Novel algorithm for improving accuracy of ultrasound measurement of residual urine volume according to bladder shape.

OBJECTIVES

To suggest a novel algorithm based on individually determined proportionality constants to improve the accuracy of noninvasively calculated residual urine volume using ultrasound bladder images.

METHODS

The residual urine volume of 116 subjects was calculated from ultrasound images obtained using a conventional two-dimensional ultrasound device at two institutions. The proposed method uses individually determined proportionality constants for specific bladder shapes. The corresponding true residual volumes were determined by measuring the catheterized volume. The accuracy of our method was confirmed by comparing the mean and standard deviation of the fractional absolute error (FAE) versus true volume with values obtained using other methods, which rely on fixed constants.

RESULTS

The calculated residual bladder volume of 116 patients correlated significantly (r = 0.94, P <0.001) and the differences from the true values were not statistically significant (P = 0.42). The mean FAE was 0.17 +/- 0.10. Subgroups divided by sex, postvoid residual urine volume (over and under 150 mL), and institution produced different results (mean FAE for men 0.15 +/- 0.11 and for women 0.18 +/- 0.12; mean FAE for greater than 150 mL, 0.16 +/- 0.11 and for less than 150 mL, 0.17 +/- 0.14; mean FAE for Seoul National University Hospital 0.15 +/- 0.10 and for Seoul National University Bundang Hospital 0.18 +/- 0.18). Our method provided better accuracy than other known formulas using a conventional two-dimensional ultrasound scanner. The latter multiply the product of the sagittal height and depth and transverse width by a fixed coefficient.

CONCLUSIONS

Our method, based on individually determined proportionality constants, improved the accuracy of residual bladder volume determinations. The proposed algorithm can easily be implemented on a personal computer connected to a conventional ultrasound imaging unit.