Optimal Diagnostic Indices for Idiopathic Normal Pressure Hydrocephalus Based on the 3D Quantitative Volumetric Analysis for the Cerebral Ventricle and Subarachnoid Space

Abstract

Background and purpose: Despite the remarkable progress of 3D graphics technology, the Evans index has been the most popular index for ventricular enlargement. We investigated a novel reliable index for the MR imaging features specified in idiopathic normal pressure hydrocephalus, rather than the Evans index.

Materials and methods: The patients with suspected idiopathic normal pressure hydrocephalus on the basis of the ventriculomegaly and a triad of symptoms underwent the CSF tap test. CSF volumes were extracted from a T2-weighted 3D spin-echo sequence named "sampling perfection with application-optimized contrasts by using different flip angle evolutions (SPACE)" on 3T MR imaging and were quantified semiautomatically. Subarachnoid spaces were divided as follows: upper and lower parts and 4 compartments of frontal convexity, parietal convexity, Sylvian fissure and basal cistern, and posterior fossa. The maximum length of 3 axial directions in the bilateral ventricles and their frontal horns was measured. The "z-Evans Index" was defined as the maximum z-axial length of the frontal horns to the maximum cranial z-axial length. These parameters were evaluated for the predictive accuracy for the tap-positive groups compared with the tap-negative groups and age-adjusted odds ratios at the optimal thresholds.

Results: In this study, 24 patients with tap-positive idiopathic normal pressure hydrocephalus, 25 patients without response to the tap test, and 23 age-matched controls were included. The frontal horns of the bilateral ventricles were expanded, with the most excessive expansion being toward the z-direction. The CSF volume of the parietal convexity had the highest area under the receiver operating characteristic curve (0.768), the z-Evans Index was the second (0.758), and the upper-to-lower subarachnoid space ratio index was the third (0.723), to discriminate the tap-test response.

Conclusions: The CSF volume of the parietal convexity of <38 mL, upper-to-lower subarachnoid space ratio of <0.33, and the z-Evans Index of >0.42 were newly proposed useful indices for the idiopathic normal pressure hydrocephalus diagnosis, an alternative to the Evans Index.

Fig 1.

Automatic extraction of CSF space. The figures in the top row show the MIP images on the T2-weighted 3D-SPACE sequence in the representative iNPH case. Light green indicates the subarachnoid space segmented automatically at a threshold intensity of >700 on the SYNAPSE 3D workstation. The other figures show the 3D volume-rendering reconstruction images of the subarachnoid space on the second line, total CSF on the third line, and ventricles on the last line. The left, middle, and right column figures show axial, coronal, and sagittal dimensional views, respectively.

Fig 2.

Division of the subarachnoid space into the upper and lower parts. The subarachnoid space was divided into the upper and lower parts in a horizontal section on the anterior/posterior commissure plane at the level of the junction point of the vein of Galen and the straight sinus. The left and right figures show coronal and sagittal views. The Sylvian fissure is typically included in the lower part of the subarachnoid space. The upper-to-lower subarachnoid space ratio was defined as the upper part to the lower part of the subarachnoid space.

Fig 3.

Division of the subarachnoid space into the 4 parts. The subarachnoid space was divided into the following 4 parts: frontal convexity (yellow), parietal convexity (magenta), Sylvian fissure and basal cistern (sky blue), and posterior fossa (light green) in the 3D segmentation. The left, middle, right upper 3D volume-rendering reconstruction images show the axial, coronal, and sagittal dimensional views, respectively. Light green in the left lower axial MIP image indicates the segmented region of the parietal convexity of the subarachnoid space, and that in the middle and right lower MIP images indicates axial and coronal views of the Sylvian fissure and basal cistern.

Fig 4.

Three-directional linear indicators for evaluating the size of bilateral ventricles. The figures are the 3-directional MPR reconstruction images of the T2-weighted 3D SPACE. On the basis of the anterior/posterior commissure line, x-, y-, and z-axes for spatial coordinates of head position were defined. The green lines show the maximum length of 3-axial directions of the bilateral ventricles. The yellow lines show the maximum 3-axial length of the frontal horn of the bilateral ventricles. The red lines show the maximum intracranial 3-axial width. In addition to the original Evans Index, the y- and z-Evans Indices were defined as the maximum length from the foramen of Monro to the anterior and superior extremities of the frontal horns (yellow lines)/the maximum intracranial y- and z-axial length (red lines), respectively. The x-, y-, and z-Maximum Indices were defined as the maximum width of the bilateral ventricles (green lines)/the maximum intracranial width on the each of the 3 dimensions (red lines).

Fig 5.

Receiver operating characteristic curves for discriminating tap-positive from the tap-negative group. The ROC graphs show specificity on the x-axis and sensitivity on the y-axis. The left graph shows the ROC curve of the parietal convexity of the subarachnoid space, the middle one shows that of the z-Evans Index, and the right one shows that of the upper-to-lower subarachnoid space ratio. The black circle points indicate the optimal cutoff points of the maximum area under the ROC curve.