Levator ani muscle (LAM) injury has been known to be highly associated with the presence of pelvic organ prolapse, and is reported in 13-36% of women after vaginal delivery. Therefore, accurate evaluation of levator ani muscle structure and function would complement generally used clinical tools and might reveal aetiological factors for LAM injury after vaginal delivery. Static and dynamic magnetic resonance imaging (MRI) has been used to evaluate the morphology of the LAM. Diffusion tensor imaging (DTI) with fiber tracking is a useful noninvasive MRI technique, which can describe microstructure characteristics and organization of LAM by means of directionality. The purpose of this study was to investigate the clinical application of DTI fiber tractography in evaluating the LAM injury after first vaginal delivery. It can provide objective evidence for clinical rehabilitation treatment after delivery and prevention of pelvic organ prolapse.Thirty-five primiparous women with six weeks after vaginal delivery and twenty-five age-matched nulliparous women volunteers as control group were included. All women underwent assessment for pelvic organ prolapse quantification (POPQ) before MRI examination, and then underwent axial and coronal fast spin-echo T2-weighted (FSE-T2W) sequence and axial DTI sequence of the pelvic floor with FSE-T2W sequence for anatomical reference using a 3.0 T MR scanner (Ingenia, Philips Healthcare, Best, the Netherlands). Offline fiber tractography of each major levator ani subdivision (pubovisceralis and iliococcygeus) and quality assessment of fiber tracking were performed on ISP V7 workstation (Philips Healthcare, Best, the Netherlands). The resultant fiber track representation of each lecator ani subdivision was independently rated using a four-point scale (good = high-quality representation of the expected anatomical appearance within expected boundaries; sufficient = adequate representation of the expected anatomical appearance, based on fiber orientation, shape and location, but presence of focal tracking distortions and non-tracking; insufficient = non-satisfactory visualization with presence of only a few fiber tracks and/or deviant fiber orientation, not found = not present).¹ With good or sufficient tracking results , fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were calculated. The primiparas women were divided into 2 groups by the existence of pelvic organ prolapse (POP): primiparous normal group and primiparous POP group. Inter-rater reliability of each DTI parameters was evaluated using the intra-class correlation coefficient (ICC). FA, ADC values among the primiparous normal group, primiparous POP group and control group were compared using an ANOVA using IBM SPSS Statistics 21.0 (Armonk, New York, USA) where P < 0.05 indicated a significant difference.Pelvic organ prolapse was found in 20% (7 of 35) of primiparas women. The quality of the 3D representation was judged as good and sufficient for 97.5% (117 of 120) of the pubovisceralis and 7.5% (9 of 120) of the iliococcygeus (Figure 1). Substantial inter-rater agreement was found for FA and ADC of pubovisceralis (ICC 0.66–0.93). No significant differences of FA and ADC values among primiparous normal group, primiparous POP group and control group were observed for pubovisceralis (Table 1).The reported prevalence of prolapse after vaginal delivery was 15.8 %.² Our finding was 20%. The possible reason could be the relatively small sample size in our study. In this study we obtained satisfactory three-dimensional fiber trajectories of the pubovisceralis while the study of iliococcygeus was more difficult. The result was similar to the previous reported study.³ Due to the complex shape, small size, much surrounding adipose tissue and the presence of air in the rectum, and thus provided a low good and sufficient fibers count of iliococcygeus. As DTI has been reported to be valuable in the assessment of skeletal muscle injury,⁴ there was particular interest in the tractability of the LAM in the primiparas women. However, no significant differences of DTI measures among the three groups. Detecting differences in DTI parameters in the LAM is consequently challenged possibly because the relatively small number of fiber tracts and partial volume effects.Twenty percent of primiparas women with six weeks after vaginal delivery had various degrees of POP. DTI with fiber tractography provided satisfactory 3D representation of pubovisceralis which could be potentially used in providing a better understanding of the in vivo levator ani subdivisions as well as their alterations after vaginal delivery.