Specific features of each of the transducer parameters condition the range of obtained anatomical information. However, none of the described transducers is appropriate for the assessment of the entire urinary bladder due to endovaginal access and the beam formation perpendicular to the organ. The bladder may be visualized only partially, the range depending on bladder filling. The assessment of the entire bladder has to be performed by transabdominal ultrasound or with the use of endovaginal end-fire transducers used widely in gynecology or urology.

In urogynecology , transabdominal assessment of the bladder is needed for the evaluation of the bladder anatomy, its shape (Fig. 7.4), wall thickness, wall continuity, post-void residual volume, in preoperative assessment, and for the exclusion of anatomical abnormalities that might result in urogynecological symptoms. Occasionally, anatomical disturbances such as urachal cyst (Fig. 7.5), bladder diverticula, or ureterocele can mimic tumors of the bladder, particularly if they are localized in the bladder trigone or bladder neck area, and in such cases they can lead to urge incontinence, lower urinary tract infections/symptoms (LUTS ), or bladder outlet obstruction. In such cases transabdominal assessment of the urinary bladder should be a part of urogynecological assessment, or any other cases with suspicion of bladder abnormalities. Moreover, if during transabdominal scan of the urinary bladder the asymmetry of the lower part of the bladder in the area of bladder trigone is observed, it may be an indirect sign of lateral levator ani defect due to avulsion.

Endovaginal insertion of the transducer can influence the position of examined organs and may limit the reliability of the dynamic studies such as Valsalva and squeeze maneuvers. Stankiewicz et al. proved that in incontinent women with no coexisting POP both ultrasound methods (pPFUS and EVUS) have the same accuracy in the measurements of urethral complex and bladder-symphysis distance (BSD) at rest and during Valsalva maneuver. The study demonstrated that in females suffering from stress urinary incontinence (SUI) and coexisting POP, the endovaginal examination is not reliable in the assessment of the urethral mobility due to alterations of anatomical relations that result from introduction of the transducer into the vagina. Endovaginal approach is more appropriate for detailed assessment of urethral morphology (Fig. 7.6), while pPFUS is the method of choice for dynamic assessment [8].

All described transducers used in EVUS allow proper assessment of ultrasound morphology of the pelvic floor organs, including the urethra and the surrounding structures on different pelvic floor sections and levels as previously defined by Santoro et al. [5, 10]:

Level III: corresponds to the mid-urethra and to the upper third of the anal canal. To facilitate the assessment of the position of these structures, a geometric reference point, termed the “gothic arch,” was defined at 12 o’clock position, specifically at the point where the inferior branches of the pubic bone join at the SP. At this level, the pubovisceral muscle (PVM) (synonymous with the term pubococcygeus/puborectalis muscle) can be completely visualized as a multilayer highly echoic sling lying posteriorly to the anal canal and attaching to the pubic bone.

Level IV: the outer level, the superficial perineal muscles, the perineal body, the distal urethra, and the middle and inferior third of the anal canal can be evaluated. To visualize these structures in their entirety, the reconstructed axial section should be tilted from the most protruding surface of the SP anteriorly to the ischiopubic rami laterally. In the same scan the urogenital hiatus (UGH) can be evaluated.

The following measurements of the urethra may be performed (Table 7.2 [11]) (Fig. 7.7):

In midsagittal plane

In axial plane of the mid-urethra

According to integral theories described by Petros and Ulmsten, between the mid-urethra and the vaginal anterior wall are located the pubourethral ligaments and the suburethral vaginal hammock [1, 2]. This hammock is attached to the endopelvic fascia, and laterally the urethra is limited by periurethral space, including the Retzius vascular plexus, and by the elements of levator ani. In the section when the levator hiatus is well visible, the paravaginal spaces can also be determined, located between the lateral border of the vaginal wall and the medial border of the PVM. Shobeiri et al. demonstrated levator ani subdivisions visible on 3D EVUS at three levels, where level 2 contained the attachment of the pubovaginalis, puboperinealis, puboanalis, puborectalis, and iliococcygeus to the pubic bone [12]. It is possible to explain deterioration of continence with time in terms of age-related connective tissue laxity of the vaginal hammock. In time, improvement in continence after anti-continence surgery can be explained by tightening of the hammock via paraurethral scar contraction [13].

Review of the literature shows significant differences in the assessment of the anatomy of the urethra, its dimensions and volume [4, 5, 11, 14–17]. Santoro et al. in the study performed with the high-resolution 3D EVUS 2050 transducer demonstrated urethral length of 38.2 mm; urethral volume of 3.06 mL; and RS volume of 0.45 mL [5]. The results published by Wieczorek et al. [11], performed with the same type of transducer (2050), reported urethral length of 41.0 mm; urethral volume 4.9 mL; and RS volume 1.2 mL. The difference in RS volume between the Santoro and Wieczorek studies results most probably from different mathematical algorithms used for calculating the volume and also the unclear borders in the sagittal section that may be interpreted differently by various operators. Similar results were also found by Shobeiri et al. in the study on anterior and posterior compartment 3D EVUS with the 8848 probe based on direct histologic comparison; urethral length was reported as 36.0 mm [18]. Additionally, Shobeiri et al. obtained the following mean measurements of other pelvic floor structures: striated urogenital sphincter area 0.6 cm²; longitudinal and circular smooth muscle area 1.1 cm²; urethral complex width 14 mm; and urethral complex area 1.3 cm² (Fig. 7.8). The agreement for visualization of structures was as follows: vesical trigone 96%; trigonal ring 94%; trigonal plate 84%; longitudinal and circular smooth muscle 100%; compressor urethrae 97%; and striated urogenital sphincter 97% [18].

The above results are concordant also with those obtained by Kondo et al., where morphology obtained in transvaginal ultrasonography was confirmed on cadaver specimens. The RS (peripheral zone) thickness was reported in various patients’ groups: continent patients, patients with SUI, and patients with urge urinary incontinence, as 2.78, 2.14, and 2.87 mm, respectively [19]. Above results are also concordant with those obtained by Macura et al. in the MR study on urethral morphology [20]. These results are slightly different to those obtained by Umek et al., who presented sagittal urethral diameter of 8.4 mm if measured in endovaginal access and 11.5 mm measured from endoanal access, urethral volume of 1.6 mL (both anatomical accesses) and RS volume of 0.7 and 0.8 mL, accordingly [16]. In the study performed by Santoro et al. with the 2050 transducer [4] involving the interobserver, intra, and interdisciplinary reproducibility of pelvic floor measurements, the sole urethral dimension measured was urethral thickness. The results were concordant with urethral dimensions obtained in all the above studies [4]. Moreover, as the study focused mainly on the reproducibility of the performed measurements in EVUS among various specialties (urogynecologists, colorectal surgeons, radiologists) and the operators’ different experiences in ultrasound diagnostics, the conclusions from the study were that 3D–EVUS yields reproducible measurements of levator hiatus dimensions and urethral thickness in asymptomatic nulliparous women. Agreement was best where landmark edges were well defined (levator hiatus dimensions) and acceptable where more reader judgment was needed (urethral thickness in the oblique axial plane). Using standardized criteria, the evaluation of these pelvic floor structures appeared to be independent from the different background training of the readers. The method appeared a reproducible technique for urethral measurements [4].

The RS may be also evaluated during intraurethral ultrasound as reported by Frausher et al. This technique provides excellent high-resolution images and allows for real-time visualization of the sphincter mechanism [21]. The RS thickness for patients with urge incontinence and for patients with combined stress and urge incontinence was reported as 3.2 mm.

However, the volumes of RS given generally in the literature seem to be significantly lower than that reported by Digesu et al. [14] and Derpapas et al. [17]. The study by Digesu et al. demonstrated the RS volume of 3.79 mL in patients with successful surgical procedure due to SUI, while in patients with failures 1.09 mL. In the paper by Derpapas et al. the RS volume was 8.88 mL in black females and 5.97 mL in white females. Both studies were performed from pPFUS access: the study by Digesu et al. was performed with the use of sector, endovaginal transducer, and the study by Derpapas et al. with the 3D/4D curved array probe [14, 17].

The differences may result from the various nomenclature of urethral anatomy treated differently by different authors. There are many controversies in female urethral anatomy that have a significant impact for understanding urinary continence [22]. Variability of obtained results among authors may also result from various frequencies of the transducers used, different anatomical accesses, and various patient groups (age, race, BMI, parity, etc.). The differences in urethral morphology and physiology between black and white women have already been reported in the literature [17, 23]. Howard et al. reported that black women demonstrated differences in ultrasonically measured vesical neck mobility during a maximum Valsalva effort compared to white females (blacks = −17 mm vs. whites = −12 mm). Functional and morphologic differences exist in the urethral sphincteric and support system of nulliparous black and white women [23].

Moreover, the diagnostics of such a small organ as the urethra may relatively easily result in variability of the measurements obtained in various techniques among the authors. Moreover, review of the literature concerning 3D diagnostics of the urethral complex shows that most researchers perform only one acquisition in axial plane to obtain the 3D dataset, which may also potentially influence obtaining reliable measurements in all three planes.

Vascularity is one of the major factors contributing to maintaining the normal function of the urethra. The vascularity, particularly the presence of cushioning blood vessels within the submucosa, conditions the normal tension of the urethral mucosal wall [24, 25]. Sphincteric closure of the urethra is normally provided by urethral striated muscle, the urethral smooth muscle, and the vascular elements within the submucosa. Each is believed to contribute equally to resting urethral closure pressure [24]. The submucosal vascular systems become engorged with blood, which causes swelling of the submucosa and decrease in the diameter of the urethral lumen [22]. Furthermore, the vascularity system is also responsible for the proper synthesis of factors that influence surface sealing of the urethral lumen.

Up to now, the assessment of urethral vascularity has mainly been based on selected Doppler parameters (velocity [V], resistive index [RI], pulsatility index [PI]), measured with pPFUS [26, 27]. Siracusano et al. [27] demonstrated the usefulness of color Doppler and spectral Doppler scans in the assessment of urethral vascularization in healthy young women, defining the RI in urethral vessels at three parts of the urethra (proximal, middle, and distal), and reporting an increased RI in the intramural part of the urethra. Also the attempts of the assessment of vascularity after intravenous contrast agent administration were undertaken [27, 28] from the transperineal approach. Siracusano et al. performed the examinations after intravenous application of ultrasound contrast in order to enhance Doppler signals from the urethral vessels, which seemed to generate good results. The application of invasive and relatively expensive diagnostic methods such as intravenous contrast agent administration seems to be unnecessary, considering the recent advances in ultrasound diagnostics, particularly in high frequency endoluminal ultrasonography (12 MHz) in urological practice, which can generate an ultrasound beam that is perpendicular to the urethra and in almost direct contact with the organ, thus enabling vascular assessment [29, 30].