The cerebral veins are formed from tributaries in the substance of the brain, then pass through the subarachnoid space and empty into endothelial lined sinuses of the dura. The visualization of these color-coded veins on Doppler ultrasound examination is a useful sign in differentiating benign extra axial fluid collections (benign external hydrocephalus) from subdural effusions. The differential diagnosis of subdural effusion includes subdural hematoma, either due to trauma or thrombocytopenia; or hygroma or empyema due to meningitis. The clinical setting will frequently help differentiate these possibilities

A subdural hematoma is a posttraumatic collection of serosanguineous fluid between the arachnoid and dura mater. There is considerable pathologic overlap between subdural hygroma, which results from trauma or meningitis. By definition, a subdural hematoma contains hemosiderin, which may be detected pathologically or by MRI. Subdural hematomas result from hemorrhage from torn veins that bridge the subdural space. Infants are particularly vulnerable due to the softness of the underlying unmyelinated brain. This occurs as a result of either accidental or nonaccidental trauma. The cause of the subdural hematoma in the abused child is direct trauma or shaking. 90% of subdural hematomas are supratentorial, and 85% are bilateral. They are second only to hydrocephalus as a cause of an enlarging head in infants and children.

Occasionally, the important distinction between benign external hydrocephalus and chronic subdural hematoma is difficult to make, particularly on CT. Subdural hematoma will demonstrate signal intensity distinct from CSF on MRI, will compress gyri, and will displace cortical veins, which may be particularly well seen on color Doppler ultrasound examination. Findings of benign external hydrocephalus are mild dilatation of the ventricular system and extracerebral CSF spaces, especially over the frontal lobes on cross-sectional imaging. The gyri are not compressed, and on MRI, the fluid collection will follow the signal intensity of CSF on all pulse sequences. Additionally, The cortical vessels pass completely through the collection, which is the key finding on ultrasound.