Axial CT recognition of anteroposterior displacement of fourth ventricle.

The recognition of anterior or posterior displacement of the fourth ventricle on axial computed tomography (CT) has proven to be difficult because the apparent position of this structure is variable and dependent on scanning angle. In most cases direct visualization of a lesion and its relationship to normal anatomic structures allows for the correct assessment. However, in some instances it would be advantageous for lesion localization to be able to identify relatively subtle displacements of the fourth ventricle. This is possible on CT by determining the position of this structure relative to Twining's line (the line between the tuberculum sellae and torcula). The position of the fourth ventricle, tuberculum sellae, and torcula relative to an arbitrary fixed point can be established in virtually all cases, and thus the position of the fourth ventricle relative to Twining's line can be determined. In a control group of 100 patients with normal CT examinations, the ratio of the distance from the tuberculum sellae to the center of the fourth ventricle and the distance from the tuberculum sellae to torcula (Twining's line) was between 0.47 and 0.53. In 66% of the cases the ratio was 0.49-0.51. THere were 54 posterior fossa masses evaluated by this technique. Determination of fourth ventricular position by this method proved to be of particular value in recognizing brainstem glioma, and in determining the site of origin of laterally placed posterior fossa masses.