Magnetic resonance imaging

Magnetic resonance imaging is derived from the signal emission when hydrogen nuclei realign to a very strong magnetic field after the cessation of an exciting radio frequency pulse, the interval being termed the "relaxation time". Various relaxation times may be assessed and images derived from the measured signals at these different relaxation times: Tl-weighted images tend to show anatomical detail of the orbit, whereas T2-weighted images - where the high signal of tissue oedema is readily evident -generally demonstrate pathological processes.

Orbital fat has a high signal on Tl-weighting, this often hindering the discernment of orbital pathology, but the contrast can be markedly improved by use of fat-suppression software programmes to manipulate the images. Gadolinium-DTPA provides an intravenous contrast, highlighting vascular lesions or tissues with leaking vessels (Figure 10.10) but with Tl-weighted images, renders pathology less discernable unless used in conjunction with fat-suppression.

MRI should not be used routinely for the investigation of orbital disease, but provides additional information to CT in certain circumstances. It is of particular value in determining the nature of optic nerve lesions in the region of the optic canal and chiasm; in demonstrating the position of the optic nerve

Figure 10.10 MRI of a patient with recent intraconal orbital haemorrhage: (a) T1- and (b) T2-weighted images, and (c) fat-suppressed T1-weighted image with Gadolinium-DTPA, showing normal uptake of contrast in the extraocular muscles on the unaffected right side. A fluid level may be seen within the lesion of the left orbit.

Figure 10.10 MRI of a patient with recent intraconal orbital haemorrhage: (a) T1- and (b) T2-weighted images, and (c) fat-suppressed T1-weighted image with Gadolinium-DTPA, showing normal uptake of contrast in the extraocular muscles on the unaffected right side. A fluid level may be seen within the lesion of the left orbit.

within large orbital tumours, where not shown on CT; in the imaging of radiolucent foreign bodies that are not ferro-magnetic. Although the presence of muscular oedema on STIR

images is suggestive of active inflammatory oedema in patients with thyroid orbitopathy, MRI used for this purpose is expensive and does not add usefully to a thorough clinical examination.

Although there are exceptions, most orbital tumours have a fairly low T1 signal, a medium-to-high T2 signal, and show variable Gadolinium-DTPA enhancement. Nonspecific orbital inflammation tends to have a medium T1 signal, with a relatively low signal on T2. Lesions containing melanin, or the breakdown products of blood, and those with lipid, fat or mucus will give high signal on T1-weighted images; examples include orbital haemorrhage, orbital melanomas, cholesterol granulomas, dermoid cysts and sinus mucocoeles.

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