Alterations in pelvic floor stiffness have been shown to be involved in various pelvic floor disorders. Although several methods are currently available, objective quantification of pelvic floor stiffness remains challenging. Novel elastography-derived strain metrics may offer a promising approach to assess mechanical properties in specific pelvic floor structures and thereby understanding their relative contribution to pelvic floor disorders. The aim of this study was to provide the first investigation of intra-rater and inter-rater reliability of elastography-derived strain metrics in different pelvic floor structures in asymptomatic women.

Thirty-one asymptomatic women (mean age 26±5.4 years) (i.e. without any pelvic floor disorders such as pelvic pain, incontinence, and prolapse) participated in this reliability study. Pelvic floor structures (puborectalis muscle, transverse perineal muscle, obturator internus muscle, and perineal body) were examined during one testing session using a novel assessment protocol designed by our group. Elastography strain metrics (cumulated major [CMajor] and minor [CMinor] strains) were quantified from 2D radiofrequency ultrasound images (Terason uSmart 3200T NexGen, Burlington, VT, USA) acquired during controlled tissue displacement, as required for strain analysis. To assess the perineal body, transverse perineal muscle, and puborectalis muscle, a linear array transducer (15L4A, 4.0-15.0 MHz) was applied to the perineal region, while passive displacement was generated by inserting a Foley catheter intravaginally and inflating it with 15 mL of water. For the obturator internus muscle, participants were positioned in side-lying, and the ultrasound probe was oriented at the level of the obturator foramen on the side in contact with the examination table. Passive hip internal rotation was then applied to induce the required displacement, with movement ranging from 0 to 30 degrees at a constant slow speed. All measurements were collected independently by two raters. Rater 1 performed two consecutive trials with an interval of a few minutes between the measurements (intra-rater reliability assessment). Rater 2 executed all measurements once (for inter-rater reliability assessment). The order of the raters was quasi-random and the raters were blinded to each other’s results. Image sequences of all assessed structures were recorded as B-mode and radiofrequency cine loops, and data were extracted during post-processing using a customized MATLAB script (The MathWorks, Natick, MA, USA). Pelvic floor structures were outlined on B-mode images, and the marked regions of interest were used to compute CMajor and CMinor [1,2]. Reliability was quantified using intraclass correlation coefficient (ICC) estimates. ICC values were interpreted as follows: <0.20 slight; 0.21–0.40 fair; 0.41–0.60 moderate; 0.61–0.80 substantial; and 0.81-1 almost perfect agreement [3].

For both CMajor and CMinor, intra-rater reliability was almost perfect for perineal body, obturator muscle and puborectalis muscle (ICC 0.81-0.90) and substantial for transverse perineal muscle (ICC 0.61-0.75). Inter-rater reliability measurements were substantial to almost perfect for obturator muscle (ICC 0.80-0.81), moderate to substantial for perineal body and transverse perineal muscle (ICC 0.56-0.66) and moderate for puborectalis muscle (ICC 0.51).

This study showed that elastography-derived strain metrics can be reliably measured. Quantifying strain metrics from individual pelvic floor structures may provide new insights into pelvic floor stiffness, improving our understanding of the role of different pelvic floor structures in the pathophysiology of pelvic floor disorders and supporting the development of more targeted management strategies.

Elastography-derived strain metric analysis may represent a reliable research tool that helps overcome the limitations of current assessment methods.

Nayak R, Schifitto G, Doyley MM. Visualizing angle-independent principal strains in the longitudinal view of the carotid artery: phantom and in vivo evaluation. Ultrasound Med Biol 2018;44(7):1379-1391.
Mercure E, Cloutier G, Schmitt C, Maurice R L. Performance evaluation of different implementations of the Lagrangian speckle model estimator for non-invasive vascular ultrasound elastography. Medical Physics. 2008; 35(7Part1), 3116-3126.
Sainani, K.L. (2017), Reliability Statistics. PM&R, 9: 622-628.