Image 1	Image 2
Image 3	Image 4
Image 5	Image 6

Clinical History: Prenatal ultrasound was suspicious for agenesis of the corpus callosum, and a glabellar mass.

Findings: Image 1 and Image 2 : Mid-sagittal SE T1-weighted ( TR=500 ms, TE=12ms) and SE T2-weighted (TR=4500 ms, TE=112 ms) images show an absence of the corpus callosum, cingulate gyrus and sulcus, and a radial orientation of the medial hemispheric gyri down to an elevated third ventricle. Note also the small focus of brain parenchyma protruding through a defect in the inferior frontal bone. The frontoethmoidal encephalocele is covered by normal skin. Image 3 : An axial SE T1-weighted (TR=500ms, TE=12ms) image shows a small frontal encephalocele and colpocephaly (see discussion). Image 4 : The axial SE T2-weighted image shows parallel lateral ventricles related to agenesis of the corpus callosum. Image 5 and Image 6 : Coronal SE T1-weighted (TR=500ms, TE=12ms) and T2-weighted (TR=7000ms, TE=112ms) images show that the corpus callosum is absent with an interhemispheric fissure which extends to the third ventricle.

Diagnosis: Agenesis of the corpus callosum and frontoethmoidal encephalocele
Diagnostic Method: MRI of the brain and clinical and ultrasound findings.

Discussion: The formation of the corpus callosum occurs between approximately 8 and 20 weeks gestation and is often associated with other brain anomalies including the Dandy-Walker malformation, Chiari II malformation, encephaloceles, anomalies of neuronal migration and organization, and midline facial anomalies. The degree of agenesis may be complete, as in the case presented, or partial.

In agenesis of the CC, the axons that ordinarily cross to connect to the contralateral hemisphere, instead form a longitudinal bundle of axons along the medial margin of the lateral ventricles called the of bundles of Probst. These bundles give the frontal horns of the lateral ventricles a cresentic shape in the coronal plane. Due to the lack of periventricular structure normally provided by the tightly packed axons in the CC, the ventricles expand into the softer surrounding white matter resulting in dilatation particularly of the atria and occipital horns, "colpocephaly" ( Image 3). Additionally, the ventricular bodies assume a parallel configuration with respect to eachother rather than concave.. Due to the absence of the corpus callosum, medial hemispheric sulci radiate down to an elevated third ventricle (Images 5 and 6).

Frontoethmoidal cephaloceles, nasal gliomas, and nasal dermoids are congenital midline nasal masses believed to result from a lack of regression of a projection of dura extending through the embryologic foramen cecum.

References:

1. Barkovich AJ, Lyon G, Evrard P. Formation, maturation, and disorders of white matter. AJNR 1992; 13: 447-461.
2. Barkovich AJ, Norman D. Anomalies of the corpus callosum: correlation with further anomalies of the brain. AJNR 1988;9: 493-501.
3. Barkovich AJ, Vandermarck P, Edwards MB, Cogen PH. Congenital nasal masses: CT and MR imaging features in 16 cases. Am J Neuroradiol 1991; 12: 105-116.

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