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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