Normal Findings

A normal unenhanced head CT scan shows midline position of the third and fourth ventricles and the septum pellucidum/fornix positioned between the lateral ventricles. The bodies of the lateral ventricles need not be symmetrical. Mild differences in the sizes of the frontal, temporal, and occipital horns are common. There should be density discrimination between the white matter (centrum semiovale, corona radiata, internal capsule, and brachium pontis) and gray matter. No focal white matter low densities should be seen in young or middle-age patients. Patients older than the age of 65 may show mild periventricular hypodensities as part of the normal aging process. Ventricular and subarachnoid space size is variable between individuals. In general, the ventricles and subarachnoid spaces are small in younger patients and increase in size with increasing age. In an older patient, the ventricles should not be dilated out of proportion to the subarachnoid space size.

Normally calcified structures are the pineal gland and the glomus of the choroid plexus in the trigone of the lateral ventricle. Calcification may be seen throughout the choroid plexus, including the temporal horns and the outlets of the fourth ventricle. Occasionally, the habenula (directly anterior to the pineal gland) may show calcification. The arteries of older patients are not uncommonly calcified. Calcification of the globus pallidus can be physiological and becomes more prominent with advancing age. Calcification in dural structures, including the walls of venous sinuses, is seen as a normal variant. Excessive calcifications of vascular or dural structures may indicate an underlying pathological process.

Arteries and veins show normal enhancement owing to the higher attenuation of blood containing iodinated contrast material. Lack of a blood-brain barrier and the tight junctions between endothelial cells, allow enhancement of the dural structures (falx and tentorium) and pituitary gland. The choroid plexus in the ventricular system usually shows mild to moderate enhancement owing to its vascular nature. This enhancement can be prominent in the outlets of the fourth ventricle, simulating a vascular lesion.

In evaluation of spine CT, the intervertebral disc is seen as having a higher density than that of the contents of the spinal canal. The spinal canal has relatively low attenuation owing to the presence of cerebrospinal fluid (CSF), particularly in the lumbar region. The nerve roots of the cauda equina may be visualized in the posterior portion of the thecal sac. Roots in the cervical and thoracic subarachnoid space cannot be visualized unless intrathecal contrast medium is present. The intervertebral foramina contain mostly fat, which outlines the dorsal root ganglion and is best appreciated in the lumbar region. In the cervical region, soft tissue density of the nerve root is seen in the inferior neural foramen, whereas in the lumbar spine the root is seen in the superior aspect of the foramen. The osseous structures should be smooth with no encroachment upon the central canal, neural foramina, or lateral recesses. Linear lucencies representing veins may be seen in the vertebral

bodies and are corticated, distinguishing them from fracture lines. The basivertebral venous plexus exits the vertebral body midway between the superior and inferior endplates and has a dorsal calcified cap that should not be mistaken for an osteophyte. The ligamenta flava are visualized particularly in the lumbar spine but should not indent the thecal sac.

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