Calcification may be seen in optic nerve sheath meningiomas in 20–50% of cases and hence the tram-track sign may be seen on nonenhanced CT scan images as a linear calcification around the nerve, but this is less common. Optic glioma arises from glial cells within the optic nerve and there is no clear separation between the nerve and the tumor hence the tram-track sign is not seen in optic gliomas. The sign helps distinguish between optic nerve sheath meningioma and optic glioma. It is seen on contrast-enhanced CT scan and MRI images, in optic nerve sheath meningioma. Other theories that have been proposed include preferential flow to the posterior circulation and transtentorial herniation partially relieving the raised intracranial tension, leading to improvement in the perfusion of central structures such as the brainstem.Ī tram-track sign is composed of two enhancing areas of tumor separated from each other by the negative defect of the optic nerve. One of the proposed mechanisms for the appearance seen in the reversal sign is distension of the deep medullary veins secondary to partial obstruction of venous outflow due to raised intracranial tension. It indicates irreversible brain damage and carries a poor prognosis. It represents anoxic/ischemic cerebral injury. It is also known as the ‘white cerebellum sign.’ It is seen in severe head injury, birth asphyxia, drowning, status epilepticus, bacterial meningitis, and encephalitis. This sign is seen on CT scan images and represents a diffuse decrease in the density of the cerebral hemispheres, with loss of gray-white differentiation and a relative increase in the density of the thalami, brainstem, and cerebellum. The lemon sign can also be falsely produced by angling the probe downward and anteriorly to include the orbit.
A mild lemon sign may be normally seen and needs to be differentiated from a true sign. Other cranial markers include microcephaly obliteration of the cisterna magna, with an absent cerebellum and an abnormal anterior curvature of the cerebellum (‘banana sign’). Spina bifida is commonly associated with ventriculomegaly and Chiari II malformations. The sign is not specific for spina bifida and has also been seen in encephalocele, Dandy-Walker malformation with encephalocele, thanatophoric dysplasia, cystic hygroma, diaphragmatic hernia, corpus callosal agenesis, hydronephrosis, and umbilical vein varix. The lemon sign may disappear as gestational age advances and hence it is less reliable after 24 weeks. It has a strong association with spina bifida and is very useful for detecting this condition before 24 weeks of gestation in high-risk patients. This sign represents the loss of the normal convex contour of the frontal bones, with flattening or inward scalloping, seen on a transverse fetal sonogram obtained at the biparietal diameter level. High tentorial insertion in children can simulate the empty delta sign. In such cases, contrast enhancement will show enhancement in the delta. Hyperdense subarachnoid hemorrhage, subdural empyema, or hematoma surrounding the sinus may result in the ‘pseudodelta’ sign on unenhanced CT scan. Both these signs represent clotted blood within the superior sagittal sinus, which is hyperdense on nonenhanced CT scan images. The triangle sign and cord sign are signs of sinus thrombosis on nonenhanced CT scan.
The sign may not be seen in the early stage (less than 5 days) of thrombosis, as the fresh clot is hyperdense, or in the late stage (after more than 2 months), as numerous channels of recanalization develop in the thrombus after 2 months. The use of multislice contrast-enhanced CT scan, with reconstructions into thinner slices in different planes, markedly improves the yield. It is a reliable sign of sinus thrombosis but is seen only in 25–30% of these cases. It is best seen on wider window settings. The sign, seen on contrast-enhanced CT scan images, C], suggests dural sinovenous thrombosis. This sign is created by a nonenhancing thrombus in the dural sinus surrounded by triangular enhancing dura as seen on cross-section.
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