Transverse proton attenuation–weighted images illustrating the MR imaging appearance of normal IONs (A, white arrows) and unilateral (B, black arrow) and bilateral (C, black arrows) HOD in patients approximately 4 months after surgery for midline posterior fossa tumors. Note that with this imaging technique, normal IONs (white arrows) are barely recognizable, but in abnormal conditions, their conspicuity is markedly increased, allowing confident identification of HOD in most cases.