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.