Orbital Imaging and Radiography
Nothing in the field of orbital diagnostics has so increased in importance during the editions of this text as the refinements in orbital imaging. The use of the computer and its assistance in processing the orbital scan, by whatever method, have played a chief role in the study of orbital bones and soft tissue contents. The computer is responsible for the evolution of imaging modalities such as computed tomography (CT) scanning, magnetic resonance imaging (MRI), radionuclide imaging, reconstructive tomography, angiography, and MRI-guided fine-needle aspiration FNA biopsy. These modalities, including ultrasonography, and their application to the diagnosis of orbital tumors are discussed in this chapter. Pertinent references to the subjects are cited from 1988 to the present, updating the references cited in the previous edition. More specific application of these modalities to the diagnosis of the individual tumors and tumor groups is noted in Section II.
Computed Tomography
Rubin and Remulla (1997) found in three patients with orbital venous anomalies resulting in retrobulbar hemorrhage that the pathologic vessel filled with blood can be imaged during a single breath hold using a scanning time of 24 seconds in conjunction with the Valsalva maneuver.
We have previously noted (in Chapter 2 of the third edition) a fascination for ophthalmologists to report calcification in an extraocular orbital tumor. Numerous single case reports were published soon after CT scan became the standard modality for the imaging of orbital masses.
Initially, each author thought that he or she was the first to report an important advance in CT scan observations. By the time the third edition was published in 1994, 19 individual tumors or tumor families with calcification were listed. Subsequently, the numerical list has expanded.
Initially, each author thought that he or she was the first to report an important advance in CT scan observations. By the time the third edition was published in 1994, 19 individual tumors or tumor families with calcification were listed. Subsequently, the numerical list has expanded.
Froula et al. (1993) also surveyed 171 CT scans that fulfilled their study criteria. Thirty-seven (22%) scans showed calcific densities. Calcium deposition was intraocular in 20 cases and extraocular in 17. Among the extraocular tumors, calcium deposition was noted in the family of vascular malformations and neoplasms, benign lesions, and malignant neoplasms. In view of the profusion of human disorders associated with calcium deposition in the orbital space, calcium can no longer be specific for a probable diagnosis, except for the large, smooth-surfaced, round or elliptical phlebolith that is pathognomic of a vascular tumor. Some malignant neoplasms, regardless of their orbital location, may show a small, poorly defined clump of calcium. Scrutinize the scan carefully to detect calcium adjacent to or within a cystic-like space void of contrast enhancement to identify the area of necrosis.
Magnetic Resonance Imaging
When this section was written for the 1994 edition of this text, we had viewed MRI as being roughly comparable in its state of development to the stage of refinement reached by CT scan 10 years previously. We posited that in time we would know whether MRI would reach the level of importance in orbital diagnosis that CT scan had occupied.
Initially, many publications described ongoing advances in technology that refined the MRI display (Atlas et al., 1988; Roden et al., 1988; Runge, 1990; Barakos et al., 1991; Higgins et al., 1992; Breslau et al., 1995; Herrick et al., 1997; Sutton, 1998; Jackson et al., 1999; Stark and Bradley, 1999). The opinions and recommendations of these authors, roughly in chronology of publication, were as follows:
More information is obtained with standard spin-echo sequences than with short-time inversion recovery sequences.
Advances have made the technology more cost-effective.
Gadolinium–pentetic (diethylenetriaminepentaacetic) acid (Gd-DTPA) has had a major impact on the use of MRI.
The high signal intensity of fat on T1-weighted MRI has limited the utility of gadolinium pentetate dimeglumine in the imaging of extracranial head and neck areas.
Fat-suppression techniques in combination with gadolinium enhancement should replace postcontrast T1-weighted spin-echo imaging.
The flexibility of MRI is derived from the signal’s dependence on not one but at least seven physical characteristics.
Imaging of the orbit and orbital apex is clearly superior with dual-phased array coil, resulting in a higher resolution than can be obtained with a head probe.
Tissue interface artifacts can be minimized with the use of spin-echo rather than gradient-echo sequences.
The use of fat suppression combined with postcontrast sequences has markedly improved visualization of orbital masses and lesions of the optic nerves.
Small, unilateral or bilateral surface coils have proved critical for the success of MRI in the orbit.
As physicians became aware of these refinements, a series of articles appeared in the literature citing the merits of MRI in a clinical setting. However, there are only a few publications pertaining to the orbit and visual pathways (Dorfman and Spickler, 1990; Bond et al., 1992; Bilaniuk and Rapoport, 1994; Davis and Newman, 1996). Their combined conclusions are as follows:
MRI provides more information about orbital apex lesions compared to CT scan.
MRI is the modality of choice for optic nerve glioma, optic neuritis, optic nerve sheath meningioma, perioptic hemangiomas, and sarcoidosis.
Orbital pseudotumor can be differentiated from metastatic disease and reactive lymphoid hyperplasia on the basis of differences in signal intensity.
The signal void of MRI is useful in visualizing the septal lobulations and patency of the vascular supply to vascular malformations.
Thrombosis in a vascular malformation can be identified without the use of intravenous contrast.
In general, the publications comparing and contrasting MRI and CT scanning were laudatory of the former. One article suggested that refinements in MRI make it the procedure of choice for the initial screening of patients with orbital tumors, thereby eliminating the cost of CT scan unless it is needed for a suspected tumor involving the bone. According to Stark and Bradley (1999), the endorsement of MRI of the orbit must be qualified because CT scan shows excellent contrast between retrobulbar fat and disease and MRI is sensitive to globe and eyelid motion.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
