Conclusions

The natural history of new imaging techniques usually starts with a first stage of amazement at the results of initial studies, with the new technique promising to answer a multitude of questions and clinicians clamoring for it. This period is commonly followed by the emergence of larger studies showing that the technique is not as universally applicable and effective as initially thought. Finally, balance is achieved when the new technique has become established and is known to be useful in a limited number of applications. The new techniques discussed in this chapter are probably somewhere in the first or second stage. Only future prospective studies of orbital tumors can show whether these techniques are really effective in managing orbital tumors and improving outcome, and whether they will be helpful in achieving "in vivo histopathology."

Newer, more exotic techniques offer great promise; however, there is not yet enough evidence from clinical trials to warrant their consideration for widespread use. Among the most promising new techniques is nanoparticle MR imaging.49 Monocrystalline iron oxide nanoparticles (MIONs) can be made to bind (conjugate) to specific peptides that bind, for example, to a specific receptor. These nanoparticles are capable of responding to a radiofrequency pulse in a magnetic field, so that they light up in MRI. MRI can thus specifically image the concentration of the receptor to which the MION-peptide conjugate binds. If a receptor is chosen that binds to specific tumor cells, tumor-specific imaging is possible, similar to antibody-specific staining in histopathology. Currently, the emphasis in nanoparticle imaging is on searching for useful peptides that also bind to the nanoparticle.50

It is important to realize that good results from tumor imaging in other tissues such as the brain may not necessarily apply to orbital tumor imaging, especially in different types of MR imaging, such as diffu sion imaging and MRS, because the orbit is surrounded by air-filled spaces, the sinuses. The air can lead to susceptibility artifacts. In addition, the scale of orbital pathology is usually much smaller than in the brain, partly because of the constricted space in the orbit; tumors often present earlier and at smaller size. Imaging techniques that can discriminate tumor tissue types in tumors 10 cm in diameter may not be able to do so in orbital tumors typically no larger than a few centimeters.

It is clear that a single technique for "in vivo his-topathology" is not yet feasible. Instead, a large number of different techniques are available, all of them suitable to a greater or lesser degree for specific tumor types in a specific context. Since the techniques are so specific, it is very important to ask the right question. This implies that history and clinical examination in addition to is-there-a-tumor imaging techniques will remain essential in the foreseeable future, since they help decide which imaging technique should be employed to further confirm, rule out, or stage a lesion.

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