FIG 6. A 20-year-old man with pilocytic astocytoma (WHO I) in the brain stem.

A, Axial T2-weighted spin-echo image (2295/90), shows hyperintense mass involving brain stem without significant peritumoral edema.

B, Tumor strongly enhances on postcontrast axial T1-weighted image (583/15), which suggests a HGGT.

C, Gradient-echo axial perfusion MR image (627/30) with rCBV color overlay map shows no significant perfusion with a low rCBV_T value of 1.14 and rCBV_P value of 0.88, in keeping with a LGGT. Very low rCBV_P value helps to differentiate it from low-grade astrocytomas (WHO II).

D, Time-signal intensity and gamma-variate fitted curves from tumoral (red), peritumoral (blue), and normal (yellow) areas show prominent decrease in signal intensity from tumoral area. Decreased signal intensity in peritumoral area is similar to that of normal gray matter.