Background and purpose Porosity and pore density (PD) are important characteristics of flow diverters (FDs), because they may influence device efficacy and safety. Reliable measurement of these parameters would seem to be required for comparisons between devices, device selection at the time of clinical usage, as well as for research purposes. Because there is no standard method of measurement, our aim was to assess the intra-rater and inter-rater reliability of PD measurements and of three different ways of measuring porosity.

Methods Six microphotographs of two fully deployed FDs were taken overlying two different millimetric reference grids: one flat and the other corrected to match the cylindrical stent. Standardized protocols for independently measuring PD and porosity according to three different methods were used by three trained observers and by the same observer twice. Bland–Altman plots and intra-class correlation coefficients (ICC) were used to study the reliability of the measurements.

Results For porosity, satisfactory agreement occurred only when the same method of measurement was performed by the same observer. Intra-observer and inter-observer agreement were poor for measures of porosity when different methods were used (with differences in the range of 5–10%, ICC <0.6 for all methods). Measurement of PD was more reliable within (ICC 0.991 (0.946 to 0.999)) and between (ICC 0.945 (0.781 to 0.991)) observers.

Conclusions Without standardization, the porosity of different devices cannot reliably be compared because use of different methods or different observers substantially changes results. Pore density seems to be more reliably measured than porosity.