Approximation of urinary bladder volume with few measurements on cross-sectional imaging

Abstract

BACKGROUND

Accurate estimation of urinary bladder volume (UBV) is important for diagnosing and managing bladder disorders. Computed tomography (CT) is often used as a reference standard; however, existing measurement methods vary in complexity and efficiency.

AIM

To determine whether UBV can be accurately estimated on CT using a single sagittal measurement compared with multidimensional measurements and reference volumetric analysis.

METHODS

CT abdomen-pelvis studies of 80 individuals without urinary tract pathology were retrospectively analyzed. Bladder volume was determined using manual volumetric tracing as the reference standard. Sagittal long axis [sagittal length (SL)], sagittal short axis [sagittal short (SS)], and transverse diameter [right-to-left (RL)] were measured, and single- and multidimensional products (SL, SL × SS, SL × SS × RL) were correlated with reference volume using Pearson correlation coefficients. Linear regression models were derived, and agreement was assessed using Bland-Altman analysis.

RESULTS

SL demonstrated the strongest single-dimension correlation with bladder volume (r = 0.92), compared with SS (r = 0.87) and RL (r = 0.68). Multidimensional measurements showed slightly higher correlations (SL × SS: r = 0.98; SL × SS × RL: r = 0.99). A simplified linear formula was derived: Bladder volume (mL) = 5 × SL (mm) - 222. Bland-Altman analysis showed negligible bias and all values within the limits of agreement for the single-dimension model.

CONCLUSION

UBV can be accurately estimated using a single sagittal CT measurement in healthy individuals. This simplified approach provides a rapid and reproducible alternative to multidimensional methods without meaningful loss of accuracy and may serve as a reference standard for validating ultrasound-based bladder volume estimation.

Key Words: Urinary bladder volume; Computed tomography; Sagittal measurement; Volumetric analysis; Bladder volume estimation; Cross-sectional imaging

Core Tip: Accurate urinary bladder volume (UBV) estimation is essential for clinical decisionmaking but often requires time-consuming volumetric analysis. This study demonstrates that UBV can be reliably estimated on computed tomography using a single sagittal long-axis measurement. A simple linear formula derived from this measurement provides accuracy comparable to multidimensional methods while improving efficiency and reproducibility. This streamlined approach may standardize bladder volume assessment in routine imaging and serve as a practical reference standard for validating ultrasoundbased UBV estimation.

INTRODUCTION

Accurate measurement of urinary bladder volume (UBV) is essential for diagnosing and managing a wide range of bladder disorders. Bladder volume provides critical insights into functional status, helps identify underlying conditions, correlates with bladder wall thickness, and guides treatment decisions. For example, ultrasound-derived bladder wall thickness and bladder weight have shown promise as noninvasive markers of outlet obstruction and detrusor hypertrophy[1].

Abnormal bladder volumes can indicate specific pathologies: Reduced capacity may suggest fibrosis or interstitial cystitis, while enlarged capacity may point to obstruction or neurogenic dysfunction. In women, transvaginal ultrasound measurement of bladder wall thickness has been shown to differentiate detrusor overactivity (DOA) from stress incontinence, with values above approximately 5 mm strongly associated with overactive bladder[2,3].

Monitoring bladder volume also allows clinicians to track disease progression and treatment response[2]. For instance, bladder wall thickness decreases with antimuscarinic therapy, and bladder scanners reduce unnecessary catheterizations, lowering the risk of catheter-associated urinary tract infections[4].

Several methods exist for measuring bladder volume. Urodynamic studies assess bladder pressure and function during filling and voiding, while postvoid residual urine volume – often measured by ultrasound – serves as a key diagnostic parameter for urinary retention, with thresholds around 400 mL guiding catheterization[5]. Accurate volume assessment is also critical in preoperative evaluations and in monitoring long-term bladder health[6].

Given its diagnostic and therapeutic importance, rapid and reliable bladder volume measurement is indispensable. In this study, we evaluate simplified formulae for bladder volume estimation, aiming to identify the most efficient and clinically reliable method.

The aim is to determine whether UBV can be accurately estimated on computed tomography (CT) using a single sagittal measurement compared with multidimensional measurements and reference volumetric analysis.

MATERIALS AND METHODS

Study overview

A total of 80 consecutive individuals were enrolled, with equal distribution by gender and age: (1) 20 females under 50; (2) 20 females aged 50 and above; (3) 20 males under 50; and (4) 20 males aged 50 and above.

All subjects underwent contrast-enhanced CT of the abdomen and pelvis. No pathology had been reported by the radiologist or in the patient’s chart, and none had cystostomy or catheterization. Participants were selected only if there was no clinical suspicion of urinary tract disease at the time of imaging.

Bladder volumes were calculated using CT images. Volumetric analysis involved manual tracing of bladder outlines on the Picture Archiving and Communication System, followed by summation of areas (3 mm slice thickness, every third slice, randomly chosen).

Measurement methodology

Measurements were performed independently to ensure reproducibility. The following diameters were obtained (Figure 1): (1) Sagittal length (SL): Largest luminal diameter on the mid-sagittal plane; (2) Sagittal short (SS): Largest intraluminal diameter perpendicular to the sagittal plane; and (3) Right-to-left (RL): Largest left-right diameter.

Figure 1 Some of the consecutive measurements of urinary bladder volume in a 17-year-old female. A: The sum of the areas (only few demonstrated here) served as the reference test; B: Measurement of sagittal length (sagittal long axis) and sagittal short (sagittal short axis) in another patient.

Composite values were also calculated: (1) Product of SS × SL; and (2) Product of SS × SL × RL.

Statistical analysis

All measurements were confined within bladder boundaries. These calculated values were correlated with bladder volume, and Pearson correlation coefficients were computed to assess the strength and direction of associations.

RESULTS

Subject demographics and bladder volume distribution

The ages of the subjects ranged from 3 years to 89 years, with a mean age of 48 years and a median age of 50 years. The distribution of bladder volumes across participants is illustrated in Figure 2 below.

Figure 2 Urinary bladder volume in the two genders based on reference test (volumetry).

Among females, 90% had bladder volumes less than 280 mL, whereas for males this threshold was 380 mL. Female bladder volumes ranged from a minimum of 5 mL to a maximum of 723 mL, with a median of 110 mL. In comparison, male bladder volumes ranged from 9 mL to 636 mL, with a median of 203 mL.

Single-dimension measurements

Analysis of single-dimension measurements revealed that the RL diameter exhibited the lowest variability, with a range of 72 mm and a standard deviation of 16 mm. This suggests that bladder volume increases the least with changes in RL diameter. In contrast, SL demonstrated the greatest variability, with a range of 122 mm and a standard deviation of 27 mm.

Pearson correlation coefficients for the single-dimension measurements were: (1) SL (sagittal long axis): 0.92; (2) SS (SS axis): 0.87; and (3) RL (RL transverse axis): 0.68.

These values indicate strong correlations, particularly for SL and SS. The correlation curve for SL vs bladder volume is shown in Figure 3.

Figure 3 Reference volume plotted against the largest sagittal diameter of the bladder (sagittal length). SL: Sagittal length.

Volume calculation formula (single dimension)

From the correlation plot, the following formula was derived for estimating bladder volume: Bladder volume (mL) = 5 × SL (mm) – 222. This equation was validated using a Bland-Altman plot comparing measured bladder volumes with those predicted from SL (Figure 4). The analysis revealed a bias of 0.001, indicating negligible systematic error. All values fell within the upper and lower limits of agreement, confirming strong concordance between the two measurements.

Figure 4 Bland-Altman plot largest sagittal diameter vs reference volume. As can be seen, there is no bias in measurements, and all values fall between the limits of agreement.

Two-dimensional and three-dimensional products

Multi-dimensional analysis demonstrated even stronger correlations: (1) SL × SS: 0.98; and (2) SL × SS × RL: 0.99. These results indicate very strong associations between multi-dimensional products and bladder volume. The correlation curve for SL × SS vs bladder volume is illustrated in Figure 5.

Figure 5 Product of largest perpendicular diameters (sagittal length × sagittal short) on sagittal plane against the reference volume.

Volume calculation formula (two dimensions)

From the correlation plot, the following formula was derived: Bladder volume (mL) = 2 × (SL × SS) + 5.7. This equation was validated using a Bland-Altman plot (Figure 6) comparing measured bladder volumes with those predicted from SL × SS. In this analysis, 4 out of 80 cases (5%) fell outside the limits of agreement, indicating minor variability.

Figure 6 Bland-Altman plot product of largest perpendicular sagittal diameters vs reference volume.

Measurement efficiency and accuracy

Although multi-dimensional formulas demonstrated slightly stronger correlations, Bland-Altman analysis confirmed that single SL measurements provide a reliable estimate of bladder volume. Given the simplicity of measuring a single diameter compared to two or three, SL offers a practical and efficient method for clinical assessments without compromising accuracy.

DISCUSSION

The objective of this study was to evaluate the utility of single- and multi-dimensional CT-derived measurements for non-invasive UBV estimation in a healthy population. Our findings demonstrate a high correlation between calculated volumes and manually traced bladder outlines, with SL emerging as a particularly powerful single-dimension predictor of UBV.

The most notable result is the excellent correlation of SL with actual bladder volume (r = 0.92), which was nearly equivalent to the two-dimensional product SL × SS (r = 0.98) and the three-dimensional product SL × SS × RL (r = 0.99).

Single-dimension accuracy and efficiency

The derived linear formula, bladder volume (mL) = 5 × SL (mm) - 222, demonstrated minimal systematic error (bias = 0.001), with all values falling within the Bland-Altman limits of agreement. This supports the hypothesis that, in a non-diseased state, the bladder expands primarily along its longest axis, making SL a highly representative surrogate for overall volume. The simplicity of this linear formula offers a practical, operator-independent method for clinical assessment[7]. This contrasts with the widely used tri-axial ellipsoid model in ultrasound (volume = length × width × height × K, with K = 0.52), which requires three measurements and a correction factor[8]. The CT-derived linear approach thus provides a simplified yet robust alternative.

Multi-dimension complexity vs benefit

Although two-dimensional and three-dimensional products yielded slightly stronger correlations, Bland-Altman analysis of SL × SS revealed that 5% of cases fell outside the limits of agreement. This suggests that the added complexity of multi-dimensional measurements does not substantially improve clinical reliability compared with the single-dimension method[7]. In high-volume clinical settings, efficiency and reproducibility are critical, and unnecessary complexity may increase measurement time and inter-observer variability[9].

The strong correlation between SL and bladder volume has important clinical implications, particularly since CT is often used as the reference standard for validating portable ultrasound scanners[10].

Integration with bladder wall thickness

Integration with bladder wall thickness is a valuable marker in diagnosing bladder outlet obstruction (BOO) and DOA, but its interpretation depends on bladder volume. The bladder wall is thinner when full and thicker when empty[1,2,8]. Accurate UBV estimation from SL allows for volume normalization, potentially refining diagnostic cutoffs, and improving the specificity of integration with bladder wall thickness as a noninvasive marker for BOO and DOA progression[1,2].

Limitations

Several limitations of this study should be acknowledged. First, although the sample size was adequate to demonstrate strong correlations and agreement between simplified measurements and reference volumetric analysis, a larger cohort could theoretically enhance precision and external validity. However, two interim analyses performed during the course of the study, after the inclusion of randomly selected additional cases, demonstrated stable effect sizes and unchanged statistical power, suggesting that further patient accrual was unlikely to meaningfully alter the primary results or strengthen the conclusions.

Second, the study population consisted exclusively of individuals without known urinary tract pathology. Structural abnormalities such as bladder diverticula, marked trabeculation, fibrosis, neurogenic dysfunction, advanced benign prostatic hyperplasia, or bladder fibrosis can cause irregular or spherical deformation[11]. Such changes violate the assumption of symmetrical expansion underlying the SL model. Prior studies have shown that formula-based methods, including the ellipsoid model, lose accuracy when bladder shape deviates from ideal geometry[12]. Validation in diseased populations is therefore necessary before clinical adoption.

Third, the retrospective, singlecenter design may limit generalizability and does not account for variability in bladder filling protocols or patient hydration status at the time of imaging.

Finally, measurements were derived from CT images acquired at a single time point, precluding assessment of intra-individual variability across different degrees of bladder distension. These limitations underscore the need for prospective validation in larger, multi-center cohorts and in populations with structurally abnormal bladders.

Future prospects

Several avenues for future research may address the limitations of the current study and further refine bladder volume estimation methods.

First, translation of the single-dimension estimation concept to ultrasound warrants dedicated investigation. While CT tracing remains the gold standard for bladder volume measurement, ultrasound is the preferred non-invasive, radiation-free tool for bedside assessment and PVR evaluation[4,6]. The concept of reducing the number of required measurements is highly relevant to ultrasound, where accuracy directly impacts management decisions such as catheterization in postoperative retention[5]. The strong linear relationship observed in this study provides a theoretical basis for exploring simplified, single-dimension ultrasound formulas[9].

Second, dedicated studies in patients with pathological bladders are essential. Conditions such as neurogenic bladder, BOO with trabeculation, prior pelvic surgery or radiation, bladder diverticula, and interstitial cystitis may alter bladder geometry in ways that reduce the accuracy of any simplified formula. Disease-specific correction factors or alternative measurement approaches may be required for these populations, and future studies should systematically evaluate formula performance across defined pathological subgroups.

CONCLUSION

This study demonstrates that UBV can be accurately estimated using a single measurement – SL – from CT scans in healthy individuals. The derived linear formula showed strong correlation and agreement with manually traced bladder volumes, supporting its use as a rapid and efficient method for UBV estimation. While this approach enhances efficiency and may serve as a reference standard for clinical and research applications, its applicability to patients with structurally abnormal bladders requires further validation.