efishman
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this is reproduced from Medscape in a review of "Imaging in Dandy Walker Malformation" and addresses your question nicely Here is the link
http://emedicine.medscape.com/article/408059-overview
Dandy-Walker malformation is best diagnosed with the help of ultrasonography (US) and magnetic resonance imaging (MRI). US may be the initial examination performed because it can be done portably and without sedation, as well as allowing multiplanar imaging.[19, 20] US, however, is limited because it is heavily operator-dependent. Abnormalities such as the gyral, dural, tentorial, and skull anomalies that accompany Dandy-Walker malformations are not clearly depicted by US.
The introduction of modern imaging techniques, specifically MRI, has radically changed the evaluation of symptoms related to the posterior fossa.
MRI is usually performed for detailed evaluation of Dandy-Walker malformation lesions and complications after the diagnosis is suspected using computed tomography (CT) and US. MRI can best define the relationship between the cyst and the fourth ventricle, and it can detect vermian rotation and the signs of vermian dysgenesis. MRI is relatively expensive. High-quality MRI scans require patient cooperation or sedation.
MRI allows surgeons to accurately view the cerebellum and associated structures, determine which form the malformation has taken, and gauge the progress of the malformation. MRI also demonstrates which space should be shunted first. Recently, MRI has been frequently used for diagnosing fetal craniospinal anomalies.
Since it can distinguish between hydrocephalus associated with Dandy-Walker and hydrocephalus associated with other etiologies, CT scanning is also useful in Dandy-Walker malformation; however, it exposes the infant to ionizing radiation. Clearly distinguishing the subtypes of Dandy-Walker complex on axial CT images is difficult. In addition, evaluating subtle supratentorial pathologies and associated abnormalities on CT scans may not be easy because its routine use is constrained by the axial plane.
The classic abnormal findings of Dandy-Walker malformation described on cranial CT and MRI can also be demonstrated on cranial sonography. US is routinely used during the antenatal period as a screening method, and it is particularly used for postnatal follow-up studies of hydrocephalus. US evaluation of posterior fossa cystic abnormalities in the newborn is best accomplished via a posterolateral fontanelle approach or through the cisterna magna posteriorly.
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