The orbit

Modalities for examination

Radiological diagnosis of orbital lesions has changed and become more accurate since the advent of CT and MRI. Instead of demonstrating indirect changes by examination of the skeleton and orbital phlebography, it has become possible with the new modalities to demonstrate, de fine the extent of and characterize the lesion. A major reason for this is the large difference in attenuation (about l00 HU) and signal between the orbital fat and for example the optic nerve. CT and MRI are highly reliable in the demonstration of expanding lesions within the orbit. The ability to differentiate between different types of lesions is, however, relatively difficult because pathology is so variable in this area. CT remains the method of choice and is particularly preferred for the demonstration of calcification, bony detail and foreign bodies. Potential advantages of MRI as opposed to CT are, apart from the avoidance of the use of ionizing radiation, that MRI is multiplanar and that it is capable of analysing the apex region, especially if there is access to fat-suppressing sequences in combination with contrast injection. The inner structure of certain lesions, for example haemangiomas, is demonstrated much more satisfactorily with MRI. Intraocular melanomas have been shown to have characteristic signal qualities.

Depending on the choice of technique with CT, the dose to the lens varies enormously (between 50 and 150 mGy). Because of the naturally good contrast conditions in the orbit, contrast medium injection does not generally need to be used to demonstrate a lesion. The value of contrast enhancement is limited in the differentiation of lesions. However, examination with contrast should always be performed when there is a suspicion of a lesion in the region of the optic chiasm or when invasion of the skull cavity is suspected. Other indications for the use of contrast medium are suspicion of a varix or arterio-venous malformation (AVM) and to differentiate between optic glioma and meningioma in the optic sheath.

In the diagnosis of tumours, plain X-ray examination has be come obsolete. Angiography is seldom indicated but if it is performed for evaluation of vascularity it should be performed with injection of contrast medium into both the internal and external carotid artery. A relative indication for orbital phlebography is orbital varices.

Anatomy

The wall of the orbit is composed of seven different bones. The superior orbital fissure is situated between ala magna and ala parva and through this passes the superior ophthalmic vein in addition to the third, the fourth, the first division of the fifth and the sixth cranial nerves. Through the inferial orbital fissure, between the maxilla and ala magna, passes the inferior ophthalmic vein. Through the optic nerve canal passes the nerve, which is surrounded by a thin fluid-filled cavity, and the ophthalmic artery. The four straight eye muscles form a muscle cone with the bulb of the eye as the base and the apex region as the tip. In addition, there are two oblique eye muscles as well as musculus levator palpebrae. The

lacrimal gland is situated in the upper antero-lateral part of the orbit. Some important anatomical structures are shown in Fig. 5.

Kjell Bergstrom and Giuseppe Scotti