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.