RNs have a variable appearance.
Can be t1 bright from copper, or completely invisible.
The ones you do see as t2 hypointense contain iron are siderotic nodules (t2 hypointense due to t2* effects).
Hcc, as with most tumors, have more water content than background liver, hence the t2 hyper intense signal. Hcc displaces iron in siderotic nodules, hence the conversion from T2 hypo to hyper