Direct assessment of levator musculature integrity during pelvic exam is both subjective and difficult. Many have proposed that measurements of levator bowl volume using advanced imaging, may be predictive of pelvic floor muscle function[1,2]. However, these methods require the use of MRI (monetarily and time expensive) to calculate volume. Conversely, transvaginal pelvic floor ultrasound (TVU) is a faster and more readily available method for capturing images of the pelvic floor musculature. A potential drawback when using TVU in this manner is that it captures data in a significantly smaller region. However, if TVU can capture enough of the pelvic floor muscles to accurately calculate levator volume it could potentially be used as a tool to diagnose pelvic floor dysfunction.

The aim of this study was to quantify and compare the volume of the levator ani musculature using both magnetic resonance imaging (MRI) and TVU. While we anticipate that MRI volumes will be larger based on the increased field of view, we hypothesized that a correlation between volumes measured by the two modalities could be established that may demonstrate clinical utility of TVU. Additionally, we intended to map the non-captured landmarks with TVU when compared with MRI.

We performed a prospective cohort study including asymptomatic subjects who volunteered to undergo MRI and TVU of the pelvic floor. All participants underwent a comprehensive interview with completing PFDI-20 questionnaire, pelvic exam including POP-Q evaluation, MRI exam, and TVU. Women who answered “no” to all PFDI-20 questions and had a normal POP-Q exam (stage 0) were included in this study.

Levator bowel landmarks: 
Using Slicer (v. 4.11) each patient’s TVU was translated and rotated such that the pubic bone was aligned with the patients MRI in all three anatomical planes. Once aligned by bony landmarks, five anatomical landmarks were identified on the midsagittal plane of the MRI. The most inferior-posterior point on the pubic symphysis, point along the curvature of the pubic symphysis 1/3 of the long-axis from the distal pubic point (~ the level of the pubovisceral muscle, refer to as the OT point, the most inferior-anterior point on the perineal body, anorectal junction, and ischial spine (projected to the midsagittal plane) were identified. Additionally, the pubococcygeal line, H-line, M-line, line connecting the OT point to the ischial spine (OTIS line), and line connecting the inferior-posterior pubic symphysis to the perineal body point (PSPB line) were drawn. Finally, the length perpendicular to the top of the TVU’s FOV to the PSPB line was also measured. Figure 1 shows all identified landmarks, their context with respect to the MRI segmentation, and the alignment of the MRI and TVU.

Levator bowel volume measurements: 
After all landmarks were identified the levator ani muscles were segmented in both MRI and TVU using Slicer. The segmentations were conducted by one author then reviewed and edited by the remaining authors. The MRI volumes were standardized to the patient anatomy using two methodologies. In the first method we removed any musculature superior to the OTIS line or inferior to the PSPB line. This method allowed for calculation of the volume that was inclusive of the levator muscle attachments. The second method removed any portion of the segmented MRI that were outside the TVU’s FOV, this allowed us to determine whether the ultrasonic probe caused any change in the observed volume or was unable to capture some of the musculature. 
The shapes were then individually imported into Blender (v. 3.0.1) to measure their volume. Using Blender’s built in shrinkwrap function – which allows an object to project itself on to the surface of another object by transforming each vertex of the targeting object to its closest vertex on the targeted object – a cylindrical shape was created and fit to interior surface of the muscles. Then using Blender’s 3D printing toolkit, the volume of the fitted shape was measured.

Twenty subjects were initially recruited for this study. However, one patient became pregnant during the study and was thus excluded. The remaining 19 patients (age = 29.7 ± 8.2, BMI = 24.3 ± 4.5) were included in the final analysis.	
We found the levator volume measured via MRI to be larger than that measured in TVU (46.1 ± 7.9 cm¬3 v. 25.6 ± 6.0 cm3, p < 0.001). However, when the MRI segmentation was limited to the FOV of the TVU, we observed volumes much closer to the volumes observed in TVU (35.5 ± 3.3 cm3 v. 25.6 ± 6.0 cm3, p < 0.001). Although statistically different these two groups of volumes were highly correlated (r2 = 0.808, p < 0.001).

Our findings confirmed that MRI captures all portions of levator bowl while TVU captures ~ 60% of it. After overlaying these two measured volumes, it was shown that the TVU includes all distal subdivisions (levator ani muscles) but misses apical sections of iliococcygeus muscles. This difference plus anal sphincter and muscle bulk behind it that are included in MRI segmentation but missed in ultrasound segmentation explains the ~ 40% smaller volume captured by TVU (Figure 2).

In conclusion, we were able to compare levator volume measured in TVU to levator volume measured using MRI and identify uncaptured landmarks by ultrasound that accounts for the differences. Levator volume is being used as a new metric for evaluating pelvic floor dysfunction symptoms and also will be a valuable variable assessing childbirth related trauma. This study demonstrates that volumes measured via TVU may are easy and safe and also may provide clinical utility associated with this measurement.

Rodrigues Junior AA, Herrera-Hernadez MC, Bassalydo R, et al. Estimates of the levator ani subtended volume based on magnetic resonance linear measurements. Neurourology and Urodynamics. 2016;35(2):199-205. doi:10.1002/nau.22691
Nandikanti L, Sammarco AG, Chen L, Ashton-Miller JA, DeLancey JO. Levator bowl volume during straining and its relationship to other levator measures. International Urogynecology Journal. 2019;30(9):1457-1463. doi:10.1007/s00192-019-04006-8

Continence 2S2 (2022) 100293
DOI: 10.1016/j.cont.2022.100293