Sentencing Rigby, Mr Justice David MacKay told him:
The jury has convicted you of the murder of this little boy. You don't need me to tell you what a terrible crime that was.
You abused that trust by punishing him physically in quite a revolting way.
It's a chilling, terrible picture. He suffered considerably before he died, I am satisfied. He was a vulnerable boy. You abused your trust.
The judge said he did not think Rigby intended to kill Rio but had a "sudden loss of temper" and "set about him, inflicting the fatal blow".
More top news
The 81-year-old revealed she wasn't asked to follow the hit TV show to Channel 4, and wouldn't have been tempted by the money.
Minister will fail to eliminate the gender pay gap in a generation if it continues to "ignore" evidence to help achieve its goal, MPs warn.
Report criticises ministers for lack of long term plan to recruit and retain teachers in important subjects such as physics and maths.