Anaplastic astrocytomas are rare brain tumors that occur in adulthood with a peak incidence between 40 and 50 years of age. These tumors tend to present in an older population than low-grade astrocytomas, but a younger population than glioblastomas. 

Anaplastic astrocytomas are WHO grade III lesions, with imaging appearances and prognosis between those of diffuse low-grade astrocytomas (WHO grade II) and glioblastomas (WHO IV), and similarly, they are classified on the basis of IDH mutation as IDH-mutant, IDH-wild-type and NOS (when IDH status is unavailable). The differentiating feature is contrast enhancement. 

Radiographic features on MRI:

Anaplastic astrocytomas appear similar to low-grade astrocytomas but are more variable in appearance and a single tumour demonstrates more heterogeneity. 

The key to distinguishing anaplastic astrocytomas from low-grade tumours is the presence of enhancement which should be absent in the latter (although one should note that variants, especially gemistocytic astrocytomas, can demonstrate enhancement). The pattern of enhancement is very variable. 

Unlike glioblastomas, anaplastic astrocytomas lack frank necrosis, and as such central non-enhancing fluid intensity regions should be absent.  

Features on differently weighted imaging:

T1: hypointense compared to white matter

T2: generally hyperintense but can be heterogeneous in cases with blood calcification

T1 C+ (Gd): very variable but usually at least some enhancement is present. The Precence of ring enhancement suggests central necrosis and thus glioblastoma rather than anaplastic astrocytoma. 

MR perfusion: elevated cerebral blood volume