A large number of new imaging techniques have developed since the advent of CT, MRI, and ultra-sonography. Some developments are essentially improvements of established techniques, such as spiral CT, which offers faster acquisition, lower exposure to ionizing radiation, and higher resolution than conventional CT. Especially in the MRI field, there has been a constant search for better and faster imaging algorithms and stronger magnetic fields to decrease noise and increase resolution. MRI magnetic field strengths, measured in teslas, have improved from 0.3 T 20 years ago to 1.5 T as the current standard for clinical imaging. These developments will gradually improve the quality of orbital imaging.
However, other new orbital imaging techniques represent real innovations that may become important in orbital tumor diagnosis and management. Although in vivo histopathology imaging of orbital tumors is not yet feasible, what can be imaged is the change in function of the orbital structures that are the result of the presence and growth of a tumor. Functional imaging, defined as that range of imaging techniques in which the aim is to extract quantitative information about the physiological function of tissues, is different from the CT, ultrasonography, and (current) MRI techniques, which all show the spatial relationships of tissues to each other. The functional changes can be either macroscopic or microscopic and are specific to a particular type of tumor. In malignant tissue, exam ples of microscopic functional changes are an increased ratio of cells to extracellular matrix, increased ratio of apoptotic to normal cells, increased concentration of the amino acid choline, oxygen perfusion, and increased capillary flow. Changes in all these variables have been imaged successfully in patients, and some of them in the orbit. Examples of macroscopic functional changes in the orbit are changes in tissue motion, or increased attachments to other tissues caused by scarring or invasive growth, which have also been imaged successfully in the orbit.
Some of the "new" techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT), have been around for decades but are new to orbital imaging and thus deserve mention.
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