750 12.6 mm (1.5–3.9 mm) at 250 ml bladder Wlling. There was no statistical diVerence between mean BWT values when the same ultrasound method was compared during the Wrst or second measurement either at 150 or 250 ml. All diVer- ences between repeated measurements were distributed evenly around their mean without any evident trend across the respective ranges of BWT measurements (Fig. 1). Mean diVerences of replicate measurements were subtle (¡0.1 to +0.01 mm). Both modalities were repeatable at both blad- der Wlling volumes since no systematic diVerences were observed between replicates. The variation of diVerences between replicates (mean § 2SD) was higher at 250 ml World J Urol (2009) 27:747–753 bladder Wlling with both modalities. However, the variation was generally higher with BS at both bladder Wlling volumes compared to the corresponding variation with US. Comparison between ultrasound measurements obtained by diVerent methods Comparison of US with BS measurements showed that the mean diVerence (US ¡ BS) was 0.23 mm at 150 ml and 0.11 mm at 250 ml bladder Wlling (Fig. 2). Mean diVerences did not diVer signiWcantly from zero (p = 0.362, 0.662 for bladder Wlling volumes of 150 and Fig. 1 Repeatability of bladder wall thickness measurements: Ultra- sound at 150 ml; n = 50 replicates (a) and 250 ml; n = 47 replicates (b), bladder scan at 150 ml; n = 40 replicates (c) and 250 ml; n = 43 replicates (d). The mean diVerence between replicates and their varia- tion (mean § 2SD) are indicated by respective lines. 95% CI of the mean (not shown) are ¡0.21 mm to +0.09 mm (a), ¡0.18 mm to +0.01 mm (b), ¡0.38 mm to +0.18 mm (c) and ¡0.15 mm to +0.17 mm (d). The presence of one outlier is indicated in a (patient 17), b (patient 17) and c (patient 16). The exact position of each outlier 123 in relation to x, y axes (not shown) is (12.37, ¡2.33 mm) in a, (12.08, ¡0.97 mm) in b and (4 mm, ¡4.20 mm) in c. There is no obvious trend of the distribution of diVerences across the range of BWT mea- surements. Both modalities are repeatable at both bladder Wlling vol- umes since no systematic diVerences are observed between replicates; mean diVerences do not diVer signiWcantly from zero (p > 0.05). How- ever, repeatability is improved at the bladder Wlling volume of 250 ml for both modalities (reduced variation)