66 Chapter 3 MaterialsandMethods Study design Patient selection and additional information about the study design are described in Chapter 2. The two observers who reviewed the CEMRA and CTA data of the 75 included patients in the first part of this study (Chapter 2) were asked to assess the “coilability” of the aneurysms which had been detected by CEMRA or CTA, and to measure the size of the aneurysms. Techniques CTA was performed on a two-slice (Elscint Dual; Elscint, Haifa, Israel) or on a four-slice multidetector- row spiral CT scanner (Toshiba Aquilion; Toshiba, Tokyo, Japan). In most cases a semi-automatic bone subtraction method, Matched Masked Bone Elimination (MMBE), was used. MRA was performed on a 1.5 Tesla Philips system using a dedicated head coil (Intera; Philips, Best, The Netherlands). The scan protocol included an ultra-short first-pass CEMRA sequence with concentric k- space filling. All patients underwent catheter DSA examinations (Integris; Philips Medical Systems, Best, the Netherlands). A detailed description of the scan techniques can be found in Chapter 2. Image interpretation and statistical analysis The CTA and MRA data were post-processed and interactively evaluated on a workstation (Vitrea; Vital Images, Minnetonka, Minnesota, USA). Both observers blindly evaluated the images acquired by both modalities, in random order. Quality of images, feasibility of endovascular treatment and diagnostic confidence were used as scoring criteria. Quality of images and confidence in scoring were both rated on a three point scale: ‘poor’, ‘moderate’ and ‘good’. Size was given in millimeters in two directions. Calculations were done on the product of the two measurements. Sensitivity, specificity and ROC curves were calculated to evaluate the capability of each modality to predict the feasibility of endovascular treatment as well as Cohen’s Kappa for interobserver agreement. For ROC curve calculations the confidence scoring was transferred to a negative value if the aneurysm was considered not coilable. The resultant values were used for cut-off points. The actual coilability of the aneurysm during the endovascular coiling procedure was used as standard of reference. In the cases where no coiling procedure was performed, the diagnostic DSA served as standard of reference. The performance of CEMRA and CTA in aneurysm size measurement was described with linear regression analysis and 95% predictive intervals. Interobserver variability was calculated using the Intraclass Correlation Coefficient. DSA findings served as the standard of reference for size measurement. Aneurysm size in the DSA images was estimated by comparison with known diameters of adjacent intracranial arteries.
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