Best Practice & Research Clinical Endocrinology & Metabolism
3Neuroimaging of Graves’ orbitopathy
Section snippets
Technique
The basic principles of MR imaging (MRI), an imaging method without ionizing radiation, are presented in a simplified manner. This method generates cross-sectional images of the interior of the body using the physical phenomena of nuclear magnetic resonance. Especially atomic nuclei with an odd number of protons (and/or neutrons) have a magnetic dipole moment (spin). Hydrogen (1H) as the most abundant nuclei in human tissue is used for medical MRI. In the presence of a magnetic field, the
Technique
Computed tomography (CT) was the first modern imaging technique, which was able to distinguish different soft tissues by measuring their different densities. CT is still routinely used in imaging of orbital and visual pathway pathology. The distinctly different X-ray absorption of bone, fat, muscles, vitreous body and lens represents an excellent natural intrinsic contrast of the orbital tissue, resulting in a high anatomic accuracy. In concert with MRI it remains the method of choice for
Orbital ultrasound and octreotide scanning
Low costs, short time of investigation and lack of radiation characterize orbital ultrasound.18, 19 Nevertheless, neither a clear differentiation regarding disease activity is possible, nor is an evaluation of orbital tissue precise enough. Ultrasound investigation of the orbit is not as effective as CT in delineating the relationship of orbital pathology to contiguous structures, nor is it reliable in imaging lesions of the posterior orbit, nor those involving the bone walls. The accuracy of
Neuroradiological findings in GO
GO is the most frequent cause of uni- or bilateral proptosis in adults.28 Orbital tissue edema, enlargement of extra-ocular muscles and increase of retro bulbar fat volume are usually noted.7, 29 This might result in extreme proptosis and stretching of the optic nerves (Fig. 2). In chronic stages, collagen deposition in the affected muscles results in fibrosis and may be associated with fat deposition. Indication for imaging has changed from primary diagnosis to control examination in the
Differential diagnosis
In most cases, diagnosis of GO is obvious: lid retraction combined with uni- or bilateral proptosis. When the diagnosis is uncertain in atypical cases, orbital imaging is recommended.5, 28 Orbital myositis, myasthenia gravis, carotid-cavernous fistula, orbital tumors and progressive external ophthalmoplegia should be ruled out. The important differential diagnosis is myositis (Fig. 8, Fig. 9), especially in rare cases of unilateral involvement of GO. This local representation of an idiopathic
Summary
Orbital CT and MRI are commonly used as imaging techniques to demonstrate pathological changes in ocular adnexa of patients with GO. Short investigation time, precise imaging of the orbital apex and moderate costs are the advantages of CT. This method delivers a significant radiation dose to the lens, which if repeated constitutes a risk for cataract development. For this reason, MRI is preferable, particularly if repeat scans are required to assess response to treatment. Precise tissue
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