and correlate with myositis-specific antibodies and/or myositis-associated antibodies.

Among 34 IIM patients whose PET/CT results were available, sensitivity and specificity of a positive FDG muscle uptake, compared to liver and mediastinum uptakes, were 37.1% and 100%, and 65.7% and 92.9%, respectively. Using multivariate analysis, higher baseline C-reactive protein and lactate dehydrogenase levels were associated with muscle PET/CT positivity. Median muscle FDG uptake with PET/CT was higher in IIM patients compared to non-IIM patients and may be used for the evaluation of extent and activity in patients with IIM.¹

REFERENCE

1. Bektaş M, Göknur Işik E, Oğuz E, et al. Utility of positron emission tomography as a new tool for muscle involvement in patients with idiopathic inflammatory myositis: A controlled study. Clin Exp Rheumatol 2024;42:358-366.