COMPARISON OF REAL-TIME ULTRASONOGRAPHY AND MAGNETIC-RESONANCE-IMAGING IN THE ASSESSMENT OF URINARY-BLADDER VOLUME

Objective To compare estimates of bladder volume obtained by conventio nal real-time ultrasonography with those obtained from magnetic resona nce imaging (MRI) by the Cavalieri method of unbiased stereology. Subj ects and methods The study comprised nine subjects (four men and five women, mean age 23 years, range 18-34) with no history of bladder dise ase. Before micturition, each volunteer underwent ultrasonography, imm ediately followed by MRI. The volunteers then voided, the true voided volume of urine was measured and the imaging protocols were repeated i n the same order after micturition. The bladder volume was estimated f rom ultrasonography using the formulae: volume -0.7 (L x TS x AP), (wh ere L is the maximum supero-inferior diameter, AP the maximum anteropo sterior diameter and TS the maximum transverse diameter) and from MRI using the Cavalieri method. For each imaging modality, the volume of u rine voided was estimated as the difference in the volume estimate bef ore and after micturition. Results The mean percentage coefficient of variation for the estimates of bladder volume by ultrasonography was 2 .17 before and 4.43 after micturition. There was no significant differ ence in the replicate estimates of each bladder diameter by ultrasonog raphy before and after micturition (P = 0.98). The MRI method consiste ntly underestimated the voided volume and the true voided volume was 7 .7 mL and -67.7 mL for the ultrasonographic and MRI estimates, respect ively, which are significantly different (P = 0.02) when assessed usin g a multifactor ANOVA. Further analysis using multiple-range tests sho wed a significant difference between the voided volume estimated by MR I and the corresponding true voided volume. There was no difference be tween the voided volume estimated by ultrasonography and the correspon ding true volume. Conclusion Ultrasonographic estimates of voided volu me were more reliable than the those obtained using the MRI method. Th is is possibly due to a delay between micturition and the acquisition of MR images after micturition, which allowed the bladder to partly re fill with urine. The empirical approach using measurements from ultras onograms provides a fast and reliable technique; ultrasonography remai ns the recommended imaging modality for estimating bladder volume.