Microsoft Word - PNE278BF.rtf

The Neuro Image Quiz, Answers T.P. ThomasP. Naidich Thomas P. Naidich, MD, Division of Neuroimaging, Children’s Memorial Hospital, 2300 Children’s Plaza, Chicago, IL 60614 (USA) This patient has a typical anterior fontanelle der-moid. On Tl-weighted images (fig. la) subcutaneous fat (1) has high signal intensity, so it appears as a bright crescent that defines the scalp. Bone marrow (2) is slightly less bright, and forms a crescent that defines the diploic space of the calvarium. This stripe is interrupted at the anterior fontanelle, the site of the lesion. No marrow stripe is observed in neonates, because their thin calvar-ia lack a diploic space. The outer table (3) and the inner table are cortical bone with few mobile protons. Cortical bone gives no signal, so they appear black. Such absence of signal is termed a signal void. The outer table may be identified specifically, because it lies between the bright fat and marrow stripes. Unfortunately the inner table cannot be identified as easily. On Tl-weighted images, cisternal CSF (4), ventricular CSF and flowing blood also appear black. On a single Tl-weighted image then, one cannot differentiate among bone, CSF and flowing blood; one cannot distinguish the precise contribution of the inner table, superior sagittal sinus (5) and interhemispheric fissure-parasagittal cisterns to the crescentic stripe of low signal that circumscribes the brain. The mass (6) is isodense with brain, so one cannot rule out an encephalocele by signal intensity alone. However, no midline pedicle of brain traverses the low signal stripe between mass and brain, making this diagnosis less likely. Nasal glioma and cysts of other etiology also cannot be ruled out, but they are far less likely choices than dermoid for a lesion situated precisely at the anterior fontanelle. The mass appears to lie within the fontanelle and adjacent bone, deep to the scalp. Posteriorly, the mass bulges over the external table. The thin stripes of high signal marrow are separated from the anterior and posterior poles of the mass by low signal cortical bone. This finding suggests that the mass sits within a well corticated – therefore chronic or congenital – defect in the bone. The deep surface of the mass lies flat against a low signal structure, suggesting that it is tightly applied to either the inner table or the subjacent dural roof of the superior sagittal sinus. With increasing T2 weighting, the signal intensity of fat falls off rapidly, so the scalp appears far less bright (fig. lb). The signal intensity of fluid spaces such as ven-