Disorders of migration

1-Migration occurs during the 8th week along radial glial fibers(RGF).

2-Damage of RGF---------Migration disorders.

3-Neurons migrate along radial fibers to reach the cortex, abnormal migration of the neurons is known as Heterotopia which means presence of grey matter in abnormal location.

4-Heterotopia has two main types, the focal type and diffuse type(band Heterotopia---giving rise to double cortex sign). So types of heterotopia are subependymal heterotopia, focal heterotopia and band heterotopia.
5-Focal heterotopia can be differentiated from tumors by giving contrast, in case of focal heterotopia, it does not take contrast as normal cortex, while in case of tumor, it will take contrast. 
6-In case of subependymal heterotopia can also be separated from tuberous sclerosis where in the latter can be calcified(in T2 image it will show a very dark signal if compared to that of the heterotopia), and before calcification it can be enhanced, while in the former, it is a normal cortex in an abnormal site so it will not be calcified or enhanced.
7-In case of heterotopia, it shows the same appearance of all sequences of MRI and do not enhance as if it is a cortex in addition, no edema or mass effect.

Subependymal heterotopia

Band heterotopia

Focal heterotopia