MRI
Besides dedicated MRI of the orbits, MRI of the entire brain should be
performed concomitantly to assess for intracranial invasion and
metastatic disease during initial diagnosis and follow up. Attention to
details in the suprasellar and pineal regions is mandatory to assess for
“trilateral” or “quadrilateral” retinoblastoma, respectively. For
patients with extraocular retinoblastoma, imaging will also consist of a
bone scan to assess for systemic metastatic disease, in addition to
lumbar puncture and bone marrow aspiration. Routine MRI sequences of the
orbits ought to be performed with dedicated thin-section, high
resolution images of the orbits (< 3 mm slice thickness)
using an optimal field of view (FOV). This includes at minimum: axial
T1WI, axial and coronal fat suppressed (FS) gadolinium-enhanced T1WI,
axial and coronal FS T2WI. High-resolution three-dimensional (3D)
imaging of the orbits using a heavily T2-weighted 3D sequence and
diffusion weighted imaging (DWI) are optional but recommended in the
evaluation of retinoblastoma. Such ultrathin-slice images offer superior
spatial resolution compared to conventional thin-sectional MRI and are
particularly useful in the intraocular tumors and optic nerve sheath
complex lesion to include perineural CSF space assessment6. It should be noted
that thin-section high resolution imaging requires longer acquisition
times and is more susceptible to motion degradation necessitating a
fully cooperative or sedated patient. As noted, resolution DWI is often
helpful. Retinoblastoma, for example, typically shows restricted
diffusion (high signal) on DWI and exhibits low ADC values which is in
contrast to high ADC values in the vitreous7. Furthermore, SWI is
helpful in detecting and differentiating blood products from calcium.