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Myasthenia gravis MRI
Introduction
Overview
Overview
Magnetic resonance imaging (MRI) is a problem-solving modality for the thymus in myasthenia gravis (MG), reserved for equivocal cross-sectional findings rather than routine screening.[1][2] Chest computed tomography (CT) is first-line, but chemical-shift and diffusion-weighted MRI are more accurate than CT for distinguishing thymoma from thymic lymphoid hyperplasia (TLH) and normal thymus, and avoid ionizing radiation and iodinated contrast.[3]
MRI
MRI
Indications and Clinical Role
MRI is not usually used for initial thymus assessment; CT is obtained first, and MRI is reserved for equivocal cases—chiefly a focal CT lesion that could be a small thymoma versus focal TLH, or when radiation/iodinated contrast should be avoided (young patients, pregnancy, contrast allergy).[1][4] For thymoma detection, CT and MRI perform similarly (sensitivity ~90%, specificity ~95%); MRI’s advantage is lesion characterization.[1][4] In addition, MRI confirms cystic lesions and defines local invasion, preventing unnecessary thymectomy or biopsy.[2]
Quantitative MRI Techniques
- Chemical-Shift MRI: Chemical-shift MRI is significantly more accurate than CT for distinguishing thymoma from nonthymomatous thymus.[3] The signal intensity index (SII) is the single most accurate parameter (opposed-phase signal drop from microscopic fat in hyperplastic/normal thymus; cutoff ~6–8%, AUROC up to 1.000, approaching 100% sensitivity and specificity in surgically confirmed MG cohorts), outperforming CT radiodensity (20 HU cutoff, AUROC 0.904) and qualitative CT reading (MRI accuracy 96.4% vs CT 86.7%).[3][5]
- Diffusion-Weighted MRI (DWI): Diffusion-weighted apparent diffusion coefficient (ADC) adds an independent discriminator (TLH/normal thymus shows higher ADC than thymoma; cutoff ~1.6 × 10⁻³ mm²/s).[5][6] Furthermore, lower ADC values predict higher-risk World Health Organization (WHO) histological subtypes and worse disease-free survival.[7][6]
On routine imaging, hyperplasia is identified in only ~18% of cases and cannot be reliably distinguished from a normal gland; at specialized centers MRI detects hyperplasia better than CT (sensitivity 68% vs 14%) but performance remains limited.[8][9]
Areas of Uncertainty
High-accuracy quantitative thresholds derive largely from single-center prospective cohorts awaiting multicenter validation, and ADC values vary with b-value protocol, limiting cross-site generalizability. The chemical-shift ratio (CSR) overlaps in early adulthood because the fat-poor young thymus suppresses incompletely; SII is preferred and age must be factored into interpretation.[10] No dedicated MG society guideline mandates MRI—its role is defined by radiology cohort studies and the ACR Appropriateness Criteria.[2][8]
References
References
- ↑ 1.0 1.1 1.2 Punga AR, Maddison P, Heckmann JM, Guptill JT, Evoli A (2022). “Epidemiology, diagnostics, and biomarkers of autoimmune neuromuscular junction disorders”. Lancet Neurol. 21 (2): 176–188. doi:10.1016/S1474-4422(21)00297-0. PMID 34958509 Check
|pmid=value (help). - ↑ 2.0 2.1 2.2 Expert Panel on Thoracic I, Ackman JB, Chung JH, et al. (2021). “ACR Appropriateness Criteria® Imaging of Mediastinal Masses”. J Am Coll Radiol. 18 (5S): S37–S51. doi:10.1016/j.jacr.2021.01.007. PMID 33858683 Check
|pmid=value (help). Vancouver style error: initials (help) - ↑ 3.0 3.1 3.2 Priola AM, Priola SM, Gned D, Giraudo MT, Fornari A, Veltri A (2016). “Comparison of CT and chemical-shift MRI for differentiating thymoma from non-thymomatous conditions in myasthenia gravis”. Clin Radiol. 71 (3): e157–e169. doi:10.1016/j.crad.2015.12.008. PMID 26774127.
- ↑ 4.0 4.1 Priola AM, Priola SM (2014). “Imaging of thymus in myasthenia gravis: from thymic hyperplasia to thymic tumor”. Clin Radiol. 69 (5): e230–e245. doi:10.1016/j.crad.2014.01.005. PMID 24581970.
- ↑ 5.0 5.1 Priola AM, Priola SM, Giraudo MT, et al. (2015). “Chemical-shift and diffusion-weighted magnetic resonance imaging of thymus in myasthenia gravis: usefulness of quantitative assessment”. Invest Radiol. 50 (4): 228–238. doi:10.1097/RLI.0000000000000119. PMID 25478741.
- ↑ 6.0 6.1 Priola AM, Priola SM, Giraudo MT, et al. (2016). “Diffusion-weighted magnetic resonance imaging of thymoma: ability of the apparent diffusion coefficient in predicting the World Health Organization (WHO) classification and the Masaoka-Koga staging system and its prognostic significance on disease-free survival”. Eur Radiol. 26 (7): 2126–2138. doi:10.1007/s00330-015-4031-6. PMID 26427698.
- ↑ Abdel Razek AA, Khairy M, Nada N (2014). “Diffusion-weighted MR imaging in thymic epithelial tumors: correlation with World Health Organization classification and clinical staging”. Radiology. 273 (1): 268–275. doi:10.1148/radiol.14131643. PMID 24877982.
- ↑ 8.0 8.1 Klimiec E, Quirke M, Leite MI, Hilton-Jones D (2018). “Thymus imaging in myasthenia gravis: The relevance in clinical practice”. Muscle Nerve. 58 (4): 707–713. doi:10.1002/mus.26075. PMID 29890786.
- ↑ Luo H, Xie S, Ma C, et al. (2018). “Correlation between thymus radiology and myasthenia gravis in clinical practice”. Front Neurol. 9: 1173. doi:10.3389/fneur.2018.01173. PMID 30697185.
- ↑ Priola AM, Priola SM, Ciccone G, et al. (2015). “Differentiation of rebound and lymphoid thymic hyperplasia from anterior mediastinal tumors with dual-echo chemical-shift MR imaging in adulthood: reliability of the chemical-shift ratio and signal intensity index”. Radiology. 274 (1): 238–249. doi:10.1148/radiol.14132665. PMID 25105246.
Editor-In-Chief:
C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Keanu Ngo[2]