INTRODUCTION
Portable ultrasound bladder scanner (PUBS) is widely used to measure postvoid residual volume (PVR) in patients with lower urinary tract dysfunction (LUTD) [1-3]. In PUBS using the traditional motorized scan (TMS) method, bladder volume (BV) is calculated by reconstructing bladder images obtained using a mechanical sector probe in 3 dimensions—a widely adopted approach [4-8]. Owing to their ease of use and minimal training requirements, PUBSs are increasingly used in low- and middle-income countries and resource-constrained environments [9].
Despite widespread use, clinical demand remains for PUBSs that are easier for a single operator to learn and more cost-effective [10]. PUBS using the manual fanning scan (MFS) method calculates BV by reconstructing scanned cross-sectional images of the bladder in 3 dimensions. As it is simpler and cheaper than TMS-type PUBS, it may serve as a viable alternative.
However, to the best of our knowledge, no studies have assessed the measurement accuracy of BV using the MFS method. Therefore, this study aimed to compare the accuracy of PUBS using the MFS and TMS methods in measuring BV.
MATERIALS AND METHODS
Patients
This prospective study involved patients who visited the urology outpatient clinic of the Seoul National University Hospital between October and December 2024 because of lower urinary tract symptoms (LUTS) and underwent a urodynamic study. This study was approved by the Institutional Review Board of Seoul National University Hospital (approval no. 2408-155-1566). The inclusion criteria were age ≥20 years with neurogenic or non-neurogenic bladder dysfunction. The exclusion criteria included conditions that could affect ultrasound measurements, such as pregnancy, abdominal wounds, ascites, or a history of bladder surgery, including cystectomy. The significance level was set at 0.05 with 80% power. A minimum sample size of 32 was calculated, and 40 participants were planned to be recruited, considering an anticipated dropout rate of 20%.
Study Design
The TMS-type PUBS (BioCon-1100S, Mcube Technology, Korea; frequency 3.7 MHz, 120° scan angle) uses a 3-dimensional mechanical sector ultrasound probe. It is configured to mechanically acquire 12 cross-sectional images of the urinary bladder and surrounding structures at 15° intervals using a motor built within the probe. This device extracts the location of the bladder wall from cross-sectional images of the bladder, reconstructs the bladder in 3 dimensions using a unique algorithm, and calculates the BV (TMS-BV). The MFS-type PUBS (BioCon-800, Mcube Technology, Korea; frequency 2.8 MHz, 120°) uses a 2-dimensional sector ultrasound probe operated by manual fanning. The device acquires up to 20 sectoral crosssectional images, reconstructs the entire bladder image in 3 dimensions, and calculates the BV using a unique algorithm (MFS-BV).
BV using PUBS with 2 methods was calculated as follows: Before free uroflowmetry, the patient was placed in the supine position, with the examiner positioned on the right side of the patient. For TMS-type PUBS, the probe was positioned 2–3 cm above the pubic bone from the midline at a 30° angle from the horizontal side of the lower abdominal skin. The probe angle was adjusted to center the bladder image on the display panel. Subsequently, BV was measured by pressing the scan button at the angle where the bladder image was most clearly visible. The total time required to measure TMS-BV in one patient was 20– 30 seconds.
For FMS-type PUBS, the patient laid down in the same manner as for the TMS-BV, with the same examiner on the right side of the patient. The display panel was directed to the right side of the patient, and the MFS probe was positioned in the same manner as the TMS. A prescan confirmed that the bladder image was centered on the display. The probe end was fixed, and the device was slowly fanned from left to right of the patient at a constant speed. Once scanning was complete, the device beeped and the BV was measured. The total time required to measure BV in one patient was 20–30 seconds. For the MFS method, the examiner performed measurements more than 20 times using the phantom before this study to ensure familiarity with the measurement method. Immediately after measuring BV with both methods, the patient voided into a uroflowmetry device in a standing or sitting position to record the voided volume. The patient was then placed in the supine position, and the PVR volume was measured by draining the urine with a 6Fr urethral catheter. The sum of voided and PVR volumes was defined as true volume (T-BV).
Statistical Analysis
To compare the accuracy of BV measured using each PUBS method, the following 4 parameters were analyzed: (1) a paired t-test was used to analyze whether a significant difference was observed between TMS-BV and T-BV and between MFS-BV and T-BV; (2) The percentage of difference in volume (PDV) between TMS-BV and MFS-BV across different BV ranges was calculated and compared using the paired t-test and Wilcoxon signed-rank test. PDV was calculated as (measured BV – T-BV)/T-BV [4]; (3) Bland-Altman analysis was performed to evaluate the agreement between each PUBS method (TMS-BV and MFS-BV) and T-BV. The differences between TMS-BV and T-BV and between MFS-BV and T-BV were calculated. The mean difference of TMS-BV and MFS-BV from T-BV was calculated. The 95% limits of agreement (LoA) were calculated as mean±1.96×mean±standard deviation (SD); and (4) linear regression analysis was performed to determine the correlation between TMS-BV and T-BV and between MFS-BV and T-BV. The results are expressed as mean±SD. Statistical significance was set at P<0.05. All data were analyzed using IBM SPSS Statistics ver. 27.0 (IBM Co., USA).
RESULTS
Forty patients were enrolled in this study, with one excluded due to mechanical issues with the BioCon-800 equipment. Therefore, data of 39 patients (30 males [76.9%], 9 females [23.1%]; mean age, 65.3±14.8 years) were analyzed.
No significant differences were observed between TMS-BV (230.9±157.9 mL) and T-BV (230.6±156.6 mL) (P>0.05), or between MFS-BV (222.6±154.2 mL) and T-BV (230.6±156.6 mL) (P>0.05). Comparison of the PDV between TMS-BV and MFS-BV across BV ranges (0–100 mL, 101–200 mL, 201–300 mL, and >301 mL) showed no significant differences (P>0.05) (Table 1, Fig. 1). Bland-Altman analysis revealed a mean difference of 0.2±50.2 between TMS-BV and T-BV and -8.0±69.1 between MFS-BV and T-BV, with no significant difference (P>0.05) (Fig. 2). The upper and lower LoA were 98.7 and -98.2 for TMS-BV and 127.3 and -143.4 for MFS-BV, respectively. Linear regression analysis demonstrated a strong correlation between TMS-BV and T-BV (R2=0.90; 95% confidence interval [CI], 0.84–1.05; P<0.001) (Fig. 2), and a slightly lower correlation between MFS-BV and T-BV (R2=0.81; 95% CI, 0.77–1.06; P<0.001) (Fig. 3).
DISCUSSION
As the population ages, the number of patients with LUTD increases, resulting in a growing demand for PVR measurements [11]. Since the clinical introduction of PUBSs in 1967, they have been widely used as an alternative to urethral catheterization for PVR assessment [12]. Several studies have shown that calculating BV by reconstructing multiple bladder images in 3 dimensions is more accurate than 2-dimensional estimation using ultrasound [13-15]. Accordingly, TMS-type PUBS devices with built-in motors in the probe have been developed and widely used to obtain bladder images at specific angles and intervals. These devices are well known for their high accuracy in BV measurement [6,16,17]. However, despite their high accuracy, TMStype PUBSs have high initial purchase costs and increased maintenance requirements due to the mechanically complex design.
We conducted this prospective clinical study to evaluate the accuracy of MFS-type PUBS as a potential alternative to TMStype PUBS, considering its cost-effectiveness and enhanced portability. TMS-type PUBS demonstrated a higher correlation with T-BV than MFS-type PUBS. However, the measurement accuracy of BV was similar using both. Specifically, neither TMS-BV nor MFS-BV differed significantly from the T-BV. When BV was stratified into 100-mL increments, no significant difference was observed between PDV for TMS-BV and MFS-BV. However, the R2 value between MFS-BV and T-BV was 0.81, indicating a relatively high correlation. Therefore, MFS-type PUBS can be used as an alternative to TMS-type PUBS for LUTD diagnosis.
In addition to urology, patients complaining of LUTS visit the obstetrics and gynecology, neurology, neurosurgery, rehabilitation medicine, internal medicine, and family medicine clinics. PUBS should be utilized in primary care centers for the early detection of LUTD [11]. Even if the measurement accuracy is slightly lower, a less expensive PUBS can be highly valuable for initial screening of LUTD in such primary care centers [13]. The MFS-type PUBS offers the advantages of being simple and low-cost, making it suitable for practical use in general clinics and developing countries. However, a potential drawback is the learning curve associated with its operation. In our experience, MFS-type PUBS is lighter and simpler than the TMS-type PUBS, and once examiners become familiar with its use, it has the advantage of ease of handling.
As one researcher measured BV using both PUBSs, we could not investigate interobserver variability. Additionally, although BV was measured using PUBS by an experienced independent examiner, we did not investigate the intraobserver variability. Therefore, further research is required to address these issues. Moreover, research on the learning curve of MFS-type PUBS is also warranted.
In conclusion, the correlation between MFS-type PUBS and T-BV was lower than that between TMS-type PUBS and T-BV. However, the accuracy of BV measurements using the 2 methods was similar.
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