Compared to the vitreous body, retinoblastoma is moderatelly hiperintensity on T1WI and quite hypointense on T2-WI. The calcified areas appear as markedly hypointense foci on gradienteecho T2WI. Also, the tumour shows moderate to marked contrast enhancement and restricted diffusion on DWI at high b values, with low ADC values. Fig. 3 Fig. 4

Three patterns of growth are recognized: 

1) Endophytic: inwards to the vitreous chamber and may be associated with neoplastic seeding in this cavity.

2) Exophytic: outwards to the subretinal space, possibly associated with retinal detachment and/or subretinal neoplastic seeding.

3) Combined (mixed endophytic and exophytic).

Fig. 5: A- Enhanced mass projecting into vitreous chamber (endophytic pattern) (yellow arrow). B- Subretinal T2 low sign mass (exophytic pattern) causing retinal detachment (green arrow). Note that there is an increase on anteroposterior eye diameter (buphthalmos). C- Both vitreous and subretinal (mixed growth pattern) T1 heterogeneous mass (blue arrow) associated with intraocular haemorrhage.

Focal thickening or irregularity of the choroid may indicate tumoral infiltration and its evaluation should be included. Fig. 6 When present, anterior eye chamber enhancement reflects neoangionesis, which is associated with more agressive tumoral behaviour (larger tumor volume and/or optic nerve invasion). Thickening of the optic disc (prelaminar) and contrast-enhancement of the nerve (postlaminar) are indicators of tumour invasion, whose presence is a definer of patient’s prognosis and management strategies. MRI can predict optic nerve infiltration when there is more than 5 mm of postlaminar contrast-enhancement, which may help surgical planning. However, histopathologic assessment is still the gold standard, because MRI can miss microscopic invasion. If tumor is found in the distal end of the ressected optic nerve, further orbitotomy should be performed to remove residual neoplastic cells.

Orbital extension (extraocular) is considered a metastasis pattern of RB. It may occur through the choroidal vasculature and sclera, or the optic nerve (as mentioned above). Fig. 7 Other forms of metastasis are subarachnoid space seeding, haematogenous dissemination (mainly to the bone and liver) and, more rarely, lymphatic. Brain metastasis, sometimes haemorrhagic, may occur in late stages of the disease. Fig. 8

Trilateral RB refers to the occurrence of unilateral or bilateral ocular RB (always hereditary form) and primitive midline neuroectodermal tumour (PNETs) arising in the pineal region. Fig. 9 Along with the pineal region, tumors may also occur in the suprasellar or parasellar regions, when considered tetralateral. Fig. 10

On the role of MRI, it is important to exclude several pseudoneoplastic lesions that also cause leukocoria in children. Calcification is the most important differentiating feature, because it is found in the great majority of RB cases and is rare in those lesions, which include persistent hyperplastic primary vitreous (PHPV), Coats disease, larval granulomatosis, retinopathy of prematurity and retinal astrocytic hamartoma, in decreasing order of frequency. 

Fig. 11: A– Coats disease. 3-year-old boy presenting with leukocoria and strabismus. MRI axial T1 and T2 images show subretinal detachment and haemorrhage (yellow arrows) without lesion enhacement on contrasted T1 images (green arrow). B– Persistent hyperplastic primary vitreous. 1-year-old boy also presenting with leukocoria. MRI axial T2 FS images show a slightly smaller left globe with a retrolental mass (purple arrow), hyperintense vitreous and a linear vessel along the hyaloid canal, also seen as an enhanced line on axial T1 FS + GD images (persistent Cloquet canal) (orange arrow). C- Coloboma. MRI axial T2/T1/ T1 FS + GD images show a focal discontinuity at the posterior eyeball wall (blue arrows). Embryologically, colobomas are due to failure of closure of the choroidal fissure.

Muscular attachment of the ocular implant is provided by fibrovascularization, which progressively appears as a peripheral contrast enhancement with low-signal on T2WI.