Mitchell staging system for avascular necrosis

Class Aosteonecrotic lesion shows signal similar to fat – high on T1, intermediate on T2
Class Bhips show signal of blood or hemorrhage (high on both T1 and T2)
Class Chips show signal of fluid (low on T1, high on T2)
Class Dhips show signal of fibrous tissue (low on T1 and T2)

Peripheral band of low signal intensity seen in all classes. Border visible on T1 in Class A and B and T2 in Class C and D.