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With the 2D-phase-contrast technique the volume flow of the CSF via the cerebral aqueduct can be quantified by MRI-means. In this study the stroke volume of CSF via the aqueduct per cardiac cycle (SVcc) is used to measure the extent of the volume flow. Normative values for the SVcc are not yet defined, however, they are indispensable for the clinical utility of this non-invasive method.The aim of the presented investigation is to evaluate, if the interthalamic width of the third ventricle is useful as a reference system for the extent of the SVcc via the aqueduct and if a normal CSF-flow can be defined.Hundred and seven patients (56 female, 51 male; age distribution 8 to 89 years) without clinical or imaging findings of a CSF-flow disturbance were examined on a standard 0.5 T MRI-scanner (Gyroscan, Philips). The measurements of the SVcc via the aqueduct were performed in a single slice perpendicular to the aqueduct in the level of its median third with a retrospective cardial-gated quantitative 2D-phase-contrast sequence. The interthalamic width of the third ventricle was measured in a transversal slice (bicommissural orientation, standard T1-weighted spin-echo sequence) in the level of the upper margin of the tectorial membrane.In 83 patients with a normal heart rate and without any further abnormalities in their imaging studies the SVcc is essentially dependent (r = 0.822) on the interthalamic width of the third ventricle (between 1 and 16 mm). Eleven patients with either a subcortical atrophy without leucencephalopathy, megacisterna magna, Dandy-Walker variant or bradycardia showed a significant increase of the SVcc (p < 0.05). On the other hand a significant decrease of the SVcc (p < 0.05) is seen in 13 patients with either tachycardia, Arnold-Chiari Type-1 malformation, relative aqueductal stenosis and/or severe periventricular leucencephalopathy. These results are in good agreement with the current conceptions on the physiology of the CSF-flow. As the above mentioned criterias of influence have ho pathological significance concerning a CSF-flow disturbance requiring therapy, we used the linear regression with y = B1*× +b0 (b1 = 22.2 ± 2.9; b0 = 43,5 ± 21.1) in all 107 patients to evaluate the extent of the SVcc (y) versus the interthalamic width of the third ventricle (x).This correlation offers the possibility to differentiate a hyperdynamic (above +3 standard error SE), a hypodynamic (below -3 SE) and a normodynamic (between ± 3 SE) CSF-flow via the cerebral aqueduct for the first time. Additional imaging findings and the heart rate must find their influence in the evaluation.