Anatomia TC ed RM della regione orbitaria

Abstract

Pur non sottovalutando il ruolo degli US, la TC e la RM mostrano un ruolo di primo piano nella valutazione delle strutture ossee e del contenuto orbitario.

Although ophthalmoscopy, fluoro-angiography and ultrasound can usually identify most bulbar lesions, CT and MR play a major role in assessing the bony orbital structures and the post-bulbar soft tissues. These imaging techniques are especially important in investigating the eyeball under conditions “blind” to ultrasound not transparent to dioptric instruments (blood in the vitreous body, cataract, corneal opacity) or to define the site and extension and sometimes the nature (melanoma) of tumours.

Whereas both CT and MR are completely reliable techniques in the study of the orbit, overall CT is decidedly superior to MR in the study of bony structures and MR is the method of choice in assessing the endo-orbital contents. This is because MR offers better spatial resolution and contrast with multiplanar views (allowing study on three planes without requiring the patient to assume uncomfortable positions for coronal CT scanning), greater sensitivity to tissue changes and hence diseases, the lack of signal from bony tissue (displaying the content of small structures like the orbital apex, nasolacrimal fossa and optic canal which are difficult to assess by CT due to artefacts) and lastly, the absence of ionizing radiation in examining children. These advantages are flanked by the use of the fast-imaging technique and surface coils (phased-array) which have overcome the limitations originally linked with MR due to the prolonged examination times and images degraded by artefacts. Nowadays, the only obstacle to routine use of MR is the shortage of MR systems in Italy.

For both MR and CT scanning, technological developments have led to significant advances with the advent of the spiral technique which reduces the artefacts of eyeball movements because anatomical volumes can be investigated rapidly, improving patient compliance (in acquiring coronal scans) and enhancing the quality of two and three-dimensional reconstructions and allowing optimum use of contrast medium as the structure can be scanned during the maximum increase phase with a reduction in the total amount of contrast administered. Although the spiral technique is seldom used to examine the orbit in adults given the small volume to investigate, it is particular indicated in uncooperative patients (children or trauma patients).