The efficient calculation of neurosurgically relevant volumes from computed tomographic scans using Cavalieri's Direct Estimator

Objective Because decision making in the daily practice of neurosurgery requires judgments about changes in the volume of spatially complex three-dimensional objects, we sought a simple, rapid, and accurate method for extracting this information from standard computed tomographic (CT) scans. Methods Volume measurements were made by using a transparent template to overlay a grid of regularly-spaced points over CT scans of randomly selected patients with hydrocephalus or malignant gliomas. The volume of an object appearing on a scan is the product of the sum of points that fell on the object, the area associated with each point, and the distance between scan slices. This method is known as the Cavalieri Direct Estimator. We assessed the potential clinical applicability of this technique by measuring interobserver correlation for ventricular volume measurements in hydrocephalic patients, correlating brain tumor volume measurements with those obtained using the ISG Allegro system (ISG Technologies, Inc., Mississauga, Ontario) (a three-dimensional reconstruction software package), and measuring the time required for volumetric analyses. Results Tumor volume estimates using this method correlated highly (r = 0.999) with those made by the ISG Allegro System. Interobserver correlation for ventricular volume estimates was also high (r = 0.968). We found that the time required for application of this technique to CT scans ranged from 4 to 10 minutes. Conclusions We conclude that the Cavalieri Direct Estimator can be easily applied to acquire volume measurements from standard CT scans and requires only a few minutes and no additional expense, making it ideal for daily use in clinical practice.