752 quality. Although these suggestions were followed meticu- lously in all patients and the BVM 6500 indicated measure- ments with good quality, invalid measurements were still delivered in 2–14% of patients after sending the data to the central sever computer. Since we proceeded with the blad- der Wlling during the central computer evaluation time span of approximately 2 min, invalid measurements could not be repeated. An integrated evaluation function inside the BVM 6500 without data transfer could have delivered the mea- surement results faster and might solve this problem in the future. Invalid measurements appeared in diVerent men and women indicating that gender or tissue properties were not responsible for missing values. Chalana et al. [12] suggested measuring BWT with the BVM 6500 between 100 and 400 ml of bladder Wlling. However, BWT should be measured according to the oYcial Verathon® handbook for the BVM 6500 device at a bladder Wlling volume between 100 and 300 ml. We evalu- ated BWT at a bladder Wlling of 150 and 250 ml to ensure measuring within the recommended volume range. Auto- matic BWT measurements at lower or higher volumes than recommended might have provided diVerent results. In con- trast to these recommendations, conventional ultrasound BWT measurements are possible at every state of bladder Wlling [14]. Questions regarding the comparability of bladder Wlling volumes between the Wrst and second round of BWT mea- surements might arise. Since BWT is dependent on bladder Wlling until 250 ml [3, 14], diVerences in bladder Wllings could result in diVerent BWT values. We emptied the blad- der completely before starting each urodynamic study, the patients did not drink during the investigations, and the bladders were Wlled at the same speed during both cystome- tries. As urine production is believed to be continuous, we assume that the same amount of urine has been added to the infused bladder volume during the Wrst and second urody- namic investigation. Although the assessment of an exact bladder Wlling volume (e.g. 150 or 250 ml) might not be absolutely accurate, a diVerence during the Wrst and second round of BWT measurements is regarded subtle and there- fore negligible. DiVerences between US and BS measurements could have been attributed to diVerent ultrasound frequencies. We used a dynamic 9-4 MHz ultrasound scanner for US mea- surements, whereas the BVM 6500 device uses a 3.7 MHz scanner. Because the US scanner was located on the skin of the lower abdomen with a distance to the bladder of only a few centimeters, ultrasound frequencies close to 9 MHz were used to image the anterior bladder wall. The resolu- tion of ultrasound images is frequency dependent. Ultrasound scanners of 7.5 and 3.5 MHz have a resolution in the order of 0.13 and 0.3 mm, respectively [14]. The use of higher ultrasound frequencies in future generations of 123 World J Urol (2009) 27:747–753 automatic BWT scanners would improve the resolution and might decrease the diVerence between repeated BWT mea- surements. The variance of BWT measurements at the same patient was lower with US and the lowest at 250 ml; therefore, determination of BWT with US at 250 ml appears to be the most precise technique and volume of all tested. However, we investigated measurement variations only at 150 and 250 ml bladder Wlling; thus, the variance of measurement results might further decrease at bladder Wllings above 250 ml. Thicker anterior bladder walls appeared in patients with bladder outlet obstruction due to benign prostatic hyperpla- sia. This result is in line with previous Wndings [1–5]. Although mean BWT of all patients in our study did not diVer signiWcantly when US was compared with BS, mean BWT of obstructed patients were lower with the BVM 6500 device. There was no patient with bladder outlet obstruction with automatic BWT measurement above 4 mm, whereas Wve patients with bladder outlet obstruction had a BWT between 4.1 and 13.5 mm with conventional ultrasound BWT measurement (mean BWT 2.79 vs. 3.42 mm). Direct comparison of ultrasound images of both techniques in obstructed patients showed that the BVM 6500 measured only the inner part of the anterior bladder wall without including the outer part. The BVM 6500 device does not correctly measure BWT in the individual patient with bladder outlet obstruction and, therefore, previ- ously published results seem to be doubtful [16, 17]. If the investigator could manually change the measurement lines of the BVM 6500 images to indicate the true inner and outer border of the anterior bladder wall, these erroneous measurements could be avoided. Our study was designed to investigate repeatability and accuracy of BWT measure- ments and included only a small amount of patients with benign prostatic hyperplasia. Therefore, the groups of dis- eased patients were too small to investigate validity, diag- nostic accuracy, or clinical usefulness of BWT measurements. A study based exclusively on patients with benign prostatic hyperplasia with or without bladder outlet obstruction should clarify this matter in the future. Conclusions This is the Wrst study on repeatability and agreement of automatic and conventional ultrasound BWT measure- ments. Our study of 50 unselected patients with diVerent types of bladder dysfunction provided evidence of good repeatability for both modalities and agreement between them at both bladder Wlling volumes tested (150 and 250 ml). Repeatability was shown to be improved at higher bladder Wlling volumes for both modalities. However, the