Imaging Modalities
MRI
MRI is superior in characterizing soft tissue lesions due to its high soft tissue resolution. It can help characterize the soft tissue lesion and assess lesion extent including regional spatial invasion and intracranial extension via the skull base foramina and fissures. This includes evaluation of invasion into a parameningeal location (nasopharynx, nasal cavity, parapharyngeal space, paranasal sinuses, infratemporal fossa, pterygopalatine fossa, tympanic and mastoid temporal bone) 10. MRI is used for initial evaluation, staging, follow-up and treatment response evaluation of orbital tumors using a similar protocol as already described for ocular and optic nerve sheath lesions. DWI sequence is routinely included in MRI of the orbits because it can address potential histoarchitectural differences between various tumors and is therefore useful in lesion characterization alongside the use of gadolinium-based contrast. DWI may help differentiate benign from malignant tumors. For example, lymphoma has been shown to have low ADC values compared to benign tumors. This may help differentiate lymphoma from atypical lymphocytic infiltration or other inflammatory processes. Similarly, leukemia, RMS and LCH will typically demonstrate lower ADC values compared to benign tumors11. However, to date, no definite single ADC value threshold has been proposed as a cut-off value 12. Single-shot echo-planar imaging DWI (EPI-DWI) is a commonly used technique. The newer multishot with readout-segmented echo-planar DWI and non-echo planar DWI may improve imaging quality, reduce distortion and susceptibility artifact from air-bone interfaces13. Furthermore, time resolved MR angiography (MRA) in which dynamic multiphase vascular imaging occurs during contrast injection is an optional but sometimes helpful technique in delineating vascular from nonvascular lesions and furthermore arterial from venous fed lesions.