Specific features of each of the transducers 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, in 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.

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 (TPUS 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. 5.4), while TPUS 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 I: the highest level visualizing the bladder base on the screen at 12 o’clock position and the inferior third of the rectum at 6 o’clock position;

Level II: corresponds to the bladder neck, the intramural region of the urethra, and the anorectal junction;

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 as 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 symphysis pubis (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 5.2) (Fig. 5.5):

According to integral theories described by Petros and Ulmsten between the mid-urethra and the vaginal anterior wall there are 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 Retzius vascular plexus, and by the elements of levator ani. On the section when levator hiatus is well visible also the paravaginal spaces can 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 endovaginal three-dimensional ultrasonography 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. Improvement in continence after anti-continence surgery with time can be explained by tightening of the hammock via paraurethral scar contraction with time [13].

The 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 high-resolution three-dimensional EVUS 2050 transducer demonstrated urethral length of 38.2 mm, urethral volume of 3.06 ml, and rhabdosphincter volume of 0.45 ml [5]. The results published by Wieczorek et al. [11] performed with the same type of transducer (2050) where urethral length was reported at 41.0 mm, urethral volume 4.9 ml, and rhabdosphincter volume 1.2 ml. The difference between rhabdosphincter volume between Santoro and Wieczorek results most probably from different mathematical algorithm used for calculating the volume and also unclear borders in sagittal section which may be interpreted differently by various operators. Similar results were presented also by Shobeiri et al. in the study on anterior and posterior compartment 3D EVUS with 8848 probe based on direct histologic comparison where 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. 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 urethra 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 rhabdosphincter (peripheral zone) thickness was reported in various patients’ groups: continent, patients with SUI and UUI as 2.78 mm, 2.14 mm, and 2.87 mm, respectively [19]. Above results are also accordant to 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 rhabdosphincter volume of 0.7 and 0.8 ml accordingly [16]. In the study performed by Santoro et al. with 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 different experience in ultrasound diagnostics of operators and 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 (LH 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 rhabdosphincter 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 rhabdosphincter 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 rhabdosphincter given generally in the literature seem to be significantly lower than reported by Digesu et al. [14] and Derpapas et al. [17]. The study by Digesu et al. demonstrated the rhabdosphincter 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 rhabdosphincter volume was 8.88 ml in black women and 5.97 ml in white females. Both studies were performed from transperineal (TPUS) access: the study by Digesu et al. with the use of sector, endovaginal transducer and the study by Derpapas et al. with 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 which 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 been already reported in the literature [17, 23]. Howard et al. reported that black women demonstrated difference 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 urethra may relatively easily result in variability of the measurements obtained in various techniques among the authors. Moreover, the review of the literature concerning 3D diagnostics of urethral complex shows that most researchers perform only one acquisition in axial plane to obtain 3D dataset, which also may potentially influence obtaining reliable measurements in all three planes.