Validation and Initial Application of a Semiautomatic Aneurysm Measurement Software: A Tool for Assessing Volumetric Packing Attenuation

NV Software.

The NV software measurement process was initiated by a few discrete steps (Fig 1). Through a simple user interface, the image region containing the aneurysm was separated manually, and an image threshold was subsequently applied to extract the aneurysm and blood vessels from the surrounding background image. The threshold was semiautomatically set by use of the discriminant analysis method, which guaranteed that the selected threshold optimally separated the image intensities belonging to blood vessels from those that belonged to the background.¹² The optimal threshold was that which corresponded to the maximal value of the Fisher linear discriminant, defined as the ratio of variance between the image intensities of the 2 classes (vessel and background) to the within-class variance.¹³ Use of discriminant analysis to determine the image segmentation threshold avoided the problems associated with varying measurements because of the arbitrary choice of window center and with the varying image quality of different imaging systems.

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Fig 1.

A, A white marker in the aneurysm and 2 markers (blue and green) in the parent artery are needed to start the measurements in NV. The markers are easily placed manually by the user. B, Height, width, neck length, and aneurysm volume are measured semiautomatically with NV. C, To measure VER, we select the type of GDC coils from the drop-down menu. D, Aneurysm VER is displayed on a green-colored meter on the right.

Aneurysm Size Measurements.

Neck length, height, and width were measured by NV. Aneurysm neck length was defined as the largest diameter at the orifice plane of the aneurysm. Aneurysm height was defined as the longest distance from the midpoint of the aneurysm neck length line to the aneurysm wall. Aneurysm width was defined as the largest aneurysm diameter perpendicular to the height line.

Aneurysm Volume Measurement.

NV virtually reconstructed the parent artery after semiautomatic aneurysm removal. The aneurysm neck plane was manually adjusted for complex aneurysm neck shapes through the manipulation of 3 points on the plane. The aneurysm volume was calculated semiautomatically based on the aneurysm neck plane.

Validation Study.

We prepared 4 silicone aneurysm models with known sizes and volumes. The aneurysm models had different shapes: round, with a bleb; conjoined aneurysms (heart-shaped); aneurysm with a daughter sac; and round without bleb (Fig 2).

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Fig 2.

Silicone models.

We compared the ground truth aneurysm volumes with the NV volume measurements and the estimated volumes based on a commonly used approximation formula (AF): [aneurysm volume = (4π/3) × (d₁/2) × (d₂/2) × (d₃/2)], where d₁, d₂, and d₃ are 3 orthogonal measurements denoting aneurysm height, width, and breadth.¹⁴ We calculated the errors of each approach.

Calculation of VER with Use of NV.

Coil volume was easily calculated based on the Guglielmi detachable coil (GDC) diameter and length data provided by the manufacturer (Boston Scientific, Natick, Massachusetts), which were preloaded into the NV software. VER was semiautomatically calculated based on the aneurysm volume and the volume of coils used during embolization.

Patient Population.

To assess the value of the measurement parameters provided by the software, we used NV to retrospectively measure aneurysm volume and VER on 3D DSA images from a cohort selected from our institutional data base of patients with cerebral aneurysms who were treated with coil embolization. A total of 295 patients with 308 cerebral saccular aneurysms underwent endovascular embolization with detachable platinum coils at our institution between January 2006 and August 2008. They included 115 patients with 121 unruptured cerebral aneurysms treated only with GDC; only patients treated with GDC^3,11 were included in this study because only volume information for this coil was available from the manufacturer. Eighteen of the 115 patients were excluded for either refusing to undergo follow-up angiography or only undergoing MRA follow-up (follow-up rate, 84.35%). Thus, our study cohort included 97 patients with 100 aneurysms treated with GDC who underwent follow-up angiography approximately 1 year after the procedure. There were 72 women and 25 men. The mean age of the patients at the time of treatment was 58.85 ± 11.57 years (age range, 31–83 years). Seventy-eight (78.0%) aneurysms were located in the anterior circulation and 12 (12.0%) in the posterior circulation (Table 1).

Embolization of Aneurysms.

Before embolization, patients underwent 3D DSA by use of a biplane flat detector C-arm angiographic system (Axiom Artis dBA; Siemens, Erlangen, Germany). 3D DSA images were reconstructed on a clinical workstation (syngo XWP VB15B; Siemens) at 256 × 256 image matrix size.

The embolization procedure was performed with the patient under general anesthesia. Systemic heparinization was administered as an initial 3000-U bolus, followed by 1000 U/hour under whole-blood activated clotting time monitoring throughout the procedure. In all patients, systemic heparinization was continued for 24 hours after embolization.

Aneurysms were embolized with GDC coils by packing as densely as possible. The percentage of occlusion at the end of the procedure was evaluated by embolized volume. VER was calculated by use of the following formula: VER = (volume of the embolization coils)/(volume of the aneurysm) × 100.

Using the classification schema reported by Raymond et al¹⁵ and Roy et al,¹⁶ we categorized the rate of occlusion immediately after initial treatment as measured on angiographic images as: complete occlusion, when the sac and neck were densely packed in any projection; subtotal occlusion (residual neck), when the sac was occluded but there was suspicion of a neck remnant or there was an obvious tiny neck remnant; and partial occlusion (residual aneurysm), when there was loose packing and/or persistent opacification of the sac or neck remnant.

Angiographic Follow-Up Studies.

Patients were scheduled for follow-up angiography at 1 year after treatment. For the evaluation of follow-up angiographies, the Raymond et al^15,16 definition of aneurysm recanalization was modified. An aneurysm was classified as stable at follow-up if the degree of occlusion was unchanged or had improved (such as further thrombosis) compared with the end-of-treatment study. Recanalization was defined as an increase in the amount of contrast material filling the aneurysm compared with the angiographic appearance at the end of the treatment, and such an aneurysm was classified as unstable because of coil compaction and aneurysm growth.

Using these definitions, we further classified the follow-up results into 3 groups: stable group, including unchanged occlusion or further thrombosis; recanalization group, including any recanalization; and re-treatment group, including recanalization with re-treatment. Aneurysms that had contrast filling at the neck of > 15% of the aneurysm height were selected for re-treatment.

Aneurysm and VER Measurement Protocols.

Radiology technologists used NV to obtain aneurysm size (millimeters), aneurysm volume (millimeters cubed), and VER (percentage) measurements for all 100 aneurysms. Two neurosurgeons who participated in the coil embolization of these aneurysms reviewed the NV measurements for accuracy.

We then compared the VERs of each group: stable group, recanalization group, and re-treatment group. We further divided the aneurysms into 2 groups according to volume and compared the VERs: The large-volume group included aneurysms with a volume of > 200 mm³, and the small-volume group included aneurysms with a volume of < 200 mm³.

We assessed the impact of aneurysm volume and VER on the probability of recurrence and re-treatment for all groups.

Statistical Analysis.

All values were expressed as mean ± standard deviation. The statistical significance of differences in mean value between 2 populations was assessed by use of the Student t test if variances were equal, as determined by F value; otherwise, the Welch t test was used. If population distribution was not a “normal” distribution, we analyzed it by using the nonparametric Mann-Whitney U test.

A receiver operating characteristic (ROC) curve was used to analyze the effect of varying VER, as a discriminating variable, on embolization outcome (eg, re-treatment vs no re-treatment). The ROC area under the curve (and its 95% CI) was calculated to determine the discriminating level of VER.

The Youden index^17,18 (J) was defined as the difference between the true-positive rate and the false-positive rate in the outcome variable. Maximizing this index allowed us to find, from the ROC curve, an optimal cutoff point, independently from the prevalence. J was defined as the maximal vertical distance between the ROC curve and the diagonal or chance line, and it was calculated as J = maximum (sensitivity + specificity −1). With use of this measure, the cutoff point on the ROC curve was selected so that it corresponded to the maximal J.